I've had my ABS go out a few times in the past and always fixed it by either replacing one of the relays (could hear the pump stuck on), or by cleaning the wheel sensors.

However, this time none of the usual tricks work. The light does not come on until you get the car moving at least 5-10mph, just creeping does not trigger the light. I figured this was an obvious bad sensor but the cleaning didn't help. Resistance checks for all 4 sensors were good, as were most of the other system checks in the manual. The only odd thing I could find was while measuring AC voltage from the sensors while spinning a wheel manually. Trying to spin about 1rev/sec, three of the sensors would read about .3-.4V, and the right rear would read more like .7-.8V. This behavior is very consistent.

I have some new sensors on the way, just curious if it makes sense to anyone else that the one sensor it bad? If the AC amplitude is proportional to speed it would make sense, but I couldn't really check over higher speeds.

-Andy M.

 

Well, some follow up research is that the amplitude is probably just proportional to the gap. The controller should only care about frequency which I'll need a scope for. The fact I see voltage from all four sensors is promising, except that doesn't answer why the system faults. Hopefully a scope trace will show a poor signal from one of the sensors. I don't really want to get all new sensors at 150/ea or a new controller (don't even want to look at that price).

-Andy M.

 

Continuing my own personal thread....scoped all four wheel sensors and got nice sine waves out of each, and all seemed to report the same frequency as best as I could manually spin the wheels at 1 rev/s (scope showed about 8V amplitude and 40-50Hz). Have done all other service manual checks and nothing fails.

Replaced rear sensors anyway and drove the car. ABS light came after I'd backed into the street and got moving forward. So still seems to be related to wheel speed only.

So now I'm down to replacing the ABS controller and see if that works. Or replace the front two sensors also even though their output looks fine. Argh.

-Andy M.

 

Did you scope at the sensors or at the ABS unit? Have you checked all your harness connections back to the computer?

-b

 

Yeah, scoped them all at the ABS unit connector back in the trunk. Signals looked nice and clean. All the ground checks look good on the computer as well. Swapping the ABS controller is pretty cheap and easy so I'll do that next. I really don't want to start continuity checking every wire in the harness.

-Andy M.

 

i'd replace the front sensors before the ABS controller. the sensors and related wiring is very finicky and doesn't take much to trip an ABS light. They symptoms you described were the same ones I experienced when I had a bad ABS sensor.

I would expect the controller to exhibit some other symptoms since it operates like a computer - reading signals from the sensors and behaving in a preprogrammed fashion. In this case, it seems like it is not getting a read on a sensor and lighting up the warning light. If it was a problem with the controller itself, I would expect the warning light to illuminate immediately.

 

I'm skeptical of the computer, too. But the output of the front sensors looks very good when I spin the wheels and watch on a scope. This is measuring right at the connector going into the computer.

-Andy M.

 

For an S5 FC, At 5-10mph the whole system auto self tests. The system checks the wheel sensors and the abs motor at this speed.

It could be your abs pump/motor since all your wheel sensors seem to be testing out OK. I'd check the ground of the ABS motor which just bolts right to the abs pump mounts. The motor also gets +12v via a fuse near the battery; it should be a 20 or 30 amp and is designated for the abs motor and nothing else.

If you need parts I have a spare abs pump I've got for parts for free plus shipping from an S5 T2. It leaks bad but the motor is good. I've also got a stock tested S5 t2 abs computer if you really think your unit is trashed, its also free plus shipping. I see you have an FD, but I've seen both units up close and they look very similar.

 

Tried a couple more things....

-pulled rear wiper fuse (this is part of circuit to ABS computer)....ABS light stayed on after car started, as would be expected
-jacked rears off the ground and ran car up to 20mph....light stayed off

So I would think this means the computer doesn't know the rears are spinning? From above, I've already scoped those sensors and even replaced both of them with same result, light comes on almost as soon as I move forward.

I'll try a new computer when it gets here in a few days. Till then any other ideas?

BTW, I did do all the pump/hydraulic unit tests in the manual and everything looked fine.

-Andy

 

Come to think of it, I don't the ABS light has ever come on when on the dyno, so not sure what jacking the rear proved.

-Andy

 

Ok, figured it out. Short in one wire going to the left front sensor. Thing that threw me was that if you measured that sensor and even scoped it, it looked fine. However, when you connected it to the computer the sensor would short out. The short was from the 315's I run up front rubbing through the harness.

Things that finally led me to that were jacking each front wheel with the car running, spinning one at a time and seeing which triggered the light. The left front wouldn't trigger the light even when I spun it, but the right would. Checking each of the 8 leads for the four sensors at the computer connector showed one as a short.

A little bit of tape to patch things up and I should be good to go. Thanks for the suggestions.

-Andy