Citroen C4 l 2004 front left ABS wheel speed sensor fault.

[Signalglow]

Citroen C4 l 2004 front left ABS wheel speed sensor fault.

Hi, ABS and ESP lights up on the dashboard.

Im getting a reading on Kph when driving on three sensors, except for the front left one.

The left front wheel speed sensor has been replaced plus the ESP module and pump, and the brake-light switch (7800)

(See schematic)

Left front wheel sensor umplugged and connector to ESP module (7800) unplugged.

Measured ohm

7002U to 7001U = IO


7002U to connector (7800) = 0.0 ohm


7001U to connector (7800) = 0.0 ohm


7002U - 7001U to GND = IO


7002U - 7001U to 70012U - 7011U - 7022U - 7021U - 7032U - 7031U = IO


MC702 to GND 0.0 ohm


ME702 to GND 0.0 ohm


Left/right front wheel sensors umplugged and connector to ESP module (7800) plugged in - car is running.

Measured VDC

7002U to GND = 14 VDC - problem here?


7012U to GND = 0.2 VDC

Cant seem to figure this out.

Help is much appreciated.

Best regards.

 

[rjenkinsgb]

What reading do you get at the ESP side of 7002U - 7001U with the sensor unplugged? If it's still near zero, it implies a short in the cable to the ESP.

Other than that, is the rotor / magnet / toothed wheel or whatever the wheel sensor reads from still intact??

As the sensors only have two wires, they are likely just small solenoid style coils, with either voltage induced by a magnetic rotor, or with a built-in magnet (or bias current?) to give a fixed field, that is modulated by a toothed wheel, as the teeth move towards and away from the sensor - the same system as an electric guitar coil-plus-magnet pickup, sensing a moving metal string.

 

[Signalglow]

What reading do you get at the ESP side of 7002U - 7001U with the sensor unplugged? If it's still near zero, it implies a short in the cable to the ESP.

Other than that, is the rotor / magnet / toothed wheel or whatever the wheel sensor reads from still intact??

As the sensors only have two wires, they are likely just small solenoid style coils, with either voltage induced by a magnetic rotor, or with a built-in magnet (or bias current?) to give a fixed field, that is modulated by a toothed wheel, as the teeth move towards and away from the sensor - the same system as an electric guitar coil-plus-magnet pickup, sensing a moving metal string.

Thanks for the reply.

The wheel-bearings has been replaced on both the left and right wheel, because they were worn out, the magnet ia facing towards the sensors.

Today i have tried to run two new wires from 7002U 7001U to 7800. After this I still get this fault-code.

BUT now, after disconnecting, and reconnecting the front right side speed sensor (w/battery off) i get the same code for this sensor now as well.

(short to ground, no signal) I have tried to clear the codes but it didn´t do the trick.

Hmmm..

Best regards.

 

[Diver300]

I would look at the voltage waveforms on pins 1 and 2 of each of the wheel sensors as the wheels are rotated. That will need an oscilloscope.

My guess is that one of those pins is connected to ground inside the "7800" device. I think that is the ABS module.

You might be able to measure the AC and DC components of the voltage if you don't have an oscilloscope.

All four wheel speed inputs to the ABS module are almost certain to be physically identical. You could swap the sensors around between inputs and see if the voltages are the same when swapped. That could point to one faulty sensor or ABS module input fault.

Do not drive the car with the wheel speed sensors incorrectly wired! The ABS sensor won't realise that they are wired wrong and the car will drive fine, but if a wheel spins with the sensors wired wrong, the ABS / traction control will do the wrong thing. The situation with the wheel speed sensors wired wrong is not only dangerous, it is hidden. If you do swap any wires, check very carefully that you have put them back correctly before driving on the roads.

 

[rjenkinsgb]

From looking at other C4 abs faults, bad connectors at both the sensors and modules seem a common problem.

It's worth giving them a dose of a good contact cleaner/lubricant spray, electrolube or similar brands, then connect, unplug and refit two or three times to scrape any tarnish from the contact surfaces.

It appears the sensors are very fragile and easily damaged when being removed or refitted; also the reluctance ring on the hub can fail, even if it is visually good...