Oddjob

Im looking at installing an ABS reset and shut off switch in a 944 series car.

First, is it necessary to utilize both a reset and a shut off switch, or can the shut off switch also be used to reset the ABS computer.

And Im not real clear on the need for the reset - what is the cause or the need to have the ability to reset the ABS control unit.

Im looking at the wiring diagram and see that the power comes from the battery to the central electric (fuze box), goes through the ABS fuse, through the relay, then splits and goes to the ABS pump/hydraulic unit and also to the control unit. Do I want to install the reset/switch in the power directly from the battery to the fuse box, or after the relay, just the power to the control unit, or also the pump?

924RACR

You'll want to disrupt power in the wire that powers the control unit (ECU). If you wished only to reset the unit, I'd recommend installing a momentary-off pushbutton to momentarily interrupt power, resetting the unit. If you wish to be able to drive with it off, you'd need a toggle; either off for off, or off-on to reset.

Reason to reset; seems that the old logic in the 951 ABS ECU wasn't too good at tolerating wheel speed sensor noise and disruption, so that when on track - well, make that off track on the gator's teeth, the noisy signal would cause a wheel speed sensor fault to shut down ABS. Having a momentary reset switch will allow you not to clear the code but simply to reset the unit without stopping (key-off/key-on, which would otherwise be necessary - these faults are reset every key cycle). So for the DE or race car, that's why you'd want it, in case you drive off track a bit and have this problem (or even expect you might).

HTH, call or write if more questions...

Oddjob

So a simple toggle (or using one of the factory toggle type switches - i.e. for the rear window wiper) installed in the power wire from the fuse box/relay to the control unit will serve the purpose of a reset and an on/off switch? No need to cut the power to the pump?

Thanks.

924RACR

Correct, that should be all you need. Again, I prefer the idea of using a momentary-off switch, for simplicity's sake, but any sort of on/off switch will work.

Geoffrey

I have access to the factory C2 Cup Car switch that was used from 90-94. The Cup car used the ABS system from the C4 model and the wiring diagram can be found at www.cupcar.net under one of the sections describing the modifications from street Euro RS. You should be able to install the switch as described above.

The Cup car has the switch on the dasboard so it can be hit off in the event of an emergecy such as a spin where ABS does not allow for the same effect when "two feet in" is applied.

Oddjob

I would like to have a complete shut off capability in addition to the reset switch, but dont want to have two separate switches for the two functions, so I think I will opt for one toggle switch that can be used as a shut off and as an off/on for a manual reset.

Geoffrey, thanks for the link but I was not able to find the reference or wiring diagrams. By the way, is the car in your signature block one of the factory S2 club sports, or is it a firehawk converted car? There was a discussion yesterday on the 944 board regarding 944S2 Club Sports and Firehawk cars.

Oddjob

UPDATE:

I installed a toggle switch in the black/red 2.5mm wire from the ABS relay to the control unit. When I flip the switch off, the ABS warning/failure light comes on. Which tells me that the control unit is still getting power from another source in order to identify a fault and light up the dash. When I turn the switch back on, the warning light goes out.

I drove over railroad tracks today and the ABS faulted, and the warning light came on. I tried to reset using the toggle and it did not work (warning light stayed on), so I had to shut the car off in order to reset the control unit.

I am looking at the wiring diagram right now and I dont see where there is another power supply to the control unit other than the blk/red from the relay (although some of the wiring is hard to trace or identify the function).

Im guessing that I will have to install the switch in the 4.0mm wire from the battery to the central electric, unless some can point me in the right direction of another power wire that is supplying the control unit?

Any help?

garrett376

Here's the site I think Geoffrey was referring to:
http://www.carreracupusa.org/Race/el...electrical.htm

924RACR

You're not entirely correct in your assumptions, Oddjob. Think about it from the safety side. If the ABS ECU turns the light on - what if the ABS ECU has no power? You now have a failure without warning the driver - not acceptable (by law). Rather, the ABS ECU turns the light off - either directly or indirectly.

It's quite possible that the dash is not recognizing that the ABS ECU has come back on and is functional, yet it is. Need some more insight from other owners of similar cars. The ones I work on, that's all now done by comm messages (over CAN or the like), and once the ABS/ESP ECU turns off they often never realize if it comes back on, requiring a ignition key cycle to reset lights - even though ABS is working just fine. Acid test here is to, of course, do a stop and see if ABS works. I suggest cycling power on the ABS on a quiet gravel road, then see if you can lock the wheels after turning your switch back on.

Geoffrey

The ABS light turns on with a connection to ground, not with +12v power. This means that when the ECU has power it holds the output high and when it looses power the output naturally grounds. The ECU only gets power from the relay and fuse assembly (60amp fuse). You can test this by grounding the ABS Dash light output and it should light. I believe there are several conditions that will light the light, sensor failure, loss of power to the ECU, etc.

Oddjob

I did try braking while the warning light was on (after attempting to reset with my switch) and the brakes did lock up - ABS was not functioning. And the ABS does work when the warning light is not on. But I will do some more checking today.

Garrett, thanks for finding the link for me. Not a very complete schematic, but it shows that the kill switch comes out the central electric for a 964. Which may be cutting the power going to the fuse? I will try to get the ABS to fault again, then pull the fuse (which is a 15 Amp Fuse) and see if it resets the ECU and if the warning light goes out (I would think it has to, but as pointed out above - not all my assumptions are on target....).

Thanks for any addtional input.

928NZ

I put a switch into the fuse line that ran to a standard switch on my center console. My car fails the Bosch BITE self test when moving off and the ABS light comes on. Using my switch to the ABS fuse I can switch off the power at the fuse and switch it on when I am moving. I now need to be accelerating hard to make the ABS set up again properly. Maybe indicating a bad earth?

924RACR

YHM...

xupkid2

Jim

I will take a look at how my ABS reset switch is wired up in my car and report back later today. I have a push button reset switch that turns it of and turns it back on. As far as the light, the light turns on immediately and also turns off immediately once it is reset. There isnt a delay in my system.

Lemming

A timely search as I want to get my ABS working in the race car. One issue that I have is that I do not have the wiring, just the computer, abs unit, and sensors. Any suggestions on the type of wire I need to use to connect the sensors to the unit?