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Discussion Starter #1
OK, so I got the SES light, checked the code - P0380. No problem, glow plug circuit. Did my research, got the factory service manual so I should be good.

First thing, check for voltage at the relay - good to go, fuse must be good. Then I started checking the plugs at the controller. Found 4 bad ones (i.e. test light to positive on the battery, check each wire, 4 did not light). Ordered and replaced the plugs (I did all 8, may as well, it's an '02 and they are a bit of a PITA). Retested using above method, I got all 8 lit up.

Fews starts later, light comes back on. Checked, friggin P0380 code again!!! OK, what else is in the circuit? Decided to replaced the intake air heater since it was only $60. Again, light comes on.

So, I have checked the fuse, replaced all the glow plugs and the intake air heater. Only thing left is the relay. So I ordered and replaced that.

And the friggin light came back on!!!

I have a Diablo, so I checked real time engine data and it looks as if I am only getting 1.9 - 2.0 volts feedback on the glow plug circuit when the key is first turned to the ON position. Once the engine is running, I get a constant .9 - 1.0 volts (is that normal?). That would indicate 2 bad plugs, however, they are all checking good. I have also checked the voltage at the plug going TO the relay and I am getting 12V at the ignition source, the glow plug relay control coming from the ECM gets 12V for about 16 seconds when the key is first turned on, then goes off and the ground is good.

I am at wits end. What else could it be?? Did I get a bad relay right off the shelf? Any way to check that? Is my ECM bad? Or is there a flash that maybe wasn't done on the truck? (I am the second owner BTW)

Any help or guidance would be greatly appreciated.

EDIT: Truck is CA emissions.
 

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glow plug help!

It sounds like you have checked most of the circuits and still having
unfortunated luck. I am assuming since your from California you have a
California emission truck. As long as both battery feed circuits show 12volts or better at controller(circuit 539 pink wire pin d, and circuit 42 red wire) that portion is ok. Use a good dvom and measure resistance to
ground with key out of ignition(connecter 2 at controller terminal b)should be real low, possibly .5 ohms or less. If all of this is ok and your feedback or signal voltage is low while glow plugs are on I would suspect circuit 506
is possibly open. This wire is light blue and runs from controller to ecm(terminal c at controller and terminal 52 connecter 1 at ecm). One other
thing to check is terminal tension at all connections related with glow plug
circuit which includes connecter c107(large 38 way in-line connecter on top of engine). Good luck, i hope this helps in some way, if not clear let me know.
 

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Discussion Starter #3
Thanks for the reply Scott. OK, I checked the schematic against what you posted and I understand where you're coming from. I'll have to wait until tomorrow since it's dark now, but I've got a place to start at least.

Ken
 

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I have the same problem only I have not gone through all of the trouble you have trying to get it to go out. I've been told that the predator will cause this code to be thrown. It's been doing it ever since I installed the predator last year sometime. I just check everyonce and a while to make sure it's the same one and just ignore the SES light. I have an 02 as well btw.
 

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i have an 02 also, ive had that code for about 2 months now, had it when it was stock and with my PPE. idk whats wrong with it aside from a bad plug or two, or the controler, or the relay, or the fuse...:eek: this problem is covered under hte federal emissions control warranty which is 5/100,000. my 5 years is up next month so im goin to the stealer armed with info. tell them to look in article 1.2 page 15&16, there its states the emissions warranty and that there is NO deductible. Ive given up trouble shooting this sh!t, let them deal with it.
 

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i just posted the same code, only mine didnt allow the glow plugs to cycle off (after starting the volt. meter never jumped backed up indicating the glow plugs never shut off) after i shut the truck off batteries were dead in a matter of minutes, I went down to the stealer and picked up a glow plug controller, the black box just behind the fuel lines on the back of the drivers side valve cover, just a few nuts and replaced in a few minutes.
cleared the codes and all is great, i read some posts on here (do a search P0380) seams to be a common issue on the 02's with about 68,000 miles average, mine has 66,000 miles and it went out.
 

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Discussion Starter #8
Well, just came back in from testing. Ground came out fine, pretty much 0 ohms resistance to battery ground. Both ignition source and relay power read 12V. I plugged everything back in (after checking the glow plugs yet again - all good) and managed to get the volt meter wedged in to the ECM return wire at the GP controller plug. Sadly enough, it read exactly the same voltage as the Diablo does at the OBDII connection; 2 V then down to 1 V after the key had been on about 15 seconds.

Now the big question - the wiring has been taken out of the equation, as has every component in the circuit. Now what? For whatever reason, the return voltage from the glow plugs is incorrect. Why? Should I suspect an open in one of the glow plug wires? That would explain why the test of the plugs at the controller keeps coming back good. If the wire had rubbed thru and was grounding itself out on the block or frame, then my rudimentary test of test light to wire would always come back good. However, would this cause the issue that I am faced with as far as incorrect voltage reading being returned to the controller and ultimately the ECM??

Sorry for the rambling, but I am grasping at straws here.
 

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glow plug help!

Been places like this before where you feel like giving up. Just a couple of
thoughts to try. First, does glow plug code set with stock program
installed only? Also being second owner how long have you owned truck and when did fault start occuring? It could be possible that incorrect
calibration was installed in truck but unlikley, but this could cause fault.
Only way to tell is with scan tool under glow plug data should read system
type-California not federal. As we both know controller is reading glow plug voltages and sending feedback to ecm to determine on time of plugs. You mentioned if wiring could be grounded and causing concern, i would say yes. To check would need to disconnect each glow plug indiviually
and disconnect from controller. Using dvom check for resistance to ground
on all circuits, while there I would check for short to voltage and ohm check each wire and compare. If any show resistance to ground, short to power, or high resistance you have a problem. If none show problems and rest of system is ok i would replace controller; you mentioned replacement of relay. Did you mean intake air heater relay or controller?
 

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for all this trouble it might justify the $80 diagnostic charge sad to say. I hate spending unecessary money but with the number of things that could be wrong i guess it might be worth it. mine doesnt really seem to be affecting anything its just annoying to see that darn light on all the time. Im gunu see if the stealer will warranty mine tomorrow.
 

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Discussion Starter #11
Yes, the code was set before I tuned with the Diablo.

I bought the truck about a month and a half ago and this problem was identified by the PO. So I got into it knowing the GP issue, thinking I could fix it. Truck runs great otherwise, all the injectors were replaced about 25K miles ago and it's ultra clean.

I have done the exhaust, air intake and Diablo tuner since I bought it.

Unfortunately, the Diablo does not display whether the GP system is CA or Fed, at least not any place I can see.

When I said controller, I did mean the GP relay/controller for CA.

I posted a VIN check request on here a few days ago and got the warranty work done to date. It confirmed the injector replacement, fuel filter replacement and a couple of other issues. No mention of any kind of reflash or ECM replacement, so I can only assume the truck was not modified in such a way that a Federal part was put into the CA GP system.

In hindsight, I suppose I should have either insisted the problem be fixed or passed on the truck. However, compared with what was out there in this price range, this truck was head and shoulders above the rest. And so far, this has just been a mild PITA for me, and has cost me a couple hundred dollars. I have a difficult time trusting the dealer, just because they hardly ever take the customers word for anything.

One more question - when removing the GP controller/relay, the factory manual basically says to remove the heavy gauge red wire that supplies the voltage from the 175 amp fuse. OK, I get it, however, there is another wire underneath that one, much smaller gauge, that runs toward the bottom of the engine. No mention of this wire anywhere I can find in the manual. The power post on the controller has a double nut setup, one holds the small diameter wire down and the other holds the main power feed down. So, what is the smaller wire? Is it possibly the lead for the intake air heater? I have not traced it down yet, and that will be the next step in my process, but I just thought I'd ask here first.

All of the assistance and repsonses are greatly appreciated!
 

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The new controller comes with the 2 small wires already attached and everything in the box is complete, factory coating grease and all, the part was less than 80 with tax, (made in japan and all)
fixed mine without a hitch. It cycled through the start up and kicked off just like brand new.
I was going to send it to the dealer, but i didnt have the time to drop it off, (my warranty is up 4/15/07), and wait, and then fight with them because of aftermarket parts or deductables,
I found out the hard way when the controller locked the glow plugs to on and killed my batteries deader than hell as soon as i shut the truck off.
 

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Discussion Starter #13
Well, the Cali truck is slightly different from the feds. My GP relay/controller has connectors along the backside of it. The wiring harness in the truck plug in and consist of 3 four wire connectors, two on either side for the glow plugs and one in the center for ignition source, ground, relay control from the ECM and feedback voltage to the ECM.

I know there's a relay for the intake air heater, but I think it's good because the Diablo reads 12V to it when the truck is first turned on and cold.

Another thing I noticed this morning, when I turn the key and wait for the "Wait to Start" light to go out, the battery light comes on and then after it starts, my voltage gauge reads around 12 V for about 30 seconds. Then it suddenly pops up to around 15V which seems to be the normal reading for the truck. To me, this would indicate that the glow plugs are cycling on, and things are working correctly. So the only mystery is why is the feedback voltage only 2V?

The last thing to note, when the truck is off and no key in the ignition, I get a reading of approx. 0.2V at the ECM feedback on the controller. Not sure if that's normal or would it indicate a problem?
 

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I had the same problem last year. I changed that fuse and the code kept coming on. I took it into the dealership and they had it over one week before they found out that the entire brains of the truck had to be replaced. I was without my truck for 12 day. GMC technical support had to get involved and they advised the dealership that they had never seen a problem like this. The P0380 code doesnt come up anymore, but my dic still says "battery not charging" when I start it up. GMC said they could not find the cause and it wasnt affecting the vehicle, so they gave it back to me.
 

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Po380!!

Sounds like your truck is nice we just need to get those darn glow plugs
working right. The other wire leaving controller would have to be a fusible
link type wire if sending voltage to intake air temp relay, but that is the only thing it could be. Also i have been thinking some on this one and if all
circuits are providing good power meaning test light is good and bright and
that ground is ok lets try something to see if ecm recognizes 12 volts on
feedback circuit. In order to do this disconnect your 4 way connecter providing powers and ground(C2). Jumper terminals for glow plug relay control to feedback wire or yellow to light blue and then turn key to run
to cycle glow plugs on. On your diablo see if it shows 12v or battery
voltage. If it does we know both wires are good and ecm can recognize
higher voltage. If it doesnt read this something appears wrong in circuit or
ecm at fault. At that point I would backprobe at ecm and see if voltage
is correct. If not I would be looking for a open or short to ground in feedback circuit. If this all comes out ok I would go back to controller
ground and physically check it for being loose(G104) and ohm check while
moving wire to see if changes at all. Still nothing then there has to be
something wrong with those glow plug circuits and again would need to check them individually and compare. I hope this helps I can't think of
much else to try. Good luck!
 

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Discussion Starter #16
I only had time to try one thing this evening and that was the feedback voltage. I got a solid 11.9 V, pretty much battery voltage. That's good in a way, I don't think I want to know what it would cost to replace the ECM. :eek:

So, it'll probably be the weekend before I'll tear into the glow plug circuits and ground. My suspicion at this point is one of the GP wires grounding out on the block. As much as I don't look forward to checking each one of those, it seems to be the absolute last piece in the puzzle.

I will update again once I have done the other two checks.

Ken
 

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Weekend Fun

I'm glad to hear those two circuits and ecm ok! I would diffently look into
main ground first then glow plug circuits. If any doubt in ground run a
wire from battery ground to controller and see if it helps. Poor grounds can cause havic. Good luck.
 

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The ecm is only $300 from vendors on the site here. The battery not charging is a sign that the wrong ecm program is in the truck. As far as the glow plug I don't know for sure. If you talk to Guy at socal - diesel he can get you an ecm and if you give him your vin he can pre-program it to your truck.
 

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Discussion Starter #19
Well, been a while since an update, and truthfully, this is the first chance I have had to tear into it again. Had to replace the vacuum pump, so this was a good opportunity.

I checked all of the glow plug circuits, and they are all OK. All wires measured .2 - .3 ohms of resistance from the controller harness to the glow plug connection.

I checked ground wire resistance at the controller and got .2 ohms from the wire to a ground point on the firewall. I would assume this is good as well.

I checked the other circuits as well, and found ignition voltage to be within spec, relay circuit voltage came on when the truck was first started and remained ~ 11.5 Volts until the plugs were commanded off, then it went to 0.

So the problem still remains that the feedback voltage only goes to 1.2 Volts max, then back to .7 Volts when the plugs are commanded off. The Diablo reads it this way as does a meter at the contoller.

I even went so far as to pull the 175 amp fuse and use a piece of copper solid conducter wire as a bypass. No difference.

I am completely baffled. All the plugs are still checking as good by using the test light from b+ to each plug at the connector.

The ONLY thing I can come up with at this point is the fact that I did not use genuine AC glow plugs when I replaced them. I used some that I found on eBay that were touted as "instant on". Would there be enough of a difference in those plugs versus AC to cause me to not get the correct feedback voltage??? I am seeing the plugs come on, by virtue of the fact that my gauge is about 11 volts at startup and about 30 seconds into running, jumps up to 14.5 volts. So they appear to be functioning, but either the relay (new) or the plugs (new, non AC brand) are not sending the correct signal back to the ECM.

UGH!!
 

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did you try putting an amp clamp on each glow plug wire,one from a battery tester works well.each one should show a draw,if it doesnt the plug is ng.on the scanner the feedback voltage should be about 5.8v when they are all good.
 
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