Porsche Enthusiasts Club Forum

Got an ABS problem I can't figure... While stationary, no problems - no warning lights. Soon as I start rolling, I get an ABS warning. So that points me towards the sensors... Check those at the axles and they're ok. Remove the plug on the ABS computer next to the driver's knee and check sensors from there and they're all between 1052 and 1056 ohms, thus proving the sensors and the car wiring. Sensors are clean so I'm a bit stumped on what the problem can be!? Any thoughts, anyone?

Wheel bearings or odd size tyres

Dave

Yup would be my thoughts, could very well be a bit of slack in the front wheel bearing. Needs a scope on the feed to see whether it is in spec.

Is there not a pump for the ABS? Maybe it has given up? My old thing doesn't have any of this new technology like power assisted steering and ABS on it so I wouldn't know for sure.

Nope pump is tested at start up & would fault then. Sensors are checked at around 5MPH for signal hence a warning as you set off.

Can't see how it would be odd tyre sizes - was ok before with the tyres that are on it, and the wheels/tyres are 17in normal fit. Hmmm, wheel bearing, that sounds a bit coincidental (see other post), however, following agressive adjustment, front brgs do not have any detectable play in them now so ???

Scope on the feed? Do you mean an oscilloscope? and breaking into the system where? Before the ABS computer? Regretably, I don't have one of those expensive bits of TE lying around...

I think that there are two relays on top of the ABS pump inside the drivers side wheel arch which can get wet and stop working. Might be worthwhile having a look there.

The relays should fault on start up.

Paul are you saying that the present tyres have been on it and the ABS ok?

If the fronts are OK, Check the rear wheel bearings.

Tyres are Goodyear F1s, 255/40/17 R, 225/45/17 F. Pretty sure I put them on a couple of years ago without the problem appearing! Rear brgs were replaced couple of years back - just checked rear offside one and no play... I assume N/S will be the same but will check. Out of interest, are you saying the the play in the brg will lead to the gap between the sensor and the toothed wheel varying and hence causing a poor reading?

yes that's what happens

No detectable play in any of the wheel bearings - it's just passed its MOT (and the guy was thorough, believe me!), but still get warning light when car starts moving (car doesn't move during test so tester never saw it... ). I presume next step is to get someone to get their ABS tester onto it...

i agree with Jim, You need to test the sensor outputs, a scope would be ideal but if you use a voltmeter...this is how i would test it

1) Disconnect the abs Ecu, connect voltmeter across the sensor e.g if testing LHF sensor and the pins are say pin 1 and 2 voltmeter goes across these pins, now get a friend to hold the voltmeter and drive car in straight line on a quite road ..record the speed and the output (note they are 2 wire magnetic sensors the output will be approx 2volts AC @ 30mph ) this will vary and you need to watch for any larger variables as you go from stationary to 30mph...worth doing each sensor in exactly the same conditions ...

2) I f all four readings are with 5% of each other try mimicking the a rough road surface to get a more unstable sensor output eg poor connections intermittent output due to movement around the base of the sensor o-ring (common issue with this type of sensor especially the fronts )

hope this helps eliminate the sensor pm me or let me have you mob as i am at Gaydon tomorrow and could pop over to you and offer some more advice

cheers

Sorry Derrick, didn't get your post until after Gaydon. Thanks for the advice, I'll try it and see how it looks. Have you done it yourself? I'm interested in whether the ABS signal is FM or AM - variation in voltage or frequency?

The variation is frequency, as the wheel speed increases so does the frequency.

No probs,

As jim confirmed the signal is frequency dependant, have a look here :- for a good trace example.

http://www.picoauto.com/tutorials/senso ... ators.html


i did some ABS work with FoMoCo and Aston's, we had some issues with high mileage cars on the front magnetic sensors, they have an o-ring at the top section of the metal casing of the sensor and water ingress used to form corrosion around the chamfer where the o-ring sits, this used to "lift" the sensor slightly and would cause a irregular output... the ECU then picks this up as wheel beginning to lock (even thought is not actually taking place) and you would get symptons like the 1).ABS flashing on or 2)a slight brake pedal vibration 3)ABS valve body chatter on application of the brake pedal.

try that test to eliminate the sensors and let us know how you get on. A BOB box to place in line to the ECU is great tool. Time eh

cheers