About two months ago I started to notice the converter unlocking intermittently during low load, steady speed cruise. This will happen about a dozen times usually in the first five or ten minutes of highway cruising. It does not appear to be related to changes in load or speed (in fact the tach will only flash about 100 rpms most times) and will lock back up almost immediately. It's a stock 85 LG4.

Any tips on diagnosing this puppy?

 

Not sure about that, other than I'm sure that you might want to check TCC solenoid and or wiring for your 700r4. Perhaps a new TC will solve it.

Its the durndest thing, mine does exactly the opposite. I get intermittent TC lockup during ONLY the first 5-10 minutes of driving, then it goes away completely...... It's rebuild time I know...

 

lube the speedo cable

 

The speedo cable can cause this? That's what my problem is, the tcc will jump on and off after the car warms up during highway cruise. Guessing that a sticking spedo cable can cause the ecu to recieve a mixed signal?

 

Wanted to close this one, who knows someone may do a search one day...imagine that.

Anyway a test light at the ALDL showed that the ECM was commanding unlock (or more accurately intermittently failing to command lock). Problem remained intermittent (would go a week or more then come back) until one day I pulled an open circuit coolant temp sensor code. Swapped a new pigtail on the CTS and it's been more than three months now and no more problem.

I'm thinking that a speedo cable bad enough to affect the VSS would manifest itself in a bouncing speedo needle, but never been there so...

 

I was having that same issue, particularly locking the converter while I was sitting at lights after I had been on the highway In my monte. I replaced the solenoid and the problem went away.