Keeping you on the road...                                 With a focus on reliability...  


    About Us        


















- Contact Us -




- Links -

Mercedes Benz






    1. Define the problem in objective terms, e.g. The CO level is too high (5%).
    2. Determine what has possibly changed or happened prior to the problem, e.g.
        Both the fuel pressure regulator & O2 sensor were replaced at the recent 60K service.
    3. Review similar problems which have occurred in the past, e.g.
        A bad vacuum line to the fuel pressure regulator caused a 4% CO problem.
    4. Evaluate all the possible causes, e.g. vacuum line, fuel pressure regulator, O2 sensor,
         temperature sensor, throttle position sensor, fuel injection unit, or mass air flow sensor.
    5. Localize the problem, e.g. Test the O2 sensor voltage & the temperature sensor resistance
         for the proper values and check the connectors & wiring for them.
    6. Substitute a known good unit for the potentially bad unit, e.g. Replace the O2 sensor.
    7. Test for the problem, i.e. Check the CO level.
    8. Repeat the process at step 4, if the problem still exists.

    Typical Sensor Values

    Temperature Sensor - Hot ~ 300-400 ohms, Cold ~ 2,000-3,000 ohms
    Throttle Position Sensor - 0-5 volts
    Air Flow Meter - 0-5/12 volts
    Mass Air Flow Sensor - 0-5 volts
    O2 Sensor - .10-.80 volts
    Throttle Body Switch - Closed ~ 0 ohms, Open > 100K ohms
    Altitude Sensor Switch - Closed ~ 0 ohms, Open > 100K ohms
    Pressure Sensor - 0-5 volts
    Magnetic/Reluctance Pickup (speed/reference) Sensor - 600-1,000 ohms, .10-1.0 volts A.C.
    Knock Sensor - > 100K ohms, ~ 100mv A.C.
    Hall-Effect Sensor - Switch = Closed ~ 100 ohms, Open > 10K ohms - Module = 0/5 volts

    Typical Ignition Coil Values

    Bosch TCI (911 black/silver) - R(primary) = .70 ohms, R(secondary) = 6200 ohms, N = 90,
       L(primary) = 3.6 mH, L(secondary) = 30 H, ~ 60 mjoules @ 6K RPM - 6 cylinders TCI
    Bosch TCI (944 black/silver) - R(primary) = .70 ohms, R(secondary) = 3100 ohms, N = 50,
       L(primary) = 5.8 mH, L(secondary) = 15 H, ~ 70 mjoules @ 6K RPM - 4 cylinders TCI
    Bosch Blue Coil - R(primary) = 3.5 ohms, R(secondary) = 8800 ohms, N = 65,
       L(primary) = 12 mH, L(secondary) = 50 H, ~ 30 mjoules @ 6K RPM - 6 cylinders TCI
    Bosch CDI (small black) - R(primary) = .30 ohms, R(secondary) = 700 ohms, N = 78,
       L(primary) = .20 mH, L(secondary) = 1.2 H
    Bosch CDI (small silver) - R(primary) = .70 ohms, R(secondary) = 720 ohms, N = 63,
       L(primary) = .40 mH, L(secondary) = 1.6 H
    Bosch TCI (Porsche 993) - R(primary) = .70 ohms, R(secondary) = 8200 ohms, N = 90,
       L(primary) = 3.1 mH, L(secondary) = 25.3 H
    MSD Blaster (8202) - R(primary) = .9 ohms, R(secondary) = 5300 ohms, N = 98,
       L(primary) = 4.5 mH, L(secondary) = 43 H
    MSD Blaster (8222) - R(primary) = 1.1 ohms, R(secondary) = 4700 ohms, N = 95,
       L(primary) = 4.2 mH, L(secondary) = 38 H
    Perma-Tune - R(primary) = .90 ohms, R(secondary) = 10000 ohms, N = 125,
       L(primary) = 4.7 mH, L(secondary) = 74 H

    Electronic Diagnostic Tool Use

    Powers/Grounds => incandescent/LED test light, multimeter set to D.C. volts
    Pickup/Ref/Speed/Knock sensors => multimeter set for A.C. volts, oscilloscope
    Hall sensors/switches => LED test light, multimeter set to D.C. volts
    CHT/ECT sensors, contact switches (TS) => multimeter set to ohms
    AFM/MAF/MAP/TPS sensors => multimeter set to D.C. volts, OBDI/II scanner
    O2 sensors => multimeter set to D.C. volts, oscilloscope, ODBI/II scanner
    TCI ignition signal => incandescent test light, oscilloscope
    CDI ignition signal => oscilloscope
    Fuel injector signal => incandescent bulb, LED test light, OBDI/II scanner
    Diagnostic Fault Codes (DTC) => ODBI/II scanner, LED code flasher
    CAN data => ODBII scanner, data bus analyzer

    Initial Checks

    First, check all the battery connections & engine grounds and also the smaller power leads & grounds.
    Then check all fuses in all fuse boxes. Solder all other connections that may have been just crimped.
    Disconnect & clean all connectors, i.e. check for corrosion/oxidation. Next, check the battery voltage
    and make sure that the battery provides an adequate cranking speed for the engine to start. Once the
    engine starts, again check the battery voltage, i.e. the battery voltage should increase once the engine
    starts if the alternator is O.K. Also, make sure that the alternator isn't overcharging.


    Always begin with a simple check for spark (15-25mm) at the main coil or at a cylinder coil.
    The use of carburetor or brake cleaner sprayed into the intake/throttle body can quickly determine
    whether the no-start condition results from an ignition problem or a lack of adequate fuel.
    Next, check for an injection signal with a small injector light. If both of these are present,
    then check for the proper fuel pressure. If both the spark and fuel injection are controlled by
    by the same control unit, e.g. Porsche DME or BMW DME, and either signal is present but not
    both, then mostly likely that control unit is bad. Check that the distributor is installed correctly,
    i.e. correct rotor orientation, and that the spark plug wires are routed to the correct cylinders.

    If any of the above tests fail, then check that both the ignition and the fuel injection control units
    (or the integrated unit) have proper +12 and ground inputs. Vehicles with a check engine light
    generally have an additional continuous +12 input. Main fuel injection relays and fuel pump relays
    can be a problem. The timing signals, engine speed and reference, should be checked for ohms
    and A.C.volts (inductive pickups) or a switching signal if a Hall pickup (a semiconductor diode
    or transistor) is used. The speed and reference sensors sometimes have been reversed in error.
    Some vehicles may have a third signal, e.g. a distributor or cam sensor. These usually must be
    synchronized with the reference sensor.

    Newer (post 1990) vehicles may have only one signal which provides both speed and reference,
    and a distributor or cam sensor which is generally used for sequential fuel injection and/or direct
    ignition systems (DIS). If either the spark or the injector signal is present, then the timing signals
    are most likely present since they are common to both the fuel injection and the ignition systems.
    Then in most cases the other system or the integrated fuel & ignition system has failed, if one
    signal is present but not the other.

    The injectors must have a +12 at one side, as the control unit provides a pulsating ground to the
    other side for operation. Inductive discharge ignition systems (early points ignition and late model
    vehicles)  have +12 at one side of the ignition coil. The ignition signal is usually a pulsating ground.
    Capacitive discharge (CD) types of ignitions (CDI) never have a +12 coil input, only a ground and
    a pulse input. The Bosch CDI unit, e.g. 911 ignition unit, produces a buzzing sound from the DC
    to DC converter when operating, indicating that its power source is present.

    A standard test light can be used when checking for a signal on an inductive discharge system,
    but not for a CD ignition. A scope must be used for this test to prevent CD damage. Use an LED
    type of tester when checking for injection pulses. If trigger points are used as an input for an ignition
    unit or a fuel injection control unit, use a standard test light connected to +12 to check for a pulsing
    ground signal from the trigger points. A weak spark (<15mm) usually results from a bad ignition coil.

    If the above tests are OK, then the problem is probably not electrical. Other inputs such as engine
    temperature, air flow, O2, TBS, i.e. throttle body switch - idle and wide open throttle (WOT)
    switches, or the TPS (throttle position sensor), will generally not prevent an engine from starting or
    attempting to start. If the engine has been altered mechanically, e.g. during a parts removal and
    replacement, the timing from the flywheel, damper pulley, distributor/spark plug wires or other
    mechanical sources may have changed affecting the overall engine timing and thus starting.

    The typical EFI fuel pressure is approximately 2-3 bars. The typical CIS system fuel pressure is
    approximately 5-6 bars. On CIS fuel injection systems, the control pressure set by the warm-up
    regulator (WUR) can significantly affect starting. The CIS-K system with the WUR has a control
    pressure of approximately 1.5 bars cold and reaches about 3.5 bars when hot. These values vary
    based on the engine, e.g. turbo or non-turbo. The CIS-KE system has a differential pressure of
    approximately 1.5 bars cold to .5 bars hot set by the electric hydraulic actuator (EHA) unit.
    The control pressure is checked at the base of the fuel distributor near the pressure regulator
    source line. The system pressure is checked at the top of the fuel distributor near the cold start
    fuel line. Hookup plugs are provided for connecting both pressure test lines.

    In addition to the WUR, the CIS-K unit provides additional fuel enrichment for cold running based
    the oil temperature switch. The fuel accumulator dampens/buffers large fuel pressure demands,
    e.g. during heavy acceleration, and thereby helps to maintain a constant system pressure. A faulty
    accumulator can cause the fuel pressure to reach a very high value under de-acceleration stalling
    an engine.

    Intermittent Running

    A number of intermittent problems may arise from; bad coils, distributor caps with radio inference
    covers (black plastic shields), fuel pressure regulators or accumulators, distributor rotors which
    breakdown to the metal shaft, or timing sensors. Additional problems may result from poor electrical
    connections, e.g. +12 & grounds, or bad relays, e.g. main fuel injection relays (small cube or DME).
    Some aftermarket capacitive discharge (CD) ignition units have become intermittent with varying
    engine temperatures. This may result from units that are filled with a potting compound that can
    cause thermal expansion problems for electronic components and circuit boards. This problem has
    occurred in other types of electronic systems where a potting compound was used.

    As with any engine related problem, always try to isolate the problem to either fuel or spark. Also
    check the potentially problematic part, e.g. fuel pressure regulator, when the intermittent problem
    occurs and not during normal running times. Try and determine how different units and sensors affect
    the engine when it's running good, e.g. disconnect the CIS K/KE Lambda unit, or an O2/temperature
    sensor and monitor the starting/running. Additionally, measure voltages and resistances of possible
    intermittent control units or sensors when the engine is running properly to establish a base value
    to check against when the engine runs poorly.

    Later model vehicles with check engine lights will usually store codes relating to a sensor problem
    or help in diagnosing an intermittent problem. Alarms, either OEM or non-factory units can cause
    problems. Also, an intermittent overcharging alternator may be a problem. Many older vehicles may
    have problems from wiring connections which were not originally soldered but only crimped together.
    These connections must be cleaned and soldered for reliability.

    Generally, most sensors do not become intermittent with the exception of air mass meters or a TBS.
    When very hot, ignition coils or some sensors, e.g. temperature, Hall/magnetic pickup, may become
    intermittent. Typically crank position (reference) sensors, and speed (RPM), and cam sensors have
    fairly high failure rates and are difficult to diagnose.

    Occasionally, the difficult intermittent running problem results from a bad fuel injection or ignition unit,
    as was the case for some early Porsche DME and BMW DME control units. A light taping on the
    bad unit may help determine if it's really bad, i.e. In some cases of an intermittent connection internally.
    Most situations will require a temporary replacement of the control unit to determine the problem,
    which is the ultimate test and generally the only test. The key point about an intermittent problem
    is that tests are only valid when then intermittent problem occurs.

    Poor Running

    A poor running situation is generally less difficult to analyze since testing can be done continuously
    with the engine running versus trying to start an engine or waiting for an intermittent problem to
    occur. Before electrical testing is begun, it is assumed that all pressures and air leaks have been
    checked. Scanners (BMW MODIC, Porsche Hammer/ST2, Snap-On MT2500, & Ottoscan),
    multimeters, or signal scopes can be very valuable in analyzing a poor running condition. Also, LED
    test lights which provide a green LED (ground level) and a red LED (+12V level) can be very
    helpful for checking levels and changing signals in quick troubleshooting. Additionally, a three gas
    analyzer, i.e. CO, HC, & O2, is generally a necessity for a comprehensive diagnosis.

    Specifically, bad spark plug wires or their routing near sensors, spark plug connectors not properly
    pushed on spark plugs, or overcharging alternators can cause strange running problems. O2 sensor
    wires which short together or to ground can cause major problems. Also, high impedance sensor
    inputs such as the O2 sensor input, are easily affected by ignition spark radiation. As in the no-start
    case, improperly timed/aligned flywheels, damper pulleys (e.g. with rubber couplings), or timing
    sensors can definitely affect a running condition. Marginal timing sensors, or their alignment, can
    result in hard starting problems or intermittent poor cold running, inadequate performance, or high
    RPM misfiring, e.g. worn distributor shafts.

    The proper fuel pressure value is very critical for an ideal running condition. Improperly adjusted
    throttle sensors can significantly affect a running condition, e.g. an idle switch not closing. Variable
    camshafts not properly setup can cause a severe idling problem. On CIS fuel injection systems,
    the auxiliary air regulator (AAR) and the WUR can significantly affect the cold idle running. Also,
    check for air intake leaks or leaky vacuum lines on fuel pressure regulators, which may cause a
    poor idle. EGR valves which are open at the wrong engine conditions, e.g. during cold running,
    can cause problems.

    A weak spark may cause problems under load conditions. A strong spark can usually jump about
    15 to 25mm from the coil wire to ground. Also, a weak injector signal the result of a bad +12 at the
    injector or a poor injector pulse can cause a loss of power as could a loss of or drop in fuel pressure.
    Another common running problem results from water and/or corrosion in electrical connectors. These
    conditions can affect the values which input sensors (temperature or TS) provide the fuel injection
    or the ignition unit. As always, bad +12 and ground connections can be major sources of problems.

    A poor running engine may contribute to an emissions test failure. A high CO (carbon monoxide)
    level may result from a number areas, e.g. a bad O2 sensor, an incorrect fuel pressure, a bad
    temperature sensor, or a faulty air flow/mass meter. High HCs (hydrocarbons) may be caused by
    some mechanical problems, e.g. leaky injectors, fouled spark plugs/weak spark, intake air leaks,
    weak cylinders, or an incorrect timing. The HCs will reach a minimum value at the optimum timing.
    High NOX (nitrous oxides) levels usually result from an advanced ignition timing, a very lean mixture,
    or a bad catalytic converter. The O2 sensor has the most effect on the CO value.

    If the CO needs to be adjusted, the internal wheel of an AFM (air flow meter) or the adjusting screw
    of a MAP (manifold air pressure) needs to be adjusted for a CO change over the full range of RPMs.
    The CO adjusting screw/knob only adjusts the CO at idle. Most AMSs (air mass sensors) used on
    late model vehicles can't be adjusted for idle or the full range without major changes to the unit.
    If the CO is less than .5 to 1.0% before the catalytic converter without the O2 sensor and the other
    inputs, i.e. the fuel pressure, the temperature sensor, and the TPS, are correct, then the AFM or the
    AMS is potentially bad. The off-idle CO should be checked at 2000 to 2500 RPMs.

    On the CIS system, the mixture adjustment affects the full range and must be adjusted after proper
    WUR setup and after the control pressure reaches the value set by the normal operating temperature.
    The CO is typically set to 1.0% - 1.5% at 2000 RPMs before the catalytic converter without the O2
    sensor by means of a gas analyzer. The CO level usually falls to less than .20% with the O2 sensor.

    Vehicles with OBDII ('96 and later) have extensive fuel injection diagnostics and monitoring systems
    to determine the effectiveness of the emissions controls. These include; the functioning of the catalytic
    converter, the O2 sensor, the adaptation of the fuel injection unit to parameter changes, e.g. air leaks
    or fuel pressure, and the functioning of the secondary air injection system. The OBDII standards
    require that the fuel injection system go thru a self-test at engine startup and cycle thru six readiness
    modes based on certain driving conditions over various time periods. Some late model vehicles,
    e.g. 2003, may have 15 or more readiness modes.

    Once this cycle is completed, readiness "flags" are set indicating that the vehicle is in an acceptable
    state for an emissions test. If the fuel injection system is reset because of an existing check engine light
    or because of an impending fault code, the readiness process must be re-initiated requiring two driving
    cycles, "trips", about 20 minutes each. No indication of the readiness status is provided to the driver.

    A bad catalytic converter can cause many problems especially with late model vehicles that have two
    O2 sensors. Early vehicles that have a bad catalytic converter will lack performance or run/rev poorly
    above idle. Vehicles with two or more O2 sensors can have additional problems such as intermittent
    fault codes indicating cylinder misfires on a bank of cylinders. This can occur during hard acceleration
    versus idle to light acceleration conditions. A problematic catalytic converter may also be indicated by
    an OBDII readiness code not being complete after the proper driving interval.

    Lastly, it's always possible that the fuel injection unit or the ignition control unit may be the problem.
    Again, a light taping on each unit may help determine if either is the source of the poor running
    condition, i.e. because of possible intermittent connections internally. Given the complexity of most
    control units, determining a poor running problem the result of a control unit, even by use of a scanner,
    is difficult. Usually, the control unit must be replaced temporarily with another unit.


copyright 2017 Systems Consulting