Bendix ABS problem

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steelcityuk
Posts: 1053
Joined: 03 Jul 2006, 21:51
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Bendix ABS problem

Post by steelcityuk »

A winding tale about the XMs ABS problem, this has been posted elsewhere
but is duplicated here just for info. I barely run the SED before taking it
off the road for over a year for the clutch and injector pump rebuild that got
out of hand. That meant that by the time I'd got the car back on the road I'd
forgot what worked and what didn't. During the rebuild a front ABS sensor was
destroyed whilst dismantling the front suspension/hubs/calipers prior for
cleaning and painting. I bought what I thought was the correct sensor (Bendix)
and fitted it. Next I grit blasted the rear arms and cleaned the rest of the
subframe and underbody before painting and waxoyling it all. Once it was all
back together it went for a MOT and passed, it was after this I noticed that
when slowing down just before coming to a complete stop the rear end would rise
up then drop. It seemed that just before stopping the ABS was activating on the
rear brakes allowing the rear to rise then applying the rear brakes again
bringing the rear end down again. It was/is very disconcerting for the rear to
shoot up just before you want to stop. Measuring the sensors impedence revealed
the 'new' sensor had twice the reading of the others so I put the problem down
to that. Further research suggested this shouldn't be a problem so I thought I
should check the rear reluctor rings (having grit blasted the rear arms) and the
fault seemingly being a rear ABS problem. The arm I took apart looked pristine
inside, the 'ring was perfect and I thought (incorrectly as it turns out) that
the rear arms are sealed once the hubs are in place, this made me think there
was no point in checking the other side... I ran the car for a year with the ABS
ECU removed because the brakes worked really well without it, in the meantime
I kept an eye out for a early Bendix sensor.

Fast forward to this year, I decided that once the weather improved I'd start
tackling the odd jobs, faulty A/C, faulty ambient temperature sensor and most
importantly the ABS fault for the MOT. Luckily I found a early Bendix ABS sensor
on Ebay. It's reading was correct with a multimeter so I fitted it. The XM was
exactly the same, I drove around for a few days with the rising rear problem
then for some reason on a drive across town the brakes became very poor and it
was possible to tell that ABS was activating on all 3 circuits then the light
came on the dash and the message on the dot matrix, braking was restored to full
power with no rising back end. On the trip home the same thing happened. A Lexia
session showed permanent faults with both rear sensors and intermittant faults
on both front sensors. Testing with a multimeter on the ECU plug shows that all
four sensors are within 1.04 and 1.06 kOhms. So if the sensors read OK
statically then I thought it must be the reluctor rings.
Yesterday evening I removed the right rear hub to check the 'ring, sure enough
just like last time it was almost perfect, I cleaned it, the sensor end and the
cavity with brake cleaner and replaced it. Last night I checked hydraulic and
electrical diagrams for more clues, I came to the conclusion that maybe the ECU
plug had oxidised in the year or so it had been disconnected despite looking OK,
maybe the multimeter probes are cutting through this crud to give me a reading
whereas the ECU pins aren't, so the idea was to clean up all the connections on
the ECU and plug with fine emery paper but before I did that I'd check the left
rear hub 'ring because I'd already got it part stripped. After work on Tuesday I
removed the hub to find grit and other crud in the cavity, they're not sealed
after all... Also there was crud on the reluctor ring and a thick paper like
sticker on the end of the ABS sensor - maybe this is a spacer that's used to set
the air gap but should be removed before final fitting. I've removed the sticker
from the end of the ABS sensor and carefully cleaned up the cavity and 'ring.
Thinking about the way the Bendix ABS system works this makes some sense (at
least in the way the XM reacted, maybe not the Lexia diagnosis), the system has
four sensors but only three control blocks - one control block each for the
front wheels and one shared control block for the rear. This would suggest that
if either rear sensor detected a 'problem' with the rear wheels all rear braking
force would be lost and the rear would rise.
not applicable
steelcityuk
Posts: 1053
Joined: 03 Jul 2006, 21:51
Location: not applicable
My Cars: not applicable
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Re: Bendix ABS problem

Post by steelcityuk »

Next on the to do list was check the front sensors, so I started on the front
left, the sensor I'd replaced just a week or so ago. Well the cable had sprung
out of one of the clips and had been rubbing on the tyre, it had worn through
all the insulation and started fraying the copper cores in such a way that the
odd strand was being pulled by the rotation of the wheel and shorting out on the
other conductors. That must have been what was causing the total ABS failure. I
soldered the ends back together and insulated and protected them using self
amalgamating tape and heat shrink tubing. The car was taken for a quick spin and
it was exactly as before - rising rear.

The Citroen manual suggests that spinning the wheels by hand will generate a
modest voltage that can be measured with a multimeter so that was my next move.
Measured at the ECU connector the rear right generated 0.45V when spun as fast
as I could manage, the resistance of the sensor was 1.06K Ohms when the wheel
was stationary. The rear left generated just 0.043V but it's resistance was
1.05K Ohms. My thoughts were that this couldn't be right so I rechecked my
readings, they were correct. Surely this could only be correct if the induced
voltage was lower because of some other fault than the sensor itself, maybe the
reluctor had lost it's properties or the air gap was wrong.

Checking the voltages and resistance the next day gave the same results, the
passenger side rear was producing around a 10th of what the other sensor did.
Disconnecting it on the rear subframe gave the same results except for a tiny
drop in resistance, that's to be expected. So I thought is it that the reluctor
ring is faulty or is the air gap too big, I could swap around the rear hubs to
check for reluctor faults but the air gap would be difficult to asses and
probably impossible to change due to corrosion. On a whim I decided to unbolt
the ABS sensor and see how seized it was, I undid the bolt and the ABS sensor
head fell off!

Cleaning up the hole with a rotary tool took minutes. I fitted the spare Bendix
Mk2 sensor I had and took some readings with the multimeter. The resistance is
twice that of the Bendix Mk1 sensors but the voltage was now the same when
spinning the hub by hand. I set the air gap using several layers of insulation
tape on the end of the sensor, bolted it up and removed the hub to remove the
tape. The readings were taken again both at the subframe joint and from the ECU
connector. Everything was put back together and the ECU connected up, it seemed
happy on the self test so that's a start. I couldn't road test the car because
it was out of MOT. The road test would have to be the drive to the testing
station. The trip to the MOT station was fine and the ABS braking system
works as it should. Shame the car failed on a spot of corrosion.

Steve.
not applicable
citroenxm
Posts: 8061
Joined: 30 Dec 2004, 23:10
Location: Somewhere in North Wales, Anglesey
My Cars: M reg Xm S2 2.1td Auto Exclusive. 269k and rising
L reg XM S1 V6 12v Manual SEi
L 94 XM 2.1 TD auto total resto

2008 Peugeot 207 Sw 1.6 16v hdi. 217k and rising
2010 Peugeot 207 SW 1.6 8v HDi 161k and rising
x 70

Re: Bendix ABS problem

Post by citroenxm »

interesting stuff steve.. the later "bulged" sensors were fitted from around 1994 in mk.1 cars and do have different resistances but are perfectly mixable.. there is also ecu changes.. xmexclusive is all clued up on this bendix stuff.. i need to get into it for my v6 sei 1993 xm has a fault where the light is on but the Abs Still Works!

nice post! ive not look yet to see if you have this on club xm. . ill go have a look now.
Sharing a pug 207 1.6 hdi Sw 16v.
M reg Xm 2.1 td auto exclusive S2 269k and rising
L reg XM V6 12v SEi auto .. Light project

A very sad...
1994 XM 2.1 d auto
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