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Glow plug controller (GPC) question

Burning oil

Burning oil

LeroyDiesel.com
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I think my PCM may be fried.
I can't get the GPC to come on. Its not getting the signal from the PCM.

So my question is: on the 1994 where does the PCM get its signal from OR what tells the PCM to send a signal to the GPC. It must get a signal to tell the GPs how long to stay on and to cycle them on cold starts right?
I have 12v to pink/black wire and good ground to black wire, but yellow wire from PCM is dead. I need to figure out what turns the yellow wire on.

I can hot wire the GPC so I know it works.
At worst case if it needs a PCM will a 95 work? (I have one of those around here)

Thanks.
 
The PCM/glow plug question Leroy is trying to ask is: does anyone know what temperature sensor the PCM uses to decide about cycling the glow plug relay? i.e. does it use the engine temperature sensor, the air intake, or does it ignore temperature sensors and just use a timer? I think it probably uses the engine temperature, but don't know for sure.

At this point I think his PCM is not working at all because it has power and ground at the correct pins but it doesn't even flash the service engine or throttle lights with the ignition on. It also does not flash any codes with pins A & B connected on the DLC. We didn't have the cable to hook it to the computer handy, but will try that next time.

I also have a 4WD 94 suburban we can try swapping PCMs with, but in the meanwhile does anyone know if his spare 95 PCM will work in a 94?
 
Its a wierd one for sure, Ive never heard anyone talk about that asspect of the GPC.
On a 92-93 The GPC controlls the cycling and duration of the GPs (its smart) The 94 GPC is on or off thats it, it depends on the signal from PCM (yellow wire). We are tring to figure out what inputs and from where does the PCM get them.

As far as PCM is has not been used since the Isuzu swap and has been unpluged & was left in the truck unmolested. So, why would it have gone bad?
 
IIRC glow temps are sent to the PCM via the temp sensor near the stats. I would think a 95 PCM would work as long as all the parameters are the same.
 
We were also thinking PCM gets signal from water temp sensor. I am guessing when water temps are in certain ranges the PCM sends a predetermined signal the GPC?? (not sure about that)
My concern with using the 95 PCM on the 94 is because of the unique IPs that 94s had on them, Can someone post up the IP number/s for the 94. I know its on the forum here, so will search and see if I can find it before someone is able to post it.
Thanks.
 
Unplug both temp sensors then try it. I cant remember which one sends temp signal. With it unplugged it defaults to max glow time.
 
ECM is not turning on with no SES light. Recheck ign on power and fuisable links. Check voltage at ALDL connector.
 
Possible short keeping quad drivers from turning on lights etc.
 
Will L. said:
Unplug both temp sensors then try it. I cant remember which one sends temp signal. With it unplugged it defaults to max glow time.
Click to expand...
I'll give that a try, but its not turning on at all so don't think it will change. WW could be onto something.
ak diesel driver said:
just for reference it was the 5068 IP that was the oddball for 94
Click to expand...
Thank you. Do you guys know is the odd ball a completely diff PCM of can the eprom chip be swaped out?
WarWagon said:
ECM is not turning on with no SES light. Recheck ign on power and fuisable links. Check voltage at ALDL connector.
Click to expand...

WW you may be onto something. I was in there under dash the other day flipping the Hydro boost unit right side up (from Isuzu swap mods) and could have bumped the ing switch?? PCM was getting 12v to one of the pins though, but my brother is the one doing the diagnostic so not sure what pin he was probing? or if there is constant and switched 12v to PCM?
 
Burning oil said:
I'll give that a try, but its not turning on at all so don't think it will change. WW could be onto something.

Thank you. Do you guys know is the odd ball a completely diff PCM of can the eprom chip be swaped out?


WW you may be onto something. I was in there under dash the other day flipping the Hydro boost unit right side up (from Isuzu swap mods) and could have bumped the ing switch?? PCM was getting 12v to one of the pins though, but my brother is the one doing the diagnostic so not sure what pin he was probing? or if there is constant and switched 12v to PCM?
Click to expand...

The chip can be swapped, ecm's are the same
 
ALLRIGHTY THEN!!!

My brother got it figured out. Turns out it was the fuseable link on the firewall. It powers the ECM constant voltage and the LP.
We tested the voltage in (and yes there is a switched and constant 12v source) we had no voltage on the constant 12v pin so suspected the firewall juntion block wires.
From outward appearence it was easy to miss, but looked alittle wierd, so he pulled on it and it came apart.
I have a temp butt splice on it now, but will repair it with a fused wire.

OH, I did put the 95 PCM on it and it fired up right away!! Then put the 94 back on and it runs great with it also. Still have to fix a small leak on the quick connect fitting on xover.
Thanks for the help.
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I would use the spare 1995 ECM with the 1994 chip and toss the 1994 ECM. Likely it is shorting out. ECM's are the same part numbers for 94-95 OBDI. I had an ECM blow on a friends 1998 at the dump and it took out this link. We caught it on fire as the TBI was also leaking gas. :firedevil: Wound up replacing the injector wires and connectors along with a TBI rebuild.

You have 2 power wires: a constant 12v memory power to the ECM to store codes and other things. Then you have the ign switched power.

Weak power, bad grounds, burned ign switches etc. shows up as the ECM refusing to go into diagnostic mode. This shows up as low voltage at the ALDL pins. SES light can pass the bulb check with low voltage.
 
Thanks WW. Its been unpluged since removal of the 6.5, so not sure what would have caused fuseable link to fry. It must have shorted on something. Anyway all is good again with it.
Runs real strong in the drive way. The little heater core connector on the xover busted (the pot metal POS!!) so I have to drill that out before I can drive it.
 
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