I bought a '95 G-30 extended to make into a camper van. It started started right up, low mileage, new transmission. When I took it for a test drive, the new trans seemed fine, went smoothly through all 4 gears, and into reverse as well.

On the drive back, after about an hour, I stopped for gas. Started the engine and immediately the "service engine soon" light came on. Banged through R, N, into O. Struggled to shift into 2nd gear on the road. I pulled over, cut the engine, panicked for five minutes, and started it up again. No problems.

Where to start?

 

stop at an autozone to get a free OBD diagnostic

 

You shouldn't even need a code reader. GM OBD1 just takes a paperclip to pull the stored codes. What's the plug under the steering column look like?

 

1995 was a transition year, which is why I suggested using a scanner. Some 94/95 GM vehicles have an "OBD1.5" system which doesn't work with the paperclip.

 

However many code readers for OBD-II won't work on OBD-I.

I'd try the jumper first anyways, it costs nothing and can be done anywhere. Either it works or it doesn't. If it works you get the code and have something to look for.

An extended I would think has a 4L80E trans. I would strongly consider re-tuning the chip or swapping the entire ECM for one from a 1993-1994 van. Federal emissions standards were reduced for trucks in 1995 and the GM answer was basically to de-tune the OBD-1 controls in a way that costs about 3 MPG, which is a lot on a vehicle that now gets about 12. It was just GM being cheap, as they were replacing not only the entire engine control system the next year but for the 1/2 ans 3/4 ton vans they got an entire new platform.

1995 vans with a 4L60E also have a one year only deal in the trans, the TCC solenoid operates in steps instead of just on/off, so just an ECM swap may not work.


As for the trans issue, I would look over the grounds. They just disturbed all that stuff with the swap and maybe one is loose or broken. My 91 G20 had some kind of work done and I found one bellhousing bolt not fully tight with the ground on it loose. It was before the computer controlled transmission though so no problems there.

The fact that it did it, then went away, suggests it could be something affected by either vehicle heat or the movement of the engine and trans, it will rock side to side a little even with good mounts. If a connection is loose, or dirty, it might cause the problems indicated.