Random torque converter unlocking @ on hwy

I have a '90 4.0, aw4 and mt torque converter will randomly unlock at hwy speeds. I got it in March of this year with 140,000 miles on it. The person I got it from didn't take very good care of it. I had numerous vacuum leaks associated with an older Jeep. I removed and or replaced all dry-rotted/oil soaked or redundant lines. I also ripped out the inoperative cruise control and rerouted the vacuum lines to eliminate the vacuum ball behind the front bumper. When I removed the cruise control I removed the vacuum line from the brake switch. Does removing the vacuum line from the brake switch contribute to the unlocking TC problem, and if so where can I hook up a new vacuum line to from the brake switch?

1. the vacuum ball also helps control the vacuum controlled heater controls so hook that back up.

2. t/c unlocking randomly can be the switch at the brake pedal. Make sure it in pushed in to the bracket far enough.

3. what redundant lines? Both my 89's had/have very little as far as ANY redundant lines.

4. never had cruise on an XJ so not sure on that issue.

Your TPS could be bad or out of adjustment. PM me with your email address and I'll send you a simple procedure to check your TPS on either the trans side or engine side as needed.

g.rude said:

I have a '90 4.0, aw4 and mt torque converter will randomly unlock at hwy speeds. I got it in March of this year with 140,000 miles on it. The person I got it from didn't take very good care of it. I had numerous vacuum leaks associated with an older Jeep. I removed and or replaced all dry-rotted/oil soaked or redundant lines. I also ripped out the inoperative cruise control and rerouted the vacuum lines to eliminate the vacuum ball behind the front bumper. When I removed the cruise control I removed the vacuum line from the brake switch. Does removing the vacuum line from the brake switch contribute to the unlocking TC problem, and if so where can I hook up a new vacuum line to from the brake switch?

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Simple answer, yes, that vacuum line to the brake switch is involved in send the yes-no, signal, via the brake switch, to the TCU (Transmission control unit) to allow the TC to engage, or forcing it to disengage. If that brake switch is not clean (it can be removed, disassembled and the contacts cleaned), and properly adjusted it will also cause your problem too, so focus on it for now. I am not sure if the vacuum is needed at the brake switch for normal TC lock up or not?????

Once you get past verifying the brake switch and vacuum or no vacuum needed issues since you mucked (LOL) with that area already, there is an electric solenoid in the AW4 that actuates the TC, it is the third one, number 3 of 3 solenoids. There is a TCU, and wires from the TPS and brake switch to the TCU, TPS, and wires from the TCU to the 3rd solenoid that can also be suspect, as well as poor grounds that can add to random operation of various stuff on the old jeeps!!!

Vacuum is not necessary for the TCU to do it's job. There are no vac lines attached to the TCU. That's just a dump valve for the cruise control. The brake switch contacts can have an effect. Do you run your power/comfort switch in power or comfort mode?

cruiser54 said:

Vacuum is not necessary for the TCU to do it's job. There are no vac lines attached to the TCU. That's just a dump valve for the cruise control. The brake switch contacts can have an effect. Do you run your power/comfort switch in power or comfort mode?

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Cruise control had a dual brake switch for the signal to the TCU that enables or disables the TC lock up by the TCU. It is a second switch, white, way up above the brake peddle. It is separate from the brake light stop switch. It has a vacuum control and a mechanical button that both operate the same switch (the white one that is way above the lower brake light switch). So the question is does that switch need vacuum or not to give the TCU the all clear signal to engage the TC, or not. IIRC, vacuum at that switch is normally active with cruise control off, so loss of vacuum may be screwing with his TC lock up.

No, the TCU does not need vacuum. It has no vacuum ports on it. The vacuum dump valve is a double safety to make sure the cruise control servo loses vacuum when the pedal is depressed, along with the electrical signal to the cruise control's "brain" losing it's electrical signal. Liability and safety issues.

Running in Comfort mode, having a blown 7.5 amp trans fuse in the fusebox, (which has the same effect), Bad Power/Comfort switch, and bad TPS on the trans side are your most likely suspects. Steer away from the vacuum causing the issue and concentrate on the above mentioned possible culprits.

cruiser54 said:

No, the TCU does not need vacuum. It has no vacuum ports on it. The vacuum dump valve is a double safety to make sure the cruise control servo loses vacuum when the pedal is depressed, along with the electrical signal to the cruise control's "brain" losing it's electrical signal. Liability and safety issues.

Running in Comfort mode, having a blown 7.5 amp trans fuse in the fusebox, (which has the same effect), Bad Power/Comfort switch, and bad TPS on the trans side are your most likely suspects. Steer away from the vacuum causing the issue and concentrate on the above mentioned possible culprits.

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OMG, READ MY LIPS! LOL, I said nothing about vacuum to the TCU or TC!!!!!

The vacuum is applied to the special brake switch, brake switch number II, it is white, and it is not the standard brake light switch, but a separate switch used on jeeps that have or had Cruise control, that sends the signal to the TCU (electrical signal), telling the TCU if the brake peddle has been pushed. From your description above:

The vacuum dump valve is a double safety to make sure the cruise control servo loses vacuum when the pedal is depressed, along with the electrical signal to the cruise control's "brain" losing it's electrical signal.

So do you know exactly what the vacuum switch on the brake II switch is doing? How it interfaces with the Cruise control E-module (Yellow box under the dash) and cruise control Mechanical module under the hood that operates the throttle while in cruise mode. What about loss of vacuum to the cruise control Mechanical module under the hood that operates the throttle?All I know for sure is that they are all somehow interconnected!!As the TCU must unlock the TC for the brakes to be able to stop the jeep with out killing the engine, or hitting something in front of the jeep!

I am not saying the vacuum is his TC lock up problem, only that it sounds like he may have created more problems ripping all those vacuum lines out, including some how confusing the cruise control computer, and TCU!!!!!

It sounds like you were saying the white brake switch is an electrical switch and an air/vacuum valve, that only dumps vacuum to the cruise control while breaking, is that right?

Edit: Does it only talk to the cruise computer? Is it the second set of contacts on the brake switch I that feeds the TCU data for TC lock / unlock? I always thought the TCU signal for the TC lockup came from the white brake switch?

That brake switch is strictly for cruise and is redundant. I don't know exactly what it does for the cruise module to get it to turn off, but it does. And then the dump valve makes sure the cruise servo has no vacuum at the same time.

cruiser54 said:

That brake switch is strictly for cruise and is redundant. I don't know exactly what it does for the cruise module to get it to turn off, but it does. And then the dump valve makes sure the cruise servo has no vacuum at the same time.

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So is the white brake switch also the "dump valve" you keep mentioning?

If it is, then he should check the primary brake switch wires, as one set of them does send an electrical signal to the TCU to disable the TC lock up. And the TPS, as you have been saying is also suspect, along with the #3 solenoid and the TCU, and the fuse you mentioned, and the wiring in between and grounds!!!!!!

Yes, the white brake switch with the vac hose on it is the dump valve.
Agreed, he should check the fuse, TPS, and sensor grounds. I'm not sure if he has the instructions I/we made up that include the sensor ground resistance checking. I've sent the instructions to so many that I can't keep track.

So, to the OP, PM me with your email address and I'll send you some simple instructons for testing your TPS on both the engine side and trans side, along with a quick wa to check the sensor grounds.

Thanks all, the trans fuse is good. The power/comfort switch I'm assuming is bad because the previous owner disconnected the switch and there's a jumper wire installed between 2 terminals. I'm aware of the heater control issue with no vacuum ball, under heavy load it just switches to defrost for a minute, no big deal, I would much rather know that I don't have any vacuum leaks. The redundant lines I was referring to are near the air filter box going to the vacuum tree on top and towards the front of the intake, they could have been previous owner installed, not sure though.

Good to hear. The jumpered switch works well also. TPS checked/adjusted yet?

I'm gonna check the TPS ground tomorrow. Interesting note, while driving this morning it kept doing it's normal thing with locking/unlocking. When it would unlock I would give it more gas and it would lock then quickly unlock. I had to stop for gas and when I got back going it was fine and didn't do it again.

g.rude said:

I'm gonna check the TPS ground tomorrow. Interesting note, while driving this morning it kept doing it's normal thing with locking/unlocking. When it would unlock I would give it more gas and it would lock then quickly unlock. I had to stop for gas and when I got back going it was fine and didn't do it again.

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Did you PM me for the TPS instructions? If not, feel free to. I just need an email address.

Yeah I got both of them from you, thanks.

How do you know you are felling lock/unlock, versus a gear shift?