Tranny works flawlessly.......but I was driving today, cruising at a steady 55mph in OD and it came out of lock-up, then about 3 seconds later locked back up and stayed there for the rest of the time and things were normal. On the way home, same route on the highway it was fine.

What causes that?

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I've had the same issue, but mine would pop in/out when slowly accelerating from a stop sign or red light. If I gave it atleast quarter pedal it wouldn't do it. My tranny also was running flawlessly.

I had my tuner look at it and take it for a ride with me and after 20 mins it was fixed. I know its got to do with your trans tables, but I didn't ask specifically what the problem was. Hopefully someone can shed some light on this subject. Id like to understand exactly what it was.

 

What is happening is the converter's clutch is slipping out of lock up even though the command is still there to remain engaged.

One of a few things can cause this:
1. Insufficient line pressure.
Solution - tune in more line pressure

2. Insufficient converter clutch size/burnt or burning clutch
-Diagnosis - Get a flashlight under the car and look at the front cover (the part that bolts to the flexplate) and see if it is discolored. If it is, the converter is toast, or at least toasting.
Solution - Sorry, but it is time for a new converter. Yank has a moderately sized clutch. Precision Industries is enormous and their multi-disc is even better.

3. Valve body wear.
Solution - a shift kit with a modified valve body to have tighter tolerance to give pressure to the clutch.
-BTW, my DeVille (and most of them built) has this exact problem in the 4T80e. The valve body isn't built to tight enough tolerances causing premature wear in the torque converter clutch valve. Too bad I have to drop the engine 10" to even pull the damned transmission pan!

As you describe this, you were not having problems with the torque converter. The converter ONLY locks in 3rd and 4th gear. Slow acceleration from a light would not engage the converter's clutch. Generally, the converter locks at 35 if you hold the gear in 3rd and at about 45 if in "D." So unless you were seeing those speeds slowly accelerating from a stop, the converter clutch was simply not part of the equation.

Hope it helps.

 

No I was at speeds of 10-20mph. The converter would lock and instantly unlock, until I got up to proper speed (55mph or so) and it would lock up like it should.

After my tuner looked at my trans tables for 20 mins he said I might get slightly worse mpg's but the problem would be fixed. Since then I think I've actually got better gas mileage, but nonetheless the problem is gone.

 

The front cover?? Flexplate bolts to the crankshaft....converter bolts to the flexplate. Not sure what you mean by the cover.

If my converter is toast though...will my fluid smell burnt? Because I checked my fluid and its crystal clear and smells brand new.

My valve body is new, 4 months old.

I did put another cooler on the car, so I have dual coolers now. Maybe thats messing with the line pressure. But this hasen't happened for 4 months since the tranny was rebuilt.....this was the first time this happened today.

Could it be a bad sensor that has to do with lock-up? Do they throw codes?

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If your tuner had the converter locking at that speed, it would get worse mileage, so making it unlocked would bring mileage up like you said. The primary reason is that when you let off of the gas, the transmission would be allowed to coast while with the converter locked at very low speeds, it would hold you locked similar to staying in too low of a gear around town. I'm glad you got it fixed. It could have taken out the converter and/or the transmission.

There are basically 3 things you can see when looking at a converter. Assuming it is sitting on the ground, the top, non-painted part is often referred to as the "snout." It is what engages into the transmission/input shaft. The next UFO shaped part is called the "primary" and is part of the fluid coupling of the converter. The bottom part is called the "front cover."

You are right that the flexplate bolts to the crank and on the other side it bolts to the converter. The part that bolts to the converter is the front cover. The clutch is on the inside of the flat part and when it goes into lock-up, the clutch presses against the front cover to take the fluid coupling out of the equation. If there has been excessive heat from slipping for whatever reason, the paint will discolor there.

The first thing I think you should do is get under the car and take a look. There is an access panel about 3" in diameter that pops off with a screwdriver. Pop it off and shine a flashlight between the flexplate and the converter (the front cover of the converter). If it is discolored compared to the rest of the converter, then it is trouble - trouble enough to take the transmission with it potentially. If it looks fine, then you need to address pressure or tuning.

Good luck. I hope it is still good. Otherwise, well, damn sorry.

 

Cool, now I got it. I'll check it out asap.

Thanks for the clear explanation....

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Back in the older days of lock-up 700R4s, (and 4l60s currently) there was a minimum coolant temperature required before lock-up was commanded. I had crazy intermittent lock-ups that focused during speeds of 45-55 mph with a 700R4 controlled by an OBD1 computer. Nothing else appeared out of the ordinary, and no check engine lights. Fast forward to a fuel pump failure, and while it was at the mechanic shop they mentioned a broken coolant temperature switch on the intake. They replaced it. Lock-up issues went away. I later found out that by having no signal, the computer calculated the defaulted "lowest" temp reading available of -30. This was apparently causing the lock-up issues during that 10mph window, and the temp gauge on the dash continued to function as normal. Now, this may have been a two sender setup, whereas our cars are not....but it is something to consider. I know we have a minimum temperature for lock up because driving your car at a consistent speed (with the motor cool) with an aftermarket temperature gauge will show an initial lock-up happening consistently at a particular temperature. On my digital gauge it happens at 113 degrees. This would be harder to observe on a stock gauge. Also, on the OBD1 setup, if mph exceeded 70, initial lockup minimum temperature requirement was discarded and lock-up was commanded.

 

Hmmm, well since everyone thinks I have a temperature issue causing my missing/stumbling 1 minute after start-up, then it clears up all day and runs perfect. Maybe they're related.

I'll be getting a full scan log next week when I get home, I'll mention that.

And from what I've been told, 112*F is when our PCM's switch to open loop....thats when I'm getting that engine break-up for 20-30 seconds......then it clears up.

Interesting........

Thanks.......

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This is an interesting and quite good idea. Certainly something to consider and look into. I wouldn't have come up with it.