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2004 gmc envoy_sle_xuv
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Helping my friend with an 04 XUV. It is presenting with the ever-popular issue of not wanting to shift, and revving high but not speeding up past ~30 mph. But it temporarily went back to working correctly when I accidentally disconnected the VSS!

This is the full story starting with the list of things I have read about in all the forums as possible causes:


3-4 clutch pack
Middle case leak in the 2nd gear apply circuit
Shift solenoid(s)
EV clutch fan
3rd and 4th actuator piston
3rd accumulator check ball
PCM wiring harness
Transmission internal wiring harness
Loose wire somewhere

I read a guide that suggested removing the GREEN trans connector to force the XUV into limp mode. This was supposed to show us whether the 3-4 pack was fried- if it was the XUV should slip and not go anywhere.

Well, my clever self disconnected the VSS instead. My friend drove it around the parking lot as planned before it occurred to me I did wrong. I reconnected the VSS and waddayaknow, it shifted as it should now! Very much the opposite of what I expected. However, he did get ABS dash warnings and I lost some credibility for pulling the wrong connector I will add that this was a while ago, so I can't remember if it shifted right immediately after I pulled the VSS or only after I plugged it back in. It now has C0201, C0550, C0055, and P0502 codes.

Now, why would pulling the VSS and plugging it back in allow the XUV to shift as it should past 30 mph (we think, we didn't get on roads to go much faster than that, but the trans acted like it was civilized again)? Also, this was temporary; after a while it struggled again. Finally, it has returned to the original state of revving past 30mph with no shift.

Is the trans briefly shifting correctly a clue that it is not mechanical? Could it have been in limp mode this whole time? Or is intermittent restored function to be expected with the likely mechanical causes of this issue?
 
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