[Velocity]

My speedometer started intermittently dropping to zero (from 60) then popping right back up to normal speed a second or so later.   I saw this happen several times over the course of a 45 minute drive.     Odometer is working fine (replaced the gears a couple of years ago) and no dash lights (like ABS) are being triggered.   I'll check for codes/errors with my durametric later, but haven't yet.

 

Possible suspects:

1. "Vehicle speed sensor" - believe I've read somewhere it can get gummed up with brake dust and need a cleaning.   Is there a specific wheel the speedometer gets the speed signal from?    Also believe our 968s do not have a speed sensor in the transmission like the 944s do?
   
2. I've read the ABS fuse having intermittent contact can cause wacky speedo error like this?
   
3. The ever-popular "bad ground" issue that can cause wacky problems with anything in the car (notably gauges)?    Any ideas WHICH ground to try and check that'd be most suspect?
   

 Any thoughts/suggestions on any of the above OR other thoughts as to a possible root cause?

 

 

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[Roboman]

Hi Velocety, on mine the whole dash would drop out and then bounce back. When removing the dash to replace the odo gears I decided to give the dash terminals, ABC, a really good clean ... that issue has since disappeared. I know you said just the speedo, well just one dirty pin will do that, and the possible fix is free so why not? And MCL advises lubing those ABC clips.



Good luck.

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[Waylander]

The speedo is driven by the drivers side (LHD) ABS sensor, the signal is multiplied by G18 the frequency multiplier in the fuse box, then through plug B in the cluster to drive the motor in the head unit, there is enough connections on that route to cause issues

The interior earths are at the ends of the firewall, head in foot well and look straight up each side, a very frustrating job

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[Velocity]

Got a chance to remove left front wheel.  I assume the sensor shown in below pic is the speed/abs sensor?    I cleaned it out as best I could with a brush, used some electric contact cleaner on it, brushed it off a bit more, then got wheel back on.    Also cleaned up the grounds under/behind the dash as best as I could.

 

Seemed to fix the issue for a couple of weeks, but today it started dropping out again - mostly at highway speeds.    Also noticed when it did drop out (at times dropping out entirely for several minutes) the odometer didn't increment (which I guess makes sense).    Seems to work more frequently at slower (<50mph) speeds.    

 

   

 

Here's a video of it happening - https://youtu.be/HV_v1TagkmU

 

No other gauges on the dash are having any issues.   As you can see from the video, the tach, and other gauges read normally and continue to do so when the speedo drops out.

 

Any other ideas, short of replacing the speed sensor entirely?   

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[tejon007]

...could be wrong, but the sensor that sends a signal that's converted to the speed displayed on the speedometer comes from the transmission and isn't very easy to reach. What you're looking at is for the ABS system...

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[Velocity]

...could be wrong, but the sensor that sends a signal that's converted to the speed displayed on the speedometer comes from the transmission and isn't very easy to reach. What you're looking at is for the ABS system...

 

I've read in multiple places that the 944 takes it's speedo reading from a sensor in the transmission, but have also read (including waylanders comment earlier in this thread) that the 968 uses the front left ABS sensor for speedo measurement.

 

One other possibly related factor I hadn't mentioned.  My antilock brakes haven't worked since I got the car.  No dashboard warning light is on, they just flat don't work (locked them up solid during a quick stop a week ago).   I'd assumed it was related to the fact I have "Big Red" brakes in the front (upgrade sourced from a 993 turbo, I think) and 18" wheels.   I have since also read elsewhere that another person with a failing speedo was able to correct it with a replacement ABS relay - seems odd, but IF the speed sensing comes from the ABS sensor, I suppose it's plausible.    I was thinking about getting a spare set of most relays I could find (and did find a set) so I'll try swapping out the ABS relay to see if that fixes either problem.   I suppose replacement sensor will be next on the list.

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[tejon007]

OK, I'm not going to disagree with Waylander and without a full diagnostic, can't tell if this is your problem. However, here is a diagram of a 968 speed sensor at the transmission:



https://www.porscherepair.us/porsche-968...ssion.html



So, that's what I was thinking about where the input from a sensor eventually made its way to the speedometer comes from...of course I was probably wrong .



Best wishes for finding the solution!

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[Velocity]

OK, I'm not going to disagree with Waylander and without a full diagnostic, can't tell if this is your problem. However, here is a diagram of a 968 speed sensor at the transmission:



https://www.porscherepair.us/porsche-968...ssion.html



So, that's what I was thinking about where the input from a sensor eventually made its way to the speedometer comes from...of course I was probably wrong .



Best wishes for finding the solution!

 

Aha - that's for the tiptronic transmission.   I have a 6sp manual so totally different transmission.      Suspect the speed sensor is on the tiptronic as part of the automatic shifting, may not be there/needed on the manual.

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[Waylander]

The sensor on the gearbox is only on the tiptronic and does not drive the speedo, its only for the gearbox ECU



If you check the inside of the lid of your fuse box you will see the relay listed,



I would suggest that the comment about the abs relay is also quite valid, but if you abs is not working at all and no warning lights, then relay replacement might be a good idea



You can tell when your abs is working you should hear a motor run in the passenger footwell as the ABS pump primes for action, this should happen before you reach 5mph, there are also two relays on top of the abs pump



The usual reason for no abs and no light is the abs pump has seized solid, the pump can be repaired using parts from a Mercedes 190e era car, you will need the motor the relays and pistons, as a exchange 968 pump is close to $3000

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