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2004 GMC Safari Wont go over 45 mph

My sister has a 2004 GMC safari van. A few months ago, the van started having a very rough shift and excessive revving. we took it to a mechanic who found a transmission leak. they fixed the leak and recommended that we use Lucas oil transmission fluid to help with the rough shift. since then things have gone from bad to worse. The van will not go over 45 mph. the engine excessively at 35 mph. After 35 mph, it appears something also goes wrong in the electrical system. At 35-40 mph, my dash goes from lit to dark. Also i have lost the use of my power locks and since this issue began, i have lost use of 2 of 3 of my cigarette lighters. What is happening here?? .

3 cigarette lighters???

Anyway, you took it to the wrong mechanic. Find a reputable tranny shop and stsrt there.

You have some electrical issues in my estimation. Try a new mechanic. I imagine you have some blown fuses and a short. Check the wires that go from the body to the drivers door. Get the battery and charging system checked also.

Bring the vehicle to someone and ask that they remove the side mount battery connections from the battery, and on the positive connection have them peel back the red rubber cover to expose the positive terminals so they can be inspected for corrosion.

Tester

With a leaking transmission that is shifting rough and overrveving, don’t be surprised if the transmission needs to be rebuilt.