Tranny - Harsh Shift

Steves95Sierra · March 17, 2009

Hope someone can help. I tried searching the forums, but couldn't find a good match, so here it goes...

'95 C1500, 305 (VIN H), 4L60e Tranny w/ about 120K 'easy' org. miles.

Just recently, she's developed a very inconsistent harsh shift from P-D, P-R, 1-2, 2-3... No DTC, and no consistency relative to driving conditions -- sometimes cold, sometimes warm. This is driving me crazy! Almost as if she's being push into limp-mode (max line pressure) at any given time.

One thing is consistent though -- after about 30-40 minutes of highway driving, the condition does improve.

Here's an example of just how inconsistent this thing is...

- Sunday morning, started out on a 45-min road trip. From a cold start, the problem was very apparent - very harsh.

- By the time I got to my destination, the problem wasn't apparent at all.

- Monday morning, on a cold start - no problem, perfect throughout all shift points.

- Monday evening, on a cold start - 'CLUNK,' it was back...

This is what I've tried so far...

- Replaced Trans. Fluid Filter (2nd filter since org. - looked normal for the mileage on it; pan fluid looked good & clean)

- Replaced VSS (typical small amount of gunk out side of the magnet - no surprises)

- Replaced TPS (even though a DVM tested fine .6VDC to 4.6VDC through throttle range)

The fact that it's so inconsistent and there's no DTC makes this very difficult to troubleshoot -- Maybe EPC or manifold pressure switch (sometimes failing/sometimes not)? Could it be as simple as a bad Engine Coolant Temperature Senor (as suggested by the GMC service manual), even though the temp gauge on the instrument cluster is reading fine?

Help!

Thank you very much;

Steve

green 788 · March 17, 2009

That's a puzzler...

I know that when I was having some issues with a 4l80 I could unplug the speed sensor, losing my speedometer of course, and the tranny would shift very harshly, but reliably.

I doubt if it's a very serious problem, due to the relatively low mileage (I call anything under about 180K low )... and also because it's an intermittent problem.

If you had bad transmission internals (such as a bad torque converter or clutches, etc.) then the problem should be present at all times.

It sounds like the transmission is indeed going into some kind of limp mode, but I'm not sure. When my 4l80 goes into limp mode, it will not shift above 2nd gear at all.

I don't know if your tranny would shift into 3rd and on to overdrive in limp mode. That's something I would check out. If you discover that the 4l60 does continue shifting through all 4 gears in limp mode, that would be more reason to suspect that it is indeed going into limp mode.

Next time it does the "slam shift" thing... try shutting off the truck for a couple of seconds, then re-start and see if the problem has gone away. If it does go away, that's even more reason to suspect limp mode (because the limp mode feature re-sets when you shut the truck off and re-start).

Try disconnecting the speed sensor, which will cause default settings (limp mode) all the time, and see if the tranny acts ENTIRELY like it does during the times you are suspecting limp mode.

There will be an electronic plug that connects to the transmission, high up on the right side (check me on that location)... and if the internal seal begins leaking around that plug, it will get wet with transmission oil and you'll get intermittent problems which can cause limp mode. You may want to check that plug as well, if you can get to it. Clean it out with some electronic degreasing cleaner if it's oil soaked, then suspect a bad seal in this area. The only problem is I don't know if you can change that seal without tearing down the tranny... you'll have to see...

Dan

Steves95Sierra · March 17, 2009

Hi Dan.., Thank you very much for responding...

That sounds like a good place to start -- and encouraging too. I'll try what you suggested -- unplugging the VSS & also the stop/restart when apparent. I'll also check for fluid leaks and will post back. The underside is kept very clean (yea - I'm anal).., so if we're leaking, I should be able to find it pretty quickly.

Thanks again!

Steve. . ..

silveradobobby · March 17, 2009

Yea check the speed sensor it can cause issue of sort. But if no change it sounds like the trans has some issues. When 4l60's and 700r's get milage on them and and are coming to an end they start bang shifting due to trash in the pump and valve body causing pressure riase and valve sticking. With 120K miles she has ran her lift for a 4l60e I give them 100k-140k. I've seen 4l80's go 300K.

Steves95Sierra · March 17, 2009

Yea check the speed sensor it can cause issue of sort.

Hey SB... Thank you for responding...

Yea - the VSS was one of the first things that got swapped out -- no joy...

Quick question tho -- since it is so inconsistent, do you think replacing the EPC & pressure switch assembly might be a reasonable step before pulling the valve body, or do you think that would be a waste of time & bean$?

Steve. . ..

silveradobobby · March 17, 2009

Yea check the speed sensor it can cause issue of sort.

Hey SB... Thank you for responding...

Yea - the VSS was one of the first things that got swapped out -- no joy...

Quick question tho -- since it is so inconsistent, do you think replacing the EPC & pressure switch assembly might be a reasonable step before pulling the valve body, or do you think that would be a waste of time & bean$?

Steve. . ..

I'm thinking waste of time and money with the milage you have on it and the problems you are have its before long before she just quits. I would run it till it quits, usually the hard shifts is not a good sign with the 4l tranny family.