ABS Sensor.

[Uncle Tom Cobley]

The car in question is a 2004 V40 1.9D (B4192T3) but I can well imagine the principle will apply to most if not all the models in this range.

A month ago I suddenly noticed my speedo had stopped working and the ABS warning light was illuminated. I slowed down immediately but found the brakes were fine; I also checked the fluid level but this was to the full mark. Investigating further I learned the speedo is driven from the ABS system and any fault in this auxiliary system automatically switches it off leaving the normal braking system operational.

Over the next two weeks I found this fault was intermittent, sometimes when I started the car the ABS light would remain on while at other times it went out as normal, but in this latter case it would come on within 10 miles or so. Always when the ABS light was on the speedo stopped working.

There are obviously ABS sensors on all four wheels along with the control unit but I do not have the equipment necessary to determine precisely where the fault might lay. I took it to the Volvo dealer for a diagnostic check and they were able to determine the fault lay in driver’s side front wheel. Their diagnostic system however could not identify whether the problem lay with the sensor or the ABS wheel (reluctor ring) on the drive shaft. I removed the wheel and tried to inspect the sensor wheel on the drive shaft but only a small part of it is visible. Although the part I could see appeared OK in truth it was not really possible to see whether there may have been a small crack somewhere. While I was there I looked at the sensor but you really can tell nothing by looking at it; it’s a sealed plastic unit with a cable running off it. I was able however to determine that it was a Bosch part, part #0265006425 (the Volvo part number is #30854299). I decided to take a chance on which part was faulty and purchased a genuine Bosch replacement sensor, it was £55 including delivery (I’m now feeling very badly indeed, I have just seen it on Amazon at £32!). This at least (or so I thought) was going to be easier to replace than the drive shaft ring which requires far more dismantling.

The manual reads “Remove the mounting bolt and pull out the sensor”. If only it had been that easy!

First I cleaned up the area and sprayed it liberally with penetrating fluid then easily removed the retaining bolt, but the sensor bracket was immovable. I tried a succession of punches and chisels and progressively harder hammering but with absolutely no success. Finally I broke the metal bracket from the plastic sensor. I then attempted to twist the protruding part of the sensor with a pair of grips, difficult with the limited accessibility, still with no success. Ultimately I broke off the top of the sensor (with the lead) leaving the remaining part more or less flush with the metal hub. What I could then see was a metal rod down the centre encased in a white polythene material with a black plastic outer casing. There was absolutely nothing left to grip as this rod easily pulled out (it was magnetic).

With what tools I had available the best option I could think of was to use a ‘Dremel’ tool with small carbide grinder and slowly grind away the remaining parts. That is how I spent the next 2½+ hours! Approximately halfway down I discovered the sensor contained many metres of very fine copper wire much of which could be simply pulled out. Then I was able to pull out the metal sensor tip that protrudes from the bottom, just short of the ABS wheel. At various stages and by various means I attempted to remove the remaining part but without success. Finally I reached the bottom (careful not to touch the ABS wheel and damage it) and was able to remove the small part that remained with a pair of needle nose pliers. I then cleaned up the hole with a small emery wheel in the tool.

With all the remnants removed I could just see the ABS wheel through the hole in the bottom and although I spun the wheel to try to see if there was any obvious damage to it again the view I had was not good enough for a close examination.

Before I (easily) fitted the new sensor, should it require removal in the future I coated it with silicon grease in the belief this will withstand the heat better than grease (probably optimistically!).

Removing the old cable and rusted up self tapping screws on the cable clamps and installing the new under the wheel arch was straightforward. Access to the cable connector at the very top of the inside of the wheel arch was tight where the cable connection is retained with a small plastic clip.

I was very fortunate in that (by chance) the new sensor has fixed the problem but it could well have been the ABS wheel. As a matter of interest, when the speedo stops working then the display for the information centre is not updated (including the cumulative odometer) and part of the information display will go blank after some miles.

I can only assume that 10+ years of heat had softened the sensor casing causing it to deform exactly and very tightly to the size of the hole, together with some corrosion and dirt through the hole for the sensor tip. I can imagine however that within 12 months or so the new sensor will also deform and be almost as difficult to remove.

I write this rather exhaustive posting to warn others that this apparently 30 minute job may well take all afternoon (and maybe part of your evening!) and to warn you to have the necessary tools readily available. Also to give you the relevant Bosch part numbers to obtain the original parts more cheaply (now even cheaper!). I have no idea what may be the cost of this sensor from a Volvo dealer but I’m quite certain it would have been very much more expensive.

UTC

[Matt86]

Agree Tom, The ABS sensor itself is a completely pain to remove, Most the time it's easier to just buy a used hub with the sensor in place!

You can test the sensor to see if it's indeed the problem using a multimetre and doing a Ohms test on it. I can't remember what the reading is meant to show but it will tell you if the sensor is fried or not. I need to do this with my Dad's Tino. He has an ABS fault too!

Sometimes it can be just dirt built up around the ABS ring and a good blast with a pressure washer will sort it. Not always though