Poor Bad Idle & cruise control not working


Senior Member
Sep 19, 2010
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I had an idle issue develop at the same time the cruise control stopped working and have resolved it so thought I would post it here in case it is of use to others at some point in future.

S124 1996 E320
5 speed auto gearbox.

Problem. Starting the car one day idle was very rough and low on cold start up. Once engine was up to temperature would idle faster than normal. So as if the idle program was on the wrong way round. Would only idle OK for a few mins if warm but had been left for 10 mins or more. Then idle would gradually creep up.

Cold idle would be 5 or less. (no miss firing just so low idle would feel like going to stall)
Warm idle would be just over 10.

At the same time Cruise Control stopped working.


There seems to be some confusion on crusie control and how it is controlled. From my understanding (could be wrong) it is different if the car has a five speed auto box. In the case of an E320 with a 5 speed auto box, there is an ECU in the passanger footwell behind the black metal plate. This controls the idling and the crusie control.

For some reason this had failed in my car. They are over £2000.00 new from Mercedes so I tried to find one second hand. None in the UK, however found one in France. It had the same part number and the same secondary number although other info on the stickers was different. There was three years + difference in the part dates and a version different. Bought it and risked it and it worked. So the ECU from an older French registered car worked perfectly in mine.

Of course I do not know what caused the ECU to fail in the first place. If it is a fault elsewhere like the wiring in the throttle body breaking up causing a short, then it is going to happen again but the replacement ECU was £80.00 so willing to risk it to find out.

Fiddly but easy job to do.

Both ECU's had matching part number of 124 545 52 32 then had T/LLR after them.

Both ECU's also had a matching secondary number of 412.227/002/018

Other numbers on the labels were different together with build date and version number. One was from French registered car and transferred fine to UK registered car.

Just put all this here in case anyone else has a similar problem.
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