My 87 has SES light on and I can't read codes, the B connector in ALDL does not have power and there is no power at 3 amp ecm fuse in or out. Engine runs but fast idles and last week i started to pull out at quarter throttle and backed off and engine shut down immediatly. Does anyone have an idea what problem might be?

 

What did you back out? The throttle adjustment screw? I don't think you can adjust idle speed at all like that. I would put it back to where it was and see what it does.

 

RON4811;
If it's running the ECM must be getting power from somewhere. With the engine off, turn the ignition to on and try shorting the A-B on the ALDL and see if you get codes of any kind. You should see the SES flash 12, 12, 12 - (your code(s) ) - 12, 12, 12. Do not start the car with A-B shorted!

 

Would that work if he was getting no power at the fuse? I think he should get it running again and then sort things out.

 

It may not but it would be another symptom. If it runs at all then power is going to the ECM injector drivers ... But it seems to me it's time to check the troubleshooting charts in the FSM ... each code set has its own chart.

 

I think you should re-read his post. He didn't say he backed anything out. He let off the throttle and the car died. At least that is the way I understood it.

 

My bad. I thought he backed something out.

 

I think the 1st order of business it to get power to the ALDL so it can be used as the diagnostic tool intended.

 

Thanks for the replies, I did not back out adjustments, was went from quarter throttle to closed throttle and engine quit. The car will start fine every time but has a fast idle when it should not, even when at operating temp. When I short A and B with key on engine off I get no codes, not even the 12. SES is on the whole time engine is running. B has no power and 3 amp fuse has no power. I am loosing power before it gets to ecm, the main power from battery to ecm is good which is what allows engine to run. I need to know how to get power back to 3 amp fuse and ALDL B

 

 

Thank you rharker, these charts are great, I think they will lead me to the problem. Just at a quick glance without checking I think the ecm is bad. I may not get to check it til next weekend but will let you know what I find. Thanks again. Ron

 

 

No, the fans only run as engine temp dictates. I.m not sure I can test ecm with the 3 amp fuse circuit dead. I think it won't work right without that circuit being live? Fuse is not blown, has no power at both sides of fuse and it is for ecm.

 

IAW - Chart A-2 on my 86 FSM the SES light stays on from the ECM when the key is on not running. If you unplug the ECM then turn the key on if the light goes out then this tells you circuit 419 is NOT shorted to ground.

Further following the A2 chart quickly sends you to a new ECM but check the 6 way IP connector terminal C for 12V on both sides and check CKT 439 (PNK/BLK) to the ECM for open circuits since those are the the circuits that goes to your 3 Amp ECM/ENG fuse.

 

Thanks for all your help and thanks for the use of your fsm, do you know where that 6 way ip connector is located? That is probably why the ecm fuse has no power. I think that would give a false indication of a bad ecm???

 

I'll take a look in the FSM ... it should be under electrical.

 

Hello ron4811,

First:
Welcome to The Corvette Forum!!

Second, I had trouble finding 12 Vdc power at several key points and it was caused by a ton of corrosion on the post behind the battery where several fusible links get their 12 Vdc power from the battery. Remove the nut on top and then clean each one of the terminals as well as the larger battery cable. Then when done check to be sure that the post has the actual battery voltage on it. After this minor cleaning I was happy to find full battery voltage at the fuse block and other places. I had previously had a bad voltage drop inside the car, after the cleaning the radio even had full battery voltage going to it. Everything works better when it gets full voltage.

Be sure that your chassis and engine is properly grounded before going to far. Bad grounds can cause havoc on a Electronically Fuel Injected car like these Corvettes with their non-conductive bodies. There are several ground straps on the C4's, I found a broken one on my exhaust one day and after fixing it my Radio's antenna suddenly started working again. The Factory Service Manual is priceless when working on some of the more complex systems on this car.

Good Luck with your Corvette, you will get it fixed and be out getting the "Smiles per Mile" that Corvettes have been known to produce!
Best Regards!

P.S. ECM failure is not all that common in the real world, I would hold off throwing Expensive non-refundable parts at it until you are sure what the problem is first. You are at the right place to get help solving this issue, hopefully inexpensively for you and your Corvette.

 

I looked through my 86 FSM pretty thoroughly in all the likely places and can't seem to find a locator for the 6 way connector. So I'll give it my best guess ... Take off your breadbox and look under there ... also while in there you can probe the ECM connection to see if there is any voltage on CKT 340, 439 and also there is a single wire connector near the battery that might be open.


This is the ECM connector on an 89 but it's the same as your 87. Over by the battery.

 

Update, I got to do some checking and my 3 amp ecm circuit is now live, I may have misread the label as there are 2 3 amp fuses next to each other with the ecn label in between them, so the ecm label belongs to the fuse above and it is live. So i disconnected the ecm to be sure that the ses light wire is not shorted to ground and it is not so that should confirm a bad ecm. So I reconnected the ecm so I could put car back in garage and now the ses light is out and the idle is now normal for the first time since I got the car 2 years ago. So I went for a 10 mile drive to see if ses would come back on and it did not, and the car ran better than it has since I got it. Almost like driving a new car. The only problem remaining is that I still can't read codes from ecm, not even 12 so I am still going to replace the ecm.Thanks for all your help. Sorry about the confusion on the ecm circuit.

 

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