Testing ABS sensor

[wobbly]

My SL has recently had a intermittant ABS problem.

This week the car was serviced, and MOTed, by my independant garage, I asked them to check this fault. But I was told the fault did not occur whilst they had the car, and because of the cars age, the diagnostic check, would not show the fault.

The garage told me, they would have to use trial and error to locate the fault.
I have checked the cruise control still works, when the ABS light is on, so I assume it is not the brake light switch. I have cleaned the ABS sensors, and checked the connections, but the fault still occurs.

My question is, is it possible to check the sensors to see if one is faulty, I have a multimeter, and any advice, would be helpful.

Wobbly

 

[grober]

Good article here on the basics of abs albeit on volvos, but principles the same.
http://www.swedishbricks.net/700900FAQ/BrakesABS1.html There would appear to be lots of potential ABS problems arising from other causes than the sensors themselves. water or damp in the sensor connectors seems a favorite along with dirty reluctor wheels.There are dedicated hand held testers you can buy but expensive.I did come across this on a bmw bike site.
A simple test for an inductive sensor if you can get at a wiring plug up the frame (assuming it works off the ABS ring) is to go across both pins of the sensor with a multimeter set at AC volts. Spin the wheel and the induced voltage will give a steady reading at a steady speed. On DC it will jump as the counter teeth go by. If the reading is below about 0.3V AC at 30rpm, the electronics will typically have a problem counting the pulses, but the only fixed rule is no voltage, no speedo. If you have no voltage, check the resistance across the two pins. Fried sensors tend go open circuit as the fine wire at the coil end melts.
values for the merc sensor might be different but principles would be the same. Sensor prices vary from about £60 -£80 +vat EACH from MB depending on age/type.

 

[wobbly]

I have tried this simple test, by jacking up the car and spinning the wheels, withe the multimeter, and get readings of about 0.4V on both front wheels, so may not be the sensors

I think I might change the brake light switch, I know this is a common failure, and its a cheap fix, if this dont work the I think I will let my indy dealer sort it out, unless anyone has any other ideas.

Wobbly

 

[GazCaff]

It is better to measure the resistance of ABS sensors than to try to measure voltage.

 

[kth286]

the abs is automatically de activated when the voltage falls below a certain level.

I guess your car is not used much and therefore the battery charge is low, below the threshold voltage level for the abs.

Give the car a good long run, or charge the battery, or replace the battery, or HAVE BATTERY ON A CONSTANT TRICKLE CHARGER SYSTEM. You can buy those for cars which are used little.

 

[wobbly]

Yes the car isnt used that much, but is always attached to a optimate trickle charger

 

[wobbly]

It is better to measure the resistance of ABS sensors than to try to measure voltage.

Im not to clever with electrics, how would I measure the resistance of the sensor?

 

[BTB 500]

Im not to clever with electrics, how would I measure the resistance of the sensor?

You need a multi-meter with a resistance (ohms) function. If you haven't got one they're pretty cheap, probably a tenner or so from Maplins etc.

 

[wobbly]

I have a multimeter.

But what are the readings I should expect to get when testing for resistance.

 

[BTB 500]

I'm no ABS expert, but I'd start by comparing readings from all four to see if one is noticeably different.

 

[grober]

Values quoted by MB for ABS sensors of a w124 of that vintage are
0.85-2.3kohms resistance and an insulation resistance better than 20kohms. Hope this helps.

 

[wobbly]

I'm no ABS expert, but I'd start by comparing readings from all four to see if one is noticeably different.

My car only has 3 ABS sensors one on each front wheel, and one on the diff.


I have just checked the resistance of the front sensors 1.03ohms on offside 0.98ohms on nearside.

Im going to change the brake light switch, see if that make any diffrence.

 

[kth286]

do you have non standard wheels and tyres on the car as that could affect things .

 

[GazCaff]

My car only has 3 ABS sensors one on each front wheel, and one on the diff.


I have just checked the resistance of the front sensors 1.03ohms on offside 0.98ohms on nearside.

Im going to change the brake light switch, see if that make any diffrence.

You need to spin the wheel while you take the reading, it should make the resistance fluctuate. If it doesn't, or the resistance reads infinity or open circuit, you have a duff sensor. Obviously, it would be difficult to test the diff sensor this way, but the readings you have all ready posted suggest that the sensors are okay.

 

[wobbly]

Nope, the car is totally standard

do you have non standard wheels and tyres on the car as that could affect things .

 

[grober]

test the sensors when warm

An ABS post on another forum indicates that a faulty OVP relay can cause the ABS light to come on though normally this is accompanied by other problems. They also advocate testing the abs sensors when hot i.e. after the brakes have been used a bit since faults sometimes only show up when they are warmed up.

 

[wobbly]

Ive had a day off work so decided it was time to cure the ABS problem.

I have previously checked all the sensors, they were ok. I have checked and cleaned all the electrical connections, and the battery was fully charged.

I have read on many of this forums threads about problems with the brake light switch, relating to ABS problems. So the brake light switch was the next item to check.
I have changed the brake light switch on my ML, it was easy, so I thought the SL would be the same, but I was wrong, totally wrong.

It took nearly 2 hours to change the switch, due to lack of room, which meant being only able to use one hand to access the switch.
And its not like the modern switches, that just clip into place, oh no this switch is held in place, by two locking nuts, try tightening locking nuts with one hand, lying on your back, feet hanging over the seat headrest, and the steering wheel sticking in your chest.
God knows how I managed it, but I eventually tightened the locknuts.

And now the good news, no ABS warning light, at least the pain was worth it.

Wobbly