S-10 ZR2 4L60E Shift Problems

JHZR2 · March 20, 2013

Hello,

I have a 98 S-10 ZR2. 60k miles, owned since new. Last few years it has seen very little service, 1000 miles per year roughly. It has always run, and in particular shifted, perfectly. Transmission properly serviced every 20k or so with correct spec Dexron fluids.

We use the truck seldomly. We used it in September on a 100 mile trip and it operated and shifted absolutely perfect, as it always has. I used it a bunch at Thanksgiving, and it started to shift really early into what I guess is OD, as it would take the engine down to 500 RPM, lugging, then shift back and forth. "shifting" through manually would make it work fine.

I found a thread here: http://www.gmtruckclub.com/forum/showthread.php/75572-4l60e-trans-problems that said that early upshifts can be due to confusion in 4wd status, and even a shorted wire on a 2wd can cause it to think it is in 4lo. I had pulled fuse #8, which made the 4x4 pushbutton selector deactivated when I was checking for parasitic draws (my battery drops voltage a bit faster than Id like, so I was checking it out). When I replaced the fuse, the truck went back to working like it always had. Perfect smooth shifting and no early shifts. The amount of mileage I put on with this "issue" was maybe 50 miles at most.

After fixing that issue, Ive put maybe 50 more miles on the truck, all local, and it shifted and worked great, as it ever has. That is, until this morning. I decided to take it on a 40 mile interstate drive. I know my truck spins right at 2000rpm at 70 MPH. I noticed when driving that it was more liek 2300 RPM. I figure that the TC is not locking up. But what makes it very odd is that after doing the interstate mileage, when I was back on normal roads, the truck has started to do a very hard 1-2 shift, which it has never done before.

So, any insight into this? All of this has happened in like four days of using the truck since last September, and all within even the same tank of gas - its not like Im driving the truck high mileage.

Rodents are a possibility (chew a wire?), but I have never had a CEL, no indicator lights, etc. 4wd engages and operates, all is well.

Any thoughts or suggestions? Recommendations on what to check or do?

Thanks!

nibael · March 20, 2013

ZR2's are cool trucks. I had a 99 extended cab and it seemed like it could go anywhere off road. It was completely covered in mud the whole time I owned it. It was great. I know this isn't helpful to your situation, but it reminded me of one of my favorite trucks. Hope you get it worked out.

00Silverado4x4 · March 20, 2013

Dispite the "low" mileage, it may be time to have it cracked open and looked over internally. Sounds like you may some internal issues that need to be addressed.

wilsonz71 · April 1, 2013

Mine did the random harsh 1-2 for a while before I lost second all together. I know the 1-2 accumulator piston is made of plastic and is known to crack and leak in these transmissions. I'm really glad I did the shift kit in the replacement trans because it had cracked plastic pistons and was just a matter of time before it blew too. Transgo makes a great shift kit with aluminum internals.

aflesch · April 2, 2013

Check the TPS for smooth response. If it looses TPS signal (even intermittently) it will command full line pressure on shifts. Check for rodent chewed wiring.

Could also be the 1-2 accumulator piston is cracked or binding in the bore. (can be fixed in the pan without R&R)