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1987 Gmc 7000 wiring info

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theronbt

Member
I have a GMC 7000 with a few wiring questions about. Any help would be appreciated.
1. How do you tell if your truck has a diff lock or not?
The manual says it does but not really sure what to look for
Really getting tired of getting this truck stuck
2. Where can I get a full wiring diagram at?
My gauges work when they want to and my cab lights flicker sometimes
I looked under the dash to see if I could find a loose wire or not and found a giant birds nest of wires
It looks like a hundred different people had worked on the wiring and I am just wanting to eliminate what I can and make sure everything else works right

I bought this dump truck for hauling gravel and everything else last year. Its been a really good truck just trying to work out all the small stuff now
This dump truck has a gallion dump body on it. It looks like plastic of some sort. Does anybody know where I can get any info on this at.
Thanks for your reply
 
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RZucker

Senior Member
1, if you dont have a switch on the dash its probably got a detroit locker. Does it always spin the same tire? I had an international S-1900 that had a detroit locker with a broken spring that would only drive with the r.h. side. 1 wheel drive was pretty useless.
2, the electrical issue may be your ground strap between the cab and frame rail. It should be on one of the front cab mounts. I had a C-65 corrode one off and it did all sorts of wierd stuff.
 
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theronbt

Member
Yes only 1 wheel spins at a time.
I believe it is 1 in forward and the other one in reverse
It is awful if the ground is soft at all. I loose quite a few jobs because I don't want to pay the wrecker to pull me out.
How do I check to see what rear end it is?
 
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RZucker

Senior Member
If youre not sure what to look for, I would take it to a reputable truck specialist, In the long run it will be cheaper than giving up work.
 
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Old Doug

Senior Member
The manual will show every option that it could have. Look at the build sheet to see if it does. Be very carful spining or carey a spare third member and axle shafts. I buy up every chevy truck i can to part out a good rear end is worth its weight in gold.If you can and realy like this truck find a used harness. The last 2 chevys that i fixed up i remove allmost a 5gal bucket of wire from both of them that some body hacked into the harness on them.Like Rzucker said it maybe a ground. Some GMs had a clip that grounded the dash lights to the dash frame.
 
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theronbt

Member
It is a single axle. No, only knob like that is for the 2 speed.
Should the rear end have wires coming out somewhere?
I am thinking only the wiring is messed up
 
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RZucker

Senior Member
If you have a single rear axle with a 2 speed, then you do not have any kind of manually controlled diff lock more than likely. My F-800s use an air actuator similar to the 2 speed actuator to lock their diffs. If you do have a detroit locker that is bad jack up one side and try to drive forward. Then do the same to the other side. Listen for clicking/popping noises when you do.
 
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RZucker

Senior Member
I guess the point I was trying to make is that a 2 speed with an external locker is rare. I have seen them in mixer trucks with 60,000 lb tandems, but not in a gas power chevy.
 
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Old Doug

Senior Member
RZucker said:
I guess the point I was trying to make is that a 2 speed with an external locker is rare. I have seen them in mixer trucks with 60,000 lb tandems, but not in a gas power chevy.
Click to expand...
I have never heared of one in a older Chevy. I think you can get them in newer trucks. If the older ones had them they would need to carry spare drive shafts.
A 2speed third member with a locker would be interesting alot of parts in one package.
 
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RZucker

Senior Member
It is a LOT of parts in one package. If i remember correctly it takes about 10-12 inches of splines on both axle shafts too.
 
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theronbt

Member
How difficult is it to open the rear end up and check to see the problem. I haven't worked on this big of rear end beforemostly 3/4 or 1 ton. Is there a common problem with these that would cause it to only spin 1 wheel.
I thought about pulling the cover off but want to know what I am looking for
 
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ValleyFirewood

Senior Member
In the glove box it should have several stickers that list the build specs of the truck. Take a look at those and see if you can find anything that would mention a locking differential.

Open diffs in these trucks are very common. I would be inclined to think that is the case with yours, though that is purely speculative.


Depending on the axle you have it can be a bit of a pain to open or fairly easy. Some axles will require the 3rd member to be removed (like on a Ford 9"), while others also have a cover on the back of the axle housing, similar to a pickup truck axle.
 
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Old Doug

Senior Member
If you cant find the build sheet there could or should be a tag on the third member. I would like to know more about a 2speed locking rear end because i have never saw one.
 
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theronbt

Member
I looked in the glove box and found these numbers for the rear axle
IHPY
Drive line ref. 32331
I am not sure if those are the numbers that I need or not
Saturday while I was spreading gravel on an icy driveway I had my bed partly up and was looking at my tires and noticed that they were both spinning. Not all the time just when I was on the ice trying to get moving.
I think it would be like a LSD. Correct me if I am wrong.
Last fall I changed the rear diff oil. Should I add any type of additive if it is a LSD
 
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Old Doug

Senior Member
Sometimes the fill plug will have a tag hooked to it or around it saying to use a limited slip gear lube. It wouldnt hurt to put some additive in it. I changed the lube in a jeep LSD and use the right lube but it would lock some times on sharp turns. I used a additive and it worked alot smoother.
 
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