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No Signal To The Glow Plug Relay

119K views 37 replies 14 participants last post by  charlie-4 
#1 ·
I searched for a while but can't find the answer.

I thought my glow plug relay went out so I replaced it. I used a test light and found that the one side that is always hot is hot. But the side that should get hot when I turn the key to the on position never gets hot. Like a dummy I did not bother to check anything else and bought a new relay. I figured it was old enough. But after putting in the new relay it still does the same thing.

So I got out my volt meter to check the small terminals on the relay and they never get any power to them. All of my fuses under the hood seem to be fine. Not sure exactly what fuse is in control of the relay.

In the morning I have to pop the hood and use a screwdriver or whatever to jump the relay. For the rest of the day it starts fine. Would be nice not to have to crawl around the truck in the morning.
 
#2 ·
one small wire should get power when key is turned on. I would double check the fuses with ohm meter. The other small wire grounds the relay from a signal from PCM and then ungrounds it to turn glow plugs off
 
#3 ·
I too would like to know what the fix is. I had the same issue and blamed it on the relays, I bought 4 before I figured it wasnt the relay. I ended up putting a switch inside the cab to ground out the relay for the PCM. I would like it to work correctly but that was a quick easy fix to get me through winter.

I was told it is either the wire from the PCM or the PCM isnt sending the signal (grounding out) with the key. I cant find any broken wires and Im not sure if a PCM reflash would fix it or a whole new PCM would be the fix.
 
#4 ·
Double check fuse 22 under hood
If relay is not getting grounded from PCM then the diode in PCM burnt out and if the small wire on relay is not getting power when key is on then double check fuse 22
 
#5 ·
So if fuse 22 is good then the PCM needs replaced?
 
#6 ·
Fuse 22 is the fuse that also controls the fuel heater in the bowl?
 
#7 ·
I think you are right. Seems I have checked that fuse before and it was fine. So I might be down to a new PCM.
 
#8 ·
Double check fuse 22 for power and double check that one small wire on relay has power when key is on. If one small wire has power than the other small wire gets grounded from PCM unless PCM diode is burnt

If no power at all to one small wire fuse 22 is bad or bad wire
 
#9 ·
Happen to know what the ohm reading should be? or does any reading mean it is ok? Like I said I think I need a PCM.
 
#10 ·
Fuse 22 is ok. My electric pump gets power from the old fuel heater.

No power to any of the small wires. I will try to check the wires and make sure there is nothing wrong between there and the PCM. If the wires are good then I guess it's PCM time.

I did have the PCM out not too long ago when I swapped out my split shot injectors to single shot. But everything was fine for a few months after I swapped all of that out.

Do both of those small wires go to the PCM?
 
#12 ·
Only the ground wire for the relay goes to the PCM. The relay gets its power on a R/LG wire that is connected into the same wire R/LG that connects to the fuel bowl heater and then back to the under hood fuse panel.
The relay gets it ground from terminal #101 on the PCM on a P/O wire.

If the PCM isn't sending the ground signal you could always place a push button switch to a ground to manually control it.
 
#11 ·
Just check for infinity reading as the bar on meter should go all the way over or read 000 on digital scale. Would be just like testing a piece of wire at each end with the tester. If you have power on one small wire when key is on, I would bet the PCM diode is fried and u need pcm. As stated you could test the wire with ohm meter at gp relay and at PCM. I don't have the pinout number at the moment
 
#13 ·
Found this diagram...



One wire goes to the PCM one goes to "Keyed Power". I am guessing the keyed power is from fuse 22. My fuel pump definately works. Maybe I will get lucky and it will just be a wire.
 
#14 ·
After some more reading I am not much closer to fixing this.

My fuses are all good. The wire to the computer is good. Is it possible for the PCM to go bad and just effect the signal to the GPR? I hate popping the hood and having to jump the relay. I only have to do it once in the morning at least.
 
#15 ·
yes if the diode fries then it will not ground the relay when it is suppose to.
In that case the computer needs replaced
 
#17 ·
Frustrating. I wonder if I did something when I pulled the PCM to put the new tunes in for my single shot injectors?



Did you verify power to the small terminal(key power)?
With the key on I tried a test light to both small terminals and got nothing. Even broke out the volt meter and it read nothing.
 
#18 ·
Only ONE small wire will have power with the key on. I would take the small wires off the relay and test it that way.
Like the diagram shoes, one small wire gets power from the Fuse #22 circuit(which you checked and said #22 is good.) So you need to check the wire where it goes to the relay and make sure you are getting power to it there.

If you are then that is Not the problem.(see #2)

If you are NOT, then you will need to trace that wire back and find the break in it.

2. If you have power on the keyed power wire, check ground on the PCM ground wire. Take the wire(purple/orange) off the relay and put the test light to that wire. Then the other end to the POSITIVE terminal on the battery. if the test light lights, your grounding circuit in the PCM is Good.

If NOT, trace the wire back to the PCM and make sure there is no breaks or check continuity. If the wire is good, them your PCM is bad
 
#20 ·
Thank you very much. I can't wait to get home and try all of that.
 
#19 ·
I was having issues with mine too. I put a new relay in as well and I am still getting intermittent extended cranks like the relay doesnt come on.

I had someone turning the key on and off for me while i was measuring the posts with a voltmeter. everything seemed to be checking out, I would here a high pitch buzzing sound somtimes when the relay is on. most of the time it didnt. I think there may be some kind of connection issue.

has anyone run their glowplug relay on a full manual switch? I might just do this, luckily I am running into this problem now and not in the chicago winter.
 
#22 ·
I had someone turning the key on and off for me while i was measuring the posts with a voltmeter. everything seemed to be checking out, I would here a high pitch buzzing sound somtimes when the relay is on. most of the time it didnt. I think there may be some kind of connection issue.
What brand and type (part number, etc.) of relay?
has anyone run their glowplug relay on a full manual switch?
The manual switch would only override the PCM switching the ground leg of the coil circuit. If that's not the problem, if it's anywhere else (cheap/defective relay, wiring issue anywhere else, glow plug resistance, etc.), a manual switch will be of no help.
 
#21 ·
luckily I am running into this problem now and not in the chicago winter.
I was thinking the same thing. Winter in San Diego gets brutal. :)
 
#24 ·
I am running a GPR109 from napa. It was doing the same symptoms with my old factory GPR. I dont know about the buzzing noise tho. I am going over to my buddies house right now to do some testing to see if we can figure it out.
 
#25 ·
Got some stuff figured out.

I cycle the key forward twice and does nothing, but then I try a third cycle, and it clicks on and works fine. It intermittently working fine or not at all.

So I am thinking the problem is lieing in the wiring from the small posts to the pcm connection. If that all checks out, then It must be a pcm grounding issue.
 
#27 ·
When the engine oil temp is above a certain temperature it wont turn the glow plugs on, so if its a rather warm day outside, to save the glow plugs the computer will not turn them on.

As for a retrofit to a mechanical glowplug timing system, old mercedes (1975? to 82 i believe 240d, 300d, 300td, 300sd etc diesel only models) used mechanical 15 second timers for the glowplugs, im sure you could retrofit one of those to turn on the large relay for 15 seconds and turn off.
 
#28 ·
I just put a chip/tuner in my 00 F350 7.3L, after I installed it, the service light came on. ran the computer and it said the glow plug control module was throwing a code...I am not able to find this piece but I have found the glow plug solenoid/relay on the top of the manifold, next to another solenoid(no idea what this solenoid does). I have tested both the old and new glow plug solenoid and both give me the same results; when the key is off there is power to the hot side, and when the key is on both little posts and the big hot post have power but the other big post does not....So if anyone could help me out this would be great
 
#30 ·
Alot of good info in this thread just looking for a little more insight. Which one of the small wires should have key on power? Neither of mine do but would like to know which one to trace first. Also the fuse 22 is good...

Also when checking continuity both small wires will show no reading at all tester says OL, while the main power wire shows nearly 0 and the wire coming from the seloniod shows 3.7 ohms. What am i missing?

Thanks in advance for the help

Edited
Google searched to find this just looked to see that it is in the obs section, while i am working on a 2001 sd
 
#31 ·
Reading this post to try to zero in on my current problem. I too am having hard start problems on my 95 Powerstroke. If I plug in the block heater it starts fine, but if not it won't start (current temps 20's to low 30's). Installed a new glow plug relay but still hard starting. Put a test light on the relay and found the following. Power coming in from constant hot (battery). With key in the on position both small terminals had power, which would suggest to me that the PCM is providing a good ground. Still no power coming out of the second large lug. Anybody have this problem? Any thoughts would be appreciated.

Thanks
 
#32 ·
If the small terminal going to the PCM still has voltage with the key in RUN, that means the PCM is NOT grounding the coil circuit. You're seeing the voltage from the battery, minus a little for the resistance of the coil. When the PCM grounds that wire, voltage at that terminal should be zero.

Try jumping that small terminal (should be pink/orange wire) to ground with the key in RUN. That should energize the relay and then you should see voltage on the "second" large terminal. Then check continuity on that wire between that terminal, and pin 101 of the PCM connector.
 
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