Mercedes 126 Repair – A Great Idle Saga

One of the most common Mercedes 126 repair challenges is a big idle. Generally, the mechanically simple mechanism of the culprit electronic, but sometimes the throttle, solves the problem. Of course, older engines are likely to suffer from a variety of suction vacuum leaks, and these can also cause idle problems. Let's look at another vacuum leak in another article.


  1. The most common cause of errors is the idle control module behind the false bulkhead next to the ABS control unit. The module processes various inputs and controls the voltage sent to the idle control valve, which physically controls the amount of air delivered to the intake holes when the throttle is closed. It is useful to have a known work unit available for diagnostic purposes. Although there is no area on the circuit board that can be referred to as a common place for cracked soldering, there are four capacitors that are prone to die with age. New units will cost more than $ 200 and reinstalled units are expensive. I strongly suspect that in most cases all rewriters will replace capacitors.
  2. The idle control valve can cause two problems. Electromagnetic solenoid may be defective, or more often the valve will be weakened with greasy deposits. (Part of the ventilation smoke from the crankcase was pumped on the valve.) Good soaking with the carbohydrate cleanser usually eliminates this disease. Be careful not to expose the valve's electrical part to the solvents.
  3. Very rarely, there is a problem with the lambda control unit, the closest thing to these cars is an ECU. The idle control unit is connected to the lambda controller and "sees" the signals. Generally, if there is a problem with the lambda control, the engine is malfunctioning (due to incorrect fuel mixes). If the lambda controller does not work at all, the most common cause is the surge protection relay (OVP) in the fuse box. You may also have a poor ground connection. The unit itself seems incredibly reliable, probably because it is in a cooler climate – beside the passenger legs.
  4. If the problem occurs only when A / C is used, a delay relay in the fuse box may be encountered, which may cause a failure of the start blocking switch increases the engine speed when the compressor is switched on,
  5. Finally, the throttle position switch (TPS) on the side of the throttle body may be defective. This is extremely unusual, so we should be grateful because replacing TPS requires removal of the entire intake manifold. Sometimes the switch can simply be unlocked and can be brought back to the correct position with a long position

TPS informs the idle control unit when the throttle valve is closed, partially open or open. Electric continuity exists on a pair of three pins, another pair, or not at all. If the correct signals are not sent, the switch itself fits perfectly; the problem may simply be that throttle connector is set to . The switch is extremely sensitive; even the smallest opening of the throttle valve is sufficient to interrupt continuity on the correct taps. So it does not require excessive pressure from the connection to disrupt the system. Make sure the throttle actually closes the stop.

Another fundamental factor to consider – and which is negligible in this context – is the ignition timing. Too many ignition advances will definitely affect the idle speed. The Gen I cars have great potential for adaptation, and the engines can look forward to pinging before they begin to explode. Ten-degree initial improvement is optimal. Keep in mind that the vacuum forwarding unit results in about 16 degrees forward movement in the idle (when the manifold vacuum is high). The preparation unit may cause problems, the main one being the abrasion of the inner membrane and the formation of vacuum leakage at this point

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