speedometer

[Boydskyking]

Hey guys
I am working on a 911 Porsche SC and the speedo is not working. how do I check it , the sending unit in rear end , the wiring etc.
Thanks guys

[Boydskyking]

an addition to my question,What I am asking is how do I find out if the speed sincer is bad on the ring gear are if a wire is bad or if the speedometer it self is bad

[emac]

Don't know the year but normally a gear which is made out of plastic brakes inside the speedo - I would bet on that
Ernie

[Boydskyking]

its a 1980 SC and what do I look for when I connect a meter to the wire that comes from the sensor is it ohms or dc or ac or what

[targa80]

Originally Posted by emac

Don't know the year but normally a gear which is made out of plastic brakes inside the speedo - I would bet on that
Ernie

I believe the gear that breaks is for the odometer and not the speedometer. I replaced my gear last year when the odometer stop counting. The gear litterly disintegrated from age.

[targa80]

Originally Posted by Boydskyking

its a 1980 SC and what do I look for when I connect a meter to the wire that comes from the sensor is it ohms or dc or ac or what

The Speedometer is fed +12 Volts DC from the red/black wire. The 12 Volts exits the speedometer on terminal 31b. 31b is a Brown/Red wire that runs through the tunnel and connects to the green wire from the sensor in the rear tunnel access hole. The sensor opens and closes a connection to ground. So the speedometer is seeing a pulsing 12volt signal. Verify that your connections to the speedometer match the wiring diagrams and verify that the Brown/Red wire in the rear tunnel access is connected to the green wire going to the sensor.

[Boydskyking]

targa80 thanks so if I take a volt meter and check the voltage on the connection between the seats where the sensor connects to the Brown/red wire and get 12 volt pulsing then this tells me that the sensor is good and the wire is good and the only thing it could be is the speedometer. Am I right
Thanks in advance

[targa80]

Just to be clear, the car would have to be in gear and the wheels turning in order for the sensor to be making/breaking the ground connection. The voltmeter should be varying between 12volts and 0volts. The variation should increase as the wheels turn faster.

[Boydskyking]

Thanks so much

[rnln]

Henry,
Do you remember where did you get your gears?

Originally Posted by targa80

I believe the gear that breaks is for the odometer and not the speedometer. I replaced my gear last year when the odometer stop counting. The gear litterly disintegrated from age.

[zedsn]

I have a brand new gear in the package that I bought for my 1977 911 for $20 shipped if you are interested.

[DRACO A5OG]

Rnln,

If only your trip meter broke, your odo will still rotate. This would be your gears.

If step-motor, both will not work.

Also, you have to get specific gears for your year speedo, but I do not think broken gears are your problem.

[targa80]

Originally Posted by rnln

Henry,
Do you remember where did you get your gears?

Our host has the gears available go to this page and look up the reference for your year .

Porsche 911 (1974-1989) - Gauges & Miscellaneous - Page 3

[rnln]

Thanks Pat, both of my trip and odo meters don't work anymore, speedometer is still working. According to Draco, it's the step motor