Mercedes 126 Repair – The high saga in slow motion


One of Mercedes 126's most common repair challenges is a high idle. Generally, the culprit is electronic, but sometimes a simple mechanical adjustment of the throttle linkage will solve the problem. Of course, older engines are likely to suffer from variable gravity inlet vacuum leaks, and these can also cause idle problems. We will examine the vacuum leaks in another article


  1. The idle control module, located behind the false partition next to the ABS control unit, is the most common cause frequent problems. The module processes a variety of inputs and controls the voltage sent to the idle control valve, which physically regulates the amount of air supplied to the intake ports every time the valve is released. throttling is closed. It is certainly helpful to have a known unit of work available for diagnostic purposes. Although there is not a single area of ​​the circuit board that we can designate as a frequent location of cracked solder joints, there are four capacitors that have a tendency to die with it. ;age. The new units cost well over $ 200 and even the rebuilt units are quite expensive. I strongly suspect that, in most cases, all the rebuilders replace the capacitors.
  2. The idle control valve may suffer from two problems. The electric solenoid may fail or, more commonly, the valve simply becomes gummed with oily deposits. (Some crankcase ventilation vapors are routed through the valve.) Proper soaking in a carburetor cleaner will usually cure this condition. Be careful not to expose the electrical part of the valve to the solvent.
  3. Very rarely, there is a problem with the lambda control unit, the closest thing that these cars have to a calculator. The idle control unit is connected to the lambda controller and "searches" for signals from it. Usually, if there is a problem with lambda control, the engine will run bad anyway (because of incorrect fuel mixtures). If the lambda controller does not operate at all, the most common reason is a failure of the overvoltage protection relay (OVP) located in the fuse box. There may also be a bad connection to the ground. The unit itself seems to be extremely reliable, perhaps because it lives in a cooler climate – next to the front passenger's feet.
  4. If the idling problem occurs only in a particular gear, the start-lock switch on the side
  5. If the problem only occurs when the air conditioner is being used, there may be problems with the timing relay in the fuse box, which is intended to increase the speed of the engine when the compressor engages.
  6. Finally, the throttle position switch (TPS) on the side of the throttle body may be defective. This is very unusual and we should be grateful, since the replacement of the TPS requires the removal of the entire intake manifold. Sometimes the switch is simply out of adjustment and can be put back in the right position with a long punch.

The GST informs the idle control unit when the throttle valve is closed, partially open or wide open. Electrical continuity exists through a pair of three pins, through another pair, or not at all. If the correct signals are not sent, the switch itself could be perfectly fine; the problem may simply be that the throttle is out of adjustment. The switch is extremely sensitive; Even the slightest opening of the accelerator is enough to break continuity through the relevant pins. It does not require much excessive pressure of the link to disrupt the system. Make sure that the throttle valve actually closes against its stop.

Another fundamental factor to take into account – and which tends to be overlooked in this context – is the timing of your ignition. Too much advance in the ignition will certainly have an effect on idling. On Generation I cars, there are many possibilities for adjustment, and the engines can do a lot of work before they start to blow up (ping). About ten degrees of initial advance is optimal. Keep in mind that the vacuum advance unit adds about 16 degrees to idle (when the vacuum collector is high). From time to time, the unit ahead can cause problems, the main one being an internal diaphragm rupture and creating a vacuum leak at this point.


Source by Richard M Foster

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