1994 7.3 idi glow plug problems

1978FrankenFord

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i have a 1994 f250 with a 7.3 idi swapped from a 1990 f250 the glow plugs used to make kind of a clicking noise before and now there is no noise and also the voltage does not go up and down like it used to. What would be a cause of this? Also my temp guage is pegged on hot and never used to do that. Any help is appreciated!
 

austin92

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If it used to click off and on, I believe that means the solenoid is bad. If your gauge is pegged hot I’d say the glow plug controller thinks the motor is actually that hot and not turning on the glow plugs. If it were my truck, I’d try the coolant temp sending units first. Might fix your gauge and glow plugs


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79jasper

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Glowplug operation has nothing to do with the coolant temp sensor.
Sounds like you're relay might be bad. But you need to get out a voltmeter and do some diagnosing.

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OLDBULL8

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If the temp sensor wire is grounded, it will peg the meter. Coolant sensors are mounted in the front of the drivers side head. upper one is the gauge sensor, lower one is over temperature.

The glow plug controller continues to stay on after the initial GP cycle but it can pulse the relay for up to two minutes depending on the ambient temperature. As hot as it's been just about ever place, the GP controller might not even turn ON. How long does your WTS light stay ON ?
 

austin92

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Glowplug operation has nothing to do with the coolant temp sensor.
Sounds like you're relay might be bad. But you need to get out a voltmeter and do some diagnosing.

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How does the controller read engine temp then? The glow plugs don’t come on if the engine is hot.


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Thewespaul

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How does the controller read engine temp then? The glow plugs don’t come on if the engine is hot.


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It does come on, the controller uses the resistance in the plug circuit to decide when to turn on. The glow plugs are PTC resistors so as their temperature increase so does their resistance.
 

austin92

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It does come on, the controller uses the resistance in the plug circuit to decide when to turn on. The glow plugs are PTC resistors so as their temperature increase so does their resistance.
My 6.9 has the old style controller and doesn’t give the wait to start light much above 130* coolant temp, sometimes 140*. On the resistance thing, learn something new everyday lol


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1978FrankenFord

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My weight to start light usually lasts about 5 seconds or less. I got it to where it makes a click right when i turn the key on and it reads the correct voltage on each side of that solenoid. And then after 5 seconds or so it kills the power to the glow plug outlet side of the relay. So would this be considered fixed?
 

Thewespaul

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My 6.9 has the old style controller and doesn’t give the wait to start light much above 130* coolant temp, sometimes 140*. On the resistance thing, learn something new everyday lol


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6.9s didn’t have the solid state controller so didn’t have some of the issues the 7.3s had but when they fail they normally fail stuck on and burn your plugs so pick your poison or just go with a manual push button
 

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