85 500sel Idle Control Value Kills Engine
10-22-2017, 09:24 PM
#1
Junior Member
Thread Starter
85 500sel Idle Control Value Kills Engine
Any help most appreciated. My 85 MB 500sel runs very good, BUT has an issue where the engine will die when the idle control valve gets current. I tried three idle control units changes for testing the reason my engine would die. I connected a small bulb to the valve and with one of the control units attached I would see the bulb light up (power to the valve) and the engine would immediately cut off. With the other two control units the light would not come on and since the valve did not get power the engine would not die, BUT the rpms would move up to around 1200rpms as the engine warmed up. Also, at times, when the engine is running fine, there are times when the engine will die when I take my foot off the gas totally. While engine is running, it is very smooth with no misfires or rough idling. Plenty of power. Any body had this problem or no what the problem could be. Any testing suggestions? Thank you for your input, Dan
10-24-2017, 12:13 AM
#3
Junior Member
Thread Starter
VIN: WDBCA37D8FA155644
10-24-2017, 10:04 AM
#4
Out Of Control!!
On your 117.963 the idle control is accomplished via N8 idle speed control unit located on the 126 in the right foot well below the floor panel. I believe that is where your issues reside, however for you to test the complete circuit I have attached your engines idle speed wiring diagram---disregard page 2 as it applies to the 119.
Last edited by Plutoe; 10-25-2017 at 12:55 PM.
11-04-2017, 07:08 PM
#5
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Thread Starter
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.
Last edited by ChamaRiver; 11-19-2017 at 01:28 PM. Reason: addition info
11-19-2017, 01:27 PM
#6
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Thread Starter
Plutoe , thanks for the CIS info. The Idle Speed Control Unit is located in the engine compartment, just behind the firewall, on left-center. Do you possibly have the electrical schematic for the N8 (Idle Speed Control Unit)? I believe that the problem is in a bad Transistor of which I marked three in red above. Thanks.
12-30-2018, 12:32 AM
#7
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1985 500 SEL w126 (11/84) - 1991 560 SEL w126
Plutoe , thanks for the CIS info. The Idle Speed Control Unit is located in the engine compartment, just behind the firewall, on left-center. Do you possibly have the electrical schematic for the N8 (Idle Speed Control Unit)? I believe that the problem is in a bad Transistor of which I marked three in red above. Thanks.
Any help would be much appreciated!!
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