My ABS light has been on for awhile and I finally have her up in the air to bleed the clutch so I'm testing the ABS system to see if I can figure it out. I pulled out the trusty multi-meter, pulled the sensor connectors out of the cylinder that holds them along with the brake wear sensors. I'm checking each ABS connection to see what is running through it and here are my readings:

Front-Right: 1.1mV
Rear-Right: 10.6mV
Front-Left: .8mV
Rear-Left: -.3mV

I'm not sure which is positive and which is negative so I checked both sides. The one with the + measurement is the one I used since the ground would test as - voltage (except for the rear-left where they both tested -).

I'm thinking the rear-left circuit has a break in it somewhere since it is giving me a - reading. I figure if it was the relay all of the sensors would give me a - or no reading. I also figure it's not a sensor since the readings are all before the sensor itself.

Since I've never really done this kind of thing before, I need help from all of yu experts out there.

Should all of them have identical readings?
What should they be reading if they work properly?
Am I close in what could be wrong?

Any help would be greatly appreciated.

 

My understanding is that they produce an AC voltage when the wheels are turned. The most common failure mode is shorting to the sensor body as they are slowly crushed by corrosion.

 

Yep, change the meter to resistance and turn the wheel, should go from infinity to 0 as you turn the wheel.

Chris

 

Are they hall effect sensors, or reed relays??
jp 83 Euro S AT 49k, just curious...

 

Could be wrong, but I thought they are standard Hall Effect sensors: in this case, there are a simple coil with a 'zillion' turns of fine wire. Disconnected, the test is for continuity, and will give a steady X ohms reading. You can use this to compare sensor to sensor.
Switch the meter to mv or ma on the disconnected sensor and spin the wheel: the toothed wheel on the hub will generate a weak AC current/voltage in the sensor (sic - Hall effect) at a frequency proportional to the wheel speed X # of teeth: ABS employs a frequency comparator to monitor frequency differences sent on to its brain ...

 

The ABS sensors aren't Hall Effect, but are instead tiny A/C voltage generators, that create a sine wave output, usually somewhere areound +/- 1 or 2 volts, when the teeth of the sensor ring pass the sensor. The ABS module compares the sine waves to see if a wheel is locking.

There are two electrical tests that you can do on the sensors - a simple continuity test with an Ohmmeter, and a comparison of the sine waves with a scope.

The problem is much more likely to be faulty connections than faulty sensors.

The ABS light should come on when starting and go off after the few seconds of self-testing. If the ABS light stays on then it is more likely to be a hardware failure rather than a signal loss from a sensor. Start with the ABS relay.

If the light goes off, but lights again within a few moments of moving off, it often indicates that a sensor which passes the start-up test is now failing to send an adequate signal. This could be dirt collecting in the ABS sensor area or a flakey connection interrupting a signal.

While the ABS light stays on, the ABS module is not operating at all, but normal brake pressure will still be applied from the brake pedal.

 

If the sensors check out with a continuiety test, look to the sensor and tone ring. Brake dust and metal filings get onto the end of the sensor and into the tone ring teeth. When this happens, the sensor output is not sufficient for the ABS controller to see wheel speed readings, and it triggers a fault.

Often a good cleaning of the sensor and tone ring all the way around will clear up an ABS light problem.

 

You can learn a lot from watching when the ABS light come on. If it comes on after driving a few feet it might be a dirty sensor. If it comes on without moving the car, it could be the relay (which is actually a small circuit and relay(s) in a relay box.) I think if a sensor wire is an open circuit (broken or disconnected) it'll come on without driving too.

-Joel.

 

Thanks for the info folks!

I know it's not a dirty sensor. I just cleaned them all off recently and it did not change the situation with the light.

One interesting thing, though. I bled the system yesterday along with the clutch since the clutch pedal was on the floor. That's when I checked the sensors so I disconnected them all to test the circuits. After I tested all of them and put everything back together, the ABS light turned off. It lasted all of last night, but came back on again this morning.

I'll do the Ohm meter tests suggested and see what kind of readings I get while turning the wheels. The voltage tests I ran were with the wheel sitting still so it looks like those were invalid anyway. I'll post the results as soon as I get to them. For right now, I have to replace a bursted water hose in the Boxster so it's back on the road for the wife.

Thanks!

 

You ohm the sensors with the wheels still, just to see if the wiring is OK. With the wheels turning, you look for AC voltage. The sensors are about 1k ohm and generate about .75-1 volt AC when driving slowly.

Here is a thread from when I tested mine:

https://rennlist.com/forums/928-forum/80557-abs-sensors-pins-on-brain.html

Good luck,
-Joel.

 

After all of the diagnostics, replaced the relay and now the ABS is working beautifully.