Slipping GMC K1500 Truck Automatic Transmissions

A slipping automatic transmission is the first sign of serious internal troubles heading your way. Most of the time this condition will lead to a rebuilt transmission. Here is a question about this condition as well as a good example of the likely symptoms.

Your website has allowed me to further educate myself about the engine and transmission side of cars. Here is some information about my trucks problem before I take it to a transmission shop or general car repair place.

Question: I have a 96 GMC K1500 w/ 4l65E Chevy transmission and 4WD. I am having slipping automatic transmission problems. If I drive like an old lady and ease up to Highway speeds I can feel all gears engage and pull, 1, 2, 3, and 4. If I take off hard from a stop or if I’m driving at 6Omph or more and kick it for passing gear I can see the RPM gauge jump to 3500-4000 and hear the engine running great but no power.

No codes and 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. I think the problem is inside the slipping automatic transmission itself.

If you think the slipping transmission needs to be replaced, would you suggest any upgrades I should ask for during the rebuild?

Answer: 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.

You can prove it with a line pressure check with a pressure 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. The only way to determine the amount of internal damage is with a tear down and inspection.

As far as removing the transmission, you mentioned your GMC 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. The factory makes 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.

If you need to purchase an upgraded rebuilt GMC transmission for your truck, let me recommend GotTransmissions.com. They supply the most thoroughly transmissions anywhere, using the latest upgrades available and American made Borg Warner clutch plates. Call us now for a free quote at 866-320-1182. Don’t forget to add an auxiliary transmission cooler too. GotTransmissions.com