Water can severely damage automotive electronics. Immediate functional failure may occur or an
electronic unit may fail in the future. Water has a more detrimental effect on the active components
(integrated circuits - I.C.) versus the passive components (resistors and capacitors). The packaging
used for the I.C. wasn't designed to be submerged in water, and only for moisture resistance with
ceramic type packages and not plastic which is the most common type for non-military applications.
Vehicles such as the 911/964/993/996 with electronics mounted on the body pan are prone to have
water damaged electronics. This is especially true for Cabriolet and Targa vehicles which may have
leaky tops. Also, some late model three series BMWs may incur water damaged electronics because
of water drainage problems for the control unit compartment in the engine firewall.
The water must be removed as soon as possible or the electronic unit generally cannot be repaired
with good long term reliability. The vehicle must not be started before the water is removed and
the unit is repaired. If this procedure is not followed, additional damage to
a unit may occur. As a
preventive measure, the source of water must be diverted and the leaks sealed.
Overvoltaging of automotive electronics can result from an overcharging alternator or a high
capacity battery charger. In most cases the damage is catastrophic to the units. Overcharging
alternators are difficult to avoid. Once an overcharging condition is discovered, e.g. buzzers
going off, bright headlights or eradicate tachs, the alternator or the regulator should be replaced.
When charging a dead battery, the battery should always be disconnected prior to connecting the
charger. The small trickle chargers of less than an amp capacity generally are not a problem.
The most damaging situation occurs when trying to start a vehicle with a dead battery by means
of the large capacity chargers which have vehicle starting capabilities. Always pre-charge the dead
battery or use a jumper battery to start a vehicle with a dead battery. Once the battery is fully
charged, the maximum "key out" current draw should be less than 70ma (.07amps) to have a good
battery after two weeks of non-use.
Also, when checking an alternator output for voltage and current, always begin by testing at the
alternator output terminal and not at the battery because of possible voltage drops in the wiring
from the alternator to the battery. Furthermore, most Bosch alternators require that the alternator
warning light function properly, i.e. light up with the key in the "run" position and be "off" when the
engine starts. If the light stays "on" after the engine starts, then there's a fault in the alternator or the
wiring from the alternator to the battery.
Reverse voltaging of electronic units will result in a catastrophic failure of the units. This usually
occurs when jump starting a dead battery vehicle or replacing the vehicle's battery. Initially, the
vehicle's alternator will be damaged and if the ignition switch is "on" all the vehicle's electronics will
be subjected to the reverse voltage. Some vehicles protect key electronic control units with reverse
voltage prevention relays, but this feature can't be relied on to protect control units from damage.
Only by carefully replacing the vehicle's battery or by properly attaching the jumper battery can
reverse voltage problems be avoided. Always connect the negative lead of the jumper battery to
the engine or the vehicle's chassis and the plus lead of the jumper to the plus lead of the battery.
This reverse voltage problem can severely damage automotive electronics beyond repair. Wiring
harnesses can be burned requiring replacement, which in many cases is very difficult. Also, fires can
be started when this problem occurs.
Vibrations to automotive electronics can be very problematic leading to intermittent control unit
functionality or eventual total failure. A major problem leading to vibration failures results from how
the control unit's circuit boards set relative to the vibration motion. If the circuit boards are positioned
in a plane which is parallel to the vibration motion, the boards are flexed more than if they are mounted
in a plane perpendicular to the vibration motion. This flexing causes the solder connections to crack
and "open", e.g. resulting in an intermittent ECU or relay unit. ECUs which use a cantilever mounting
system are more problematic than systems which eliminate flexing of the ECUs.
This problem occurs in most Porsches and in pre-1990's BMW's, since the control units are mounted
horizontally on the body pans or above the glove compartments. Vibration is not as problematic with
later electronics, as most automotive electronics use surface mount technology (SMT) parts. These
types of parts attach to the circuit boards in a much more reliable method. Large types of components
such as power resistors or relays still present a vibration problem. The mounting method for control
units which include these components can be a source of intermittent problems. Another example of
this problem is the Bosch CDI unit used on the early Porsche 911s which was mounted on a bracket
that transmitted vibrations to the unit resulting in intermittent running problems. A similar intermittent
problem occurs with the Porsche cruise control ECU (911/928) the result of its mounting bracket.
Also the use of a 'daughter' board to modify/tune ECMs results in a less reliable ECM which is more
prone to intermittent type of failures.
Additionally, the larger power control relays, e.g. a 3.2 Porsche 911 DME relay, are affected by the
mounting method (cantilever) and the resulting vibrations which can cause intermittent starting/running
problems. Similar failures occur with early Porsche cab top control units because of the large relays
used and the cantilever mounting used for this ECU also. Relay failures also occur because of contact
oxidation which results from the current level switched over time. This type of problem was common
for the Porsche cab top control unit in the 964/993.
The key engine parameters affecting detonation are; cylinder temperatures, octane levels, engine loads,
and ignition timing. Without knock control systems, which maximize/optimize the ignition system's
timing advance, ignition systems require conservative ignition timing maps to avoid detonation,
i.e. whether via a distributor or an ECM map table. Additionally, the location of the spark plug affects
detonation. A problematic example is the air-cooled Porsche 911 engine where the spark plug is located
offset from the center of the combustion chamber. This offset spark plug location results in an increased
probability of detonation and engine damage, requiring a sub-optimal ignition timing advance reducing
potential torque, and thus requiring even more conservative ignition timing advances whether the engine
is stock or modified, e.g. increased compression ratio (CR).
When the CR is increased to achieve additional torque, and thereby addition horsepower, the ignition
timing must be reduced or the octane level of the fuel must be increased. If an increased octane level is
used to avoid detonation, the selected octane level must take into consideration the worst case cylinder
temperatures and engine loads for the new level of the increased CR. Another option is to reduce the
ignition timing to provide for a greater margin of safety before detonation occurs. This, though, will
reduce the torque and potentially diminish any gain achieved by the CR increase. To minimize the value
of ignition timing reduction needed to avoid detonation and still achieve the benefit of the increased CR,
the cylinder can be twin plugged. This modification will result in ignition timing values that should not
compromise the increased CR benefit, i.e. the torque increase. Thus allowing for the maximum ignition
timing advance, i.e. as near as possible to that prior to the CR increase while avoiding detonation,
which is always the goal in achieving maximum torque and resulting horsepower. As with increasing
the octane level, the worst case engine temperatures and loads must be considered when setting the
maximum ignition timing advance.
Performance enhancements for digital fuel injection systems involve the replacement of an EPROM
chip or the reprogramming (flashing) of an EEPROM chip within the fuel injection unit. The modified
chip data must be determined thru the use of a dynamometer to maximize the torque over the full
RPM range. Just modifying the chip data by using a computer to re-map the chip will usually result in
torque peaks and losses, and thereby usually provides little to no overall performance enhancement.
Generally, most fuel injection map changes without modified intake air flow or exhausts yield very
little in a performance increase. This becomes even less effective when an oxygen sensor is being
utilized. Some performance chips, though, may disable the O2 sensor input to achieve more throttle
response. The only real performance increase results from changes to the ignition maps by advancing
the timing. This usually becomes less effective with fuel injection systems that utilize knock sensors.
An analysis of a number of performance chips' fuel and ignition maps has provided insights into
what actually is modified. All the performance chips analyzed had basically the identical fuel maps
as the stock/original factory chips. The significant differences were the "pushed" ignition maps. Some
performance chips had ignition advance values exceeding 50 degrees, where the maximum BTDC
value for a 911 Porsche should not exceed 40 degrees for octane ratings and fuels available today.
Pinging or detonation can occur for non-knock sensor systems when ignition maps are advanced
beyond a few degrees, or when knock sensor ignition systems are "pushed" beyond the knock control
to achieve the desired performance. This may result in some possible engine damage. Furthermore,
changes to the fuel injection system may result in increased levels of emissions, e.g. CO & NOx.
These new levels can cause catalytic converter problems or cause emissions test failures. Additionally,
systems with OBDII diagnostics may incur additional problems with emissions testing.
When considering the replacement of a stock chip with a performance chip, dynamometer test results
should be provided by the chip supplier of the "before and after" torque curves. Also, the "before and
after" emissions levels should be provided, e.g. the CO and NOx levels. Without this data, evaluating
and using a performance chip becomes much more of a gamble. Additionally, the replacement of stock
fuel injection maps, e.g. a performance chip change, will usually require an increase in the octane level
to avoid pinging. The pinging which occurs may be inaudible, thus causing unknown damage to the
engine which makes using a performance chip even more of a gamble.
Battery Current Drain
The typical vehicle battery current drain should be less than 100 milliamps (.10 amps) and typically
is less than 50 milliamps. This current drain will result in about a two week period without the engine
being started frequently or having an external charger to maintain an adequate charge level for starting.
Some early Porsche climate control units, e.g. Porsche 964/993, had a problem of not fully turning off
once the ignition key was removed, causing the battery to have inadequate capacity to start the engine.
To determine which ECU or what area of the vehicle is causing the excessive drain, the drain must be
monitored by using a clamp-on amp-meter or connecting an amp-meter in series with either battery
lead. Then each fuse of the vehicle is removed and replaced one at a time as the current drain in
monitored. Once the drain problem is localized to one particular fuse, each ECU on that fuse must be
further analyzed as the possible source of the excessive drain.
Some ECUs on a CAN bus system may not fully power down, i.e. enter the 'sleep' mode, and cause
a battery drain problem. The ECU that is causing the drain may not be the actual primary cause,
but failed to power down because another ECU on the bus may be communicating with that ECU,
resulting in the other ECU being the problematic ECU in the excessive battery drain.
Emissions failures result from excessive levels of CO (carbon monoxide), HC (hydrocarbons), or,
NOx (oxides of nitrogen). Some emission regulatory facilities just require static tests (an unloaded
engine) and others require a dynamic emissions test (a loaded engine via a basic dyno). Late model
vehicles ('96 and later - OBDII) require an additional initial test check of the OBDII readiness
states. A failure of the readiness states being complete results in the emissions test being aborted.
The readiness states consist of non-continuous (at startup) and continuous (while driving) tests.
Completion of the tests may require additional driving of the vehicle or a mechanical correction to
the emissions system on the engine. Some non-continuous readiness states can be run using vehicle
A high CO level can result from a bad fuel pressure regulator, a bad air flow meter or air mass sensor,
a performance chip, a bad temperature sensor, or a bad O2 (oxygen) sensor. A HC level failure can
result from a bad fuel injector, a weak cylinder, bad spark plugs or ignition wires, an intake air leak
or a bad O2 sensor. A NOx level failure can result from a too advanced ignition timing (installed
performance chip), a lean fuel mixture, or a weak catalytic converter. The typical values for each are;
CO < 1%, HC < 100 ppm, and NOx < 500 ppm. The level of CO2 (carbon dioxide) which results
from the catalytic converter reaction is a measure of the effectiveness of the catalytic converter.
Typical values of CO2 are 13 to 15 percent.
Lastly, a bad fuel injection unit or ignition control unit may be the cause of any emissions test failure.
The above mentioned possibilities, though, should always be checked before assuming bad control
units or performing other costly repairs.
Diagnostic equipment, e.g. OBDII scanners, data are essential for diagnosing late model vehicle faults,
but can result in costly replacements of non-faulty vehicle components, e.g. MAF sensors, ECUs,
when not supplemented by further diagnostics. Some situations may occur where diagnostics do not
indicate any faults but yet a driver displayed fault may exist, e.g. "Brakes: Do Not Drive". Here and
in most more complex problems, a full understanding of the overall vehicle systems is necessary.
Thus in most situations, further diagnostics are always required besides just reading the DTC and
the replacement of its indicated faulty component.
As an example, the DTC may indicate a faulty MAF sensor because the TRIM value has reached its
limit, but the actual problem may be the fuel pressure regulator. In another situation, the DTC may
indicate a shorted actuator to ground, but the output driver of the ECU providing the DTC may be
the actual problem. Also, assuming that an ECU is faulty because of no CAN communications,
where the CAN gateway may be faulty or another ECU may "hang" the CAN bus, can be misleading.
Thus, the DTC data should never be considered as an absolute in determining the source of the
actual problem. The DTC data should always be considered as a troubleshooting starting point for
further diagnostics to eliminate other possibilities.
DTC data can be supplemented by reading the actual values, i.e. live data, if the diagnostic equipment
has that functionality. A conclusive determination of the faulty component may require the use of a
multimeter and/or an oscilloscope. Comparative data from a known good component can be used.
The "acid test", as always, is the replacement of the potentially bad component with a known good
spare component and the subsequent elimination of the DTC fault. This may not always be possible,
though, e.g. a costly/unavailable component and/or special coding of an ECU. But only when additional
supportive data are determined, should the component be considered as the actual problem source
and then be replaced.
CAN bus communications between ECUs in a vehicle can be problematic for a number of conditions.
First, the bus might not have the proper termination of 120 ohms, measured without power and
between the CAN high and CAN low pins with no ECUs connected to the bus.
Next, one of the ECUs could be 'hung', thereby preventing any data transfer between ECUs.
This requires that the bus be powered-down completely by momentarily removing power to all
the ECUs on the problematic bus. This will allow the ECUs to re-initialize and re-establish
proper communication once power is re-established.
Another condition can occur when an ECU totally fails where either or both CAN lines short
to 5 volts or to ground. When this occurs, the CAN bus loses communication between ECUs.
This condition requires that each ECU be removed one at a time and the bus re-tested to
determine the bad ECU. The differential signal between CAN high & CAN low should be 5 volts,
where CAN high & CAN low are the inverse of one another.
The CAN bus lines may also become shorted, e.g. a metal object piercing the bus wires, or either/both
wires getting shorted to ground or to a main 12 volt supply line. This condition requires that all the
ECUs be removed from the bus and the bus wires analyzed for the problem, e.g. test for 120 ohms.
CAN communications can also fail because of a problem with the central gateway ECU which may
be located in a standalone ECU, the instrument cluster (VW), or in the ignition switch module (M/B).
The central gateway ECU controls access to the various buses from the diagnostic connector (DLC).
If no communication exists with some ECUs, it may be a failure with the central gateway ECU and
not the end systems ECUs.
Encapsulation, i.e. the use of potting compounds, can result in intermittent functionality of electronic
modules as a result of thermal expansion and contraction. This occurs in some aftermarket ignition
modules (Porsche CDI units) where the unit fails at high temperature but functions again once the unit
cools. Encapsulation becomes most problematic for hard setting compounds versus soft compounds
used just to protect the circuitry from moisture or other corrosive elements. Once a module is fully
encapsulated, repair is very difficult to essentially impossible.
Magneti Marelli SAE701A - The BMW M1 ignition unit is an inductive discharge ignition unit which
uses CMOS logic ICs and a bipolar PROM to determine ignition timing digitally. A number of hybrid
circuits are also used, e.g. digital to analog. The unit uses a 120 degree reference signal and a RPM
speed signal. The unit's other input monitors engine vacuum via a MAP sensor. Given the number
of discrete components used verses a micro-controller ignition design, the SAE701A has a higher than
normal failure rate for an electronic ignition unit of the 1980s.
Valeo Clutch ECU - The Mondial T ('93) Valeo clutch ECU (150393) can be problematic if the battery
is jumped, or if a battery charger is used with the ignition key "on" or to start the car. The typical failure
mode is the inability to disengage the clutch via the clutch actuator, not allowing the engine to start.
The ECU may also fail to engage the clutch. The ECU uses a potentiometer to provide the feedback
position of the clutch actuator in the clutch actuation process. Powering the clutch actuator directly when
troubleshooting should always be done with the ECU disconnected or the ECU will be damaged.
Magneti Marelli Dinoplex AEC101 - The Dinoplex ignition unit is a capacitive discharge ignition unit
initially used by Ferrari in the late '60s where spark energy is stored in a capacitor as opposed to an
inductive discharge system typically used in all automotive applications previously. The capacitive
discharge ignition afforded an ignition unit with fast spark voltage rise times increasing the ability to fire
fouled spark plugs. The Dinoplex design is simple in that it comprises a D.C. to D.C. converter charging
a capacitor to about 350 volts, a silicon controlled rectifier (SCR), and various resistors, capacitors,
and a few diodes. Later Dinoplex units added small signal transistors to provide better SCR triggering
and a tach signal. The Dinoplex is vary similar in design to the Mark Ten ignition unit produced by a U.S.
company, Delta Products of Colorado. Given the simplicity of the Dinoplex design, the unit is reliable
and rarely fails, except when the vehicle's battery is charged or jumped incorrectly. The units are not
totally sealed from moisture and water, resulting in reduced reliability.
LIE ECU - The fuel/ignition ECU can have a failure mode where it fails to start its bank of cylinders,
verses the other bank having started immediately. The delay may be as short 10-20 seconds or could last
for 5-10 minutes or more. Once the bank does start, the engine runs normally. If the engine is shutdown,
the lengthy start-up process re-occurs. This problem can be resolved by repairs/modifications to the
LIE ECU PCB circuitry.
Bosch CDI - The aircooled Porsche 911s (1965 - 1998) have numerous electronic problems with
various ECUs. First, the Bosch CDI units used thru the production of the 1983 911SC had failures
because of intermittent operation, over-voltaging alternators, and component failures. Notwithstanding
this, many of the original Porsche CDI units are still in operation after 40 years.
DME ECM - Next, the later 911 3.2 Carrera integrated both the ignition and fuel functions into the
Porsche DME (Porsche Motronic) ECM. This unit was very reliable from the standpoint of internal
component failures, but was unreliable the result of circuit board intermittence. The mounting of this
ECM on the floor of the vehicle resulted in continuous road vibrations being transferred. This location
resulted in the ECM also being susceptible to water damage as was the Porsche Lambda control unit
used in the 911SC. Additionally, the 911 3.2 DME ECM failed the result of an overcharging alternator.
These three failure modes comprised the majority of all the 911 3.2 DME ECM problems resulting in
no-starts and intermittent running conditions. As a result of inadequate and mis-guided troubleshooting
procedures, this Porsche DME ECM was usually assumed as the main source of a running problem,
when the actual problem was elsewhere, e.g. a power problem (DME relay), or a pickup sensor
(speed/ref). As long as the engine always started, the likelihood of the DME ECM being bad was
Cruise Control - The Porsche cruise control ECUs for the aircooled Porsches, like the early Mercedes
Benz cruise controls, were not reliable the result of intermittent circuit board connections. This resulted
in a complete failure to function or the selected speed not being held constant. The main source of the
unreliability was the result of where and how the ECU was mounted. Like the Porsche 911, the Porsche
928 cruise control had reliability problems. It was the same unit used in the 911 thru 1987. The 928 cruise
control ECU had the same failure modes as the 911, e.g. surging and intermittency, and also failed
because of circuit board vibrations. This unit like the early 911 ECU controlled a vacuum actuator versus
a motor in the later 911. The Porsche 944 cruise control ECU which used a motorized actuator also had
intermittent problems, again the result of its mounting and vibrations.
Cab Top - The Porsche cabriolet top (Porsche cab top) ECUs used in the late 911SC thru the 1998
993 have reliability problems the result of intermittent circuit boards also. As with the Porsche cruise
controls, the mounting of this ECU resulted in vibrational stresses being applied to
its circuit board.
The intermittent unit generally caused the cab top not to lock/unlock properly, e.g. requiring multiple
depressions of the open/close switch. Also, an intermittent wiring harness which bends at the top frame
can be problematic. Additionally, the locking motors must each send a non-grounded signal (SL/SR) to
the ECU at the completion of a lock or an unlock cycle, or the opening/closing operation will fail and
in some cases may require multiple depressions of the top switch to fully lock the top in the closed mode.
If the locking motors incur any binding or are not aligned properly, the internal brass gear may break.
The switches (ZL/ZR) close (ground signal) when the cab top reaches the windshield to activate the
locking motors. Once the top is in the middle, it should move up or down independent of the SL/SR
and ZL/ZR switches. In this position, the UP & Down limit switches and the SL/SR & ZL/ZR switches
can be easily tested with an ohmmeter, and the complete functionality of the cab top simulated/tested.
Furthermore, the oil pressure switch (P, 964) and the additional hand brake switch (Hd, 993) inputs
LH ECM - The later Porsche 928 had problems with the fuel injection control unit (928 LH control unit).
This unit generally failed because of being over-voltaged thru the improper use of a battery charger.
Additionally, this unit was poorly designed without any internal over-voltage protection. The failure
resulted in no injection pulse or continual pulsing without the engine cranking, and/or no fuel pump relay
CCU 964/993 - The Porsche 964/993 has a microprocessor controlled climate control unit (CCU) which
is very reliable. The main elements of the system are the control head with its two knobs, two sliders, and
3/4 buttons controlling modes, servo motors to divert air flow and blend cold/hot air, temperature sensors,
and front & rear fans. The typical failure mode of the 964 CCU & the 993 CCU is their failure to shut-
off once the ignition switch is turned-off. If the wiring to the engine temperature sensor or the sensor is
bad, the CCU may fail to shut-off keeping the rear blower on. This can be checked by measuring the
resistance between pin G10 & pin G18 of the CCU. If the resistance is about 1K, the blower will remain
on. If the resistance is about 2K, the blower should be off. The oil cooler temperature sensor should also
be checked by measuring between pins G12 & G18. If the low speed blower resistor for either the engine
fan or the oil cooler fan fails, problematic CCU functioning may occur. Another major problem with the
CCU is a servo motor failure the result of a worn commutator at its end of life, i.e. excessive usage or
binding in its movement. A less common problem occurs when the rear fan fails and the front fans only
operate when the temperature knob is set to fully cold, usually a fuse, the thermal switch, or a bad fan.
All CCU fault codes need to have been reset to prevent abnormal functioning, e.g. cycling of the blower
fans. The early 993 CCU (993 659 047 00) can be used in the 964 with a minor modification (jumping
pins G1 & G19) in some models. The later 993 CCU (993 659 047 01) requires a number of internal
modifications to the CCU to function properly in a 964 and jumping pins G1 & G19 in some cases.
Alarm Unit 993 - The 993 alarm ECU is very reliable with the exception of being over-voltaged from
a battery charger, water damaged, or having a wiring problem. A door lock/unlock unit in each door can
be a problem source. The alarm ECU controls power to the DME ECM preventing the engine from
starting, timing the interior lights upon entry/exit, powering the alarm horn, flashing the lights when
arming/disarming, sensing the doors/trunk/radio/engine hood sensors, and locking/unlocking the doors.
The Porsche diagnostic tools (Hammer/PST2/PIWIS) can be used to determine if any fault codes exit,
and whether any sensor inputs are preventing an arming condition. The alarm ECU has an external fuse
located between its two connectors.
Immobilizer 993 - The 993 immobilizer (993 618 159.02) may be difficult to code for remote control
fob replacement when the ignition switch has aged. Either the buzzer switch or the actual three position
electrical switch may prevent the access to the entry of the four digit immobilizer code. Use of contact
cleaner may facilitate a short term correction of their electrical functionalities. If this fails, either the
buzzer switch or the ignition switch needs to be replaced. Another option is to use an external immobilizer
coder separate from the vehicle. The only purpose of the alarm light is for immobilizer coding. An alarm
unit fault indicated by a double flashing of the door LED will prevent the coding of a remote control fob.
Traction Control 959 - The 959 traction control ECU (959 618 130 00) utilizes a self-test once the
ignition switch is turned on resulting in the ECU turning on all four traction mode lights. If the self-test
fails, a buzzer will sound and the "Traction" light (top light of four mode lights) will turn on at which
point the ECU enters the "Limp Home" mode. The "Central Warning" light will also be turned on. If no
mode light is displayed, an ECU fault exists and the vehicle is in "Limp Home" mode. If the self test
passes, the last traction mode selected will be indicated by its mode light. By using an external diagnostic
tester, a switch box with a connector, one can enter the diagnostic mode and display the fault codes via
a flashing "Traction" light. Fault codes can be erased while in the diagnostic mode after all fault codes are
read by pressing the brake pedal. The typical failure mode occurs the result of incorrect battery jumping
or charging of the vehicle. As a result of this failure, the traction control ECU will fail the initial self-test.
This failure mode may also prevent the reading of any fault codes, i.e. a total Porsche 959 Traction
Control ECU failure. This ECU has the Porsche designation of FCU (Function Control Unit) interfacing
with the motor controller (959 618 131 00) designated the OCU (Output Control Unit) which drives
the pump motors assembly DPA (Double Pressure Actuator). The OCU provides feedback signals
to the FCU as does the DPA.
HVAC 959 - The 959 HVAC (959 653 021 00) typically has a problem controlling the heating water
valve and the auxiliary water pump. This usually results in no control of the cabin temperature, resulting
in excessive heat.
CCU 928 - Additionally, the 928 Porsche had problems with the climate control unit, i.e. mainly the
compressor not being activated. The 928 climate control unit usually was damaged the result of a bad
compressor clutch or an electrical short in the wiring, which damaged the internal relay. This relay also
failed over time the result of excessive oxidizing of its contacts because of being underrated for the
application. Other than this, the 928 CCU had basically no failures. 928 climate control problems were
commonly mis-diagnosed as being the result of a failed CCU, when the actual problems were elsewhere.
When troubleshooting automotive electronics, extreme care must used or the electronics may be
damaged. The use of non-electronic test lights (incandescent bulb types) are to be avoided. Resistance
measurements should always be checked with all power sources removed. Supplying a test voltage
to a control unit input should be done thru an appropriate resistor or a test light to prevent excessive
and damaging currents.
Electronic control units can damaged by improper engine ground connections. Also, all ground
connections for a control unit must be present or an alternate ground path may damage another
control unit. When testing motors, relays, or injectors, control unit output connections should never be
connected to +12 or ground without disconnecting the control unit. The CD ignition's output should
never be tested with any type of test light, only with a scope, nor be subjected to +12 or ground.
Also, fuel injector signals should be tested with a LED type of test light or a scope.
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