No Lock-Up, TCU C10 says brakes are on

Hi All,

I noticed that my wife's DD (XJ, '92, l6, 4.0 HO, 155K, ltd edition) stopped locking-up (torque convertor). Also, the cruise control is no longer functional. I don't know since how long exactly since I haven't driven the XJ in a week or two (my wife doesn't use cruise and doesn't notice transmission lock-up). However, my first thought was that maybe a brake switch got stuck (assuming that that would affect both the cruise and torque converter, right?). I checked the TCU C10 connection to see what the TCU was being told and voila........it thinks I'm hitting the breaks even when I don't touch the brake paddle (constant battery voltage on C10). So a simple fix? Maybe, but I'm at a loss....please help!

First, there seems to be a single switch on the brake paddle which consists of a little white plunger that extends when the brake paddle is depressed. Now, the brake lights work perfect. So the obvious questions are:

1) Is the brake light switch the same as that talks to the TCU? Then how can the brake lights work OK and is the TCU being lied to? If so, I should just replace the switch unit on the brake paddle? Can I diagnose that switch with my multimeter? How?
2) Are there multiple switches on the paddle? I can see only one and checked several times (my wife says this is no guarantee:laugh:, but still...). If so, where are they located and what do they do?

Thanks for any help!

Tim

The C-10 and the switch is on the ground side of the circuit, so with no brakes applied you will get voltage. With the brakes applied the voltage in the C-10 pin to, Lite blue w yellow strip, to splice, to Lite blue, to the switch, should have little measurable voltage, with everything connected. The switch completes the ground circuit.

It's been forever since I've had to troubleshoot an XJ brake switch, so my memory may be faulty. But I'm pretty sure it is a pressure switch in the brake system.

There is a diagram on in this post, that may be helpful. It's for the Renix but likely identical or nearly the same as your 92.
http://www.naxja.org/forum/showthread.php?t=921316

Thanks 8mud! I followed the directions in


http://www.transonline.com/transdigest/magazines/1997-10/Shift Pointers/index.html

(at least if I didn't misunderstand), which I got from this link

http://www.naxja.org/forum/showthread.php?t=916747&highlight=lock+up


I thought I could simply hook the negative lead to a ground (that's how I interpreted the transonline.com link). I've very little experience with wiring etc so I might easily have oversimplified the measuring procedure. I have tried to read the diagram you referred to (http://www.naxja.org/forum/showthread.php?t=921316) but so far I've been unable to print it in a readable format (too coarse). I don't have my FSM here, but as soon as I do I will scan the diagram that's in there so that we may discuss it (is that OK with FSM forum rules??). Or maybe you can comment on that transonline.com link wiring explanation so that I understand where I screwed up?

Thanks again, really appreciate the input!



Tim

dutchjeep said:

Thanks 8mud! I followed the directions in


http://www.transonline.com/transdigest/magazines/1997-10/Shift Pointers/index.html

(at least if I didn't misunderstand), which I got from this link

http://www.naxja.org/forum/showthread.php?t=916747&highlight=lock+up


I thought I could simply hook the negative lead to a ground (that's how I interpreted the transonline.com link). I've very little experience with wiring etc so I might easily have oversimplified the measuring procedure. I have tried to read the diagram you referred to (http://www.naxja.org/forum/showthread.php?t=921316) but so far I've been unable to print it in a readable format (too coarse). I don't have my FSM here, but as soon as I do I will scan the diagram that's in there so that we may discuss it (is that OK with FSM forum rules??). Or maybe you can comment on that transonline.com link wiring explanation so that I understand where I screwed up?

Thanks again, really appreciate the input!



Tim

Click to expand...

The 88 schematic I looked at said C/10 to ground through the brake switch.
The 92 schematic I looked at said C/!0 to ground through the brake switch.
The 96 schematic I looked at said C/10 to ground through the brake switch.
Though some of the grounds take different paths.

Here is a 96 schematic that may be helpful.



Here is one for an 88.



Here is one of the rings around Uranus

Sorry it took me so long to get back. This weekend I finally got to really look into my newly acquired FSM and started work on trying to understand the wiring.

So problem is solved. Cruise and T/C lock-up work again, I'm happy!

For future reference for others (I don't know how general this is; works for my '92):

* My FSM wiring diagrams suggested that C10 on the TCU (with ignition ON) should indeed switch between 0 V and battery voltage when the brake is depressed: I found battery voltage all the time....no good.

* Measured resistance across brake switch (on paddle)....bad connections.

* Cleaned switch (pry open, use contact cleaner)........much better.......not perfect (90% operation)....mechanical wear.

* Bought new switch......installed....works!

To verify initial assumption I checked that the C10 port on the TCU really does indeed give 0V when the brake paddle is up and battery voltage when depressed. It does now. Completes the circle, done.

I'm glad I finally bought that FSM, great investment!