My '99 Sierra with 124K will go in and out of lock up while cruising at 55mph. There is no shudder, just a 150(approx) rpm drop and rise about every 20 seconds. It seems ok at 65 and >. Can I bypass the lock up with a toggle switch for my 16 mile daily commute to work? If so does anyone have a schematic?

 

there's a reason it's locking/unlocking. Could be a simple solution. Try finding the cause before masking the symptom?

 

Check the fluid level, if thats fine scan for codes and see if you have a P0300. The converter wont lock if it detects misfires.

 

The converter may be bad

 

wouldn't it throw a code like transmission component slipping?

 

Good advice here. Get a scanner on it and see what is going on. Might just need to edit the tune a little or could be a bad sensor. More than likely a electrical/computer problem.

Chris

 

I had a trans cooler crack on me and I lost nearly all the fluid while I was driving and not a single warning light or code popped up.

 

Could be as simple as a fluid and filter change
If the filter is plugged you will get un-locking when driving

 

Are you sure it's the TC locking/unlocking? You might be feeling the A/C compressor turning on and off while you're driving. If you have a scan-tool available, you can moniter it to find out. Otherwise, try the same drive with the A/C fan switch turned off to see if it still happens.

 

I have no trouble codes, fluid isn't terribly old but is full, A/C is not cycling. I will change fluid and filter then get it on a Tech II as soon as I can borrow one and see what is going on.