4L60e free-wheels when slowing below O.D./3rd shift point

[rhino]

Make

Chevrolet

Model

Caprice

Year

1994

Miles

1500

Engine

LT1

Newly rebuilt 1994 4l60e that works perfectly except for this one problem.

My tune has the 3rd/OD shift point at 46mph, converter has already locked at this point. This shift point is assuming very low load acceleration. The problem comes when slowing from speeds above 46mph, as in just taking your foot off the gas. When the vehicle speed reaches 46mph, the trans kicks out of OD and free wheels as long as you are decelerating. The engine is at idle rpm (ie 750) now. If, say, at 38mph you want to begin accelerating again, you will need to increase the rpm to match the speed/gear combination in effect right now. Example: at 38mph, trans would be in 3rd/locked, so engine would need to come up to 1800rpm before 3rd/locked would re-engage and allow you to continue accelerating. Trans would shift to OD at 46mph as normal and as your speed increased, you would have compression braking if you decelerated. Until you hit 46mph again, and then OD would disconnect and you would free wheel all the way to a stop or until you accelerated again. I don't think this is a design feature.

Any comments or suggestions would be appreciated.

Dave

P.S. The vehicle info given is for the engine, trans, PCM, harness. (donor car).

 

[billr]

Remind me, you do have the stock PCM controlling the trans, correct? How about the VSS, do you have the stock-type VSS from the donor connected to the PCM?

 

[grcauto]

Who programmed it? They can program the TCC to lock at a different value.

 

[rhino]

Remind me, you do have the stock PCM controlling the trans, correct? How about the VSS, do you have the stock-type VSS from the donor connected to the PCM?

Yes, I'm using the stock PCM. Yes, the VSS is stock and single signal wire connected to PCM. But just to confirm; VSS is sending correctly to PCM as the speedo is correct; no problem shifting to OD on acceleration, disconnect is on deceleration.

Good to hear from you again billr.

 

[bp042665]

ok sound's like the 4/3 down shift valve could be the problem try to drive it into o/d to 50 mph and do a full throttle down shift and then come to stop and do it about 4 more time's and see if that help's

 

[rhino]

Who programmed it? They can program the TCC to lock at a different value.

At first my reply was going to be that I'm not having a problem with the converter lockup. But if I think outside the box a little, could the converter completely unlock and cause the free wheeling, as opposed to a gear change uncoupling?

The lockup occurs at 36mph while the trans is still in 3rd gear, so the shift on acceleration and free wheel on deceleration (46mph) all happen after the TCC is locked.

Tune was done by Ryan at Sinister Performance, I trust his work. We worked out all of the shift points and TCC lockup before I sent him the PCM and they are all on the money.

Dave

ok sound's like the 4/3 down shift valve could be the problem try to drive it into o/d to 50 mph and do a full throttle down shift and then come to stop and do it about 4 more time's and see if that help's
[/QUOT

ok sound's like the 4/3 down shift valve could be the problem try to drive it into o/d to 50 mph and do a full throttle down shift and then come to stop and do it about 4 more time's and see if that help's

Thanks bp04665, I'll give that a try.
Dave

 

[grcauto]

Is there a datalink? Do you have any way to see what the PCM is actually doing? Sounds like our resident tranny guy may be onto it.

 

[rhino]

Thank you all for your suggestions, I've been told that my TCC unlocks at the same speed (46mph) as the OD/3rd shift has been programmed and feels like something has disconnected. If I manually shift into 3rd above 46mph the TCC doesn't unlock until 36mph, which is the normal lockup speed.

It will be going up on jack-stands for the winter pretty soon. I have a list of other things I need to do, so for now, I won't be doing anything to it.

Dave

 

[bp042665]

i did forget that when you let off of the trottle the converter will unlock but you are on the right track