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Premium Member
1991 560 SEL
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98 Posts
Discussion Starter #1
With time on my hands, I've been working through the long set of troubleshooting / diagnostic steps in the factory manual for the electronic idle stabilization on my '91 560. In the engine service manual, it's 07.3.0711 – 112/4-6. I have a high idle issue that doesn't appear to be caused by the usual bugaboos (no vacuum leaks), so I decided to work through all the points in this list. I've either just found a smoking gun or - more likely - I am missing something obvious.

My question: Is there some trick or prerequisite to testing connections in the large wiring harness that attaches to the CIS-E ECU? I want to test the connectivity between the CIS-E ECU and the relevant idle components, e.g. pin 9 from CIS-E > pin 2 on the idle stabilization ECU. I am getting no connectivity on several of the circuits. I've used both a micro-fine probe on my multimeter lead to poke right in the middle of the CIS-E "pin" (really the female harness point) and I've also tried a probe right next to it (there's a bit of a notch). No connectivity. For my sanity, I tried connectivity between pin 2 of the CIS-E harness and ground point W11, which succeeded. Any tips on the process here? In the factory manual, they of course assume you are hooking up a special harness and connector box, which I don't have. Is there something in the design here that makes this necessary? Some other weirdness that I'm missing? The wiring diagram doesn't show any intermediary connectors, i.e. a secondary harness that could be breaking connectivity because it's disconnected.

If I'm not missing anything, then I'd also love to hear whether others have had connectors / wires fail in this big harness. My car is in quite good shape; I haven't yet found any wiring that's gone bad in it in 2 years of wrenching on it. I shudder to think about re-soldering this thing...

Appreciate your input!

Thanks,
Robert
 

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Outstanding Contributor
1989 560SEC, 1989 560SEL, 1995 E420
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4,494 Posts
Okay, that's weird. If I understand you correctly, in your example you're testing the wire itself that runs between the CIS-E computer to the idle stabilization ECU. That wire should run directly between the two and you're not getting anything.

Considering the condition of your car, it seems highly unlikely that the wire is damaged. I'm more inclined to think the wiring diagram you're using is off? What if you probe pin 2 on the ECU and just see if it connects to anything on the CIS-E computer?
 

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1990 420 SEC
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351 Posts
Did some measurements on idle control before I got it working on my car. Wasn't a wiring problem. One thing I noticed. Temp signal from CSI-ecu is only one tenth of what manual suggests. The fault was with valve. On my car at least the idle controller limits max current for the valve to 1 A. That wasn't enough to control idle when engine was warm.
 

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Premium Member
1991 560 SEL
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98 Posts
Discussion Starter #4
@luckymike , thank you. Yes, you got it right. On the CIS-E side, I'm testing from the harness itself, i.e. pushing the multimeter probe against the female metal contacts that would connect to the male contacts on the CIS-E computer. I did try exactly what you said - one side of the multimeter to pin 2 of the idle control unit, then ran along the full set of CIS-E harness connectors. No connectivity. I'm going to a) review the wiring diagram and make sure I'm reading it right and that it's the right one for the car, b) try testing more of the connections between the CIS-E harness and other components to see if my way of testing is valid.

@Heikkif , thanks for the note on the temp signal. The valve I have on the car is a rebuilt unit that I believe is functioning properly; I do have another one (unknown condition) to swap in just to try. On the amperage test, it is drawing .96A, so right at the top of the range, and still not limiting the idle down to spec. That may indicate a larger problem elsewhere (somewhere air getting in, despite my tight vacuum system), assuming the valve is working properly.
 

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1990 420 SEC
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351 Posts
Either the idle controller is not able to lower idle or it does not want to do that. I tested the idle valve with an external voltage source with adjustable current limiting. I found out that with enough of current I could bring down idle to desired level. Current needed was more than 1 A. I adjusted the valve a bit and now it works. Engine has good vacuum and I do not suspect leaks.
If one has two valves and an adjustable current source idle controls could be tested this way:
Connect the extra valve to electrical connection of the idle system so that you can measure control current (or voltage over the coil.
Control the installed valve with external source. As car is idling adjust current and observe how RPM and idle control current from controller behaves.
 

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1991 560sec. 1991 560sel 1969 280SL
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1,265 Posts
If you’re not sure of which wire and or pin to check continuity of the Idle control module an easy way to test it is to unplug it and see if affects idle.
 

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Premium Member
1991 560 SEL
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98 Posts
Discussion Starter #7
Quick update - I still have not figured this out. Because I have some time on my hands, I created the attached annotated / highlighted wiring diagram for the circuits that connect to the idle speed control unit, plus some others related to the signals that the CIS-E control unit might use in conjunction. The diagram is so tiny in the service manual that I needed to enlarge it anyhow. My car is a '91 560 sold originally in CA, so this should be the right diagram. Since I'm at it, I'll check the other connections between the CIS-E and idle speed control unit.

Engine Wiring Diagram High Res - Highlighted.jpg

f one has two valves and an adjustable current source idle controls could be tested this way: Connect the extra valve to electrical connection of the idle system so that you can measure control current (or voltage over the coil. Control the installed valve with external source. As car is idling adjust current and observe how RPM and idle control current from controller behaves.
@Heikkif , that's a great idea - I understand what you're going for there. I wanted an excuse to get a variable DC power supply...now I have one. :)
 

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14 Posts
The CIS isn't connected from its 9 to the IDLE's 2 which also tells the fuel relay box a temp to activate the cold-start injector. The fuel relay box might think its 14F and the cold start injector keeps spraying fuel. Start the engine warm and then unplug the connector to the injector and see if the idle drops or unbolt the injector and see if its continuously spraying which is more dangerous. Also see if you EGR is stuck open. If you've 1000% sure of an open circuit then use splice lock clamps to make a wire repair. I use eautorepair.net for diagrams.
 
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