Anti-Lock Brake Warning Light (964)

[rhoadesco]

I just changed the battery in my 964 and since that time the anti-lock warning light is on in my car. Since I had to remove the spare to get to the battery lock-down is it possible I bumped something to disable the anti-lock brakes or maybe there is a reset if they lose power for some length of time (like a battery being disconnected)?

Any ideas?

Thanks,
-DR

[kurquhart]

I turn off the battery on my 964 all the time w/o that light coming on, so that is not the problem.

If you take the black plastic cover off of the ABS hydraulic unit (between battery and gas tank), there are two relays (black cubes) that you can check for proper seating.

To the left of the spare, there is a large cable connected to the ABS control unit. This would be hard to dislodge, but worth a check.

Other causes of ABS lamp might include:
1. Failed ABS relay R34 in the fuse box
2. Failed hydraulic boost pump relay R51
3. Dislodged, dirty, or failed wheel speed sensor(s)

The last of these is fairly common; have been working around the wheels (e.g. brake bleed) lately?

[Steve Grosekemper]

The relay R34 has an over-voltage protection feature that is really sensitive. Jump starting/charging can cause it to fail.
Switch it with a known good unit.
If that is not it you should take it to a shop that has a Bosch ABS tester.
The tester makes very quick work of locating the problem.

[LUCKY DAVE]

Steve,do you know which shops in town (north county maybe?) have such a tester? I have an abs issue also, and I already checked the obvious.

[martinreinhardt]

This is an interesting post.

Because at the last autocross during Jennifer's timed runs, the 964 stalled in the pit lane for no reason. The red "!" , the repeating beeping sound and the ABS light were on. The car wouldn't start, so I switched off the battery cut off switch to reset everything. After switching it back on, it started again and ran like nothing happened. Jennifer finished her timed runs without any problems.

This is the third time this happened, but I start to wonder if my ABS and or Booster is working at all. To stop the car it requires a lot of pressure (standing) on the brake pedal.

[LUCKY DAVE]

At the last ax, I had a surprise failure of the abs function, resulting in a high speed spin (with a student in the car.....my bad ).
When I got back into the hot pit lane, I shut down and restarted the car to reboot the computer, and the problem went away, I think (I stayed away from the abs for the rest of the day). It was about this time it occured to me that the warning light doesn't work, hmmm.
Anyway, our glitches might be of similar nature. Now if we only knew the cause.....

[martinreinhardt]

1. Failed ABS relay R34 in the fuse box
2. Failed hydraulic boost pump relay R51
3. Dislodged, dirty, or failed wheel speed sensor(s)

The relay R34 has an over-voltage protection feature that is really sensitive. Jump starting/charging can cause it to fail.
Switch it with a known good unit.

A R34 relay is $80 I wonder if the trickle charger fried some the electronics. As I noticed some relays in the fuse box get hot when I keep the cut off switch on while having the trickle charger connected.

As for the R51 boost pump relay, I think the only 964s equipped with it are the C4's and 1992 Carrera RS and Cup cars OR mine is missing.

Great tip on the speed sensors cleaning, all four were covered with metal shavings and dirt.

[kurquhart]

A R34 relay is $80 I wonder if the trickle charger fried some the electronics. As I noticed some relays in the fuse box get hot when I keep the cut off switch on while having the trickle charger connected.

Swap with a known good one to diagnose. Mine is at Mirage; you can borrow it as long as it is back by Friday morning.

As for the R51 boost pump relay, I think the only 964s equipped with it are the C4's and 1992 Carrera RS and Cup cars OR mine is missing.

Correct. The original post did not specify, although now I see C2 in the signature.

Great tip on the speed sensors cleaning, all four were covered with metal shavings and dirt.

This is quite commonly the case.

[LUCKY DAVE]

As usual, it doesn't HAVE to be complicated.
Persistant, but ordinary, VOM sleuthing revealed a bright, shiny, well greased, perfect looking, high resistance connection. The pins needed remedial crimping/tensioning.
Problem solved.