Strabo

I must have missed something. I renewed the battery and brake pads in my new 964. No problem. Except that now the ABS warning light comes on (with the gong) and does not go off. Also, the speedometer no longer works.

I reviewed various threads on the 964 board. The system might have read low voltage during the battery swap, but after several hundred miles that would seem to be a non-issue.

And the speedometer..??

Rich W

Speedo is fed via the ABS system by a sensor at the left front wheel. I recommend you double check your work area in the vicinity of the left front wheel for any connections that are not properly made.

Strabo

Wow, who would have guessed? I'll take a look around the sensor. I suppose that if the ABS is shut down for any othr reason, the speedometer also shuts off, correct?

Adrian

Dear Strabo,
If the ABS unit has totally failed and you have a 1991 and up model your speedo will also fail. 89 and 90 models have their own speedo pulse sender.
The four speed sensors provide a composite signal which then goes though a converter before it gets to the speedo. If you have just one failed sensor the ABS will fail but will not fail the speedo. It will continue to provide a speed output until no sensors are left or the control unit blows up.
The warn gong suggests a major system failure. The gong will not sound with an undervoltage condition. There is a new 16 page pdf file in the technical section of rennlist on this subject.
I have reviewed your post again and I really think you need to check fuse #16 and relay R34. One other check you can do is to drive over 80 kph and see if the rear spoiler opens. This will tell me what is powered. Not working does not necessarily mean not powered. My first impression is nothing to do with speed sensor. I suggest you have a failed ABS control unit or a failed power circuit to the unit. One key question is, when does the ABS light come on. It will come on with the igntion turned to on. It will remain on for 2.5 to 5 seconds (depends on model)and then go off. If the ABS warn remains on then you have an electrical failure in the control unit or the hydraulic unit. If the light goes off and come son when you are driving in straight line, a wheel speed sensor has failed.
Ciao,
Adrian
911C4

Strabo

Well, after trying all of your kindly offered suggestions (not the >80kph suggestion, as it is am RSAmerica with a fixed tail), we put the Bosch Hammer on it and -- voila! -- both ABS and speedo started working again.

I wish I could say that I understood why..

Anyway, many thanks to all for your interest and suggestions.

Strabo

Adrian

Dear Strabo,
The Bosch hammer reset the ABS control unit. This is unusual but it does happen. There are a couple of faults which are known as latched faults and require resetting. These are related to the ABS hydraulic unit and detected faults.
Ciao,
Adrian
911C4

Strabo

Adrian,

I read your 16 page description of PDAS and ABS, and found it very clear. Nice work! It did discuss latched faults. I have thought about what I might have done to trigger a fault and have identified one possibility:

In changing brake pads, I pushed the pistons back into the calipers to make room for the thicker new pads. To do this, I did not open the bleeder valves on the calipers. I simply uncapped the reservoir so that fluid could flow upstream from the caliper to the reservoir.

Could this have changed hydraulic pressure sufficiently to triger a fault?

Strabo

Adrian

Dear Strabo,
I do not know what kind of 964 you have. I assume you have a C2 because you only mentioned the ABS warn light.
The ABS and brakes system of the C2 and C4 have no pressure warning system. Only hydraulic the brake boost system has a pressure warn system.
If you had power off then I cannot see what has happened. If you had power on I could see that you may have caused a problem by pressurising the brake lines by pushing the caliper pistons in. However this is only speculation because I was not there. I have only heard of brake pad changes causing an ABS problem once before. The main issue is to ensure the following,
All work is done with power off and battery disconnected to be totally safe.
Brake fluid levels are not permitted to fall. If you look at the brake fluid reservoir you will see that each circuit has its own chamber in the reservoir. The chamber walls are between 50-60% of the total depth of the reservoir. If you empty one of these chambers you will get problems. The return lines go to the centre section and it is is possible if the level is low enough that the returned fluid will not get back to the circuit.
Ciao,
Adrian
911C4

Strabo

Thanks Adrian,

My car is an RS America. I guess this is one of those unexplained events -- we are mystified, but not dissatisfied with the outcome.

Strabo