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post #1 of 12 (permalink) Old 03-19-2018, 11:41 PM Thread Starter
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Glow Plug Timer?

I have a '95 F350 that I am assuming that the glowplug controller is not functioning properly. I do not think the problem is with the plugs or the relay because (1) I replaced the relay a few years ago when I started having cold start issues but there was no change, and (2) it starts fine if I turn the ignition on and jump the terminals of the relay for 20 or 30 seconds. (I have no issues if it's plugged in, but that's not always possible).
So my question is: What controls the timing of the glowplugs? Is it the PCM? If that's the case, is it simply a matter of replacing it?
I've had this issue for more than 10 years but am finally getting to the age where I would prefer not to jump through all of the hoops to start the truck on a cold day.
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post #2 of 12 (permalink) Old 03-20-2018, 12:56 AM
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Re: Glow Plug Timer?

It sounds like you have another bad relay and need to test it to see if it is bad.

Does the relay activate when you turn the key on? In other words do you get voltage across both of the large lugs for up to 2 minutes after turning the key on?

I would start by testing the relay on the truck. You'll need a digital volt/ohm meter to do this. Turn the key on to start with the wait to start light coming on and then test both large lugs of the relay. You should see less than 0.3 volts difference between the two lugs with the relay on. Any more and you have a bad relay

If the relay doesn't come on check and see if you are getting 12 volts on one of the smaller wires (I don't remember the color), it the wires are still attached to the relay you will get the voltage on both of the smaller wires. If you are getting the voltage then the ground signal from the PCM is bad. This is controlled by a couple of sensors. The engine oil temperature (eot) sensor and the barometric pressure sensor.

And yes the PCM controls the relay after getting readings from those sensors.

96 F250 XLT 4x4, 4:10 gears, 5 speed, Tymar intake and HX hose, downpipe, pyro and boost guages, coolant filter, Luc clutch
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post #3 of 12 (permalink) Old 03-20-2018, 01:05 AM
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Or it could be wired in wrong, I had the same exact problem changed the glow plugs and relay to figure out the the wire coming from the solenoid from the passenger fender was on backwards not letting them get power.
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post #4 of 12 (permalink) Old 03-20-2018, 01:13 AM
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Re: Glow Plug Timer?

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Originally Posted by Macher7.3 View Post
Or it could be wired in wrong, I had the same exact problem changed the glow plugs and relay to figure out the the wire coming from the solenoid from the passenger fender was on backwards not letting them get power.

That would be the way that the wires would be connected on the fender mounted solenoid and if the OP can jump the terminals with a tool and get it to work then it isn't wired wrong on the fender mounted solenoid.

96 F250 XLT 4x4, 4:10 gears, 5 speed, Tymar intake and HX hose, downpipe, pyro and boost guages, coolant filter, Luc clutch
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post #5 of 12 (permalink) Old 03-20-2018, 07:25 AM Thread Starter
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Re: Glow Plug Timer?

I appreciate the information. I am out of town but will test the relay when I get back Friday.
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post #6 of 12 (permalink) Old 03-20-2018, 01:33 PM
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Re: Glow Plug Timer?

it could also be something a simple as the parts monkey gave you a starter relay and not a glow plug relay. the starter relay is case grounded, glow plug relay is internally grounded. they look the same, but are not interchangeable.

to test the relay wires, with a volt meter check power to terminals on GPR. one large one should have 12 volts at all times. with engine cold and key on, one small one should have power and the other should be grounded for at least 90 seconds.
if this is what you get, the relay is bad or the wrong one. if no power or ground in the small wires, the problem is in the PCM, or temp sensor.
if

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post #7 of 12 (permalink) Old 03-20-2018, 08:20 PM
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Re: Glow Plug Timer?

I also doubt that the OP has the wrong relay in the system.

If it was a case grounded relay the glow plugs would be on all the time, at least until they burnt themselves or the wiring out.

96 F250 XLT 4x4, 4:10 gears, 5 speed, Tymar intake and HX hose, downpipe, pyro and boost guages, coolant filter, Luc clutch
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post #8 of 12 (permalink) Old 03-20-2018, 09:00 PM Thread Starter
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Re: Glow Plug Timer?

I actually was wondering if perhaps they gave me a starter solenoid instead. Like I said, it's been a few years (10 or more), but I do remember thinking it was slightly different. At the time I remember actually thinking the glowplugs just used a starter relay.
They definitely have not been on all the time. I don't think they would have lasted this long. When I jump the relay with a screwdriver after about 30 seconds the motor actually coughs. That's how I know the plugs (at least most of them) are still working.
As I said I'll be back in town in a couple of days. I have a good meter. I'm not the best at using it but Youtube is a wonderful thing.
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post #9 of 12 (permalink) Old 03-20-2018, 09:08 PM
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Re: Glow Plug Timer?

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Originally Posted by Bugman View Post
I also doubt that the OP has the wrong relay in the system.

If it was a case grounded relay the glow plugs would be on all the time, at least until they burnt themselves or the wiring out.
not if the power wire is hooked up to the ignition side of the relay and ground is hooked to starter trigger side.

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post #10 of 12 (permalink) Old 03-21-2018, 01:10 AM
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Re: Glow Plug Timer?

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Originally Posted by grounger View Post
I have a '95 F350 that I am assuming that the glowplug controller is not functioning properly. I do not think the problem is with the plugs or the relay because (1) I replaced the relay a few years ago when I started having cold start issues but there was no change, and (2) it starts fine if I turn the ignition on and jump the terminals of the relay for 20 or 30 seconds. (I have no issues if it's plugged in, but that's not always possible).
So my question is: What controls the timing of the glowplugs? Is it the PCM? If that's the case, is it simply a matter of replacing it?
I've had this issue for more than 10 years but am finally getting to the age where I would prefer not to jump through all of the hoops to start the truck on a cold day.
If you have to jump the relay then the relay is bad or hooked up wrong check it with a test like the glow plugs get power for 90 seconds after the key is turned on

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