Electronic speedo in an early 911 with newer 915 trans

[wjdunham]

Google isn't telling me the answer so I will ask here, sorry if this has been covered before but I'm not finding the right info.

I am about to swap in a 3.2 and 915 trans with an electronic speedo send and I need to know how to hook up the speedo signal to my new electronic speedo.

The speedo I bought has some notes on the wiring headers as to which wires were connected to them in the later car this came out of:
In order across the connector:
Red
Black
Black
Brown

I assume Brown would be ground. What goes to the two Blacks and Red connectors? I have a wire from the speed sensor on the transmission running up to the speedo, and I assume I can ground the speed sensor near the transmission and ground the Brown speedo wire near the speedo?

Thanks for any help.

Bill

[jbell959]

Hey Bill,

You don't need the connection next to the red. Just 3 connections.

Red - Switched 12V. You can use the same feed that provides power to the tach.

Black - This is one of the wires coming from the speedo sensor on the transmission. It doesn't matter which one.

Brown - This is ground and also the second wire from the sensor.

That's it!

[wjdunham]

Originally Posted by jbell959

Hey Bill,

You don't need the connection next to the red. Just 3 connections.

Red - Switched 12V. You can use the same feed that provides power to the tach.

Black - This is one of the wires coming from the speedo sensor on the transmission. It doesn't matter which one.

Brown - This is ground and also the second wire from the sensor.

That's it!

Thanks! I guess that makes sense on the speedo sensor wires. It's a Hall Effect sensor so doesn't matter which wire is ground and which gets connected to the Speedo input?

[jbell959]

Originally Posted by wjdunham

Thanks! I guess that makes sense on the speedo sensor wires. It's a Hall Effect sensor so doesn't matter which wire is ground and which gets connected to the Speedo input?

Correct.. it's basically a magnetic switch that's opening and closing to ground.

[wjdunham]

Originally Posted by jbell959

Correct.. it's basically a magnetic switch that's opening and closing to ground.

Do you know what the voltage level of the signal coming from the transmission is. I wanted to hook up my waveform generator to the speedo input and test it out. I can generate a square wave of various frequency and voltage and see if I can move the needle.

[jbell959]

Originally Posted by wjdunham

Do you know what the voltage level of the signal coming from the transmission is. I wanted to hook up my waveform generator to the speedo input and test it out. I can generate a square wave of various frequency and voltage and see if I can move the needle.

Technically, there is no voltage level coming from the transmission since it's just opening and closing a path to ground. However, I typically test with a signal generator using a 9v square wave to calibrate them.

This should tell you if you have an issue, but a better way to test it would be to trigger a transistor with your signal generator to simulate the opening and closing to ground.