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96 GMC 4x4 image

Slipping Automatic Transmission

96 GMC K1500 4WD image
96 GMC K1500 4WD

A slipping automatic transmission is the first sign of serious internal troubles heading your way. More often then not this condition will lead to a complete transmission overhaul. Here is an email question about this common condition as well as a good example of the likely symptoms.

A visitor writes, Thank you for your website. You have pushed me to further educate myself about the mechanical side of cars. Here is some information about my trucks problem before I take it to a transmission shop or general car repair place.

I have a 96 GMC K1500 4WD. I am having slipping automatic transmission problems.  If I drive like grandma and ease up to Highway speeds I can feel all gears engage and pull, 1, 2, 3, and 4. If I take off fast From a stop or if I’m driving 6Omph and stomp it for passing gear I can see the Rpm gauge surge to 3500-4000 and hear the engine running great but no power at The wheels.

I have to let off the gas for the transmission to begin to pull again as it falls back to the higher gear. My auto scan tool from Actron says I have no codes set in the computer. I have purchased the all data online auto repair manuals for my trucks.  I have gone through the related info to become familiar as possible with this problem. After a physical inspection, my fluid level is correct and no burnt odor. I have no leaks, the wiring harnesses and connectors look fine.

Because I do not have a code, can I assume that my, vss, tcs and other related sensors have no failures or intermittent shorts? My truck has over 120,000 mi on it. From my research I’m thinking the problem is inside the slipping automatic transmission itself (clutches slipping etc.?)

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I would greatly appreciate any advice you could give me based on your 25yrs of experience in this area. I am a service tech in the industrial laundry machinery business. I have a mechanical/electrical background but not in the auto field. As they say, I know just about enough to get into trouble.

If you think the slipping transmission needs to be replaced, would you suggest any upgrades I should ask for during the rebuild? By the way, I’ll be letting the shop do the removal and installation as well so any warnings in this area would be great.  Thank you for any help you can give.

Answer to slipping automatic transmissions

96 GMC 4x4 image
96 GMC 4×4

With the high miles on your 96 GMC K1500 4WD and the age of the unit it sure does seem that internal problems would be the most likely cause of your problems. Worn lip seals allowing fluid pressure to bypass the clutch apply pistons often create a slipping condition.

This can be confirmed with a line pressure check with a mechanical gauge connected to the test port. A stall speed test is where you power brake the vehicle (apply the gas and brake at the same time) and will verify the pressure bypassing condition.

In most cases the only way to solve this is with a complete transmission overhaul were you replace all internal seals and wear items like clutch disks and bands. Also sometimes when the trans has been slipping for a while new friction plates are required to be replaced do to scoring on the face of the hard metal discs. The only way to determine the amount of internal damage is with a tear down and inspection.

Related Reading:  Manual Transmission Diagnosis

As far as removing the transmission, you mentioned your Chevy truck is a 4×4. A good thing to check when you get the truck back is the transfer case seal. The shop will have to separate the transfer case from the back of the transmission on your model. They make a gasket for this sealing area. Often shops will just use rtv silicone. This is not as long lasting as a fiber gasket and tends to leak sooner or later.

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