84 Carrera Speedometer Wiring

[yelcab1]

I have this speedo being wired into an early 911. I think it's a Carrera 84 911 speedo. There are four terminals and two light sockets. I can figure out the light sockets, and the two + and - terminals (12 and Grd). Where does the speed signal go (from the gearbox)?

[MBAtarga]

I've got pics of my SC speedometer wiring handy - but I don't think it matches up with your wiring.

[yelcab1]

Oh yes it does match up,

[wazzz]

Make sure you connect speed signal (from gearbox transmitter) to terminal 31b, which is the input for speed signal.

So on the pic edited with text above, tag "Speed signal" is wrongly pointing to third terminal and should instead point to second terminal from left.

Terminal A is not an input, but an output for optional units such as cruise control (if present), convertible top control unit (if present) and heater blower relay, for a specific function relevant to Carrera 3.2 only.

Signal to 31b also goes back to the tachometer for upshift indicator, if present.

[yelcab1]

OK, so I fixed my issues. And WAZZZ is correct, terminal 31B is where the speed signal is supposed to go to.

Full disclosure: this is a 71 chassis, with a 915 gearbox, so the speedo has to be the 84 speedo. The wires came loose and I could not remember how I wired it. It turns out, the two wires from the speedo sensor on the 915 gearbox are wired such that one wire goes to ground and the other goes to 31B.

Terminal A is left open.

[spuggy]

Originally Posted by wazzz

Terminal A is not an input, but an output for optional units such as cruise control (if present), convertible top control unit (if present) and heater blower relay, for a specific function relevant to Carrera 3.2 only.

^^^ This.

For completeness, terminal A also drives the "30,000 miles, time to service OXS" counter tucked away in the dash on earlier cars.

[917_Langheck]

That speedo has a May 1978 date stamp on it. Not an '84 Carrera speedo. May have been in one, but it wouldn't have been original.