Audi A4 ABS Sensor and MAF Troubleshooter and Replacement

In our Audi A4 Convertible project, we’re outlining some of the later issues we had with the car, mainly misbehaving ABS and traction control lights.

Audi A4 ABS Sensor and MAF Troubleshooter and ReplacementThis was down to two things: one was a defective nearside rear ABS sensor that diagnostics brought up – but before that we had a problem with a traction light that kept flickering on. This turned out to be the aftermarket MAF (mass air flow sensor) we had fitted to replace the dead original. Once we’d binned that off and fitted a Bosch unit, the problem was solved and the Audi went better as well. Faulty MAF’s can be hard to diagnose as a defective – there is often no fault code. You can unplug the MAF and if the car drives much better you’ve probably found the fault but that will bring on an ABS and/or traction control light. Though in our case, we fitted a cheap MAF and it wasn’t long before the traction and ABS lights were glowing again – this time we did have a fault code and this pointed us to a faulty MAF

Troublesome task

Working to free-off most ABS sensors is the usual charming job it is on all older cars – a story of rust, broken bolts and so on. We are indebted to Parkside Autos in Worksop (01909 506555) for lending us a ramp for no less than four hours as the Audi fought us to the death.

Removing the rear hub was fun (not) as the bolt is silly tight (threadlocked from the factory) – it’s a standard thread by the way and undoes anti-clockwise.

  1. On saloon and Avant estate models it is simple enough to lift the rear seat base out and locate the inner end of the sensor. On Convertible models, however, you have to partially remove the rear trim panel (it’s on clips) and wrestle the foam anti-drum panel out. Hard work!
  2. Here is our new ABS sensor from GSF – a Bremi made item costing less than £40 with a discount code applied. Bosch ones are a bit more but the cheaper ones seem to do the job. Make sure that you enter your registration number and get the right sensor. Ours was part number 648VG0540.
  3. Here is the sight that delights all mechanics – a rusty Allen bolt holding the sensor in. There was very little chance of this coming out – it laughed at the Allen key and with limited access we had no luck hammering in a spline drive. Vice grips wouldn’t touch it either. Time to get medieval.
  4. Here we have oxy acetylene on the job to get the Allen bolt red hot. That ought to get it loose but it did nothing. We tried again, melting some of the ABS sensor in the process though that bolt was having none of it. Just how tight are they? This is getting personal.
  5. Next, we just gripped the plastic and steel body of the actual sensor itself and snapped it off, leaving the inner part still wedged in the rear hub – no problem as we’ll whip the rear disc off and drive out the remains. With more space, we tried again with the grips and out came the bolt.
  6. The disc is easily removed – two 13mm caliper bolts to release the caliper and pads and then a single Torx bolt securing the disc to the hub. Ours was a T20 but they differ. With that out, the disc slides out as shown from within the caliper carrier that does not need to be removed.
  7. With the disc off, we can just see about half of the ABS sensor. It might drive out easily from here and you can certainly give it a go – but don’t damage the edge off the wheel hub as it has the ABS trigger ring built-in. We need to clean the hole up, so we elected to remove the hub.
  8. Now you need a serious breaker bar and a 19mm Allen socket to get the hub centre bolt out. It is tightened to 200Nm plus a whopping 180° and it’s worth buying a new one. The factory bolt is threadlocked making it hard to unscrew – make sure you do this on reassembly.
  9. Next, pull the hub off and use an old socket extension to drive the old sensor out. The hub rusts around the sensor and grips it with some tenacity which is why they almost never come out without a fight. Clean the hole with a file adding some grease so the new sensor slides in easily
  10. This is the offending ABS sensor that we snapped off earlier. You can see here the corrosion marks around the sensor that was a mixture of a plastic body with an alloy retaining ring. Back in the good old days, ABS sensors used stainless steel bodies and came out far easier.
  11. Here’s the serrated ABS trigger ring teeth machined into the back of the hub. Before refitting, we gave this a really good clean-up with a wire brush to remove any flaky rust and thus ensure that the ABS worked with a new sensor fitted – you don’t want to be doing this job twice.
  12. With the new sensor fitted to the hub and the disc and caliper reassembled, we can get on with the other fun aspects of the job. Start by slackening the two outer Torx bolts that secure the ABS sensor conduit as well as a single 10mm retaining nut. The old sensor cable can now come loose
  13. It’s off the car completely here, but this photo shows the ABS sensor conduit that fits to the wishbone. Remove it from the old sensor and fit it to the new one after you have given it a good clean-up. You can see the centre bolt hole, and the hooks either end that clip to the two Torx bolts.
  14. With the car up on a ramp – it’s hard work lying on your back – remove this rear under tray section that is secured by a couple of 10mm plastic nuts, a couple of small Torx screws and some plastic rivets. With this removed we can see another part to be removed – isn’t this a boring job?
  15. It’s not really essential, but by undoing one 10mm nut you can pull the alloy exhaust heat shield partially out of the way and give yourself more room for the next – and hopefully final – hurdle. Those geniuses at Audi could never had just routed the sensor cable through the floor…
  16. …no, they simply had to route it through a box section where the upper and lower sensor holes don’t line-up just to make the job really interesting. At this point, cut the end off the old sensor and tape it to the end of the new one – the idea is to draw the new cable through with the old one
  17. Here we can see the connector plug and the hole in the box section. Unplug it, and with the new sensor CORRECTLY ROUTED and taped to the end of the old one, slowly and carefully feed it into the car. Plug it in, fit the sealing grommet and test for warning lights.
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  1. The MAF is easy to renew. Start by removing the two small crosshead screws on the front slam panel that secure the air intake pipe to it – then remove it. It’s an easy push fit into the air filter housing and there is a prong onto which the idle control valve rubber mount fits.
  2. To remove the MAF from the air filter box, undo the single hose clip and pull the rubber air intake pipe from the MAF. Then disconnect the typical nightmare VAG connector plug – ours had already been butchered. They’re just awful. The MAF is now ready to be removed.
  3. Do this by using a long crosshead screwdriver and remove the upper and lower MAF securing screws. The lower one is easily dropped and lost forever. If you manage to retrieve it, tape it to the screwdriver for refitting. Shown here is the large rubber MAF sealing O-ring.
  4. Here’s the Bosch original (0280 218 063) with the first new MAF that wouldn’t communicate with the car and caused a misfire. You could be lucky with an aftermarket MAF that works properly but chances are, you won’t. They’re very sensitive and have to be perfectly calibrated.
  5. Bosch MAF’s are sold on an exchange basis with a £30 surcharge on the old unit so we guess Bosch reconditions the outer casing and fits a new element. With ours fitted, the idle misfire disappeared and the fault codes cleared, restoring performance.
  6. Refit the new MAF as the old one came off – use a bit of lube such as washingup liquid around the big O-ring. The intake pipe seen here has steel wire reinforcers on the inside where it fits into the front panel intake pipe. Make sure these are positioned correctl
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