1989 XJS.

Maybe I spoke too soon when I thought I'd conquered the brake/ABS problems on my differential swap.

ABS is not working, brakes feel normal, but the light is not on. It comes on with key and goes out after a few seconds but isn't illuminated despite no ABS. both fuses are good.

Where to start the troubleshooting?

 

Hi Andy

What makes you think the ABS isn't working?

If the light comes on with the Key and then goes off after a few seconds.

That is what I'd expect to see if the ABS was working.

 

I locked up the tires braking.

 

Hi Andy

Unless She does that every time, then you're guess is as good as mine, unless you were trying to Brake on very loose gravel.

 

New information for this fault.

I have the stock 89 ABS wheel sensors in the 94 cage that I installed. I previously had a problem that was solved by rotating the wheel sensors 45 degrees. After this I installed them "permanently" by placing some strapping over them to hold them in place. Then I suddenly didn't have working ABS. ABS light comes on for a few seconds at first and then goes out. No fault indicator.

The manual has a list of ABS faults and which warning indicator correlates. Inoperable ABS with no ABS light is listed as a "broken sensor." So.... I bought two used rear wheel sensors for the rear and replaced the stock ones thinking that maybe my strapping broke the stock one or interfered with their ability to operate (don't really know how that would happen but I'm ruling out variables).

New (used) ones installed and used a zip tie to anchor (avoiding previous metal straps) and same result. No ABS but light operates normally.

I ohm'ed the front sensors, both have continuity, ohm'ed the rear ones, also have continuity.

Any ideas? Brakes work fine just no ABS.

 

Tried measureing the signals they generate?

The rear rings looks different. See if you can compare the ring signals between your old axle with ABS sensor and the new axle with the signals. They MUST be the same. If they aren't, then that is your problem. The same being that the gaps in the signal must be the sams0e as the old axle had...

I think as well, that later ABS systems were with different ECUs. Last chance would be to source an ABS ECU from a 94 and see if that works... The later axle nust have an ECU wirh different mapping to process the differences in gaps bewteen the ring teeth.

 

I'm off to Amazon for an oscilloscope !

 

Maybe silly but do the 89 sensors send a signal similar to the one of the 94's (voltage, ramp-up, etc.)?
Grabbing an oscilloscope is a good idea

 

So the ABS lamp goes out, that suggest the wheel speed sensors are working. The ABS system knows to operate when the brake pedal is depressed. Not uncommon for the pedal switch to go bad or to just be stuck. Also could be a burned or pump motor. It's a long shot but after years of doing this type of stuff I know to accept the seemingly impossible.

Also the ABS motor could be defective, easy to static test,disconnect and apply 12V and listen.

Get a wiring diagram for the ABS system, locate the wire that goes to the ABS computer and check for a ground when the pedal is depressed signal using a test lamp.

 

UPDATE!

I followed above advice and brake light switch was working properly.

So I decided to go back to the beginning and ignore the newly created variables of switching rear ends. Always smart to start at beginning...

Started re-reading manual and followed an online guide about the ABS system I discovered that the reset procedure for any ABS repairs is to drive the car above 19 MPH and after thinking back to my previous test drives I realized that maybe I didn't break that speed in my neighborhood.

Quick drive up to 40 MPH and low and behold-I got an ABS light! Woo-hoo!

So, start testing for codes.

Got code 44 for a left rear wheel speed sensor- no signal or speed comparison problem. Well, that wheel sensor is a used replacement that I bought but it ohm'ed out so I think that's okay. Not ruling it out since I changed the rear end and messed with wheel speed sensors but for now assuming its okay. But another potential fault for this code is also a control module failure.

Also got code 21 which is a main valve failure. A lot of options for that code too but one reason is control module failure. I didn't do anything with the ABS system in engine bay or elsewhere so that would be a coincidence for it to fail at same time.

So, with two codes that have a potential source in the control module failure a new used control module is on the way. Will report back when it arrives.

If anybody has any other insights give 'em to me.

 

No personal experience with XJS abs, however these documents may be helpful if you don't have them already.

 

Just put in new/used ABS control module and still no ABS.

Gonna pull codes again and see what they say.

Disappointed.

 

Does the 94 rear have the same number of teeth on the reluctor wheel?

 

So, start testing for codes.

Got code 44 for a left rear wheel speed sensor- no signal or speed comparison problem. Well, that wheel sensor is a used replacement that I bought but it ohm'ed out so I think that's okay.

Also got code 21 which is a main valve failure.

If anybody has any other insights give 'em to me.[/QUOTE]
When you are measuring impedance rotating this wheel, the impedance will change. It says what the sensor is work. And measure from Abs socket.

As I remember code 21 I got too, but ABS works .properly .

 

Thanks for this tip- read my new update in post below cause something weird is happening.

 

Update:

I installed new/used control module and no ABS. So, I pull the codes and get code 27 which is for the rear valve. Okay, doing a little research to see what that means and decide to install original control module.

So this is where it gets weird: When I reinstall the original control module (CM here out) I get codes 21 and 44.

When I install replacement CM I get code 27. This code seems to have come with the replacement CM since I don't get it with original CM.

I don't think the CM's are correctly reading the codes or it seems that the CM's are not clearing old codes. procedure for clearing codes is to make repairs and drive car above 19 MPH and use brakes.

Don't know if this is related but I don't get an ABS light with either CM.

Any insights?

 

Update:
I read in the manual that until a repair is made the same code will appear until fixed and no other codes will appear until first codes are repaired. Keep that in mind so that you fix things step wise.

So, I decided to start from scratch and install everything as originally installed. This includes original ABS module, all wheel sensors and I also installed them with original mounting hole location (earlier i turned them 45 degrees and thought I'd fixed the problem but all that did was disable ABS with no light).

Original problem reappeared, ABS activation all the time and a code light. So, pulled that code, got a new code for right rear sensor. Ohmed the sensor-good. Then I got sophisticated and followed manual advice to read voltage for sensor at ABS module while turning tire at at 1 Revolution per second and I noticed a variation in voltage produced, varied from .4 to 1.1 volts, this appeared to be in spec but something was wrong and manual said, incorrect sensor gap or damage to sensor or ABS reluctor (also called ring or tone ring).

Measured the gap- good. checked the sensor-no damage. checked the reluctor wheel all the way round. Discovered that when I drilled out damaged wheel sensor that I drilled too far.

Idiot. (first pic).

I didn't think that the damage would have that big of an effect, there's still half of the teeth. But I guess it does.

I REALLY didn't want to remove the hub and so had the idea to repair the reluctor by welding the damaged part ON THE CAR THROUGH THE WHEEL SPEED SENSOR HOLE! Like arthroscopic surgery! And then grind the weld down to match the original profile. F*cking genius! (Second pic). All those sensors are are magnets-how perfect does that reluctor have to be??

It didn't work. ABS activated all the time (same as before). Now, I didn't think the previous damage was enough to cause problems but I coulda swore that my welding and grinding created enough metal to activate the sensor properly. But no dice, didn't work--but it was genius!

So, I bought a new reluctor wheel-new one was wrong part. Got frustrated and went to pull-a-part and found a 1990 XJ6 and pulled the hub off to see if any chance parts are same between 1990 XJS and 1994 XJS-they are! Same ABS reluctor wheel and ABS sensors have same type electrical connector.

Installed new/used reluctor wheel and holy-sweet-Jesus and fixed it!

So, moral of story is, do what the manual says and any damage to ABS tone wheels/ reluctor wheels matters. Also, 1990 XJ6 ABS parts are similar to 1994 XJS parts (but not similar to 1990 XJS parts).

 

At least you got it fixed, and learned something along the way - so double bonus.......