80 sc speedo not working

gorskined · 10:50 AM

Hello all
I was hoping someone could point me in the right direction as to where to start trouble shooting.
my speedo in my 80 Sc is not working its connected to a 1980 915/62 via the little round puck with two wires and not a cable.
The lights in the gauge work the puck is clipped to the side of the transmission it appears to be correct .i would love a little back ground on how this system works. The dash was completely apart. i may have something crossed would love to see a picture of a speedo connected correctly or a wire diagram to confirm my wiring . also is there a resistance reading for the puck ?
Ps my high beam light are always on but the lights works correctly .

Thanks in advance
Regards Ned

spuggy · 01:57 PM

Quote:

Originally Posted by gorskined

i would love a little back ground on how this system works. The dash was completely apart, i may have something crossed would love to see a picture of one connected correctly or a wire diagram to confirm my wiring . also is there a resistance reading for the puck ?

There's a plastic carrier inside the trans with a number of magnets on it. There's a switch on the side of the transmission that closes as each magnet passes under it.

There's a connector somewhere with two spades that goes to the switch on the transmission. IIRC, both the wires are the same color. I seem to recall that it's in the tunnel, it may be under the coupler access plate. But I don't recall ever having a reason to remove that plate, so probably start by looking under the shifter boot first...

Disconnect the connectors that go to the switch, use a DMM switched to ohms (or continuity/beep) on both terminals to test the switch itself. The switch should open/close as you rotate the rear wheels. It's either no/very low resistance (closed), or infinite (open).

In use, one side of the switch is fed 12V, and the other (return) goes to the speedometer. It really doesn't matter which way round they're connected, but checking that one or other gets a 12V feed with ignition switched on would probably be useful.

The switch outputs a 12V square wave for the speedometer when the disk in the transmission is rotating.

The speedometer has 4 terminals. The ones for power are quite clearly marked +, - and are usually at opposite ends of the strip of terminals. Terminal 31b is input; this should be connected to the return from the transmission reed switch.

Terminal A is an output for the OXY/EGR system counter/warning light, perhaps cruise control and/or some other junk. It being connected or not should not affect operation of the speedometer in any way.

Like this

from this thread: 84 Carrera Speedometer Wiring

MBAtarga · 10-07-2024, 03:36 PM

Pictures of my SC speedometer wires are in the link above.

gorskined · 10-08-2024, 03:37 AM

Quote:

Originally Posted by spuggy

There's a plastic carrier inside the trans with a number of magnets on it. There's a switch on the side of the transmission that closes as each magnet passes under it.

There's a connector somewhere with two spades that goes to the switch on the transmission. IIRC, both the wires are the same color. I seem to recall that it's in the tunnel, it may be under the coupler access plate. But I don't recall ever having a reason to remove that plate, so probably start by looking under the shifter boot first...

Disconnect the connectors that go to the switch, use a DMM switched to ohms (or continuity/beep) on both terminals to test the switch itself. The switch should open/close as you rotate the rear wheels. It's either no/very low resistance (closed), or infinite (open).

In use, one side of the switch is fed 12V, and the other (return) goes to the speedometer. It really doesn't matter which way round they're connected, but checking that one or other gets a 12V feed with ignition switched on would probably be useful.

The switch outputs a 12V square wave for the speedometer when the disk in the transmission is rotating.

The speedometer has 4 terminals. The ones for power are quite clearly marked +, - and are usually at opposite ends of the strip of terminals. Terminal 31b is input; this should be connected to the return from the transmission reed switch.

Terminal A is an output for the OXY/EGR system counter/warning light, perhaps cruise control and/or some other junk. It being connected or not should not affect operation of the speedometer in any way.

Like this

from this thread: 84 Carrera Speedometer Wiring

Thanks for the great explanation.