the Mercedes threw a bit of a wobbly last week, flashing up the ABS, ESP and BAS warnings on the instrument cluster on startup. While this wasn't terminal, it also wasn't ideal. However, I had a flight booked so YOLO'd it up to Heathrow & back for my pre-booked parking without totalling it into the back of a truck.

The situation required some brain power though.

Prior to my trip away, I'd got the trusty Delphi scanner on the car to see what the craic was. This highlighted code C1134 'Rear Right Wheel Speed Sensor; Implausible' (Intermittent) which gave me something to work from. In this case, rear right was the drivers side rear. Each side has an ABS sensor in the hub and through the power of MATHS they talk to the ECU and it detects meaningful imbalances and then moans about them basically. The Delphi proved its worth by visualising this. I got the real-time data thing for both speed sensors and readout in KPH and got this which shows the issue with what the ECU is seeing

The top graph is the rear left sensor, you can see that the readings are nice and smooth showing acceleration, cruising and deceleration at whatever interval it is (possibly 2 per second?). The bottom trace is the rear right, which has random dropouts all the time. Not at any regular interval, suggesting that the issue lay with the sensor rather than the ABS ring being cracked - which my simple brain said would show up as being a regular anomaly as it rotated. Also replacing an ABS ring on a W203 merc looks like a right old carry-on and I was keen to eliminate other possibilities first!

Just before I left, I ordered a replacement ABS sensor. Because I'm king of the mingebags I refused to buy a brand new one from Mercedes because it was £65 + VAT, Autodoc had a vast selection of options from about £10 upwards but my eye was caught by the whispered promises of an ebay seller who was selling a 'genuine mercedes' sensor (used) for £22 delivered. Guaranteed* working. I decided that the combination of low price and swift delivery was a winner and clicked 'buy it now'. Then went on holiday for a 5 days.

The instrument cluster errors I got looked like this

And then the ABS one once you got over 30mph. The ABS light stayed illuminated the whole time too. The codes could be 'clicked off' but came back on every restart.

Having got back from my holiday, I saw that the sensor had been delivered to my house in my absence. I had some time this morning to go and see about replacing it. First of all I wanted to check the fault code again to make sure things weren't getting worse.

So thats the same code which is a good thing. I cleared the code then got the car up on the lift to commence work

Popped the hubcap off the rear drivers side wheel

This is hardly exciting stuff, but I will wander a bit OT here and say how pleased I am with how well the steel wheels I bought 3 years ago are holding up. They were pretty grotty but after a thorough clean-up and respray with cheap enamel paint they're not showing any signs of rust coming through. Anyway, I digress. Wheels off too.

The sensor lives behind the brake backing plate, roughly where the arrow is pointing. its held in with a E8 torx head bolt. I was able to get a 3/8" ratchet and e8 torx socket on it and to my surprise it undid quite easily. Its in a really exposed place but let go in that (rare) reassuring way that tells you its been done up properly, but hasn't seized. Not the 'finger-tight-someones-been-here-before' sort of tight, and not 'this-is-totally-seized-and-the-bolt-will-shear' tight. I've rattled on about it before, but every job I've needed to do on this car has played out the same - its actually well built. This is a terrible picture of the offending sensor sitting in the hub

The idea is you undo the torx head bolt and carefully remove it, then wiggle the sensor out the hole in the hub. As you can see, it looks quite a lot like this is an original 22 year old part so its had a good innings. I carefully pulled the wire out of the various locating clips first, tracing it all the way back to the connector on the main body wiring loom, above the rear axle

Its the top/left one here. There is a metal tang that holds the connectors into the loom block thing which you need to ping out with a flat blade screwdriver then the sensor connector pulls out

with this end off, I could return to the hub end. The sensor was jammed in quite tight so I had to resort to careful application of penetrating oil and gentle levering with a flat-head screwdriver, but eventually the sensor popped out the hub recess it lives in. Pull the sensor wiring through and TA-DAH

I wanted to be careful removing the sensor in case that wasn't the issue after all and I'm pleased to say I managed not to break it. There was no muck or swarf on the end of the sensor and a closeup suggests this is MB-branded and likely original so its done well!

Next I opened up the parcel to check the contents

Part number looks right, visually looks good. OK a promising start. I checked the lengths of the 2 next to each other and they both had the same amount of cable, and the rubber protector boots in the same places so all looked good. I put the sensor into the hole in the hub and threaded the wiring through the various clips to keep it routed out of harms way, all the way up to the loom connector

I gave the connector a blast with some electrical contact cleaner for good measure and then push the connector home, and then pressed the metal retaining tang thing back into position to keep it all seated. The 'New' sensor is the bottom on in this pic.

This is some of the wiring routing to give an idea of how comprehensive it is. Before refitting, I put the little e8 torx bolt into my vice and cleaned up the threads with a wire brush, added a smear of copper grease to them and then did it back up to keep the sensor firmly attached to the hub.

So now the moment of truth, lets start it up

ooooooOOOOOOooooohhhhHHH! Look mum, no errors! This is a good start, but I wanted to be sure so put all my tools away, reattached the wheel and dropped the car down off the lift. I hook up the Delphi scanner, set up the live data readouts for both rear wheel sensors and went for a bit of a drive to see what happened

That looks a LOT better. Both sensors track the same line, with no random dropouts any more. I'm fairly confident that the issue was indeed the aged sensor and that the reluctor ring in the hub is actually OK - which is a big relief! No fault codes appeared and the car seems happy once again. I got up to 55mph later in the day and no ABS warnings came up so I'm tentatively calling that fixed.

If this sensor breaks I might bite the bullet and get a brand new, genuine MB one now I know that was definitely the issue, but for a leisurely hour of my life and twenty quid, I'm happy with that. The interesting* bit was using the live data readout capability on the Delphi scanner to understand what was upsetting it, diagnose more accurately, and then 'prove' it was fixed afterwards. Well worth the £30 I spent on the scanner about 5 years ago, I'd say! Its definitely saved me its purchase price ten times over during that period, vs having to take my cars to a garage to be plugged in for diagnostics.

TL;DR - fat man replaces a sensor on his car with one from a scrapper. slow clap.