ABS quandary

Tishabet · 05-25-2006, 06:05 AM

Well, I know next to nothing about ABS, so I was hoping someone with more experience than I might be able to suggest where I should be looking.

The symptom:
I start the car, everything is fine. Once I start driving, I make it a short distance (average maybe 15 feet) and the "ABS" warning light and master alarm light turn on and stay on. They both go out if I restart the car, and stay out until the car starts moving again, once again about 15 feet average.
So it would seem that the ABS system isn't having problems until the wheels start sending some signal.

So the question is, what should I be looking at? Does this sound like a malfunctioning ABS sensor? If so, how do I determine that the sensor is bad?
In the past I had an ABS issue which turned out to be a relay. In that case, the fuse which worked for the ABS would blow immediately, meaning the ABS and master alarm light were on whenever I started the car. In my current case, the light doesn't come on until the car starts moving.

Any ideas, insights, advice etc would be appreciated. Thanks guys!

theghosttanker · 05-25-2006, 06:35 AM

you pretty much have guessed it right...either a brake sensor, or the speed sensor for the car is bad. My factory manual tells me that if there is more than a 17% discrepancy between the speed sensor and the wheel sensors, then the ABS will be disabled, so it could be either wheel sensor or the vehicle speed sensor giving you trouble. But most likely it's a wheel sensor, at least in my general experience with ABS systems...

Tishabet · 05-25-2006, 07:16 AM

thanks for the response theghosttanker. Speed sensor meaning the sensor for the speedo?
How do I determine which sensor is bad? Will I need an oscilloscope?

924RACR · 05-26-2006, 05:45 AM

Failing "BITE" check - pump motor initialization after drive-off - is your most likely issue here. The WSS (wheel speed sensors) are checked for continuity on key-on, so you're already passing that if the lights reset after bulb check. It's also rather unlikely to set a WSS failure so quickly after drive-off, while still passing the key-on check.

So I'd be suspicious of the BITE check. That's when we run the pump motor and cycle all the valves. So I'd be looking for a bad pump motor relay or fuse (if it's separate). When BITE check works, you should hear the pump motor and valves cycle (if everything else is quiet enough... I'm trying to remember the speed, wanna say 15kph is pretty standard (though not universal).

Bosch Chassis Controls (ABS/TCS/ESC) engineer for 7 years here...

HTH...

bazar01 · bazar01's Garage

Do you even hear the ABS pump cycle?

Hey Vaughan I sent you a PM. I also have some issues with my ABS that maybe you can shed some light on these issues.

Thanks.

924RACR · 05-27-2006, 04:50 AM

Back at ya... I'm wondering if either you may have changed wheel/tire sizes and it doesn't like it, or it's maybe failing BITE check... also codes would help... I don't have an ABS-equipped 944, but I would like to know how to pull codes - can anyone point me in the right direction? Not to mention a list of the interpretations?

jsb944 · jsb944's Garage

Cool! Vaughan....you can help us all! My ABS light and "!" light come on shortly after startup ( before moving the car but after the bulb check) and stay on. I have not cracked a manual yet but will keep an eye on this thread! Thanks for lending advice!

best

Joe

924RACR · 05-28-2006, 05:44 AM

Cute - Joe, you've got one of my old projects, in fact - the MDX (did the ABS on that).

Not enough coffee in me yet to tackle any of these this morning; I was up very late last night, getting the quintessential 944 Experience - belts, seals, waterpump and oil cooler seals on my 924S - so recovery is slow and painful this AM. (sure glad I did it though - these engines are so much smoother when the balance shafts are timed right! LOL)

Joe, for your case, it's not even making it to that test after drive-off (BITE check), so there's some test it's failing after key-on. It turns the lights off after bulb check, that's good, but then some other failure... I'd be checking relays or sensor continuity. Sensor continuity might be easier (just check resistance, look for an open circuit - no need for an o-scope).

Anyone out there have the info on a) how to pull codes, and b) the list of the code meanings?

Tishabet · 05-30-2006, 01:17 PM

I believe you need the Bosch Hammer to get these error codes from a 944.

autovia · 02:30 AM

Having exactly the same problem as Tshabet - wheel sensors check out,main relay and fuse checks out.Everything has been cleaned up.Warning light comes on after 10 feet forward or backwards and stays on.Disconnected front sensors and warning light mainly stays off !!Speedo works ok - does this eliminate the speed sensor ? Everything normal at key on.
Any ideas or advice - getting desperate - Thanks

1989 944 2.7