transmission hunts when in OD

[thackmansr]

Recently bought a 1998 3500 flatbed with 51000 miles from the company that I work for. It was a construction truck and somewhat abused. The ABS light would not go off. I found that the brake wiring harness on the drivers side had been broken. Fixed that and got the ABS light off. Truck runs good but the transmission hunts like crazy when in overdrive and I encounter any kind of grade. Will down shift then upshift then down shift contiually. No check engine lights. Have changed trans fluid and filter. Drained torque converter when we changed fluid. Had this problem before the fluid was changed so it's not new. Any ideas?

[bobcat5544]

Its normal for the transmission to hunt, unless your at 3/4 to full throttle

[thackmansr]

This doesn't seem normal. I had bought two of these trucks. Identical. Have sold one already. This one drives completely different. When I come to a grade it will shift back and forth continually until I get it up off the grade. Sometimes will hunt like this even on no grade and steady throttle. But, I hope that you're right.

[bobcat5544]

When the trans hunts it's getting info from the pcm to do this, the pcm is watching the tp sensor, engine load sensor, ect sensor, and map sensor. to tell the trans when to up and down shift. If one of these sensors are acting up then it could cause your problem.

Also you have to look at is the truck load or unloaded this will make a big differance.

The only way to tell if it is normal is to hook up a scanner and do a live sensor monitoring and is if any of them drop out or act funny. Also look at the shift solenoids is if one is acting up..

In truth I beleive you will be chasing a ghost, but i'm not there to test drive it my self so for me telling you it is normal is like telling a blind man the sky is brown.

[mmmikkke]

Repeated shifting (hunting) will get the clutch(es) hot, so you should manually select "3" when otherwise the truck would be hunting, until you find the cause of the problem.

 

One thought I had is that if you have a blocked cat, your TP will be higher than otherwise necessary, and this might be causing the hunting. There are other symptoms to a clogged cat, you should check for them.

[dewfpo]

As others have said, it may not be your trans acting up. If the engine can't handle the grade with the load you have put on it, the trans will down shift. If you have access to a scan tool you could check for pending codes or an engine sensor not responding as it should. A sensor could be misbehaving but not so far out of whack that it kicks out a code.

 

DEWFPO

[Frozen001]

My opinion on a transmission hunting like that is that is will soon me heading for a rebuild. You said you had another truck that did not have this problem. Where they the exact same truck?? My guess is that the one that did not hunt had a different rear end gear, more suited to hauling where the one you have now is a compromise between hauling and gas mileage.

[TEE]

I agree...not normal. Something is causing it to hunt.....TC not locking up right?

[thackmansr]

Thanks for the thoughts guys. The two trucks that I bought were as identical as you can get. they were fleet trucks, ordered the same day, with the same specs. Everything was identical, right down to the tool boxes. One key difference is that the truck with the trans that is acting up was used to tow a Horizantal boring machine daily. The other truck towed a simple fiber splicing trailer sometimes. I just have a gut feel that the clutch packs are burned, but will go through the diagnostics to make sure that it isn't simply a sensor on the engine.