nile13

it's been raining pretty hard in Boston in over the weekend and I had an autocross in the rain on Saturday. Sunday night I went to start the car. The car started and ran but ABS seemed to have been pulsing, I could feel it in the pedal. That stopped a minute or so later but what remain is ABS light on and no speedometer or odometer this morning.

Now, it sounds like either the speed sensor issue or ABS brain issue based on previous posts here. I checked fuse 16 and it's fine. Have not had a chance to look further as the car was taken to the dealer for latch motors recall this morning.

So, where do I start? Have them do the hammer reading first ($62)? Or should I start looking right at speed sensor? It's the left front wheel sensor, correct? Are all 4 sensors the same? Any other ideas? I'm a bit surprised that odometer was inoperable, I'd expect speedometer and ABS, but not odometer.

2Many Cars

I'd start by pulling each wheel off and checking the connectors to the ABS sensors. I was getting a random ABS light that ultimately was traced to a dirty connector. With all the rain it's possible you're getting a short and if so the connector is a weak spot where it could be happening. You can't tell if the connections are good just by looking at them - mine looked tight and clean but I pulled them apart and cleaned them up anyway and it fixed my problem. Hopefully it will fix yours. Forget about taking the sensors off, they'll most likely break if you try to remove them. Just clean up and reseal the connectors and see what happens.


As to the odometer I'm at a loss. Sounds like a coincidence to me as I believe it's mechanical with no tie to the brakes at all.

ppressle

Perhaps some water in the abs brain or connector? I believe the speed signal goes though the abs brain.
My Durametric reader can read the brake codes, a few dealer reads and you could have bought your own.
Pete

BTW, Still enjoying the steering wheel... Thanks!

berni29

Hi

If you end up pulling the speed/abs sensors please let me know, because if they are the old metal bodied ones and are corroded onto the holes you will have fun getting them out.

All the best

Berni

nile13

Thanks, guys.

I do understand why the speedometer stopped working - the signal comes from one of the sensors. I do suspect that it could be a connection issue, especially with all the water. What bugs me is the odometer. It has new gears as of a few thousand miles ago and ran fine for a while. I'm wondering where odometer signal is coming from, though.

So... which sensor am I looking at? Front left? And are the sensors physically the same on teh front and rear wheels?

PS. Pete, I'm enjoying mine too. Great feel. I just got a perforated Momo Competition for my autocross appliance and Polish re-cover feels better than Italian original.

nile13

Bump for more opinions.

GC96

Don't the odometer and speedometer share the same input signal from the same sensor? I haven't looked at the 993 manual/wiring diagram so I could be wrong, but in other vehicles I've worked on with a similar speedometer/odometer system they both used one input signal from the vehicle speed sensor which is usually mounted on the transmission. The speedometer uses the frequency of the waveform to determine the speed and the odometer is just basically a pulse counter of sorts. On newer vehicles manufacturers have started using the ABS wheel speed sensor instead of a dedicated vehicle speed sensor, but I'm not sure that's the case with the 993.

GC

nile13

I'm fairly certain that 993s use ABS speed sensor on the wheel (which one?) for the speed signal instead of transmission signal.

911PERVY

Pretty sure the speedo only gets one input, the internals in the speedo then do the miles trip and mph. I would try brake cleaner spray on the sensors

thealphabeta

I have an issue with my speedo - it sometimes just stops working. If I tap the face of it then it kicks back into life...weird

GC96

I believe that systems that use the ABS sensor usually look at more than one sensor (probably all four) and do some sort of averaging of the wheel speeds to determine the vehicle speed.

GC

nile13

GC, from what I've heard it's one sensor. I'll try to dig out my 993 schematics book.

Pervy, do you know which sensor off top of your head? For some reason left front is stuck in my head as the one sending signal (logical, as it's the closest one to the ABS brain, I believe).

geolab

100% sure, signal speed in manual is left front sensor, in tiptronic it is the transmission shaft regards

nile13

Geo, thanks for confirmation!

Any plan on how to diagnose things by myself? Or should I simply start with taking the front wheel off and cleaning the sensor (with brake cleaner?)?

Thanks!

berni29

Hi

When I had problems with my Tip box it was the RH (from memory) front ABS/speed sensor that caused it. That was just the Tip box though not the speedometer which worked fine.

Berni