99 GMC Jimmy Transmission problem?

[Lisa J]

I'll start with a little history: 2 years ago, while going up a hill, my GMC Jimmy dropped down to 5 mph and shot up to 5RPMs. Had it towed to a mechanic (or so he claimed!) and they couldn't find anything wrong with it. It has never done that again. Now, if I drive a considerable distance, and then come to a complete stop and then accelerate again, it will go up to about 10mph, jerk harshly, and then continue to accelerate. There is a 'clunk' sound coming from under the truck at the same time that the jerking occurs. The RPMs will go up to 3-3 1/2k, and then after the clunk and the jerk, they will drop back down. Every mechanic I've asked is completely clueless as to what it could be. My service engine soon light comes and goes. When it is present, and it is read, it says 'trans slip at 45 mph' It only does it if I drive a considerable distance (over 20 miles away from home) and only if I go above 50mph. If I stay in town, don't ever go above 50mph...it will never do it. It also doesn't do it all the time. A new problem is that there is a grinding sounds coming from my back end...it almost sounds like I am dragging something behind me, but there is nothing touching the ground. The truck just hit 140,000 miles.

 

[Molson02536]

I'll start with a little history: 2 years ago, while going up a hill, my GMC Jimmy dropped down to 5 mph and shot up to 5RPMs. Had it towed to a mechanic (or so he claimed!) and they couldn't find anything wrong with it. It has never done that again. Now, if I drive a considerable distance, and then come to a complete stop and then accelerate again, it will go up to about 10mph, jerk harshly, and then continue to accelerate. There is a 'clunk' sound coming from under the truck at the same time that the jerking occurs. The RPMs will go up to 3-3 1/2k, and then after the clunk and the jerk, they will drop back down. Every mechanic I've asked is completely clueless as to what it could be. My service engine soon light comes and goes. When it is present, and it is read, it says 'trans slip at 45 mph' It only does it if I drive a considerable distance (over 20 miles away from home) and only if I go above 50mph. If I stay in town, don't ever go above 50mph...it will never do it. It also doesn't do it all the time. A new problem is that there is a grinding sounds coming from my back end...it almost sounds like I am dragging something behind me, but there is nothing touching the ground. The truck just hit 140,000 miles.

For the hard shift problem with the clunk, Check TPS for smooth and linear electrical response over the entire range of motion. If not, replace. Also replace your Boost valve, it sounds like it's weak and the EPC is trying to compensate for it by raising the transmission line pressure.

Visit a local trans shop and have them put a gauge on the pressure test port and go for a drive to verify proper line pressure. Base pressure at idle in P,N,OD should be around 70psi and around 140psi in R. Max line pressure should be at least 190psi at WOT in 1st or 2nd, 225psi is optimum. If base pressure is low, trans or pump is worn out. If max pressure is low, check EPC solenoid. If reverse is slow to engage, and line pressure does not come up to at least 130psi in reverse, replace the boost valve with a Transgo 0.490" boost valve (easy fix in pan), stock i think is 0.470"

As for the grinding noise, if your lucky, it's just going to be the U-joints from your drive shaft. At worst it's going to be the overrun clutches are applying due to a cracked or leaking forward piston. Overrun clutches will be worn out after 30 seconds of this behavior. Car can be safely driven in D: Horrible noise in 4th and feels like the brakes are on.

Good luck and keep us posted.