Testing ABS Sensors With A Multimeter

[rg]

Hello Folks,
This relates to my "other car", but hopefully will have relevance to Citroenists as it deals with diagnosis of faulty ABS sensors.
The ABS Light on my Vauxhall Monterey (Isuzu Trooper) comes on intermittently, usally after a turn has been made. Full left lock (followed by recycling) often cures it.
There is a useful guide to DIY ABS testing at
http://www.topbuzz1.carenthusiasts.co.uk/
Taking a meter to each sensor today revealed the following values in ohms on the "2000" scale:
Nearside:
stationary: 1024
rolling 985-1052
Offside:
stationary: 1059
rolling: 940-1221
I'm ashamed to say that my physics theory is not that good! I take it that these figures point to a partial failure of the <i>nearside </i>sensor, by the fact of it's lower resistance?
Can anyone confirm this?
Just for the record, I rolled the wheels by hand. If this method works, I will have saved the £30.00+ at the main dealer for a 10 minute diagnostic check, and I will be most pleased!
Cheers
rg

[johntaylor]

can not help you with the physics but I can confirm that most sensor problems emminate from the nearside front wheel as it gets the most abuse from kerbs etc. Make things simple by testing the resistance on all four sensors and eliminating the rogue resistence???

[AndersDK]

The sensor would be a dead simple coil, of which you are measuring the total ohmic resistance of the wire.
When you turn the wheels, the toothed cam passes the tip of the sensor and thus induces a small current in the coil. This current will disturb your ohmic measurement !
The value of these sensors are most commonly around the 1000 ohm (1Kohm) mark, with quite some tolerance allowed for, i.e. at least +/- 15% typical. Add to this the usual un-certainty in your measurement setup (probe connection resistance, test lead resistance, voltmeter accuracy class, etc, etc.).
As you got readings between 940-1059 ohms, you have simply checked continuity in the sensors, which is the ONLY meaningful test you can do. Spinning the wheels while testing will just add to your level of confusion [}:)]
The measurements you have done tells you the sensors are good.
You should look for vast deviations in the above figures, i.e. disrupted (hi-ohmic) or shortcircuited (few 100 ohms or less) sensor cables. This will typically show up if you fiddle around with the sensor cabling while testing like you did.
Another good & simple method testing a sensor is connecting the sensor cable to the line input of a cheapo cassette player or radio. The good sensor & cabling will produce a speed related humming when you spin the wheel.
A common problem mostly overlooked, is a loose connector block on the ABS control valve block itself. This will also set off the warning lamp, as the continuity to the electrovalve coils are constantly checked.

[jeremy]

I suspect your sensor has a partial break in the screening and therefore you need to watch your meter while you tug and twist the lead to the sensor, especially where it flexes. I have used the audio method outlined by Anders and found the defective sensor lead which showed up as a crackle while I drove along and the suspension hit bumps. Incidentally when listening don't be confused by bumps which make a considerable change in the hum.
jeremy

[PowerLee]

Hi
Just to add to the confusion, I had the intermittent ABS light fault on my Pug 405, I checked sensors, cables & even rang Bosch automotive for ideas.
I found the problem in the end, It was a slightly lose earth wire for the instrument panel that was putting the ABS warning light on.

[tomsheppard]

Just a point! Anders had a typo and of course he meant use the line input of the cassette deck!