TRACS/ABS: 70: Quality on rear ABS sensors

[SwedishBus]

Apologies if this seems a daft question, or has been covered elsewhere - I have done a search and cannot find the information I need on the forum;

I have recently had an intermittent fault on the Rear Lefthand ABS sensor signal, which seems to come and go from day to day. As it was not going to go away of its own volition, I opted to inspect the sensor harness and multiplug under the boot floor, and to be frank I couldn't see anything amiss. I removed the affected disc/drum on the RNS wheel, checked the sensor nose and tone/reluctor ring, and it was just as you would expect on a Volvo - perfect. No debris, all clean with no corrosion, etc. etc.

I opted to buy a set (i.e. a pair of sensors + leads + multiplug) of rear ABS units, which at around £25 seemed too good to be true. They are branded Lemark, appear to be of good build-quality, but are not exactly the same as the Volvo originals.....

....not least that they don't work! All the time I was buying these, something was nagging me at the back of the head that these NEVER work, but until I fitted them (without dropping the fuel tank any amount - long story and a long job....) I hadn't realised just how bad they were going to be! Neither rear wheel works now, with the same ABS fault codes appearing on either wheel, and "live data" over the scantool showing no recognition of any signal. Beforehand, the "faulty" LH sensor was reliably giving me a speed signal on live data as soon as the wheel was rotated, so logically it was just an intermittent fault on the harness, etc. that I was looking-for. I haven't scoped it yet, but the multimeter check for resistance shows a very different story from "old" to "new" sensors.

The original Volvo sensors are polarity-sensitive, and either show open-circuit or around 4 MOhm depending on which polarity you use.

The new sensors are an order of magnitude lower on resistance; around 1kOhm and no polarity-sensitivity to testing. In other words, chalk and cheese.

I'm taking it that I'm not the first to be taken-in by this problem, but is there a good supply of pattern ABS sensors out there for the V70 (it's a 1998 V70 2WD 2.5 petrol 10-valve with a "Mark 20" ABS controller), and if so, are there two or more types of sensor? Or are these just a cheap and not very cheerful way to part V70 owners from their money?

[SwedishBus]

I think I may have stumbled on the answer, unless anyone knows differently;

Whilst looking for a replacement sensor set for the rear axle, I found a good site that covers just brake electronics. They list two different part numbers for the sensors on a 1998 V70 2WD, and if the illustration is anything to go by, then there definitely appears to be two completely different construction units out there. The multiplug is the same, but the whole sensor nose is very different, but it still fits into the same sensor bore in the rear brake backplates.

I have ordered a set of the "other type" sensors (now £45 a set, but still not too bad when compared to other manufacturers out there) which appear identical to my old OE Volvo ones that came out of the car. I will measure them when they arrive, and see how they compare to the previous set I received earlier this week.

[SwedishBus]

I have the correct sensors now, and they are chalk and cheese when compared to the first set I got.

The "wrong" sensor on the left of the photo is a different design altogether, yet mechanically it fits perfectly (same hub bore, same ABS multiway connector under the boot floor). It is listed as being compatible with the 1998MY 2.5petrol V70, but it is very different to the "correct" sensor on the right of the photo. This matches perfectly the sensors that came out, and other than the fact they read a slightly lower impedance when tested (they each indicated around 1.0MOhm, not over 4MOhm), it all looks good to go.

V70_B5252FS_ABS_sensors_1998.jpg

I'm hoping that this isn't down to dry joints at the ABS controller, as it would be just a bit coincidental that the RNS sensor was the one to "fail" intermittently, just after that was the corner that I did all the brake hydraulic hose work on a couple of months ago - 2hoses right next to the ABS sensor cable.

[SwedishBus]

It looks like it was the sensor (RNS) that was at fault. No recurrence of any ABS light or codes over 10 short journeys of rough-road driving for 2days now.

Maybe the old sensors were just that - old. They were both reading a lower impedance than the new parts I photo'd (just ver 1MOhm) and maybe that was part of the reason they failed?

Or maybe I was just a bit rough around the sensor cable trying to get into the brake hydraulic unions to change the hoses over.....it was a bit tight in there with the locking-pliers at the time!!