thermo switch sd25

[ten10]

I just bought an 83 nissan pu sd25. It has 108000 miles on it and ran home fine (90 miles). I started it up and drove it a few more times, then it quite starting. I am being told that either the glow plug control unit or the thermal switch is bad. The glowplug control unit i can find 500 bucks. the thermo switch i can't locate anywhere. can someone help me with specs on what the thermo switch should be metering so it can be ruled out or not?
any help is needed.

[asavage]

The thermo-time switch is located on the thermostat housing. Pic from an SD22, but should be the same. The sensor you want is the lowest one, with the two-terminal connector in nylon:



From the 1984 720 Factory Service Manual ("FSM"), page EL-43:



I would not go investing in either the sensor nor the GP controller until you've diagnosed this with a voltmeter and a helper. The fusible links are the most likely trouble spot, followed by any heavy connection to the GP bus. The GPC is, by and large, not a high-failure item, nor is the temp sensor. But the GP bus connectors and fusible links fail often.

Lastly, there are alternatives to the OEM GPC. See this thread and especially the end of this thread.

I can confirm the price range of replacing that later GPC (~$450), and there are three valid part Nos. for various date ranges of the SD25 in the US 720. The sensor is about $40 and is the same for all listed SD25s (11023-G7000).

[ten10]

The GPC is, by and large, not a high-failure item, nor is the temp sensor

Thats what I was begining to think after reading hours and hours of posts and not a lot of talk about it. Thanks for your help.
When you say heavy connection to the GP bus are you referring to electrical drain?

[asavage]

On the older, SD22 glow system (which is different from yours in a couple of ways), there are large, 8mm spade terminals that connect the GP bus (that is, the wires on the GPs themselves) to the harness at the firewall, as well as up near the battery. They fail very frequently after 20+ years, moreso in the Maxima diesel, but I've seen two fail in the 720 diesel as well, and the inline fusible link near the battery also corrodes and fails.

Put a voltmeter on No. 1 GP, watch it while someone else turns the Ign. to ON. See above 10.5v? If so, don't bother checking connections or the fusible link. But I'm betting you don't see anywhere near that much voltage, or none at all. Turn off the Ign., move voltmeter probe upstream to the first GP bus connector, repeat test, then move probe to GP relay output, repeat, then to GP relay input, repeat, then fusible link, repeat.

If you aren't even hearing the GP relay "clunk" when you turn the key to ON, you might still have a fusible link problem, a failed GP relay, a GPC failure, or something else entirely, but look at connectors first, as they have proven to be a frequent problem.

I don't know your background or experience/training, but you might want to enlist the help of someone who's comfortable performing/interpreting these simple tests. If you haven't done much electrical troubleshooting, you can become frustrated by seemingly contradictory results.

[ten10]

Alright since my truck had driven to the mechanic and quit working there I took him all the information you provided me. He didn't seem to thrilled about me challenging his diagnosis- but whatever, he said he would look it over and meter the connections. I left without confidence but have been very busy with work lately and have had time to do nothing about it (that and i am a contractor with no mechanic tools). He called and said that he was getting the proper voltage and that it definitely was the GPC. I was still uncertain, but with fuel ever rising and my alternative work truck getting 12 miles a gallon i figured that I would spend 470 dollars in fuel by the time work slowed enough to do it myself. so I bought the GPC and gave it to him. He called a couple days later and the first thing he said was "the GPC was definitely bad, but..." he had also put in a used bigger battery and now it was working. Also he went and replace, for free, a tie rod- gave me a lube and oil job- and some other suspension part im not remembering right now, (Sorry for the length of this story) I get it home and drive it all day. Next day, nothing. I look under the hood and go striaght for the fuseable link, its toast and i decide its time to dive into this I notice the wire connecting the glows looks old. I remove the metal fuel lines to remove the coolant hose and sure enough it looks melted and when i get the #1 plug wire off its is frayed and bare. I replace this with new 10 gauge wire and put everything back together and rig a fuseable link and after a few on and offs it primes and fires up. I just took it out for a spin and am thrilled. I need to redo the fuseable link (solder it and all- like i have read on other threads) but I just want to say a long winded thannnk you Al for all your help and thanks for this site... and do you think that the other GPC was bad? or did him messing with the wires temporarily fix something?

[asavage]

. . . do you think that the other GPC was bad? or did him messing with the wires temporarily fix something?

Can't really say. Really, for this old GP system, a voltmeter is all that's required, it's not complex like the later ones.

Glad you have a happy ending to this problem