Testing ABS sensors with multimeter

[Shayne]

I'm assuming one is dead and i need to work out which one , visually the rear seems more probable so jack up and spin the wheels having disconnected the plug and putting multimeter prongs onto sensor pins .

Should i be testing for continuety , voltage or resistance ?

Paper clip under the bonnet gives me nothing .

 

[karl2000]

As a starter for 10, I'd try a resistance test across them all and see if theres one thats reporting a different value!

 

[Shayne]

I don't have enough hands to keep prongs on pins and spins the wheel alone so i will have a cuppa while penetrating oil does its work on the next closest connector .

 

[Shayne]

Front drivers side harness fail rather than sensors I think which disappear up inside the wing so would I be right to assume these with my fingers on are the other end ?

 

[Shayne]

Seems unlikely the loom often fails so does anyone have one to post out ? will have to wait for it to come from China otherwise .

 

[Towpack]

The sensor is just a pick up coil so should have a continuity/resistance between the pins.
0.7 - 1.1k for the rear sensor and 0.87 - 1.27k for the front according to the FSM.
There should be no continuity between any pin and the vehicle body.

 

[Shayne]

Happy to admit i don't know what i'm doing with a multimeter and this kind of stuff bores me , but i believe i found continuity shown at an equal 7 or 8 something at the rear sensors and an equal 12 something at the front , which alters some when i spin the wheels .

Ignition on i then put the multimeter pins to the harness at each wheel and only the drivers front wheel give nothing to read .

 

[Towpack]

The sensors should generate 0.5v or more when the wheels are spun at 12.4mph. The FSM specifies checking with an oscilloscope, bit it should be possible with a decent multimeter set to a low enough range. Short of actually jacking the vehicle up with all 4 wheels off the ground, spinning the wheels at that speed will be the difficult bit!

 

[Shayne]

4 sensors make numbers and only 3 wires to sensors do regardless of how i set my multimeter so i need a new wire rather than a new sensor .

Can't find one in stock and no way i'd pay what Roughtrax want , Only reason I'm fixing it is because its easier than removing all the ABS components . The ABS fuse lives i the glove compartment so i can find it and put it back in the fuse box at MOT time .

 

[karl2000]

No way to look at live data on these? They normally give speed for each wheel?

 

[Shayne]

Paperclip in diagnostic port is supposed to make the abs light flash indicating which wheel is the problem but i get nothing and the light just stays on solid .

 

[Michael80]

Not sure if this would cause your problem, could it be the sensor on the transfer box thinking it’s still in low?
Also have switches on the front and rear axles.

 

[Towpack]

ABS is, indeed, disabled (dash light on) in low range with the diff locked but then the CDL dash light would be on.

 

[Shayne]

I’ve asked before about that and got no answer , my 80 was modified before I bought to so I have a dash CDL switch and can use the low box without locking the centre .

transfer box was in neutral when I was testing with multimeter .

Also when I bought the truck the front diff had been changed and the wire I now think needs replacing was trapped when the diff faceplate was bolted back on .

 

[Towpack]

ABS is disabled when the CD is locked, wether your in low, high or neutral. You can use low box with ABS when the much discussed CDL switch mod is done.

 

[Shayne]

Thanks for clearing that up for me Towpack i had kind of ruled out the transfer mod as a cause because when i untrapped the wheel sensor wire abs was fine .

Is it fair to assume the wiring is unlikely to fail so one from a breaker should be ok ?

 

[Towpack]

I suppose the only way wiring fails is either through mechanical damage or water ingress at connectors.
From the info in the FSM, the sensors seem easy enough to test so one from a breaker could be an option.
You could try shorting out the wiring to the suspected faulty sensor with a resistor of the correct value and see if it fools the ABS ECU into thinking the sensor is good and turning the ABS light off.