Hi all,
Working my way through the 32 checks of the ABS systemdetailed in the XJS service manual to establish why my ABS light is on. I have a few more checks to complete but havea query regarding Test Nos 11 to 14 which relate to checking of the wheelsensors.

• The manual says that the recorded voltage at theECU, when the road wheel is jacked up and rotated at 1 rev /second (you shouldhave seen my wife’s face when I explained what she had to do whilst I laid inthe boot with a mustimeter!), is 0.15 to 0.7 V for front wheels and 0.04 to 0.1V for rear wheels. Why the difference? (This is important below)
• I am recording between 0.502 and 0.518 volts atmy ECU for all 4 wheels without them rotating. Is this normal as I would have expected 0 volts for a static wheel or doI have duff sensors? Also, if the limitsin the manual are correct then my rear sensors fail as they exceed the voltagerange without any rotation. I have onlytested one of the front wheels so far whilst rotating - the voltage clearlychanges with wheel speed and varies 0.53 to 0.54 volts so not much differentfrom the static reading.

Apologies for length of post and thanks for reading. Any thoughts appreciated.
LeeP

 

My book is packed and I can not find it ATM, so I can not look up these tests. Was this AC or DC volts, this makes a HUGE difference.

 

Hi Warren,


Pretty certain it is DC as it will be coming from the car's battery.


Really just looking to understand if my results are normal or not. Has anyone else carried out these tests and if so what voltage did they measure?


Cheers,


LeeP

 

I found my ROM and looking at the circuit the voltage at the ABS CM is coming from the sensor so it will be AC. The voltage you are seeing at the CM (0.5xxV) will be the DC offset not the sensor signal.

The reason for the voltage being different front to rear will be because its AC and the number of teeth on the reluctor wheel.

My ROM does not have the service checks can you post the service checks so I read them. Then we will look at fault finding.

 

Hi Lee,

The AC voltage may vary slightly depending on how fast the wheel is spun, mostly due to the response of the multimeter.

Another thing I would check (and I would do this last) is the AC voltage for tests 11-19 (I know tests 15-19 say Ohms) but check the AC voltage on all these pins with the wheels STATIONARY and the car running, this test will tell you if you have any noise induced into the sensor cables. This can cause havoc with small signals.

 

Many thanks Warren.


Have checked again and I am getting DC voltage at the wheel sensor terminals (tests 11 to 14) not AC. I am told my multimeter automatically indicates if AC or DC and it is definitely saying the latter.


I get 0.51 to 0.53V without rotation and 0.52 to 0.55V with rotation so not much more. I am surprised I am getting any voltage without wheel rotation. No recorded voltage on tests 15 to 19.


Unless anyone has any bright ideas I may pull a sensor and clean to see if it effects the voltage above.


Cheers,
LeeP

 

The DC voltage is normal, it is a DC offset and will always be present, although will vary between cars and wheels.

You need to lock the meter to AC range, if the meter is in auto and the DC voltage is higher than the AC the meter will not read the AC voltage.

You need to ignore the DC voltage. The ECU will ignore this as well.

Its the AC voltage that the ECU uses to determine wheel speed.

 

Thanks Warren - much appreciated. I've clearly much to learn!


Will hunt down another meter if I cannot lock the current one in AC.


Cheers,
LeeP