Speedometer upgrade, No more broken cables or angle drives Options

[ClayPerrine]

I found something online that I wanted to try:



I bought one and wired it to my factory 911 speedometer.

I wired it like this:



The white wires are from a 12v DC transformer, used to bench test it. The Positive is on the left, and the negative is on the right. The Autometer module is wired to the same positive and negative lugs, and the purple wire from the Autometer is connected to the speedo input. The other speedo input is jumped to ground.

Running the calibration procedure in a bench test, found here , I was able to get the speedo to read exactly 80 mph, and the odometer was working too.

The next step is to test it in a vehicle.

The heavy wires from the positive and negative go to an old cigarette lighter plug that I will use to test it in my truck, comparing the output with the truck's speedo, and my android app "gps speedometer".

If everything works as I expect, I can remove the speedo cable and angle drive from my car. I will probably plug the speedo output in the gearbox to prevent any gear oil leaks.

I will update this when I run the second test.



I will keep you updated on the results.

[ClayPerrine]

Not sure I understand how it would work with a stock 914 Speedometer. Isn't it a mechanical gear driven device needing the cable to operate.

(IMG:style_emoticons/default/confused24.gif)

Yes, a stock 914 speedometer is mechanical. A later model 911 speedometer is electronic. It uses a disk with magnets on the side of the differential inside the transmission, and a "puck" (aka a coil of wire) to generate a small pulse of electricity when the magnets pass by. That is sent to the speedometer, where it is interpreted by the electronics to indicate the speed of the car.

I replace the puck and the wiring with the GPS box. The GPS box output sends a pulse to the speedometer. Grounding the other side of the signal input gives it a full circuit, and the speedometer responds just like it was hooked to the puck.


Update: I took the test rig out in the truck this evening. I put the GPS antenna on top of the dash, and the speedometer in the trash can pointing up. It worked! I realized that the transformer I used in the house to calibrate it may not have contained a full wave rectifier, and that induced pulses into the speedometer. It affected the calibration.. it was off by 2 mph. The update was as smooth and as quick as a standard cable operated speedometer. The only drawback is that if you jump in and take off real quick, the GPS box needs a couple of seconds to find the satellites, so the speedometer will appear dead for 15 seconds. Then it jumps up to the proper speed.

When I built the wiring harness for my dash, I included power and ground wires for an electronic speedometer. So this weekend I will be installing it into my car, and removing the speedometer cable and angle drive.

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