My glow plugs often times do not come on. Normally, on a cold start, when I turn on the key the voltage drops down to about 12 volts give or take, the glow plug light goes off and it start right up on cold mornings. But lately, the voltage doesn't drop, it stays really close to 13 and you can crank and crank and crank.....Anyway, here's the strange part, every single time that you think it is not going to start, the last time you try to start it the voltage drops down like the glow plugs are engaging and it fires up like a champ.

I have changed the GPR, put brand spanking new batteries in it, tested the charging system and it charges around 13-14 volts or so with every single thing on that can turn on in the truck. I do not think it is power to the plugs being a problem as you can jump the relay and it will always fire right up. This truck has no codes stored in the computer according to schucks, no SES light, and runs flat out perfectly once started.

I am at a loss. Someone a few weeks back told me that some oil temperature sensor turns on the glow plugs, but our local ford dealership doesn't have a clue what I am talking about. Also, someone mentioned checking the MAP which I have been told will throw a code, which my truck doesn't have.

I am frustrated, you guys have any ideas?

 

IIRC the dlow plug relay gets it's signal from the PCM via a ground. Check to see if it is loose.

 

Not to sound like a moron, but are you certain you changed the GPR and not the relay for the AIH (that sits right beside the GPR)?
Put a meter on the GPR to see it is closing the relay.

 

Steve,

Thanks for your help. I think I changed the right one. It is the one towards the back of the engine, right? No matter anyway, I changed them both just to be sure The GPR has 12 volts everywhere except for the wire that supplies the glowplugs. When it actually is switched on, all is well so there is no need to check it, and you hear that audible click when it works.

Does anyone know where the ground is? I just changed out the parking brake mechanism, it isn't over there is it? Just curious, if the pcm is grounding the circuit and it makes sense that it does, if I had a ground problem wouldn't I have a whole slew of other weird things going on? Like I said, this thing runs just perfect once you get it started.

 

Put a volt meter accross the little terminals. Turn the key on. Let us know what your seeing. Also, what model GPR did you put in? The ford one?

 

Glow plug operation is dependent on oil temperature and altitude (and if battery voltage is over 16v, it will limit on time). The GPR receives a key on signal from the ignition switch, and a ground signal from the PCM. With the key on you should see voltage at both small terminals and one big terminal always. When the PCM activates the GPR, you should see ground at one small terminal and power at the other small terminal and both big terminals. If one small one doesn't change to ground, then you have a ground problem through the PCM or one of the sensors is out of range. I don't believe the MAP sensor will keep the GP's from coming on at all, it will extend the time if it is at high altitude. The oil temperature can however, so if it things the truck is warm it won't turn the glow plugs on. However, it sounds like you have an intermittent problem with either the power or ground signal. I would wager the power signal, either from the fuse or the ignition switch itself. I'm guessing this isn't the 1978 in your profile, so depending on the year depends on where the fuse is for the GP signal.

 

I put the ford GPR in it. It is an expensive little guy.... Across the little GPR terminals there is around 12 volts with the key on all the time. As it was mentioned, this relay is not being grounded for whatever the reason. Anyone have the part number for that oil temperature sensor that my friendly Wasilla Ford Parts Dealer doesn't know exists? If it is not hideously expensive I am going to buy one and give it a try. Thanks for your help guys

 

The truck is a 2000 ford f350 dually 4x4 crew cab. I have owned since 2001 and loved it. It has 84000 miles on it. I will check the fuse you mentioned if I can find it. But, if it is in either one of the two fuse boxes it is good as I have checked them all. Any ideas where it is at just for verifcation purposes? I do have power to the relay all the time.

 

Just try and unplug the EOT before starting. Without the sensor, the PCM should go to default and energize the GP system to max time. If your GPR works every time, I would say that your EOT sensor is bad and giving the PCM a false reading.

 

I have not. I will try it and keep you posted. Thanks for the idea.

 

The gpr will only come on with less than 167 degree coolant and air temp below 45 degrees. The voltage on the glow plug side of the gpr when energized should read the same voltage as the batt side. I just changed on that didn't work that was showing 2 volts less, seems to have fixed my problem. By the way I used a gpr relay from Auto Zone for $15.99, part # F494, the mount is 90 degrees off, works fine.

 

Soo, the black probe on one little terminal, and the red probe on the other little terminal, at all times is 12 volts? If thats the case the solenoid should always be on, atleast the way solenoids work, all your voltage drop is accross the solenoids coil?

 

The terminal with the rubber cover is the batt. (12v or more) other large terminal (0v) until ignition turned on. Both small terminals have 12v until ignitiion on and pcb changes 1 terminal to gnd and energizes the relay. This should give the large terminal, the one that had 0v, the same voltage as you haveon the rubber insulated large terminal. You will also hear a click as it activates.

 

This is why I had asked to check accross the two little terminals. (coil) With what you said above, the reading accross the two terminals would be 0 volts, since they are both at 12v. There is no voltage difference between, them they are at the same potential, and the coil is not engaged. If he measured both sides, just like you stated, then both sides should be around 12 volts, expecially if the solenoid isnt working. But, if there is 12 volts ACCROSS the two coil terminals, the coil is engaged. One side is pulled low, and the other side is hot.

 

I agree what you said, you get same results , maybe less confusing.