700R4 issue - long

[cosmophone]

I have a 1996 Tahoe 5.7L with 160K miles. About 2 months ago it started what I will describe as intermittently downshifting for a fraction of a second or maybe a second and then upshifting again. It's either that or the torque converter (or whatever does this) goes out of lock up and then back into it. This happens when it's in Drive or 3rd gear. I see an uptick of 200-300 rpm on the tachometer and, of course, hear it in my Flowmaster duals. There is no consistency to this action and it can happen when the transmission is cold or warm, although it appears to happen more when it's warm.

Shifts from 1st to 2nd, 2nd to 3rd, 3rd to 4th and into overdrive are firm, i.e., no slippage, even if I am on the throttle hard. I have babied this vehicle for its first 14 years but in the last two years I have been pulling a trailer. I have made about ten round trips of 330 miles each way through OK/AR hills. The trailer weighs when loaded from 4000 to 8000 pounds. I keep the transmission in 3rd gear when pulling the trailer. The upshift/downshift issue occurs without the trailer and with the trailer.

I had the transmission fluid and filter changed after it started doing this and thought the problem was fixed but I was wrong. After the transmission work was done I noticed the fluid was a pint low so I filled it to the correct level (warm and running in Park) but the problem remained. By the way, the transmission is not leaking. The shop that did the work just didn't put enough fluid in it. Yesterday I overfilled it by a quart after reading some posts on forums. that made no difference and, no, fluid did not come spewing out the fill tube because of the overfill.

As this transmission is electronically controlled, I suspect there is an issue with the electronic control rather than a mechanical issue within the transmission. At least I hope this is the case because, as an electrical engineer, electrical issues are much easier for me to fix than automatic transmissions.

If you have had a similar experience or knowledge specific to this issue please respond. If you know a way to troubleshoot the electrical control, i.e., disabling torque converter lockup or isolating electrical components/modules are that are involved with the shift control please let me know.

I apologize for the long post but I wanted to include as much detail as possible. Thank you.

Brian Harris
Plano TX

 

[retorq]

Put a real time scanner on it and watch it closely while someone else drives, my temp sensor caused what you described not long ago ... it would go to -40, come out of OD and then go back in after it had a normal temp reading.

Also you have a 4L60E not a 700R4, they are close but there are some major differences.

 

[cosmophone]

I don't have access to a real time scanner but will try to find one. The problem is getting worse. It is happening often, whether it's warm or cold, sometimes once every few seconds and at all different speeds. I found if I drove in 3rd gear at 70 mph it would not go into lockup. This was while on a 300 mph trip pulling my trailer. 9 mpg, ouch. I will disconnect the wire that locks it up until I decide what I am going to do with it. I know, I know, this causes the trans to heat up but it's than totally frying what's left of the lockup clutch in the torque converter. Thanks for the correction on the trans type.

 

[SunlitComet]

if your lock up is erratic try looking into a defective brake switch on the brake pedal. it is one device that determines to disable lock up. it can also kill off your cruise control.