c4cab0

Last night after filling up with gas I reset my odometer. It registered the 3.4 miles to
home but when I made another store run the
light for the spoiler stayed on. I then noticed that the speedometer wasn't working.
Where does the speedometer drive from,the distributor or the transmission?
This a 90 C4Cab.
I haven't noticed any odd sounds or a loss
of power.

neil williams

Had a similar problem recently.

On my 90C4 the feed comes from the gearbox. There is sensor which I understand is very easy to get to. I am sure if you know what to look for you can check that it is connected and I am equally sure someone like Adrian can tell you.

My experience is get then to check this before anything else. On my car the fault was intermittent, so they pulled the speedo and wasted $250 to decide there was nothing wrong with the speedo.

If you are lucky they will swap out a sensor from another 964 and should be able to prove that is the problem ASAP, which what they did finally with my car.

cheers

Neil

Adrian

DearC4,
The sensor for a 1990 C4 comes off the gearbox. It is called a pulse sender. The spoiler warn light circuit uses a speed signal as well but via the speedo head itself. The failed odometer, failed speedo pointer and failed warnlight (did not go off at 7kph) strongly suggests a speed sensor input failure. These sensors themselves rarely fail. However the wiring attached to them does. I would first be checking that the connector on the rear of the speedo is secure, check all the wiring is intact, no pins have popped out and then I am afraid it is underneath and check the speed sensor on the gearbox. I would say from previous experience (that normally means it is something else when you say such things) that the plug on the sensor is damaged or corroded or both,
Ciao,
Adrian
911C4

PS: I do have to advise you though that an internal failure within the speedo can cause exactly the same effect. However we always start with the easiest and the less costly troubleshooting first.

horst

Comment & Question:

Comment- I have found that many times unplugging & plugging back in, undoing grounds and re-tightening cleans contacts a bit and sometimes gets things working again

Question- I don't have my owner's manual in front of me now, but I wonder why my ownners' manual for my 930 (euro-85) cautioned that the odometer should only be re-set with the engine running?? (to make sure, I usually punch it as I drive off)

c4cab0

Thanks for the help on my speedometer problem.
I will be back in Texas in a couple of months,then I will check out these listed solutions. I am amazed at the knowledge of
Porsche owners about these wonderful cars.
Adrian must spend many hours researching
Porsche problems. Thanks again

Adrian

Has become a second unpaid job but it keeps me off the streets,
Ciao,
Adrian
911C4

PS: If you are reading this thread and you have a 1991 model 964 or up, then please disregard. You have a different system installed for the speedo. If you have a Tip, you need my book.

RobEvo5

hi, Adrian

I have just posted a similar problem but with a 1991 model which you have indicated runs a dirrent system. My speedo works once the car is warmend up after say doing 5 miles up the road. But I can't locate the speed sensor to replace or check connections. At first I thought is was a magneto sensor near the flywheel, but I now think this is perhaps a TDC sensor. Where is the sensor on the 1991 C4's ?. Any help would be gratefully appreciated, as this small problem is driving me nuts.! Is there anything else I can check other than the connection to the sensor?

RobEvo5

hi, Adrian

I have just posted a similar problem but with a 1991 model which you have indicated runs a dirrent system. My speedo works once the car is warmend up after say doing 5 miles up the road. But I can't locate the speed sensor to replace or check connections. At first I thought is was a magneto sensor near the flywheel, but I now think this is perhaps a TDC sensor. Where is the sensor on the 1991 C4's ?. Any help would be gratefully appreciated, as this small problem is driving me nuts.! Is there anything else I can check other than the connection to the sensor?

Adrian

Guys,
I responded to this problem via another post. If you have a 1991 model year and up 964 your speedo signal comes from the PDAS/ABS (C4) or ABS (C2) circuit. If you have a failed wheel speed sensor you will also have a PDAS/ABS failure (C4) or ABS failure (C2). Therefore the problem is either the connector to the speedo or the speedo itself. 9 times out of 10 the problem is the speedo.
Are there any other problems? Rear spoiler warning. Failed On board computer (if installed) mileage functions? Odometer failed if in the speedo (without OBC)?
A failed speedo pointer and everything else is working is not uncommon. MIne does this occassionally. A slight tap on the side gets it working again.
Ciao,
Adrian
911C4