Glow Plug Relay/Circuit

trucknorris · Oct 21, 2017

My glow-plug circuit was working fine for last 3 years, until last week. I didnt bother looking into it because the truck would start w/o glow plugs... it was hot outside. But now that cold front is coming, I want to fix it.

What I have done so far:
+12v is present on high side at all times
+12 is present on low side with ignition on
Resistance was good on all 8 plugs. (0.5 ohms)
Cleaned grounds on both batteries.

Interesting Tidbits:
The Wait to Start does not light up; other lights light up during cranking.

There is no relay cycling/clicking noises with ignition on.

It's acting as if I have a bad gp or bad ground. What else to check?

Going to check if I can manually cycle the relay by grounding the relay.

Thewespaul · Oct 21, 2017

Sounds like your relay went out, I would start there

trucknorris · Oct 21, 2017

I was able to manually jump the relay. The relay works. Does anybody know what resistance should be normally between white and ground? I am getting 10.3 Ohms

trucknorris · Oct 21, 2017

Question - if a wait to start bulb burns out, would it affect the glow plug controller from working?

IDIBRONCO · Oct 21, 2017

I don't think so. I think that the light is lit with power from the controller, not the other way around.

trucknorris · Oct 22, 2017

Yeah, just making sure the logic board on the glow plug controller would still operate with the light in open circuit.
If the logic board is not affected by the WTS light, then I think my controller is toast.

Can anyone test resistance on a connected glow plug controller/relay between the engine and white lead (-) connected to the glow plug relay?

I'm getting 10.3 Ohms; key off.

How much resistance should be on the brown wires to the GP? I had about 0.8. Should have wrote down resistance between the yellow and the green bus wire.. It was about 600 kOhm.

OLDBULL8 · Oct 22, 2017

Make sure the Black wire on the GPC has a good ground, not corroded or rusty.
Take a jumper wire from the battery Pos + and jumper it to the Red wire on the GPC, that should energize the relay, with a VOM set for 12VDC + you should get that 12VDC on the output side to the GP's of the relay, NO 12 volts, then the relay is bad, make sure you have 12VDC on the battery side of the relay first.

If the temp is around 70*F or above with the key ON, the GPC might just cycle the GPC for only a second, the colder it is, the longer the GPC cycle, then a ON/OFF cycling of the GPC.
Even one bad GP will cause the GPC to short cycle.

Make sure the Green and the White wires have a good connection.

If you have to replace the relay, make sure it's for a GPC and not a starter relay/solenoid.

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trucknorris · Nov 5, 2017

The relay is fine. It gets power on both sides, low and high. What's not working is ground/controller. I cleaned the black ground connection. It was fine.

The relay never clicks, like it should do if there is a bad gp. It never cycles, not even on initial key-on.

Manually grounding the relay energizes the relay, and allows high side to power all glow plugs. All 8 glow plugs get warm.

I'm pretty sure controller is toast. What a bummer.

franklin2 · Nov 5, 2017

Do what I and many others do; Take the white wire off and tape it. Take a new wire and hook it where the white wire was, and run it into the cab and hook it to one side of a momentary pushbutton. Hook the other side of the button to a good ground.

Push and hold the button for a few seconds to glow the plugs and then start it.

trucknorris · Nov 5, 2017

That's the plan at this point.

Glow Plug Relay/Circuit

trucknorris

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Thewespaul

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trucknorris

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IDIBRONCO

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trucknorris

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OLDBULL8

Good Morning Ya'll.

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