GM-GMC: GMC K1500 Quad Cab 4X4 Transmission, vacuum problem, gmc 1500

Question
Hi Harry,
I have a 1993 GMC 1500 Quad Cab 4x4, 5.7L OD V8 automatic transmission, 49,800 original miles.

The truck sprung a pin hole size leak in the transmission line and I lost a considerable amount of fluid before I was able to get off the highway.  I saw the leak and was close to a service station and bought a couple of bottles of transmission fluid.  I still had 75 miles to go to get home.  I was able to get up to @50 mph with the tach about 3000.  I stopped a couple of times and added more fluid to make sure I didn't run dry.  I took the truck to a mechanic.  He fixxed the leaking line and cleaned the pan and put in new fluid.  But then on the test drive, the engine wouldn't shift out of 2nd gear. He said the transmission was shot.  My husband has always used Lucas in his diesel transmissions and never had trouble.  He suggested we drain a little fluid and add a bottle of Lucas.  The truck shifted much better, in fact seemed to run up to 50 mph with no trouble but wouldn't shift anymore.  Do you think adding another bottle of Lucas would help? I need another opinion or advice on what else may help. Could I have a vacuum problem?

Thank you,
Ruth Martin


Answer
You can try more Lucas, but it is likely that your mechanic is correct.

The transmission in your vehicle is either a 4L-60E or a 4L-80E.  The key point is the "E".  It stands for "Electronic" and that is how the transmission is controlled.  Vacuum is no longer used for that purpose on GM vehicles since the TH400 was discontinued.

Often, after severe fluid loss (and transmission overheating), the draining of old fluid and the addition of new fluid causes the transmission to go into "shock" and it never recovers.

I would take the vehicle to another transmission shop for an evaluation.  I think that you will find the answer to be the same "rebuild necessary."