Another GP relay question

It seems like there are tons of threads dealing with GP's and cold start issues but I haven't been able to find the answer to my question. My 88 7.3 idi has been taking awhile to start in the last month or so, it probably takes around 15 seconds of cranking in the cold to get it running. I replaced the GP's with ZD9's less than a year ago, so I went straight to the relay with my multimeter. Key off at the hot terminal I'm getting 12.5 volts, key on at small terminal with red wires I'm only getting 9.5 volts. Does this mean I have a bad relay? I hope I don't have to replace the whole controller.

Grady J said:

It seems like there are tons of threads dealing with GP's and cold start issues but I haven't been able to find the answer to my question.

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I love it when people say that. Lol
There will be a pretty good voltage drop as the glowplugs pull the system down, but that's a little much.
Check the hot side vs the output side (both large terminals) while the glowplugs are on. You'll have to move quick or setup two volt meters.

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79jasper said:

I love it when people say that. Lol
There will be a pretty good voltage drop as the glowplugs pull the system down, but that's a little much.
Check the hot side vs the output side (both large terminals) while the glowplugs are on. You'll have to move quick or setup two volt meters.

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Hot side drops to just under 10v after I turn the key on and then starts to climb up. output is pretty much 10v as well, same with the small terminal with the red wires. Does that sound normal?

Roughly. The small terminal doesn't really matter much. How about voltage at the batteries while glowplugs are on?
The relay sounds okay though.

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79jasper said:

Roughly. The small terminal doesn't really matter much. How about voltage at the batteries while glowplugs are on?
The relay sounds okay though.

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Batteries go from 12.6v with key off, 11.8 with glow plugs on.

Okay....
So I would look at the power wires feeding the glowplug relay. Follow them from the fender solenoid. Iirc they're either yellow or black with orange tracer. One of the fusible links could be bad. Or, the harness connector could be fried. (Pretty common)
Imo, a 2 volt drop from the batteries to the glowplug relay is quite substantial.
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Voltage changes quickly in the first second or two that you turn on the gp's. Been a while but sounds right to me. After fifteen seconds of cranking they're cold again. Probably that's the point fuel is primed again due to an air intrusion issue.

79jasper said:

Okay....
So I would look at the power wires feeding the glowplug relay. Follow them from the fender solenoid. Iirc they're either yellow or black with orange tracer. One of the fusible links could be bad. Or, the harness connector could be fried. (Pretty common)
Imo, a 2 volt drop from the batteries to the glowplug relay is quite substantial.
Sent from my SM-G892A using Tapatalk

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I was finally able to get around to working on this issue this weekend. I followed the two yellow wires to the starter solenoid and it looks like the previous owner did some tinkering, they even cut out the fusibile link on one of the wires. I just decided to delete the two wire setup and run one 4 gauge wire from the GP relay to + battery terminal with a 175 amp megafuse. I then re-tested the voltage drop at the relay and it now only drops 1 volt. I would say this solved the issue, cranking time has at least been cut in half. Thanks.