# ABS Sensor fail: Wiring or Unit?



## Harper (Sep 14, 2019)

Hi All,

I hope someone can shed some further light...

*Quick Q*: Does 0.47v on both Sensor and Voltage Supply pins on the J104 connector imply an issue with the circuit (regardless of whether ignition on or off)? Front Left and Front Right show 0.00v for both respective pins and Rear Left has 0.07v on each.

*Details of issue:*
Control Module Part Number: 8N0 907 379 H
Component and/or Version: ESP ALLRAD MK60 0101
Software Coding: 0022549
Work Shop Code: WSC 01236
VCID: 41F5C6075891
1 Fault Found:
00287 - ABS Wheel Speed Sensor; Rear Right (G44)
012 - Electrical Fault in Circuit

*Actions performed thus far:*
- Replaced ABS Sensor: no change.
- Checked Fuses: all good.
- Had Audi Dealer carry out £60 Diagnostic check directing them at possible wiring fault; they reported all ok - needs new ABS Unit (at a cost of more than double the car's value!)

As well as the answer to my QuickQ can anyone direct me to other possibilities/diagnostic actions to perform.

Anything and everything much appreciated - even telling me it's a Lost Cause...

Cheers,
Michael


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## silkman (Jul 29, 2004)

Do you have VCDS? It could be more than one sensor has failed but the error gets cleared when you turn off the engine.

What is the actual problem when driving? Do you have ABS light on? always?


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## John949 (Apr 12, 2017)

Not sure quite what you mean by voltage supply pin. The sensor has two wires, neither of which are connected to a supply voltage. The sensor works by generating a voltage difference between these two wires as a tooth passes the sensing element. Google 'Zero crossing detector circuit' if you want to know how it works. The sensor wires are AC coupled to the ECU circuitry so measuring voltages between the sensor wires and ground will not give useful readings.

It is not unknown for sensors to be supplied faulty from new so it's probably worth swapping the rear ABS sensors over to see if the fault swaps sides. If it does then you have your answer.

If it doesn't then you need to check continuity between the ABS sensor connector pin and the relevant pin on the ABS connector. Is this what Audi have done already?

If the continuity is OK then a new ABS unit is needed. These can be had off eBay very reasonably. I got one for £20! Physically fiting it is not too bad (but do buy yourself a couple of flare spanners if you haven't got them). The hardest part is bleeding the system afterwards. You need to use the brake bleeding procedure in VCDS and about 3L of brake fluid. When I did it I found the procedure extremely confusing because it kept telling me to close the bleed nipples without having told me to open them first.

You could try one of the many ABS repair places advertised on the net but be sure to explain your symptons to them as they mostly do pressure sensor repairs and your car does not have snsors inside the ABS unit.

I have also heard that ou can replace just the electronics module to save breaking the hydraulic connections, but on my units this didn't seem possible, apparently some have visible screws holding the electronics module onto the casting.


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## Skeee (Jun 9, 2009)

John949 said:


> ....................... it's probably worth swapping the rear ABS sensors over to see if the fault swaps sides. If it does then you have your answer................
> If it doesn't then you need to check continuity between the ABS sensor connector pin and the relevant pin on the ABS connector. Is this what Audi have done already?


 This seems a good next step.


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## LesRSV (Jul 5, 2017)

A faulty abs sensor can sometimes be found by unplugging each sensor and measuring resistance(around 1k from memory but don't quote me) any one that is wildly out is likely to be faulty. There can also be a fault in the abs module that has some very fine wires that connect pcb to socket and can barely be seen by any normal person :lol: these have been known to fracture and cause abs lamp on.I had this on b5 passat and managed to remove the module from the pump without disturbing brake pipes etc. and car can still be used.Mine was sent for repair to someone with better eyesight and a steadier hand than me and cost about £100.Similar to this pic-


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## John949 (Apr 12, 2017)

Forgot to add that you should also check that the individual pins and sockets in the connectors haven't been pushed back.


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## Harper (Sep 14, 2019)

Hi Guys,

many thanks for the replies so far. Just to clarify: I am measuring small amounts of voltage on contacts 42 and 43 (Right Rear ABS Wheel Speed Sensor G44) on the disconnected T47 connector to ground. VCDS reports an Electrical Fault in Circuit on G44, but no errors for the other wheels; and there is no voltage on the other wheels' respective contacts.

My current hypothesis (pun not intended  ) is that the ABS Unit carries out regular self-diagnostic checks including at ignition on, detects this voltage whilst stationary and so raises an error. I infer this because I cannot clear the error (using VCDS) whilst stationary (as there should be no voltage) but can clear it whilst moving (as there should be voltage).

Audi have checked the sensor and continuity between the ABS sensor connector pin and the relevant pin on the ABS connector; and have declared that it must therefore be the Unit that is failing. But they didn't check between the contacts in the disconnected T47 connector and Ground.

I accept that this is not an area I fully understand so am simply basing my diagnosis on observations that may or may not be relevant...

Cheers,
Michael


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## miTTzee (Dec 8, 2015)

Hi de HI Harper - not sure I am able to help with your current problem
regarding the ABS - but found this link which may be of some assistance.

Not sure if it would be relevant to what you need, if not it "May" be useful
to many of us looking to solve that elusive problem we get.






Best regards - miTTzee :wink:

...


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## StuartDB (Feb 10, 2018)

My memory is pretty poor! But.... I am sure, I previously found a thread which highlighted and demonstrated a repair for the actual ABS circuit board under the bonnet. You needed to carefully cut open the box and remove the glue / sealant everything is held in and repair any dodgy connections.

It might have even been from a different car?

I'll have a search.
| | | _

Maybe this one?


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## miTTzee (Dec 8, 2015)

Hi de O Harper - my bad :? but I seem to have posted the wrong link with
regards to your problem with the ABS module.

So please have a look at this link as it might also help with your problem.

This link should help with the intermittent fault.






Sorry for pointing you in the wrong direction in my first post, my intention
was to try and help.

Best regards -- miTTzee :wink:

...


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## Harper (Sep 14, 2019)

Thanks for the replies guys!

StuartB: my efforts thus far point to an issue with the wiring, not the unit - but thanks anyway.

miTTzee: great videos and whilst they don't really help me at this time they do highlight that error codes can sometimes point you in the wrong direction ;-)

I won't have time during the week but next weekend I intend to swap over the wiring to the connector (between the erroring sensor and a known good one) to see if the problem moves and confirm or otherwise who is more accurately diagnosing the problem: me or the Audi Dealers.

Cheers,
Michael


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