1996, torque converter electric lockup issue, please advise!

Long post, everything I know so far....

Hello Tacoma World! New member, but been wrenching for the last 20 years.
I just joined, to hopefully solve this specific issue with my brothers Truck.

I sent this to gearcruncher, but realized he hasn't been on here for some time, so I hope there is still help to be found.

I apologize in advance for the long read hereafter. Hopefully I will provide all the information I have at this time. Haven't found a thread with my exact problem, been reading through a bunch of posts the last couple days.

It is a 1996 Tacoma, 3.4L V6, Automatic transmission, 4x4, ext. Cab. The transmission tag says 30-40LE. Let me know what other info I need to provide, or other ways/things to test.

We did a big servicing before he took possesion of it and drove down to Utah from Eastern Washington yesterday.

The service included - greasing drivelines/ball joints/tie-rods, oil flushes and refills throughout all diffs/did transmission pan drain first time, then dropped pan and cleaned magnets & screen second second drain/engine flush/power steering flush, timing belt kit+water pump+ thermostat, all other belts, cooling system flush and refill, replaced front brake calipers and pads, adjust rear brake shoes & e-brake and replaced one rear wheel cylinder+ Flushed and bled brake system. Cleaned and greased warn hubs, New ngk iridium spark plugs, sprayed cleaned throttle body, spray cleaned MAF sensor, adjusted all cables on throttle body, cleaned up a bunch of electrical grounds and battery connectors, cleaned and oiled k&n air filter (cleaned MAF sensor again after about 200 miles of test driving), as well as a section of exhaust repair to extend pipe to back of truck.
Added a smidge of stop leak to Power steering and engine oil, added trans-X to transmission before first fluid change, then added risolone transmission treatment after second change.
Truck has 240,000 miles on it after all that is said and done.


Now the issue:
At highway speeds, everything at operating temp, the electric clutch in the torque converter will engage and disengage in 3rd and 4th gear between 45-65 mph, about every 5-10 seconds, it's pretty consistent. I don't know if this is bad, or slightly/extensively wearing out that electric clutch, etc. it is however, very annoying! Haha
It Does this with or without cruise control on, if on a flat piece of highway I notice it will do it if I let up just ever so slightly on the throttle, and will re-engage if throttle is pressed down just slightly, as well as holding throttle steady, perhaps load changes slightly. But I don't know what is causing this.

I did a stall test (had to have it in 4wd, because no matter what it spins the rear ties in 2wd), it maxes out at 2000-2050 rpm's, engine runs smooth under this load, no weirdness.

Did a brake tap test, keeping the throttle steady, just barely touching the brake pedal would cause the TCC to disengage, same outcome several more times I tried, no shudder or anything at all.

Runs, shifts and drives fine otherwise except for a barely noticable slight vibration between 25-45mph, would not have even noticed it except for a smooth stretch of highway I was on. Was getting about 20 MPG the last time I topped off (95% highway). No codes were pulled at all at AutoZone, no dash lights on, no noises.

If you made it this far without falling asleep, congrats! And thanks in advance for your time, have a great day!

(***Update- CEL popped up on my brother halfway to Utah, he had it checked, it was for the catalytic converter, which wasnt there then we got it. No other codes or dash lights.
I also advised him to add lubeguard synthetic atf protectant and the instant shudder fix as a precaution, which he is doing today, I don't expect any change.)


-Nick

 

The P0420 is common, check for exhaust leaks in front of the cat before you spend any money on sensors.

Check the brake switch... it could be loose and causing your TCC issues. The signal from the brake light switch is used to disengage the TCC internal to the PCM

TCC issues can be a real bitch to figure out. If it was me I would start with the connector on the top D/S of the trans. There is a "thing" called AC ripple and it causes TCC to do as you describe but I have never heard of it in Tacomas. One fix is to clean all the ground wires. There are 3 ground connections on Tacomas, Battery to block, battery to inner fender well and on the P/S head rear to firewall. The PCM grounds are just in front of Toyotas black connector on the upper intake for the 3.4. These might "look" ok but after a tech or 2 has rested his hand there the wires break internally. Give each one a a wiggle indivdually where they are crimped onto the eylet to test internal integrity.

This is what i found on my 96 4X4. I had to buy the whole harness from the wrecking yard as the connecotor was not avalible by itself. Obviously when the trans was pulled out at some point in her life they forgot about this plug, broke it and rigged it.

 

I don't understand the torque converter, but when my throttle to transmission cable was adjusted too tight, it would suddenly accelerate on its own at some point when I was cruising steadily at the speed you describe.

 

Okay, I will relay this info to him. I meant that the catalytic converter was missing when we got it, no codes at that point, but it popped up on him driving down to Utah from Washington.
I'm assuming the ECU picked up parameters out of spec on the rear O2 sensor, and dumped extra fuel in trying to clean up the CAT that isn't there, say no change and threw the code (which would explain a 2-3 mpg drop during one fill-up he had). I think the sensors themselves are fine.

I will have him check the brake pedal switch, those grounds (I think I cleaned most of those up, expect the points on the block and head), and the trans connector.

I did get a reply from sonnex transmission company, describe everything to them, they said for a on/off switching like this it is usually the TPS, or the solenoid itself, they also suggested the valvebody issues with line pressure and wear, but they said if it's the valve body, usually there is a chattering or other similar noise and a code.

I know how to test the TPS, if it is bad, does it need to be adjusted to any specific voltage at open/closed positions, or just slapped on there like the old one was?
Or for that matter, move the old.one around for kicks and giggles and see what it does?

Might be a week or so before he gets around to doing anything, he is working two jobs.

I will reply when I have new info.

 

Okay, Update. Since he is strapped for time, I ordered him a new TPS, he installed it and it seems to have fixed most of the problem. He says it is no longer locking and unlocking like it was before at highway speeds, But he said he can still get it to unlock/lock after he accelerates up to speed and just barely backs off of the throttle when cruising.

I think that may be normal, but I told him to video record the dash cluster next time he was on the highway, (and to check his wiring). Seems it's 90% resolved though. Thanks for your input everybody! I'll update again once he has time and sends me a video.

 

Final update;

He has driven it more on the highway, and the locking/unlocking TCC issue is fixed. IT WAS THE TPS!
Thanks for all your help everybody, I'll try to convince him to go over more of the wiring and check that out, everything seems to be pretty solid on this little truck...
Now if only I could make it get 25MPG combined....

Take care and have a happy new year!