Working on a 95 chevy G30 van with a TBI 350/4l80e. It has two problems that i think may be connected. First, the trans is in limp mode. Has only 2nd and reverse. Also, the check engine light is constantly on. Like it only goes out when you turn the key off. Along with that, when you jumper A-B on the aldl port, the light doesnt flash. Just stays on. Any ideas? Thanks!

 

Get a REAL scan tool connected, and write down all the codes. Once you've figured out what the codes mean, look at the data stream for sensors or outputs that would set those codes.

You'll want a service manual so you know what the procedure is to diagnose the problem that caused the code to set.

 

So as an update. Changed the ECM and now the CEL works properly and can check codes. Only codes are 12, and an ABS code which i dont remember the number of. But now, the van runs quite poorly. So im wondering if previously, the ECM was bad and put the whole truck into limp mode. With the previous ECM, the van ran well but the trans was in limp mode with only 2nd and reverse and the CEL light was on constantly. Now with the new ECM, it runs bad enough that it cant even be driven around the block to see if the trans shifts now. Hunts for idle and just runs bad and rough over all.

The van had been sitting for about 5 years up until a couple weeks ago. The fuel was drained from the tank and 5 fresh gallons were added. Initially it wouldnt run but it was determined that the pick up in the distributor was bad. Upon replacement it ran just fine with the previous ECM.
So thats where we're at now.

 

Need to re-learn valid fuel trim and IAC data? What did you learn from the data stream of the original ECM?

Cat converter plugged with carbon?

A differently-defective ECM?

 

I suspect something to do with it sitting so long. Possible fuel related? Iac related? Probably would be best to do a zero* timing reset and an IAC relearn. Probably also a new o2 wouldn't hurt to ensure the relearned fuel trim tables are accurate. He says it runs good cold.

We never got data from the old ECM. The new ECM is brand new from napa.

 

I have the same problem and wonder if you ever got it resolved.

 

WHAT VEHICLE???

Connect a scan tool, find out what the computer is trying to tell you.

Check for "codes"--current codes, history codes, pending codes--but more importantly, check the data stream. Verify EVERY sensor, and all the computer outputs. Look at the fuel trim numbers, and O2 crosscounts.

How old are the usual "tune-up" items? Cap, rotor, plug wires, plugs, air filter, fuel filter, O2 sensor(s)? Verify the PCV system and initial ignition timing. What is the fuel pressure?

 

1. Limp mode 4L80E will be 2nd gear

2. No reverse

Both are trans issues. Don't see the reason for the ECU being the issue.

The 4L80E trans has a known issue of the internal wire harness leaking into the connector on the side of the trans. Pull the connector on the side of the trans and look inside. There should be no trans fluid. If so you'll need a new internal wire harness. Easy DIY job but requires drop the pan. While in there I'd at min change both shift solenoids A&B and turn the force valve up 1/4 turn clockwise. as well as new filter and fluid Dex VI

Clean both ends of the connector out with electrical contact cleaner.