I had an issue with a truck not starting cold last year. Tested the GPR, and replaced it, it tested bad. After replacing the GRP, it still wouldn't start. The wait to start light worked, so I was thinking that everything was okay. I bench tested the GPR, it's definately good. So to start the truck right now, I just ran a ground to the GPR, and hold the button like an old time truck...and start after holding the button for 30 seconds.

I really want this thing to work on its own like it's designed to, does anyone have any ideas of what I might look into? It starts and runs great when I manually run the GPR, but I'm thinking it might be the pcm.

BTW, it's a 2001, 142k.

 

Your relay's might not have necessarily been bad. When you tested your GPR's, did you only check to see if power was being transmitted between the two larger posts?

Because you should only see 12v constant at one big post (closest to the passenger fender), and with the key on you should see 12v at both. Now with the key on, you should see 8v at the smaller post (closest to the firewall) and 8v at the other small post.

Your "wait to start" light is a good indicator that your PCM is good and GPC (Glow Plug Controller) is working properly. How long does the "wait to start" light stay on before shutting off? It should stay on for a minimum of a couple seconds (like 5 or so if it's warm out) and shut off after say 20 or 30 seconds (depending on temperature). But in reality, your glow plugs are commanded on for a minimum of 10 seconds, and up to 2 minutes depending on outside temperature (the "Wait to Start" light does not stay on for the whole duration).

I went through this EXACT problem on my buddies 95', (I wasn't seeing 8v at the two smaller posts) it was the GPC harness from the GPC to the GPR.

 

Key off, I checked the larger post that has direct battery voltage, it read 12.7. The opposite larger post read nothing. After turning on the key, checked the voltage again at both posts. They both read the same as before the key was on. That is what prompted me to replace the relay. I never did bench test the original one, as I was 99% sure it was bad. I took it off and tossed it in the dumpster.

But now that you mention it, I will have to test the truck's 2 smaller posts. I think I read somewhere that it was controlled by a ground signal sent to the GPR. That's what prompted me to run a wire to manually switch the ground.

With the key off, should I see anything at the 2 smaller posts?

Will repost on Monday and let you know what I find at the GPR for voltage again.

Thanks for the great info!!

 

No, you will not see any power on the smaller posts with the key off. You should see 8v supplied by the GPC with the key on, engine off OR running on both sides of the relay. (small posts)

A relay (if you don't know this already) is an electromagnetic switch. It uses a small, low amperage signal to control a high amperage circuit. The low amperage circuit being your signal from your GPC, using only 8v, controlling the high amperage circuit, being your 12v.

If you do not see 8v on the two smaller posts, it means that the signal from your GPC is not reaching the GPR to switch the relay on. If you can take test leads from your battery + and -, and touch them to the two smaller posts and the same time, and you see 12v appear on the other big post on the relay, your relay is fine.

 

I'm 110% sure the relay is good due to grounding the terminal and it working. I just need to reconnect the wire to the relay and disconnect the temporary switch I wored up to check for the 8v signal.

I'm guessing that one of the wires is probably corroded through as I've had nothing but headaches with electrical problems due to the previous mechanic probing wires for whatever reason, and not sealing the little hole he created in the wire's insulation. But I looked as much as I could, and could not see any bad wires at the time I was troubleshooting the problem. I was also under the gun to get the truck started because it was snowing and it's a plow truck...

Anyhow, more info tomorrow...

 

[QUOTE=ken_man_1;863920] I'm guessing that one of the wires is probably corroded through as I've had nothing but headaches with electrical problems due to the previous mechanic probing wires for whatever reason, [QUOTE]

THE "MECHANIC" WAS PROBING WIRES?!?!?!?!: ouch:

I'm sorry but the NUMBER 1 thing you learn with ANY type of electrical training is that you ABSOLUTELY DO NOT PROBE WIRES. for this exact reason

But to answer your question, yes. If you are manually controlling your relay and the glow plugs are working, then I'm pretty confident your harness from your glow plug controller to your relay is hosed.

 

Before I go totally nuts, where is the GPC? Then I can go in with an army instead of just a scout.

 

I will be honest with you, I am not 100% on 99-03 7.3's. Although I do belive it's on the passenger side of the top of the motor.

I'm more familiar with the "OBS" 7.3's where it is located under the dash (94.5-97)

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P0683 - Glow Plug Control Module? - Ford Truck Enthusiasts Forums

^ found it on google for ya

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and just my opinion, if the previous mechanic was probing wires, you might as well just order the harness from Ford or International. Chances are the one wire you need (if not more) are in the same condition. Just my 2 cents

 

I hear ya. The issues I've run into were all on the driver side where the 4x4 relays are (shift on the fly), and of course, the trailer wiring. I've had to repair wires on many trucks he put his golden screw driver in. I've gotten most of the problems squared away, as I have good record keeping in place, and I've scoured his records and looked into everything he did. But I think if I get 1 more issue, I won't be repairing it with a sealed butt connector...

 

There is no glow plug controler on most 7.3's... Only the cali models and the excursions have a glow plug controler...

You have a glow plug relay.

There are two of them, one closest to the turbo is the glow plug relay, one closest to the front of the motor is for the intake air heater.

Make sure you replace the correct relay.

The WTS light has NOTHING to do with the glow plug relay, or controling the glow plugs.

It is an idiot light plain and simple.