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Thread: 2000 GMC Z71 Differential/Tcase noise...HELP!

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    2000 GMC Z71 Differential/Tcase noise...HELP!

    Hey folks, my 2000 GMC Z71 SLE with 5.3L engine and pushbutton 4 wheel drive is giving me fits! It has 135,000 miles, but has only been in and out of 4 wheel drive a few times each year. I live on a big ranch and only use it when needed. Lately the "service 4wd" light comes on while driving down the highway in 2 WD. It doesn't always come on, but when it does, it stays on until I restart the engine. If I put it in 4 WD (high OR low) it sometimes has difficulty engaging. Once engaged however there is a HORRENDOUS banging noise coming from underneath the front end when I try to turn in either direction. Once I pulled a guy out of the mud in 4 WD Low and it banged loudly going straight! I have replaced BOTH drive axels and all of the control arm bushings (they were shot) to no avail. My question is...IF the front differential is positrac...would it make this noise due to low/contaminated oil? Is the positrac friction disc? Could the noise be eminating from the Transfer Case? Would the "service 4WD" message indicate a problem with the differential, or only the Transfer Case? It hasn't been used in 4WD enough to wear anything out so I'm hoping it's relatively minor... Wishful thinking? I appreciate ANY input

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    bradrhale's Avatar
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    have you checked the joints on the input shaft for the front diff also?? on that year model, they have had a problem with the srv 4wd light coming on. i believe it was related to a ground wire in the switch.

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    OK...the U Joints on both ends of the input driveshaft from T case to differential seem to be good. Auto parts store guy suggested "splines" in the shaft similar to CV joint could be cause. There is about 4 inches between the nose of the T case and the yoke on the U joint coupler that is covered by a boot similar to the CV boot. Could this be the problem? It looks to definitely be the "weakest link". If so, how do I check it...there are no leaks or sloppiness. Side note: The "banging" noise also manifests itself on straight ahead hard acceleration. (Sounds like jumping gears) I suppose that turning under accelleration just exacerbates the situation quicker. Also, on startup first thing in the morning, the 4 WD buttons work in ALL cases as they should. Drive the truck about 5-8 miles and the "Service 4WD" light comes on after which it will NOT shift into anything other than 2 WD. Broken actuator? I couldn't see any loose wiring. These MUST be two seperate issues

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    Everything above would indicate thatt you have more than one problem. These trucks are known for having bad switches and the switc itself makes the light come on. The fix is to replace the switch, or repair the solder joints inside of the switch. This will take about 20 mins tops to repair. the broken solder joints are painfully easy to see.
    The stearership will hit you for around $150.00 for a new one, and a salvage unit from the yard might result in the same problem. This is a good reason to try to fix it.
    That being said, I think we need to really look into this noise. It is possible that the splines could be dry and could be causing some of the noise, but I kinda doubt it. I realy fear that there is something broken or worn, like the t-case itself.
    As I type this, I am also questioning the fluid levels of the case and front diff. What typr of fluid is in these?
    Does the "pop" noise occur when you are in 2wd, and are the fuids proper and at the right level?
    Does the 4wd actually work the way it should?

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    Senior Member tjzz4's Avatar
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    I`m not sure about chevy but ford uses two speed sensors on their auto 4x4 cases to detect front or rear wheel spin. A bad sensor or loose tone ring will cause the t-case to try to lock into 4x4 on hard accel or when turning and it makes a noise similar to what you are describing. Willl also light the malfunction light. Are your tires matched????
    Last edited by tjzz4; 10-25-2007 at 11:47 AM. Reason: sp

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    First things first...the "pop" only occurs in either 4 WD mode. It isn't quite as pronounced on a straightaway acceleration as it is on a turn with acceleration. It seems to be eminating more from the T case area. Went to the dealer today to get fluid to change out the T case. It hasn't been done since new. The service manager said I MUST use the dealer approved fluid as it is teflon based and anything else could trash the case!. $12.99 a quart and they had to order it...go figure. I hope to get it on Sat. so I will know by Sunday if that is the problem. When engaged, the 4WD works properly...I'm thinking it slips when the "pop" is heard, but I can't really verify that. It's a completely stock truck save for the tires which are oversized, but all properly matched. tjzz4...this occurs only AFTER it is locked into 4WD. I'm banking on the fluid as the 4WD has seen so little use it's hard to believe internal damage is the cause...but...I could be wrong...wouldn't be the second time.........Oh...75K30..I assume the switch panel just pops out of the dash...perhaps with the "persuasion" of the proper sized screwdriver?

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    Yeah, the little switch does come out with a little help from a screwdriver or something of the likes.
    Anyway, before you get too far along, if this thing is popping in 4wd, i would take a little time to inspect and troubleshoot a couple of things.
    First of all I would check the front differential to make sure the spider gears (side gears) are not broken and the noise you are hearing is them tearing up the diff even further.
    Next try to isolate the problem down to a specific area. If your diagnosis is correct, and it is in the t-case, everytning still leads me to believe that it is the chain.
    Since during a turn, the front and rear wheels turn at different rates, one of two things must happen;
    The ground has to have a limited amount of traction so the tires can slip, or
    one of the axles has to give, usually a tire will "bark", and the truck will hop.
    Now if the truck is engaged into 4wd on hard dry ground, it will take a real toll on the case, and it could be damaged. If the tension on the chain is not tight anymore, it can skip and, well, we know this to be not so good.

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    Bad news for me... changed the Tcase oil. Old oil was black/dk brown but didn't smell burnt and was almost a half to three quarters quart low...no signs of any leaks though...there WAS residue on the overflow tube...hmmmmmmmmm. Anyways...pretty new blue fluid installed and pretty much NO difference...but then subconsciously we all KNEW it woudln't be THAT easy I guess. I haven't checked the front diff, but the noise really seems to be eminating from the Tcase. I can listen leaning out with the door open...suicidal I know, but I gotta end this one way or the other I suppose I'll put it on jacks to make absolute verification then suck it up and tear into the case. I'll keep ya posted if you're interested

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