devilfish

Hi guys.

Im looking for some fresh imputs on a ECM trouble I have.

Its a vette -91.

I get a erratic SES light from time to time. I dont get any codes. The only thing datamaster says when this happens is "lost connection to ECM, reconnecting".
When this happens the idle RPM increase, SES light goes on. Then 2-3 sec later the SES light goes off, and the datamaster scan continues, and the idle goes down to normal RPM again.

This is happens more during long idle sessions then when cruising.

I tried 2 diffrent ECMs, 4 diffrent chips includning the stock chip, and also my romulator. I looked for bad groudning, bad censors, but I cant se anything out of the ordinary. Drives my nuts.
1 theorie I have is that I get a powerdip from the battery/alternator. That is next to test. I will hook up a multimeter and se if I get random drops in power.
Other then that Im lost?
Throw me a bone here, anything else you whant me to look for?

Thanks

Toehead

sounds like it is going into ALDL mode.

Does it do it without the datamaster connected?

devilfish

good point, I will test that

Toehead

Any updates?

devilfish

Yes. I have tried to run the car without the aldl cabel in place. But its the same thing. BUT what I did now whas to unhook all the grounding from the ecm to the block. It whas alot of oil and dirt on that connections. So i cut it and re did all of the groudning.
And so far so good. I have not yet test drive the car ( due to bad weather ), but I have let the car idle for 20 min without any SES light. And that is a new record. So I hope it whas the groudning that whas bad.
But I still think its strange that the car dident die complete if the ECM lost ground.? Doesent the ECM trigger the fuelpump relay? And if ECM lost connection would that not kill the fuelpump as well as the ignition?

cheers

Toehead

Yes, if it lost it completely.... But all it takes is a noisy ground to screw up EFI.

devilfish

Update.

I drove the car today, and the damn blinking SES light whent on from time to time again So no change.
The trend is the same. As long as i keep the RPM up and drive the car, its fine. But when i come to a hold/stop and the RPM drops the SES light goes on/off randomly. I fail to se any logic. I dont know what to test next. Can a intermintent faulty censor cause this?
Also i verifyed that I have no voltage drop from the alternator. Its between 13.99 to 14.2 all the time.
Regards..

devilfish

Update..

I solved it!

2 things where "wrong".

First it whas bad groudning. When I sorted that out, the SES light whent on more rarely then before.
So then I tried to reinstall the orginial chip, and behold no more SES light!
But the probleme still existed with the romulator.
SO that made me think about electronic interference.
As the ECM on the vette is under the hood the romulator is more exposed to electronic interference. What I did whas simply to shorten the cabel between the romulator and the ECM. And that made the trick!

Thanks for all the feedback

cheers