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Discussion Starter #1 (Edited)
Hey guys I have a 96 7.3l. This only happens when the motor is cold. About 10 seconds after the motor starts I can see the battery gauge on the dash surging up and down about 1 sec. intervals. I can hear the idle increasing with voltage at the same time. This last for about 30 seconds and then everything returns to normal. It does this every morning. If the motor is warm it doesn’t do this at all. I know it’s normal the glowplugs continue to operate after the motor starts until the gpr kicks off but this issue is new because it is as if the gpr is cycling on and off for about 30 seconds. But again it does it every time the motor is cold and never does it when it’s warm so so I’m not sold that’s it’s the gpr. I haven’t replaced the gpr yet. I wanted to get some feedback before loading the parts cannon. The glow plugs are about 2 years old and are motorcraft. The valve cover connections visually look good. The batteries are in good condition with clean terminals. The alternator is recently replaced but this problem existed before I replace d the alternator. I’ve also heard that the PCM controls when the gpr gets turned off but I don’t know how I would go about diagnosing the PCM. Any suggestions would be greatly appreciated. Thanks.
 

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I've seen the same issue reported before, but I don't recall a resolution to it. Just me, I think I would throw a Western Plow 56131k relay (or a White-Rogers 586-902 relay) on it and see what happens. The Western Plow unit is a lot cheaper than the W-R, but both are excellent (personal experience with both on 2 trucks). I guss you could see if there is some kind of voltage variation on the small control wires of the relay to check if the PCM is somehow cycling the relay. The PCM provides the ground to activate the relay. Cheers!
 

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I had it happen on my truck just once. I believe that it was the relay for causing it since I went to the Stancor White Rodgers a week later and haven't seen it happen since for the last 20 years

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Could have sworn this was normal operation.
Iirc, the explanation was it's to regulate charging voltage on start up. Basically to keep from frying anything/burning out bulbs.
The other WAG I've seen is to help clear startup smoke, by keeping the glowplugs hot for a bit.

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I suppose one thing to check before ordering the new GPR would be loose connections on the current GPR.

EDIT:
I always watch the Voltage on my 2002 at cold starts and it does not bounce. It is a steady draw until the Relay shuts off.
 

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FWIW, I've never had fluctuation on either of my trucks. The OBS trucks do not vary the voltage for glowplugs (at least intentionally). Cheers!
 

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Discussion Starter #8
Just an update. I had access to a spare alternator and replaced my other. The battery voltage is a steady 11.2v while the gp are on and 14.4v thereafter. When I swapped the pulley on my old alternator the paperwork said it was very important for proper operation to torque it correctly but never gave a torque value (Bosh alternator) or... maybe I put too much dialectic grease on the connections because I cleaned those up or... just a faulty alternator. Either way I’m glad it’s working correctly now. If I get bored I might swap the original alternator back to see if the problem comes back with it but I doubt it.
 
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