Torque Converter Won't Unlock

87 with supposedly a 88 4.0 and aw4. np231 and 4.10s to match the 31's. Unknown mileage on the motor and trans. Fluid changed to atf+4, with cooler and remote filter.
TPS is adjusted correctly. I'm going to double check the TV cable cause it didn't seem to set right the second time. Also Power and Comfort mode doesn't change it but does change shift points.

The jeep has been doing it since I got it (2 years) but under normal throttle it locks up in 2nd at an appropriate time and stays locked through 3rd and 4th. WOT it locks up right away in 3rd gear and also down shiffting into 3rd locks up exactly like at WOT. Never unlocks in 4th. Any ideas? I only found stuff about it not locking up. Thanks

It should unlock when you press the brake pedal. Does it do that?
I'm thinking it must or the Jeep would die at stops.

I believe it's not unlocking when braking but unlocking because of the tranny downshifting. I have a good amount of engine braking when going down hills and can get on the brakes, let off and it will still be slowing itself. Though if I put it in 3rd gear it should be unlocked and actually probobly brakes similair to any other car in 3rd but 4th is the one that is unusually dragging. Also a lot times letting off the throttle in 2nd it will shutter like a manual and then unlock I believe or maybe just shifting into 3rd.

I don't mind the engine braking, it's helps control the speed around town and on highway hills.

in 4th it should be locked up fully..in 3rd should be the coupling phase...could be the 4th element on your torque converter could be going..causing the shuttering...did this happen when you changed the fluid to atf+4?

My 87 locks up in 3rd at about 38 to 45 mph, and unlocks (shifts and relocks) for about 1 second, before it shifts up or down. It stays locked with my brakes on, till the speed is down to about 23 mph and 1200 rpm, then unlocks. I have a custom idiot light (red LED) on mine to show when there is power to solenoid #3. I also have a toggle switch on mine so I can switch it manually. Mine never stalls. I am not 100% sure if my brake switch signal to the TCU is working, as I was once told the TC should unlock when the brakes are applied, but I have not 100% verified this.

Get rid of ALL the ATF +4, AW4 only likes Dex III. May take several transfusions to get it all out.

Ecomike said:

I was once told the TC should unlock when the brakes are applied, but I have not 100% verified this.

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there were some changes in the TCU between '87 and '88 and I think the lock up functions were one of the changes. IIRC, Cruiser discussed them in a thread 6ish months back and the better TCU's are in the '89 or '90 XJ's.

Shorty said:

there were some changes in the TCU between '87 and '88 and I think the lock up functions were one of the changes. IIRC, Cruiser discussed them in a thread 6ish months back and the better TCU's are in the '89 or '90 XJ's.

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I discovered a bleed off diode to ground (IIRC) in the brake switch wiring between the 87 TCU and the brake switch on mine many moons ago. Have not gone back to dig into that topic yet either. It is hiding inside the wiring harness about 18" from the TCU as I recall.

Interesting, looks like I need to dig more useful info out of Cruiser54!!!!!

The first 4-5 years I had the 87, the TC only showed a 50-100 rpm drop when it locked, but the rpm rose like it was not locked when I gave it a little gas (that was why I added the LED light, for troubleshooting). I think that TC was just worn out, or the AW4 had a fluid pressure issue, or both, which did finally kill it four years later. I plan to replace all the seals someday......it never had any clutch plate material in the pan, the O-rings just died after 24 years, and stopped sealing I think. My new one locks like it means it, LOL, a good 200 rpm drop, and if I go to say 1/2 WOT in 4th, it stays locked, and the engine RPM barely moves)

They changed the TCU in 88 because the 87 was too "busy" for customer's liking.
Grab an 88 to 90 TCU from the boneyard.
Also be aware that if the 7.5 amp "trans" fuse is blown you're stuck in Comfort mode and also have no reverse lights.
Additionally, the TCU shift logic circuit grounds at the engine dipstick tube stud!!
Refresh your grounds.
I'll attach a little write-up that I did as a result of my wife's 88 shifting up too soon. This is something that really made a difference in her Jeep.

Cruiser’s Trans Plug Refreshing

Over near the trans dipstick tube are 2 rather large connectors. One is black and the other gray. These 2 connectors carry all the info between TPS, TCU, NSS, speed sensor, and trans solenoids.
Unplug each one, visually inspect for corrosion or bent pins, spray them out with electrical contact cleaner and plug them back in.
Revised 3-28-2012

That reminds me, when I first got the 87 in about 2004, it upshifted at 1200 rpm and I was in 4th gear at about 25 mph at WOT, LOL, took 3 minutes at WOT to reach 60 MPHs!!!!!

The 87 TPS test said the TPS was fine, 3 years later I found the 89 TPS test changed, and I discovered the TCU side of the TPS was bad all the time. I learned a lot about renix debugging that problem.

Good idea Mike. Wouldn't hurt to check the trans side of the TPS.

AFTER the ground refreshing and trans connector refreshing......

Yea I've done the grounds but will check the plugs and tps reading on the trans side (new tps). I might try the newer Tcu also just to see the difference.

My friends 89 mj seems to shift much later when on it compared to mine that doesn't care if your trying to rev it. Also on the highway his unlocks then locks everytime he gets on the gas in 4th. Is that normal? It never stays unlocked for any time. I don't notice anything else distracting transmission wise.

Trans switch in Power or Comfort mode? If in Power mode, does it light up?

Trans fuse in fusebox powers that switch. Is it blown?

It works. The difference is noticable and doesn't change the torque converter behavoir. The same for the Mj.

Super mud said:

It works. The difference is noticable and doesn't change the torque converter behavoir. The same for the Mj.

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Just making sure it works.