Speedometer Problem

[TD2015]

Although the car is now back together and running fine the Speedometer (or more particularly the odometer) is now causing me a problem.

Prior to the "rat attack" the odometer (it's actually in kilometres since it's a LHD car) worked perfectly but since the car's return it has not moved. The speed registers, the gear changes register (it's a tiptronic) but no odometer. The garage had it out and sent it down to a local speedo back street guy who confirmed that there must be a chip problem within the unit itself.

I think this a little too coincidental but Adonis assures me the wiring and cable is fine and to prove this he put it up on the ramp, spun the wheels as speed and sure enough the computer tripometer works fine - which suggests the cable from the gearbox / wheels must be working.

So, any ideas and - if it is something inside the speedo unit - then does anyone know someone who can fix or, if not, anywhere one can buy a second hand tiptronic (kilometeres only) speedometer?

Cheers,

Jon

 

[clyde]

Sounds like the speedo head itself is broken and it's just one of lifes amazing coincidences Jon. It's not necessarily the type of thing you would know for certain was working before unless you were checking on a daily basis though is it?

I know they can be overhauled but no idea of costs

 

[Dave Wilkinson]

It's a problem common to the 964 too. Apparently one of the gear wheels has a habit of becoming loose on it's shaft - usually encouraged if you happen to press the reset for the trip meter when moving. I realise that you don't have the trip meter in the OBC version, but the internal workings are pretty much the same.

I found on mine that the problem was aggravated by hot weather and when the weather cooled down the odometer reverted to working again. There is a place in the States that sell the gears and I've seen many reports on Rennlist where people have had success in repairing the speedo themselves. There are a couple of places in the US that will repair them (Hollywood Speedo and Palo Alto Speedo if my memory serves) but I'm not sure if anyone in this country does it. It might be worth ringing Reap Automotive (they advertise in PP) - I know they do gauge face refurbs and it's possible they may do internals too.

Regards

Dave

 

[TD2015]

Dave,

You may be right. However I found this on the Pelican Parts forum:

"I was told to replace a relay in the front trunk central electronics box which handles the signal from the ABS left front wheel sensor that goes to drive the odometer in a 1996 tiptronic 993 coupe. I looked and could not locate this relay but saw one which is just left of the 2 relays for the headlights. By description it is a tall rectangular relay with "VDO" markings on the label and have 4 wide spades and 1 thin spade at the base where it plugs into the bottom of the relay box. The symptom of this relay needing replacement is a speedometer which works but the odometer reading is no longer moving."

It certainly sounds like the same problem I've got, although my mechanic thinks not!!

I should know more tomorrow when he checks the relay.

Does anyone out there have any more info re this relay i.e. does it exist?

Jon

 

[Mark Elder]

Well it does sound like someone with a tip has practical experience of the relay making a difference....but on my (manual), when I had a problem with my speedo, there was just one faulty cable which, when broken, stopped both the odo and the speedo.

I suppose there could be another signal which is also required for the odo...

 

[TD2015]

All I know is the speedo on the 993S is pretty sophisticated. I only say this because I drove the car whilst the speedo was being checked over and the info on the computer re speed, trip miles / kilometres etc all registered zero which means the info must come via the speedo. Also, the back spolier does not work if the speedo is disconnected i.e. it obviously rises when a relay from the Speedo detects the car is above 100 kms per hour.

Dave's cog theory makes sense - and since the unit was out of the car for several months due to the rat problem it could have been knocked or something. I'll definitely investigate this if my hoped for simple to fix odometer relay theory falls flat.

Jon

 

[clyde]

Interesting one Jon, I really can't see what would be different about the tip that would mean the speedo head needs separate feeds for the speedometer and the odometer but as Mark says, the guys on Pelican are usually on the ball, so there must be something. [8|]

Look forward to the update after your mechanic has had a look

 

[frf]

Unfortunately, I had a serious problem with my speedometer which provided me lots of information on how the speed information is obtained and passed by the ABS system to the speedometer. In fact, in ROW cars, the speed information for the ABS system is provided by the rear left wheel ABS sensor (if there is any doubt about this, just try an easy trick by disconnecting that ABS sensor). This information, after being processed by the ABS system is sent to the speedometer via the Central Informer. The speedometer sends this information to the OBC in order to be used to calculate speed and every related information like consumption, next gas station stop, etc. If the speed signal is not being received by the speedometer, it will stop working together with the odometer, as well as the OBC. As in your case the speedometer is working and only the odometer is stopped, it seems that this is clearly a problem with the odometer itself - usually a well known problem with a gear tooth. With me the problem was in the speedometer itself, which had to be replaced. It was a Mercedes-Benz electrician that, after having tested the speedometer with an oscilloscope, discovered that when ground connected the speedometer was reducing to almost 0 volts the speed signal being provided by the Central Informer. I hope this will help you trace your problem.