ABS Sensor Change

Category: Maintenance
Published on Saturday, 24 November 2012 01:34
Written by Colin S
Hits: 14642

It starts with the ABS light coming on intermittently at first, and then staying on. The usual cause of this is a broken wheel speed sensor, so the first job is to run some diagnostic tests to find out which one. Here is the article about ABS Diagnostic Trouble Codes.

The first step is to ascertain if your car has an OBDII port or not, mine being a Mk2 doesn't but has the Mazda port under the bonnet.

Having bridged the appropriate ports, GRD and TBS, the ABS light flashes when you turn the ignition on and you count, a long flash is ten and a short flash one so as in my case a long flash followed by a short one is error code 11 which is front right wheel sensor.
I then unplugged the front right sensor and checked it with a meter and got an open circuit, which sort of confirmed the diagnostic test. The next step is to sit down, take a stiff drink and order a new sensor. You will need the drink when you see how much they are!

Sensor mounting at rear of upright.Front ones are usually only available new or complete with the upright as it is almost impossible to remove them without damage. Rear ones can be bought second hand and are much easier to change as they don't sit in a hole through the upright, although the plugs can take a bit of getting at.

Mine duly arrived, and the first task was to plug it in to the appropriate socket and see if the ABS light went out as it should. If it doesn't then some more investigation may be called for. Mine did go out confirming the sensor was the problem so I could then set about changing it. This should be a simple job that should only take about an hour BUT......

First task is to chock the wheels, crack the wheel nuts, jack the appropriate corner up and get a couple of axle stands under it so it can't go anywhere. The job will take considerably longer if it is punctuated by a trip to A & E so please be careful!

Once in the air the wThe bolt head after a little pressure-oopsheel can come off and you should see the ABS sensor bolted to the back of the upright with the lead running off and up through the inner wing. There are two M6 10mm head bolts on the inner wing, and two brackets onto the suspension held by M8 12mm head bolts. These I had soaked with penetrating fluid several times in the week before and they did actually come undone OK.
The M8 12mm head bolt holding the sensor was another matter however and despite soaking, the head snapped off when I applied a reasonable amount of pressure. The sensor showed no sign of moving either so it looked as though brute force would be required. If this does happen it may be worth removing the caliper, disk and hub to get better access. I decided against this as I didn't want to disturb too much (and the hub nut requires about 160Ft/lbs.)

I managed to remove the sensor by knocking a wedge between the back of the sensor and the cable bracket (to avoid mashing the upright ) In my case the wedge consisted of an old wood chisel. This eventually prised off the body of the sensor but left the centre still in situ.Forcing the sensor off using a wood chisel as a wedge between sensor and bracket.. I managed to remove this by knocking the remains through and ended up with a small pile of bits including a magnet and a load of fine copper wire. The sensor wiring brackets were then undone, the wiring unplugged from the socket in the engine bay and the wiring removed.

The remains of the securing bolt were then tackled. I did try and use stud extractors on it but it was too far gone. I filed the top flat to get a good surface for the drill and managed to just get a nut on the remains. This meant I could drill down the centre of the nut with the correct size drill (6.8mm) and the nut centered the drill nicely. Once drilled down a bit the nut could then be removed and drilling completed. Be careful not to go too far as you could go through the rear brake shield, guess how I found that out!


Once the bolt was drilled out I ran an M8 x 1.25 tap down to clean out the remains, and fortunately the trick with the nut had worked and the drill had gone down the centre so once cleaned out the threads were ok. If they are a real mess then you may have to resort to thread inserts.
Rear sensor body off.Rear sensor body off.

Rear sensor body off.











The hole for the sensor now needs to be cleaned out. I tried the new one for fit and ended up using a small grind stone to clean all the rust off until it fitted without forcing it. I also cleaned off the mounting surface, and the surrounding bits of the upright.

The new sensor can now be fitted. I applied Copper grease on the mounting faces to try and prevent them rusting up again. I used a new A2 Stainless Steel M8 flange bolt with Copper Grease to secure it. The brackets followed and again I replaced the bolts with new Stainless Steel ones and Copper grease. The plug was fed through the hole in the inner wing and the plastic grommet fitted, this just clips into place.
Rear sensor body off.

The plug can then be re-connected. At this point I turned the ignition on and crossed my fingers. The ABS light came on......and then went out again as it should phew!

I did a final check on all the bolts, wiped down any greasy finger marks and the re-fitted the wheel, took the stands out and let it down off the jack. I then collected all the tools scattered around the car cleaned them up and put them back in the garage.

Having scraped the muck off my hands (never have got used to wearing gloves) it was time for a quick spin which confirmed that the ABS light is behaving itself. Haven't had a chance to test if the ABS actually works yet, I will have to find a nice gravel car park somewhere.........

If you have any questions or comments, head over to the relevant thread on the forum here.




More Images:

Rear sensor body off.Rear sensor body off.


Rear sensor body off.

Rear sensor body off.

Rear sensor body off.