TC will not lockup. MT2500 says Brake is pressed

Hi all,

1988 XJ 4.0 4x4 AW4 195deg thermo Cruses at 205deg

TC will not lock up automatically. I can do it manually through a switch, but the TCM will not do it.

Snap On MT2500 scanner says the brake is pressed.

Does anyone know which sensor tells the AW4 TCM that the brake is pressed?

My brake lights are off. The instrument panel brake light is off.

Can't figure it out. Thanks.
-Clay

On your Renix XJ, there are two separate switches, one for the brake lights/cruise control and another for the transmission controller. The white plastic one is the one for the transmission controller while the metal body switch is the one for the brake lights/cruise control. If you have cruise control, the metal bodied switch will have two sets of terminals on it. The plastic body is threaded, yours may just need adjustment...

Jim Malcolm said:

On your Renix XJ, there are two separate switches, one for the brake lights/cruise control and another for the transmission controller. The white plastic one is the one for the transmission controller while the metal body switch is the one for the brake lights/cruise control. If you have cruise control, the metal bodied switch will have two sets of terminals on it. The plastic body is threaded, yours may just need adjustment...

Click to expand...

x2. There are switches on the brake pedal. The white switch up higher is the you want to check. It will also have a vacuum line on it that relieves pressure on the cruise control solenoid when you hit the brake.

Thank you Jim and Lawsoncl...
I did not realize there were two switches. I will look to adjust the sensor. I appreciate the help.

There is really only one switch module located on the brake pedal. Inside it has two separate switches with two similar connectors. If you need a schematic, I have the full XJ factory wiring and troubleshooting manual and can scan a couple of pages for you.

https://www.cherokeeforum.com/attac...brake-light-switch-88-xj-image-3000137197.jpg

The white plastic body at the top right is the switch for the tranmission controller as well as the cruise control vacuum switch that lawsoncl mentioned.

The brown connector going off the right side of the image is for the brake lights.

The brown connector in the center of the image is for the cruise control.

Jim Malcolm said:

https://www.cherokeeforum.com/attac...brake-light-switch-88-xj-image-3000137197.jpg

The white plastic body at the top right is the switch for the tranmission controller as well as the cruise control vacuum switch that lawsoncl mentioned.

The brown connector going off the right side of the image is for the brake lights.

The brown connector in the center of the image is for the cruise control.

Click to expand...

Beat me to it. The switch closes the contacts when the plunger extends. The switch could be out of adjustment and just pushed closer in the mount. It could also be dirty inside and it can be taken apart be carefully pushing in the tabs on the sides.

With the key-on, you should see 12-volts on the lt-blue/yellow.stripe wire and ground on the black wire. If the switch seems fine but your not seeing 12-volts on the wire, there is a resistor wrapped up in the wiring harness near the trans computer. I've seen that get corroded or damaged and cause the trans computer to think the brake is applied as well. It's a "pull-up" resistor and one side is connected to +12volts, and the other to the wire going to the brake switch.

Clarification: With the key-on and the plunger pushed in on the switch, you should see +12 and ground on the wires. With the plunger out, both should show ground.

Checked it out tonight, and the white plastic switch was barley in the mount. It came right out in my hand. I turned on the scanner and the switch is good.

Does it thread in, or just pressure fit by me pushing it through the hole? I don’t want to break anything. If thread in, should there be a lock nut on it?

It's a pressure fit... I believe the official calibration procedure is to push it in as far as possible and then apply the brakes; the sensor slips within its mount to auto-calibrate much like the trans cable. Internet rumor as I don't have the official manual.

Jim - Thank you. I'll do that and hope for the best!!!