Symptoms of a bad/failing ECU?

[87ChevyR10]

Just curious what one would expect to experience from a failing/bad ECU.

Thanks

 

[HotRodPC]

Ohh man, the sky is kinda the limit on that one. Could be a no start. May or May Not throw a code, poor performance, lost it's PROM programming, Burning smell or smoke coming from the kick panel etc etc.

Might be easier to esplain your symptoms and see if we can eliminate or confirm a bad ECU.

 

[chengny]

Just curious what one would expect to experience from a failing/bad ECU.

Thanks

An illuminated SES light and a scan that reveals a DTC 55.

 

[SkinnyG]

If the ECU is getting all the correct signals all the way to its input, but the output is drunk.

 

[1987 GMC Jimmy]

Or a failing diagnostic circuit. You may not get a code at all. I've also had experience with the ESC circuit messing up. There are many ways that it can mess up as aforementioned.

 

[87ChevyR10]

I was just wondering because my truck has 2 types of cold starts: 1 where the idle will increase as it should or where the idle sits around 6 or 700 RPM, drops to about 500 rpm, begins a mild surge and idle roughs. Last year, I replaced the temp switch, MAF and rebuilt TBI which altered the start up idling to a degree. Before, the truck would go high idle no matter what. Now, when it's hot out, it doesn't high idle even though it feels like it is trying to causing the rough surging idle. After warm up, engine is fine. Engine has always had the off idle surging which folks here have said that could be from a stuck IACV. Even then, it does not exhibit these symptoms all the time which is why I was thinking maybe the ECU is going bad.

 

[1987 GMC Jimmy]

I would check for vacuum leaks and suspect the IACV as sticking or failing. You can try a relearn. Jump the A and B pins on the ALDL, turn the key on for thirty seconds, unplug the IAC connector, turn the key off, undo the jumper, reconnect the IAC, and start her up!

 

[Bullittben]

I had similar issues once and it was caused by a cheap ignition module from a discount parts place. If it started and ran rough, I would switch it off and restart until it ran right. Issue was solved by putting a GM module in the distributor.

 

[87ChevyR10]

I forgot about this thread.

The CEL does not light up when key is on as it should. Instead, it flashes during starter cranking. What would this indicate?

 

[chengny]

I forgot about this thread.

The CEL does not light up when key is on as it should. Instead, it flashes during starter cranking. What would this indicate?

Here is the diagnostic procedure for "No SES Light". The left hand side of the flow chart on page 2 deals with a "No SES Light but engine still starts" condition:

You must be registered for see images attach

You must be registered for see images attach

Couldn't find any info on what "SES flashing while cranking" would indicate. That might even be normal operation. I've never had a truck with an ECM, so I can't comment on that. Maybe someone else - with an ECM equipped truck - can confirm whether or not the SES is supposed to flash while the starter is operating.

 

[1987 GMC Jimmy]

I can confirm that it does flicker in those aforementioned conditions.

 

[87ChevyR10]

I can confirm that it does flicker in those aforementioned conditions.

What about with key on no crank? Does yours not illuminate for a bit then shut off? Maybe I am wrong on how this works.

@chengny . Based on the flow charts, it wiould point to an open CKT 419 which could lead to bad ECM. Truck doesn't exhibit any problems. Runs strong. Kinda drinks the gas though. Looking like 10 to 12 MPG even on the highway.

I was mainly concerned about not seeing the CEL light before startup. I thought I had read it was suppose to light up briefly before cranking.

 

[chengny]

What about with key on no crank? Does yours not illuminate for a bit then shut off? Maybe I am wrong on how this works.

@chengny . Based on the flow charts, it wiould point to an open CKT 419 which could lead to bad ECM. Truck doesn't exhibit any problems. Runs strong. Kinda drinks the gas though. Looking like 10 to 12 MPG even on the highway.

I was mainly concerned about not seeing the CEL light before startup. I thought I had read it was suppose to light up briefly before cranking.

No, it's not supposed to light up briefly before cranking - it is supposed to come on and stay on until the engine is running. But I suppose - if you are just starting the engine normally - you don't linger in the RUN position. You just skip right through RUN and go straight to START. That would give you a brief flash of the SES light.

With the key turned to RUN and allowed to just stay there, the SES warning should remain steadily illuminated. Then (as I just learned), when the key is moved to the START position, it should flash - until the key is released back to RUN. At that point - with the engine running - it should only be lit up if there is a DTC stored in the memory.

Maybe disconnect the battery negative and unplug/plug the harness connectors to both banks of the ECM. Reconnect the battery and check SES lamp operation. But really, if it's running good - if it were me - I wouldn't give it another thought.

 

[87ChevyR10]

No, it's not supposed to light up briefly before cranking - it is supposed to come on and stay on until the engine is running. But I suppose - if you are just starting the engine normally - you don't linger in the RUN position. You just skip right through RUN and go straight to START. That would give you a brief flash of the SES light.

With the key turned to RUN and allowed to just stay there, the SES warning should remain steadily illuminated. Then (as I just learned), when the key is moved to the START position, it should flash - until the key is released back to RUN. At that point - with the engine running - it should only be lit up if there is a DTC stored in the memory.

It doesnt light up when in RUN position. I’ve left it there before to power stereo.

 

[1987 GMC Jimmy]

No, mine comes on and stays on until the vehicle is started. You should absolutely see it before startup.

EDIT: Missed Jerry's last post there. But yeah, sounds like there's an issue there if it's not staying on. And also, there's no pattern to the flicker like pulling codes or doing the field service mode flashing just to clarify. I think it's just a byproduct of the power drop.