brain buster

I recently bought a '92 bravada brought it home and discovered that it would run beautifully for about 10 miles and then I would have to head back to the house. Needless to say it has reached the point that it will not run at all. If I do manage to get it to start I have to pump the accelerator frantically and constantly to keep it running and it runs like crap even then. I have worked with it for awhile now and have found that if I swap out the ECM with one out of an unknown year and model Blazer (at least I think it was a Blazer) that I bought from a junkyard it will crank up and run fine. The only problem here is It burns a lot of gas and I can't use the a/c. When I turn the a/c on while driving it around it drives the tranny crazy and causes it to constantly shift up and down:screwy:. I would like very much to salvage the truck but I am at my wits end here and am about a hairs bredth from parting it out. ANY ideas here would be greatly appreciated.

Hmm, ever think about getting the proper pcm?, if the wrong one seems to help.

The ECM may be the same on both vehicles but the chip inside is unique to the vehicle. If you still have the old ECM you can try swapping the EPROM chips inside the ECM and see if that fixes it.

I agree with Eric, certainly worth a try.

Hey Maxwedge, Tony DePillo has a site,

http://honkinhemi.com/

I neglected to say that I have replaced the old ECM and PROM with a reman ECM and a brand new factory PROM, but it still will not run with the original equipment. The junkyard ECM has a different PROM in it so they will not swap.

Is the Check Engine light on?
Does it come on momentarily then go off after first turning the keyswitch on?
Do you know how to check for stored DTC's?

I'm kinda leaning towards the TPS.

the engine light is on until I try to start the truck, it goes off while the truck is trying to run and then comes back on as soon as the truck shuts down. the trouble code just flashes a code 12 repeatedly with no other codes in the mix. If by TPS you mean throttle position sensor I pray that you are correct. I haven't tried that yet and that's why I am looking to y'all for a direction to go in. So far that's the only suggestion that has been made. If you think that may be the problem I'll give it a shot. Thanks for the help.

This is odd. Mine and Eric's '92's are pretty good about setting a code whenever there's a sensor issue.
Since it's acting up after the engine is warmed up, it's in closed loop and all the sensors should be active. Those not within parameters should have the ECM setting off the Check Engine light and storing DTC's.

Sorry racejunky but it may end up requiring a look with a Pro shop type OBD I scan tool.

Just to make sure there is no misunderstanding about the problem The truck does not have to warm up before the problem occures. It is there from jump street. I don't know if that makes a difference or not but If so maybe it would help to know that. I have an OBD II but I don't think it will fit the data link plug. do you know of an adapter or will I need to accquire an OBD I in order to run a check on it?

So we're still at starting, running, and driving fine until the AC is turned on, then the trans has a shifting fit?
If so, what does the voltmeter show with the AC on?

Some of the Actron's just need a cable.

the only way it will start, run and drive is when I have the ECM from the junkyard installed. with the original ECM it will start but only momentarily and it runs rough for that short moment. But yes when I have the ECM from the junkyard installed the a/c drives the tranny nuts when I turn the a/c on and am driving around. It seems to work fine if I'm sitting still and the truck is in park. I can drive the thing all over if I don't use the a/c it just gets horrible fuel mileage.

Electrical noise from a questionable alternator makes the transmissions act up on Dodge Diesels.

What does your voltage look like w/AC on?

I will check it and see, if I remember correctly I think the guy I bought it from said He had just replaced the alternator. With all these new electronic gizmo's I suppose it could be anything. That's why I deffer to someone with more knowledge of these things.