Disable ABS
 

Anyone have insight on a method to disable the ABS but maintain the speedometer function? Pulling the fuse kills the ABS but also kills the speedometer.

My race class rules require I run 23.5" diameter tires in front and 25" in the rear. At times the ABS can work fine but with some tires the difference is just enough to give it fits (and when it kicks in, it's not good). So, I've learned to run without the ABS crutch, but really would like to have a functional speedometer and odometer.

is the turtle a C4? (can't remember)
the speedo uses the abs wheel speed sensor, you might be able to cut out the circuits from the ABS computer to to the ABS pump so that it sends a "dead" signal. I would disconnect it at the pump.
Not 100% sure if that would work (might require a resistor to fool the abs computer into thinkng the pump is active).

just a thought.

or you could replace the speedo with a an autometer gauge (or add a seperate digital readout) same with the odo (an hour meter would probably more appropiate?)

Tom, why don't you machine some ABS rings with fewer teeth in the front? You have 45 in the rear now which cannot be changed so you would need 42 teeth in the front to maintain the same percentage difference between the 23.5 and 25" diameter tires. This should keep the ABS functioning. I've done this in my racecar where I'm running a 640mm front and a 680mm rear with the 993 racing ABS system.

I don't think you want to change or interrupt the valve circuitry to the ABS pump as was suggested since there are multiple lines for the solenoid valves.

You may try to interrupt pin 25 which is the stop light pin. Basically when the brake pedal is pressed, it sends 12v to the ABS controller letting it know the brakes are on. I have not determined if this will prevent the ABS from working, but it is easy enough to try in a parking lot by disconnecting the brake light switch at the pedal.

You may also try to interrupt pin 15 wich is from the alternator. It tells the ABS controller when the engine is running. I have not tried either of the above suggestions so try at your own risk.

Geoffrey: Rich and I discussed the "machine a new ring" option, but it didn't seem like it could be done for a reasonable fee. It would certainly be the way to go if I was going to have a 996 cup car ABS installed. I'm not sure it's worth it just to get functionality from the stock C2 ABS (guessing that to machine new rings would be a few hunder bucks or more). Your suggestions are certainly worth a try - thanks. You didn't happen to make "extra" 42 tooth rings for your car did you? Unfortunately for me, my car is the main experiment for the new class as I'm the only guy starting with a 964. The rest have all started with early 911 tubs and don't have ABS so the tire sizes are not an issue for them.

Rob: Yes, a timer would probably be better than an odometer anyway. Right now, I simply estimate it. The Turtle started life as a simple C2. I had thought of simply pulling the "chip" that sits on top of the ABS controller out and see what happens then, but figured it would be similar to pulling the fuse.

Right now the most likely scenario is to completely remove the ABS/stock brake system and go to a Tilton dual MC system (which still means figuring out a way to get a speedometer). I guess I'm just going to have to wait until next year when I'm most likely to add MoTeC or AIM data aquisition and ignore how Porsche designed it. I don't really need a speedometer anyway since I just putt-putt around the track.

Pull the ABS relay, not the fuse.

Tom, I just don't really see the issue. Yes, I machined 41, 42, 43, 44 teeth rings, but at this point I only have 1 set of each. It wasn't a big deal and as you say, only a couple hundred bucks and I may have some blanks around somewhere. FWIW, the stock C2 ABS is terrible on the track and if you wanted to, you can install the 996 Cup racing ABS system and use it with the Tilton MC setup. The only problem is the $3200 you'll have to shell out to PMNA for the unit.

You can use the MoTeC SDL and ADL for wheel speeds, either from the ABS unit as a digital signal, or directly from a wheel speed sensor. I've done it both ways.