Porsche Enthusiasts Club Forum

just getting a few things out of the way for a new mot. its not up til dec but would like to do it before the cold sets in my question is, would it fail the mot with the abs light on? its a 1991 s2. the brakes are working fine.

is there a way to switch the light off?

My abs light was / is on, and it passed the mot because the brakes work fine. I'm still trying to work out what's wrong. I've changed both back sensors and the light's still on. No you can't switch it off.

The ABS does two checks the first is an ECU check when it first is turned on by the ignition & then at around 5-6MPH when you start moving it does a sensor signal check. If it is on all the time while the ignition is on it is probably the ABS ECU. These can be checked & repaired, try gosling BBA Reman. Sensors can be checked with a multimeter.

Hi

Yes it SHOULD fail the Mot with the light on..... i.e ABS LIGHT ON is legally an automatic MOT fail...

Plus....light on means ABS NOT WORKING! i realise the brakes work but Porsche would not have fitted ABS unless they thought it needed in an emergency....there is a LOT of difference in stopping distance between ABS and no ABS....

All the best Brett

Hi John

It passed either because he didn't notice it or he was being easy on you...not because the brakes worked fine...lol

See my later post...

All the best Brett

I have seen this argument online on many forums for years and it has always irritated me
(a little) as my T5 has passed with an ABS light on 3 years running. So my opinion has been that it isnt a fail as you can understand. Since so many folk feel otherwise I thought right ! I'm going to find out though it wasnt easy

Below extract from the MOT testers manual of guidence :

The Testers Manual
3.4 Anti-Lock Braking Systems
Information Method of Inspection Reason for rejection
This ABS inspection applies to all systems fitted as 'standard', and to 'optional' systems fitted.
When testing vehicles that have ABS fitted the road wheels should not be allowed to rotate clear of the standing surface with the ignition on as this can cause the ABS system to indicate a fault which may require specialist equipment to rectify

Missing components
Where a vehicle is presented for test with an ABS component obviously missing but, follows the correct sequence of operation, the defect must be brought to the attention of the presenter.

If the vehicle is fitted with an anti-lock braking system, check that

a. a warning lamp is fitted
b. the lamp illuminates
c. the lamp follows the correct sequence of operation
d. does not indicate a fault.

Note: The sequence varies with the type of system. Refer to VSI, the manufacturers or other reliable data, eg purpose produced charts, books, etc. 1. The warning lamp
[color=#FF0040]a. is missing
b. does not illuminate
c. does not follow the correct sequence
of operation
d. indicates an ABS fault.

Issue Date: 10th January 2005 [/color]

It appears that I for one was always wrong in my belief ! , but hey I'm quite happy to prove myself wrong before the wife does !. Best I repair her ABS ecu this week.

Regards Mas

Hi

I knew I was right because I had the light on and failed lol...

Its usually one of the sensors or the sensor cable shorting out (in my case)

It depends how the light comes on...

IF you drive a few yards and THEN the light comes on and wont go off, its NOT the ECU....ITS A SENSOR OR CABLE...

TEST for this at the ECU plug....

IF the light comes on without moving its either the ABS ECU or possibly (but unlikely) the pump.

All the best Brett

It must be a fail otherwise the Ministry would not supply the testing stations a chart to tell you how the light should work.

If the light comes on while driving it can also be a wheel bearing or tyres

mine goes off when the engine is started, then lights up when i drive off (at bout 5 or 10mph). i have a metre to check the voltage (i think in this case it is ac and not dc, is that right?) is there a black and red wire? and do i just spin one wheel at time with the ignition on? what os the full procedure?
thanks in advance.

That is a sensor probably. Use a multimeter to check the resistance of each sensor all should be the same I cannot remember the resistance at the mo. The signal from them is a square wave best checked with an oscilloscope. But resistance is a good step.

As said, definately a fail if it doesn't light up when on ignition (but not running) and if it's lit up when it's running. It seems you guys know a few friendly MOT testers .

Whilst I'm here I best ask, does the ABS cut in quite late compared to most cars, as when I initially tried to make it cut in (on quiet roads near me to get an idea for the car) I really had to push the car to its limits. On previous cars it went crazy earlier than I thought (but the brakes were also way more over-assisted too.

Hi

Dont forget these are Porsche brakes so are bloody good in the first place....on my 928 S2 they are excellent and the ABS only comes in at the final "oh shit" moment lol...but it is VERY VERY good

You can get it to activate but you will need a wet greasy road and need to do a high speed emergency stop...

All the best Brett

just done a copy and paste of the following, which i will be trying at the weekend. i cant believe how expensive these sensors are. someone is taking the micky somewhere


Jack up the car and take off the road wheel. The ABS road speed sensor cable should be visible within the wheel arch. Locate the connector (usually 2-pin) and disconnect it. Behind the wheel hub should be the road speed sensor, which is bolted on and near the crown wheel.

With your DMM set to ohms, connect the probes to the connector, road speed sensor side (so it measures the sensor and not the ABS system). Take note of the value with the road wheel stationary. Now rotate the wheel hub and you should observe the resistance changing. It may not be a smooth transition, but it should change.

If the resistance value does change, then the sensor is OK. If it doesn't, then you may be looking at a new sensor, or there is a broken wire in the small amount of cable form leading to the sensor.

Hi

Umm that will work but....

Its damn site easier to test the resistance at the ABS ECU....as ALL the sensor wires end up there...

You just disconnect the huge ABS multiplug and look at the workshop manuals for the pair of pins (number and location in plug) and test there, both with and without someone spinning each wheel for you....

All the best Brett

The only advantage of checking at the wheel end is that A. it enables the state of the sensor, connector & cable to be checked & B.allows the connector to be cleaned as a bad connection could be the issue.
Getting some switch cleaner from Maplin is a good idea as a Porsche owner.