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Torque converter won't stay in lockup

gone huntin

gone huntin

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My '90 Suburban has a 700r4 that is about a year old. I've been noticing that after driving it for a few miles it starts kicking out of lockup then it won't lock at all. What determines if it stays in lockup? It's tbi and doesn't have the vacum switch on the firewall that the older trucks had. The tv cable is adjusted properly, and the fluid level is correct. Lockup works normally for the first few miles of driving, I am watching my tach to verify this.
 
It is controlled via a solenoid inside the trans. The solenoid could be on it's way out.
 
I know that the TCC operates lockup inside the trans. I'm asking what outside controls could cause this.
 
There is a single power wire from the ECM to the trans that goes to the pressure solenoid for the TCC. If the lock up works for a few minutes it's doubtful there is a problem with the ECM or wiring to the trans. The solenoid is a common failure point.
 
I have this same problem but mine has the vacum switch on the firewall. Do I have the same solenoid?
 
yes, this is the solenoid that engages lockup. It might have some odd little wiring preesure connector(s) that could come loose.
 
the ECM/EST controller under the dash, vacuum switch, brake switch,

mine in my 91 started goin haywire and locking unlocking over and over down the highway at 198k miles so i just unplugged it

sometimes its just how it is

been running beautiful without it

no worries

the $$ and work involved to repair it just isnt worth the 200 or less RPM difference cruising
 
82k10short said:
the ECM/EST controller under the dash, vacuum switch, brake switch,

mine in my 91 started goin haywire and locking unlocking over and over down the highway at 198k miles so i just unplugged it

sometimes its just how it is

been running beautiful without it

no worries

the $$ and work involved to repair it just isnt worth the 200 or less RPM difference cruising
Click to expand...

The heat will KILL the trans pretty soon.
 
OK, after giving myself a headache reading up on this I'm even more confused than before. At the ALDL connector under the dash I checked for signal at the "F" terminal. I have power(12+) there until it goes into overdrive then I have ground until I let off the gas, then it goes back to 12+. The test diagram showed that I should have 12+ at the "A" terminal of the ALDL, but I have ground at termianal "A". Until I get this figured out I decided to run it in drive instead of OD. That's when I noticed that it will lockup and hold lockup just fine in 3rd. Why would lockup work fine in 3rd, but not 4th?
 
gone huntin said:
OK, after giving myself a headache reading up on this I'm even more confused than before. At the ALDL connector under the dash I checked for signal at the "F" terminal. I have power(12+) there until it goes into overdrive then I have ground until I let off the gas, then it goes back to 12+. The test diagram showed that I should have 12+ at the "A" terminal of the ALDL, but I have ground at termianal "A". Until I get this figured out I decided to run it in drive instead of OD. That's when I noticed that it will lockup and hold lockup just fine in 3rd. Why would lockup work fine in 3rd, but not 4th?
Click to expand...

Depending on tire size and gear ratio you might need more gear to keep it in OD lock-up. I run 35's with 4.56's and depending on what speed the trans goes into OD lock-up determines if it locks/unlocks or "hunts" between 3rd and 4th gear.
 
It's got 33's and 4.10 gears. I sold this truck a year ago and just bought it back. When I got it back it was doing this, but never acted like this before I sold it. It doesn't hunt between 3rd and 4th (it did when I put 35's on it, so they came back off)it just won't hold lockup. One more thing I noticed was right before it stops working all together, when it comes out of lockup, I can mash the gas a little further and it will lockup again. Then shortly after that it drops it all together. I'm gonna drop it off at the tranny shop this morning to see what they find.
 
Agree with the pressure solenoid. IIRC The pressure solenoid works in 4th or overdrive only. That is why it locks up fine in third.
 
They kept it tonight to order parts. They're gonna replace the pressure solenoid and the lockup solenoid and see what that does. It's just out of warranty, but the manager said he'd cover that for me. He's a pretty good guy, used to be my MAC man years ago.
:D
 
4X4HIGH said:
The heat will KILL the trans pretty soon.
Click to expand...

Is there any proof to this? I know heat kills transmissions but if the transmission never overheats then how would it damage it? Thats like saying if you never go over 40 mph your trans will die due to the TC not locking up.... :confused:
 
FastMaxx said:
Is there any proof to this? I know heat kills transmissions but if the transmission never overheats then how would it damage it? Thats like saying if you never go over 40 mph your trans will die due to the TC not locking up.... :confused:
Click to expand...

Convertor slippage builds heat and that heat WILL kill the trans. The only way to avoid damage is to have the trans converted to a non lock-up style (which uses a special convertor).
 
I got it back this afternoon and it's working great. They replaced the lockup solenoid. When they removed it they found it was cracked in the plastic neck and it broke in half. They had to fish out the remaining piece. I'm so glad to have my Suburban back and to have it operating as it should. Thanks guys for all of your input, I really appreciate it.
:bow::waytogo::waytogo::waytogo:
 
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