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700R4 No Lockup When Hot

longbedder

longbedder

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The title pretty much says it.

I've got a 700R4 with manual TC lockup control via a dash toggle switch. If the trans is cool it'll lock right up at speed (and hold it). If I've been running around a bit and the trans is warm it will not lock up. It's of course self-defeating at that point because without lockup it won't cool down. I don't have a temp gauge installed, so I can't comment on exact temperatures involved.

My first thoughts regarding culprits are:
-hydraulic pressure gets too low when hot (pump clearances opening up?)
-Failing torque converter clutch
-Solenoid doesn't work when hot
-Something in the valve body failing when hot?

Anyone have experience with this?
 
What year trans?

Keep in mind that the lockup is an AND function that relies on hydraulic and electric signals. It will help to figure out exactly which setup you have.

There is a lockup valve in the valve body based on what gear you're in and TV pressure (can't remember if governor is involved at all). That valve provides the lockup pressure. The solenoid prevents that pressure from venting to the pan, when it is engaged. There are also some pressure (electrical) switches involved. Typically the tranny gets 2 12V sources. 1 is constant (terminal B) and 1 is the switched signal (terminal A) from the factory vacuum switch. The constant hot is switched by the 4th pressure switch, so power from terminal B would do nothing if that switch is failing. From either terminal, the solenoid is switched by the "downshift switch" which opens when the tranny is moving from 4th to 3rd. If that switch was failing you would also lose lockup.

A nice troubleshooting tool is to connect to trans terminal D. That provides a ground path when the hydraulic lockup pressure is present (assuming your model has this switch).
 
Blue85 said:
What year trans?

Keep in mind that the lockup is an AND function that relies on hydraulic and electric signals. It will help to figure out exactly which setup you have.

There is a lockup valve in the valve body based on what gear you're in and TV pressure (can't remember if governor is involved at all). That valve provides the lockup pressure. The solenoid prevents that pressure from venting to the pan, when it is engaged. There are also some pressure (electrical) switches involved. Typically the tranny gets 2 12V sources. 1 is constant (terminal B) and 1 is the switched signal (terminal A) from the factory vacuum switch. The constant hot is switched by the 4th pressure switch, so power from terminal B would do nothing if that switch is failing. From either terminal, the solenoid is switched by the "downshift switch" which opens when the tranny is moving from 4th to 3rd. If that switch was failing you would also lose lockup.

A nice troubleshooting tool is to connect to trans terminal D. That provides a ground path when the hydraulic lockup pressure is present (assuming your model has this switch).
Click to expand...
The trans is a replacement from Bowtie Overdrives - I'm not positive that it's a later style, but I'm reasonable confident that it would be.

When I got the trans I bypassed the pressure switch setup on the valve body so that I could lockup on command in any gear at any speed. It worked great since new in 2006 that way, but lately has been exhibiting the symptoms I refer to in the OP.
 
I'm guessing the TCC solenoid is failing. One other thing to check is that you have a good clean ground from the trans to the frame.
 
lockup O-ring on the tail shaft is going bad. when the fluid heats up it wont hold the pressure. thats my guess
 
longbedder said:
The trans is a replacement from Bowtie Overdrives - I'm not positive that it's a later style, but I'm reasonable confident that it would be.

When I got the trans I bypassed the pressure switch setup on the valve body so that I could lockup on command in any gear at any speed. It worked great since new in 2006 that way, but lately has been exhibiting the symptoms I refer to in the OP.
Click to expand...

Even if you bypassed the 4th pressure switch, you still rely on the lockup valve supplying pressure, so all the pre-requisites for that still apply.

I say start with the easy stuff. Check out all the electrical stuff and make sure it's doing what you think it's doing. Then replace the solenoid - it's just a pan drop. If you don't have a pressure gauge, find a tranny shop that will test drive it with a gauge so you know if overall pressure is dropping off. After that you're getting into trans pulling territory.

If you do drop the trans or slide it back to change that O-ring, a trans shop can pressure test the TCC in the TC to see if it's good. Because a TC swap is easy at that point. I have had 2 go bad on me. 1 failed at like 10,000 miles, I assume the rebuilder didn't actually replace it. The other was failed in a truck I bought, but it had a lot of miles. There was no lockup cold or hot, though.
 
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