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Old 11-04-2017, 06:50 PM
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Question 85 MB 002 545 33 32 Idle Speed Control Schematic?

My 85 500sel dies at times when slowing down or just sitting Idling. Also when idling cold it runs at around 600-700rpms, but as the engine warms up it progresses to around 1100-1200rpms. Other than that the car runs, and accelerates perfectly. I have switched out three different Idle Speed Control Units and find that only one with send voltage to the Idle Speed Control Valve when starting up, but it kills the engine (every time) within a second of starting. The other two ISCUs will let the car start and run, but as stated above, engines dies at times.

I have done research on this issue and apparently there are three "transistors" that cause the issue I am having, but even though I watched a YouTube video on the problem transistors that need replacing it was not made clear by the person exactly which three transistors need replacement. He said you could Google for a electronic schematic of the ISCU to troubleshoot for the bad transistors but I could not find the electronic schematic I need so I can fix these ISCUs. ANY help would be greatly appreciated,. thank you.

I have marked in "RED" three (three leg) transistors that may be the ones the video was referencing, but only because the person that did the video did at one point mention a 3-leg transistor.
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85 MB 002 545 33 32 Idle Speed Control Schematic?-mb-idle-speed-control-2-marked.jpg   85 MB 002 545 33 32 Idle Speed Control Schematic?-mb-idle-speed-control-1.jpg  
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Old 11-04-2017, 07:40 PM
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The high idle issues are typically caused by broken/cracked solder connections and bad traces on the board. The stalling/dying is unlikely to be related to the idle air solenoid, if it loses signal or if the board fails, it defaults to an un-energized position and you get a high idle condition.
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Old 11-05-2017, 02:59 PM
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As Diseasel300 stated, it is usually cracked solder connections that are the source of problems with that idle control module and they are usually around the leads of the rectangular capacitors (pink, green and blue ones in your first photo). You usually can't see the cracks without removing the lacquer conformal coating that coats the board.

Electrolyic capacitors can also be a problem at that age. You have 4 radial and 2 axial on that board.

I have probably fixed 25 of those modules and have never needed to change those small transistors. Once I had to change the larger one under the metal clip because it was roasted from a shorted idle solenoid.

Last edited by Rick76; 11-05-2017 at 03:17 PM.
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