Troubleshooting Glow Plug Relay Circuit?
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More Turbo
Joined: May 2012
Posts: 542
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From: New Milford, CT
Troubleshooting Glow Plug Relay Circuit?
Ok, for a friends truck. It's a 1996 F350 7.3.
He has a hard time starting it in the morning when it's cold.
I grabbed a test light and checked the glow plug relay for him.
From what i've been reading, one big post is power from battery, one small post gets power when the key turns on, the other big posts sends power to the glowplugs, and the other small post is controlled by the ECM to ground/power the post.
As for the one that's controlled by the ECM, his always has power. It never gets grounded, so the glowplugs never come on.
I've read that the ECM looks at data from the air temperature sensor, and the oil temperature sensor to determine when to ground the relay to power the glowplugs.
I'm thinking that one of these two sensors is bad, or there's a short in one of the wires, and the ECM isn't getting any data from it therefore not turning on the glowplugs.
Or am i totally heading down the wrong path?
Thanks.
He has a hard time starting it in the morning when it's cold.
I grabbed a test light and checked the glow plug relay for him.
From what i've been reading, one big post is power from battery, one small post gets power when the key turns on, the other big posts sends power to the glowplugs, and the other small post is controlled by the ECM to ground/power the post.
As for the one that's controlled by the ECM, his always has power. It never gets grounded, so the glowplugs never come on.
I've read that the ECM looks at data from the air temperature sensor, and the oil temperature sensor to determine when to ground the relay to power the glowplugs.
I'm thinking that one of these two sensors is bad, or there's a short in one of the wires, and the ECM isn't getting any data from it therefore not turning on the glowplugs.
Or am i totally heading down the wrong path?
Thanks.
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More Turbo
Joined: May 2012
Posts: 542
Likes: 1
From: New Milford, CT
I know the glow plugs are functioning because if you jump battery voltage across the relay and to the glowplug stud and hold it there for ten seconds the truck will instantly fire up.
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More Turbo
Joined: May 2012
Posts: 542
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From: New Milford, CT
I figured if it was open then there wouldnt be anything at the relay coming from it.
i ain't rite
Joined: Mar 2005
Posts: 65,505
Likes: 5,563
From: Marlboro Mental Hospital.
correct.
if you want to get him going until you find the problem, you can bypass the pcm with a momentary contact push button switch connected to the ground side of the relay, then run the wire into the cab to the push button switch. the other side of the switch to ground. tell him to key on, hold the button down for 15 seconds, then start the truck.
if really cold he may have to up the time to 20 seconds.
if you want to get him going until you find the problem, you can bypass the pcm with a momentary contact push button switch connected to the ground side of the relay, then run the wire into the cab to the push button switch. the other side of the switch to ground. tell him to key on, hold the button down for 15 seconds, then start the truck.
if really cold he may have to up the time to 20 seconds.
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