Idles high after being in gear.

[BigRedOne]

Looks like my cable won't be here till Sat. I'm going to be in Daytona for the spring "Turkey Rod Run" check out some kool rides and cruise the swap meet. I guess It will be next week before I get back at this.

 

[DerekTheGreat]

I've never had one of these with an automatic. But, I have noticed my '89 Fireturd does increase the idle when put in gear. Is there something which senses and sends a signal to the ECM to let it know it's in gear? If so, maybe yours doesn't detect that it isn't in gear when you slap it back in park. I know you've said everything has been replaced, including the TPS, but does it read accurately when swept through the entire range? I had a problem with increasing IAC counts as my truck just sat there. Turns out the TPS was faulty and sending erratic voltages to the ECM, which in turn caused it to open the IAC. However, unlike your truck, it never held a constant idle speed once the TPS started acting up. Since everything with your truck is consistent, I suspect the PROM chip as well. I'd try a different one in possible.

Another solution for me would be to just swap a stick shift trans in there. NV4500, if you can find one. Or the dump truck ready SM465. I too, would find it hard to put a carburetor on it. In fact, I just couldn't.

 

[BigRedOne]

I've never had one of these with an automatic. But, I have noticed my '89 Fireturd does increase the idle when put in gear. Is there something which senses and sends a signal to the ECM to let it know it's in gear? If so, maybe yours doesn't detect that it isn't in gear when you slap it back in park. I know you've said everything has been replaced, including the TPS, but does it read accurately when swept through the entire range? I had a problem with increasing IAC counts as my truck just sat there. Turns out the TPS was faulty and sending erratic voltages to the ECM, which in turn caused it to open the IAC. However, unlike your truck, it never held a constant idle speed once the TPS started acting up. Since everything with your truck is consistent, I suspect the PROM chip as well. I'd try a different one in possible.

Another solution for me would be to just swap a stick shift trans in there. NV4500, if you can find one. Or the dump truck ready SM465. I too, would find it hard to put a carburetor on it. In fact, I just couldn't.

Yea the computer is seeing it switch between p/n and gear and back. That's what I thought also but it seems to be working. Weather or not the PROM is seeing it is another story.

 

[Keeper]

Bad data from coolant temp sensor?

 

[PlayingWithTBI]

Weather or not the PROM is seeing it is another story.

The EPROM (Elecronically Programmable Read Only) chip doesn't see anything. It just stores data for the ECM to access. If the ECM is seeing the inputs, it's using them to do whatever it's supposed to do, if everything else is good.

 

[BigRedOne]

Bad data from coolant temp sensor?

Had same thought last week. It has been replaced. No change.

 

[BigRedOne]

The EPROM (Elecronically Programmable Read Only) chip doesn't see anything. It just stores data for the ECM to access. If the ECM is seeing the inputs, it's using them to do whatever it's supposed to do, if everything else is good.

I understand that the PROM is read only but if the ecu is seeing the p/n switch change and shows that in the scanner that doesn't mean the PROM isn't faulty or hasn't been corrupted in a way that responds with incorrect data then the ecu will will respond incorrectly and no code will be set. also have read where they can get grounding issues within the PROM and that will give false data. Correcting me if this is wrong. Not many answers left at this point.

 

[PlayingWithTBI]

I understand that the PROM is read only but if the ecu is seeing the p/n switch change and shows that in the scanner that doesn't mean the PROM isn't faulty or hasn't been corrupted in a way that responds with incorrect data then the ecu will will respond incorrectly and no code will be set. also have read where they can get grounding issues within the PROM and that will give false data. Correcting me if this is wrong. Not many answers left at this point.

Yes, that's a possibility. These old 2732A chips have been known to go bad but, not very common. One reason I suggested going to a flash chip and flashing your own from a known good .bin, or maybe have someone "burn" a new (old stock) 2732A chip.

I was responding to how your post was worded, the PROM doesn't "see" anything, semantics but, I didn't want anyone thinking is was computing/calculating something

 

[1998_K1500_Sub]

Yes, that's a possibility. These old 2732A chips have been known to go bad but, not very common. One reason I suggested going to a flash chip and flashing your own from a known good .bin, or maybe have someone "burn" a new (old stock) 2732A chip.

Isn't there a checksum? The ECU should detect if the code in the ROM's corrupted and go into limp mode. The trouble is, I don't know for sure if it does (although I can't imagine the firmware designers would not do this). I've got a $0D code dump (16197427 ECU) around here somewhere, I guess I could check.

Flash chips are nice, I've used them, but I when I looked I didn't find any that were spec'd for temps less than 0C... not exactly automotive temp range. What Moates sold me at the time were rated 0-70C, maybe he or someone is offering better, now. Again... I've used 0-70C flash ICs in IL / northern WI for years without obvious issue, but... it's something to keep in mind.

 

[PlayingWithTBI]

Isn't there a checksum? The ECU should detect if the code in the ROM's corrupted and go into limp mode. The trouble is, I don't know for sure if it does (although I can't imagine the firmware designers would not do this). I've got a $0D code dump (16197427 ECU) around here somewhere, I guess I could check.

I haven't seen any checksum parameters in the 7747 ECM. Later PCMs have that.