Hello,

My '92 XJ6 has been giving me some trouble with the ABS system. The Anti Lock light came on a few weeks ago when driving, went out for a few days, and is now on, and currently comes on every trip.

When I turn the key to Ignition and wait for the bulb-check to complete, the light is off, and stays off even after starting the engine. Once you pull away, however, the light illuminates shortly after. Some times its when hitting 10 or 15MPH, sometimes when hitting 45 or so.

If I turn the ignition off and start then back on, the light stays off as I've mentioned above - until I take off again.

I've found the brown PM4 connector near the ABS ECU over the wheel well in the trunk. I've shorted the Brown/Pink and Black wires with a jumper, and turned the key to Ignition, but I can't get the light to blink. Wouldn't the past codes be stored in memory even though the light is off when I try to check the codes?

I have the PDU, and the diagnostic plug used for the TCM & Engine Control Module is the same as the diagnostic plug for the ABS computer. Can the PDU pull ABS codes directly? I haven't found an option for it in the PDU "Toolbox," just the Data Logger.

-Nick

 

Hi Nick,

I'm not an expert on the ABS system, but a few things I might check if I had your symptoms would include:

1. Wheel speed sensors - clean the sensors and reluctors, test the sensors for resistance or open/shorted condition, inspect the wiring harnesses for chafing or other damage.

2. Test the accumulator following the procedure in the manual.

3. Check the wiring and connectors at the electric hydraulic pump and the combined low pressure/warning switch and confirm that the electric pump runs but not continuously.

4. According to the 1991 Electrical Guide, the ABS system uses two ground/earth points. One is on the left side of the trunk behind the carpeted trim panel, and the other is in the engine compartment at the "left/rear power ground." It might be worth cleaning the threaded studs, all eyelet/ring terminals and star washers and flushing them with zero-residue contact cleaner and allowing them to dry before reassembling them.

5. I don't know if there's any connection between the Anti-Lock warning lamp and any of the DTCs, such as FF68 indicating a problem with the road speed sensor, but it might be worth checking your VCM to see if any DTCs are stored.

Hopefully one of our ABS experts will respond.

Cheers,

Don

 

Nick,

If your light extinguishes after startup and only comes on when travelling then I think you would assume an ABS wheel sensor first rather than an accumulator. The light extinguishing after startup means that the accumulator is reaching pressure from first start. At this stage, a sensor problem is not known as the wheels are not rotating. As the car moves, if a wheel sensor is then "thought" to be out of range, the light would illuminate. The fact that restarting turns out the light would make me think it's not the accumulator and far more likely to be the sensors. You can check the integrity of the sensor wiring by performing the defined diagnostic tests in the handbook. However, the fact that the light is erratic may mean that the test doesn't show up anything.

As Don has suggested, check the seating of the sensors. It doesn't take much in the way of dirt or muck under the head to confuse them. Pull them from the hub. check and reseat. Then check the integrity of the first wiring plug joint back from each sensor.

Good luck

Paul

 

Thanks Don and Paul,

I've done the accumulator test recently and I remember having many many more pumps available before a hard pedal than required, and I've never gotten the Anti Lock and Brake light at the same time from low accumulator pressure before. After the car sits overnight, the Brake light illuminates for maybe 5-10 seconds before extinguishing and a couple seconds after I can hear the pump audibly kick off.

It makes a lot of sense that an iffy wheel speed sensor would cause the light to come if that sensor was giving an out-of-range reading once the car was in motion.

I would assume, however, that the fault code would be stored in memory, and could be retrieved after the ignition is cycled, by means of the diagnostic connector.

I've checked my PDU manuals, and it turns out the ABS DataLogger function requires the use of a XJ40 Teves Breakout Cable, which I don't have. I only seem to have the XJS Teves cable... The breakout cable connects between the ABS harness and the ABS ECU to read the input/output signals. If I had the cable, I'm sure I could see the reading from each wheel speed sensor while driving, and could instantly determine which was causing a problem. I'll have to figure it out another way, I guess.

I'll be sure to check and clean the sensors and wiring at each wheel.

I just wish I could figure out why I can't pull the codes :/

-Nick

 

So, I'm glad to say the problem is solved. I went through the diagnostic steps and found I wasn't getting a resistance reading for the LR speed sensor.

Turns out the wiring was to blame. The insulation was cracked about an inch after it passed through the trunk grommet. The braided shielding was actually separated through. A quick repair to the brained shielding and new insulation fixed it in little time.

Nothing like a free repair.

Thanks for the help,
Nick

 

Nick,

Good job tracking down the fault - glad the repair was easy!

Cheers,

Don

 

Well done Nick!

A good diagnostic approach is always the way in my view! And good that you didn't just start changing parts as many people do in an attempt to cure the problem!

Paul