C24FUN

My '90 C2 has been showing an occasional "Check Engine" light accompanied by a irregular idle speed. I hooked up my Scantool and it shows code 1212: Intermittent Idle Speed Contact Ground Short.

I believe this refers to the "Throttle at Idle Position" switch attached to the intake manifold (photo and diagram attached). I have checked the switch operation and the wiring. The switch does change state when the throttle is closed and pin 2 of the harness plug does show a good ground connection.

My question is really to do with the wording of the fault code. "Contact Ground Short" is a bit ambiguous. Does anyone have a more precise description of what the ECU might be reacting to that would cause this fault code? I assume it is not seeing the correct combination of input conditions to keep it happy when the engine is at idle but what exactly?

Any help is appreciated.

Cheers!

Jim G.

klr10

I'd check pin 52 at the ECU (Wt/Br). This should be open when idling and go to ground as soon as you hit the throttle. It could be this wire is grounding out somewhere.

C24FUN

Yes that would check the continuity of the whole circuit. Worth a look.

I'm guessing the ECU sees the RPM at idle levels but does not see the correct idle speed indicator signal. If it is supposed to see open circuit at idle and instead sees a ground that would make the description "Idle Speed Contact Ground Short" logical.

My concern is more that the ECU sees it as an intermittent fault and what exactly causes the fault. Since the problem (check engine light, variable idle) occurs only when the throttle is closed (at idle) and disappears while driving (off idle), perhaps the wires are shorting in the harness somehow? Intermittent problems are always a pain to troubleshoot!

I will do some more checking with the VOM.

Thanks,

Jim G.

C24FUN

I remembered reworking the 3 pin connector at the throttle end a few years back (OK, maybe 8) so I took a closer look at the contacts. It looked like they were a little misaligned so I carefully bent them back into position and tweaked them to give a snugger fit. No repeats of the Check Engine light in the past couple of days so maybe success!

Jim G.

C24FUN

I had a repeat of the Idle Switch Ground CEL and had to revisit this circuit. In tracing out the ground wiring for the Idle and Full Load switches I discovered a discrepancy from the Workshop Manual drawings that explained why I was having trouble diagnosing the problem. On the circuit diagram it shows the ground from Pin 2 of the throttle position connector going directly to Ground Point 4 on the chassis. This is not the case, at least in my '90 964. The ground wire actually goes directly back to Pin 30 on the DME as shown in the modified drawing attached.

As a result, if you remove the connector from the DME to test the signals you will not see any ground on Pins 52 or 53 (Idle/Full Load respectively) since the ground comes thru DME pin 30. If you don't know this it can obiously mess with your head as ground continuity appears and disappears as you troubleshoot!

Anyway I eventually determined that my problem was just a loose pin on my 3 pin throttle position connector so I re-tweaked it and I'm good to go.

Cheers!

Jim G.