Typical! MOT due tomorrow and my ABS warning light has started staying on every time the car is started and gives the warning beeps.

Following the diagnostic procedure in Adrian’s excellent book I’ve found the following:
All wheel speed sensors read 940 – 960 Ohms at the ABS ECU multiplug.
Measuring the resistance of the solenoids at the ABS unit (Part No 0 265 200 052 and 964 355 75500) gave:
Front left open circuit
Front right 1.6 Ohms
Rear 1.6 Ohms.
From this I deduce that the front left solenoid in the ABS unit is defective or has a break in the wiring.

Has anyone had similar experiences / any advice as I assume the unit is very expensive from Porsche.
Is the unit reconditionable here in the UK?
Help!
Ian

 

Dear Ian,
Whilst the solenoids do fail in the ABS hydraulic unit it is a very rare thing. I would suggest you have a relay problem or a corrosion problem.
On the top of the ABS hydraulic unit there are two relays. Pull them both out and check the pins and connectors for corrosion.
If you find everything nice and shiny you need to check the connector.
The big black thing is the connecotr. Pop the plastic cover off and check the connections. If they are okay find a Torx head fro your screwdriver.
Remove the screw holding the main connector down. Then you lift up the connector and check for corrosion again.
Use the pins at the connector to check again the solenoids to ensure you do not have a break between the control unit and the hydraulic unit.
I am assuming you did check the pins at the control unit for corrosion. They can turn Kermit real quick in winter.
I hope it is not the hydraulic unit itself because these are extremely expensive and most often are changed for no good reason.
They are rebuildable but finding a place to do is not easy.
Good luck,
Ciao,
Adrian.

 

Thanks Adrian.
Pins on control unit and relays all clean.
I'll investigate under the hydraulic unit main connector tonight.
Thanks again,
Ian

 

My ABS light comes on from time to time and I found that once the engine RPM gets above 3600 (i.e. even for a split second) RPM the ABS light goes out. I suspect I have an alternator / voltage problem and this is consistent with Adrians suggestion to check your connections as the hi resistance of a corroded connection will have the same affect in that supply voltage will set off the voltage sensitive ABS sensor.

Hope this helps
Bill
90C2

 

My ABS light wouldn't go out until I revved the car for a good few seconds.
Went round cleaning (as many as I could find) earth bonding points. Now goes out straight away.

 

"Whilst the solenoids do fail in the ABS hydraulic unit it is a very rare thing."

Adrian is absolutely correct. Over the last ten years, I've yet to see that unit go bad.
Obviously, the best way to diagnose the problem is by using the Hammer or the
PST2/3.

The ABS system does a basic static test of all elements at engine startup, e.g.
the wheel sensor resistance, relay functioning. So, if the ABS light goes out at
startup-up, before moving, all elements are "there". If the light comes on after
moving, then a wheel sensor may have the proper resistance but generate no
voltage or maybe misaligned.

The above are the very basics. A Hammer or PST2/3 still should be used. The diagnosis
must be very accurate, as all the elements are costly with exception of the relays.

 

Thanks for the suggestions.
I think there may be a mistake in the book on page 409. I re-checked the solenoid resistances to the Porsche Workshop Manual and all is well. When I went back to the book I noticed a difference in that ABS solenoid front left is said to be across pins 1 and 2 (I'm assuming diagram on page 404 is being referred to). The Porsche Workshop Manual says pins 1 and 4, which seems to be correct. Pins 1 and 2 have 12V across them with ignition on.
The warning reverted to being intermittent and at one point I thought I had found the cause, as the main positive battery lead was loose! However after a week of no warnings started to give me confidence that I had a cure, plus a successful MOT pass, the warning is now back on.
Just to reaffirm that if / when I get the warning it only comes on at start up and stays on when all the other lights go out. Warning bleeps start about 3 seconds after the engine fires. I can cancel the bleeps but the ABS light stays light for the whole journey. The warning never starts during a journey so I think it must be something monitored in the initial system checks.
Back to the manual and the multimeter and then, if this turns up no problems, it looks like I'll need to find someone with a hammer.
Ian

 

Darth,
If you're still reading this post - I had exactly this problem. Replacing my clock (the one that tells the time) solved it. Strange but true.

 

Hi Dave

Interesting! One other thing I noticed when my ABS light comes on is that the battery is discharging or in other words the alternator isn't charging indicated by a battery terminal voltage below 12 volts (or there abouts). When I rev the engine and the moment it goes over 3600 RPM suddenly the alternator starts charging indicated by a battery terminal voltage above 13.8 volts (or there abouts).

Bill
90C2

 

I think ABS lights can often be caused by low voltage. I seem to recall Phil Raby had a problem when he installed some flash headlights. I guess in my case, the clock was somehow sinking voltage but don't know why.

I know my alternator output is in range - but at the lower end of the scale. Maybe it doesn't take much to tip it over the edge.

 

Indycam.
When ABS light is on, the ABS is definitely not functioning and I can lock the wheels at will.
Regards
Ian

 

I had intermittent ABS warnings in my car. Me & my mechanic pulled the connector off, sprayed it down with Wurth contact cleaner and a toothbrush. That did the trick for me!

 

Looks like I had an intermittent problem with the R34 ABS relay. Some days all was OK and others I’d have the ABS warning. Jump wiring the relevant pins with the relay removed and the ABS light / buzzer stays off all the time and the system works as it should. New 928 615 124 01 relay from OPC (cost me £53.31 which seems a lot for a relay) and the problem is fixed. Complex system, simple solution!

 

My light started coming on intermittently a few weeks ago. Now it is on permanently.

What is the suggested order in which to check what to make a diagnosis?

Thanks in advance!

Piers
RHD 1990 C2 Targa

 

Leave it off. The ABS sucks IMO. It slows the car more slowly and is overly intrusive.