what causes my converter to unlock while cruising down the highway...

[BOTTLEDZ28]

1998 Tahoe. 350 with 4 wheel drive. Driving down the road cruising at 70 around 1700 rpms and allof a sudden the rpms will jump up to 2000 at what seems like a flip of a switch. I feel it in the seat and look down at the tach and it then goes right back to 1700 rpms. It is the converter unlocking and locking but what is causing this if Im not hitting the brake pedal. On my 7 mile highway trip to work, it does it maybe once or twice. Its been doing it on and off for over 3 months but today it did it and didnt go back to lockup until I came to a complete stop at a light and drove again.

 

[kennythewelder]

It sounds like you are having transmission issues and not torque converter problems. My 97 (350- 4l60e) doesn't reach full lock up till between 80 and 85 MPH. I have 342s in the rear end. I had a 93 V6 4.3L with a 4l60e and 342s in the rear, and it did the same thing. At WOT it almost feel like it goes into another gear, at full lock up. When I slow from lets say 85 MPH, there is no slipping. You may be having some ECM issues. I would have it checked at a trans shop.

 

[BOTTLEDZ28]

Your converter does not lock up after a certain speed so your locking up after 80 mph means something is wrong. It can lock up at 50 if there is no load on the truck and its in 4th gear.

 

[Parentnoia]

I'm having a similar issue but in the opposite direction. I'll be cruising at any speed, mainly highway speed, and the RPMs will dip for a second. Not even long enough to notice on the tach unless you keep an eye on it.

 

[thunderstruck]

Check your brake switch


Sent from my Nintendo 64

 

[tunedbytuna]

I would check the brake switch too

 

[kennythewelder]

I guess I wasn't clear on my last post. What I am talking about is the lock up clutch inside the torque converter. Here is a video on how a torque convertor works. It is the lock up clutch that I am feeling, there is nothing wrong with my truck. I hope this helps you understand what I am talking about. The 4l60e is an ECM controlled transmission, so if there is an issue with the trans wanting to shift when it is not suppose to, it very well could be what is controlling the shifting to start with. As mentioned here, it could be as simple as the brake light switch. The ECM is picking up something, that is telling the trans to do what it is doing IMO, from what you are describing. https://www.youtube.com/watch?v=pTfipsejqS0

 

[Kemit]

You can test to see if the brake switch by lightly touching the pedal to see if the tcc is unlocking. Do this at around 55 or so on a flat area. It should be a direct cause/effect, like flipping a switch.

First question is: are there any codes? Misfires over a certain amount/time will cause the tcc to unlock. The ECM will do this in the background when there are enough misfires to cause the tcc to unlock and still not flash the CES or throw a code. However, if you have a p0300 set or pending, that's definitely the culprit.

 

[BoxerPitMix]

I had a very similar issue on my old ZR2. The torque converter would lock/unlock occasionally and it gradually got worse and worse until it just continually did it non stop. Turned out it was a bad valve body. I didn't fix it in time and it ended up cooking my torque converter and overheating my transmission to the point I had to rebuild it.

 

[someotherguy]

You can test to see if the brake switch by lightly touching the pedal to see if the tcc is unlocking. Do this at around 55 or so on a flat area. It should be a direct cause/effect, like flipping a switch.

Will not necessarily prove a bad brake switch. There are several sets of contacts in there, and they are leaf switches. I've opened a few up and found the leafs had lost their tension and practically broke in half, touching each other when they shouldn't.

Brake switch is cheap...$20 or so. Easy to replace, other than the hassle of getting the clip off, which isn't bad at all once you know the trick. The round style clip on OP's '98 has a split in the middle with two half-circles, put snap ring pliers in there and gently spread it and you can slide it right off the mounting stud. Way easy.

Richard