garrett376

My coupe started to do something very odd. When I am slowing to come to a stop, at about 3-5mph, just before I stop completely, the speedometer will shoot up to 30-40mph, and the ABS will fire (at least I think that's what is happening because I can feel a thump in the brake pedal). It's odd because it only happens when I am almost at a stop (just a couple mph), and when the brakes are squealing like hell (I have these terrible Porterfield race pads).

Is it possible that the brake squeal is at such a frequency that it triggers the speed sensor to screw up? Just a thought, because the problem appears related to the squealing. If there is no squeal when I stop, it's fine. If I make a bad squeal, I get the speedo jump. It's never done this before yesterday, and there are no warning lights of any kind, and everything else works just fine. I am stumped.

Any help ideas? Thank you

garrett376

Just a note... the same problem occurs when rolling backwards, too! Even though I roll backwards, the speedo will flip up to 40mph or so.

I don't have any non-squealing brake pads to compare, or I'd try to rule that out first.

garrett376

Based on my troubleshooting, although it seems hard to believe, my ABS and speedo problem appears directly linked to the squeaking of the front left brakes. The brake pads are Porterfield R4 race compound and sqeak so bad, that I honestly need ear plugs - city buses don't even come close to how loud my car is when I come to a stop from 3 - 0 mph.

Based on a tip from JasonAndreas, the speedometer receives its speed input from the front left speed sensor. I took it out to check it - it wasn't that dirty, but I cleaned it anyways. It also tested within spec with an ohm-meter. After cleaning it, I went around and cleaned up all grounding points for the ABS and PDAS. Now, I never got an ABS warning light, but I thought I'd just rule out that possibility. Took it for a test drive after first cleaning the speed sensor - same problem. Took it for a test drive after cleaning the ground points - same problem. (keeping with a good experimental model here...)

Here are the clinchers:
1. if I spray my front left brake rotor with water so it won't squeak (right side still squeaked but not nearly as much as that front left one), the problem CANNOT be reproduced!
2. When the brake pads are cold, they do not squeak. I cannot get the problem to occur!

Boy, that's strange, don't ya' think?! It appears that the squeaking from the pads somehow throws off the sensor - somehow sends a spike because the speedo will shoot to 60mph when I squeak really loud, and just to 20-30 if it's an average squeak!

Sounds like I need a different brake pad, eh?

garrett376

For those interested in what the back side of the wheel speed sensor looks like (when dirty) - here it is! It had metallic shavings on it since the sensor itself is magnetic. I cleaned it up with brake cleaner, cleaned off the o-ring, and screwed it back in - there's 1 allen-head screw holding it in and the plug that hooks on the shock tower..

garrett376

Your idea is as good as any, Indycam!

I am at a loss for any other reasons...

JasonAndreas

Maybe your squeaky brakes are causing the abs ring gear to vibrate? During production (and a few times for the 993) Porsche changed it a bunch of times. They added a washer, they reinforced it and removed the washer, they changed the way it was centered, etc. etc.

Bill Wagner

Here's a theory, and it may not be far fetched although it may sound like it:

When two dissimilar metals come in contact with one another, they produce a tiny electromotive force (emf), or voltage.If you don't believe that, do a web search for "thermocouple", which is a device used to take advantage of the electrical potential difference that exists between two different metals as these crimped metals vary with temperature (in my first job as an electrical engineer we used thermocouples to detect heat being induced via pulsed RFI). If the two metals are in static contact (not moving) the voltage is a DC voltage. If the two metals oscillate against one another, they produced an alternating current, but it's usually pulsed, not sinusoidal, but it does, never the less have a frequency.

Brake noise is caused by vibration. In most cases, dust gets between the piston and the backing of a brake pad with bad or missing damping materials. The noise occurs as the pad "slaps" against the piston rapidly a tiny amount, over and over again (distance travelled in this is probably in terms of micrometers, not centimeters). This usually occurs at low speed because the full force of the pistons against the pad doesn't exist. In any case, assuming your damping material is bad or missing, it means that the metal of the piston is contacting and separating from the metal of the back of the brake pad very rapidly, and this oscillation is inducing a tiny alternating current into the braking system, which in turn is likely inducing a signal into the brake disk because of their dissimilar metals between the piston and the back of the brake pad. The speed sensor is then detecting the signal being induced onto the disk as a "speed", thus your speedometer shoots up.

This wouldn't occur with all pads because the amount of EMF that two dissimilar metals produce would depend on the nature of the metals. Most metal to metal junctions produce levels of EMF on the order of nanovolts, but some can produce millivolts, which can easilly be enough to be picked up by your speed sensor.

If this theory is true, the solution is simple: Put some damping material on the back of your brake pads.

Just a theory...

jrotsaert

I have the exact same issue on my 964RSR race car. It is the pads. Don't worry about it!

garrett376

hey jrotsaert - nice to know my car is not crazy!!! I changed to some PFC97 which squeal much less, and I don't get the problem anymore. It is fascinating that it would do this....