I found something online that I wanted to try:

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

I wired it like this:

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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.

I drive my DD every day with the speed showing in my GPS app. It's generally very good and helps me figure out the speedometer error.

But once in a while, around tall buildings or while driving past certain high-electromagnetic-interference facilities (ahem) in the DC Metro area, it goes a little haywire -- like showing 25mph one second, 78mph the next.

Also, it won't function in tunnels if that's a concern in your neck of the woods.

Still curious though, since it's purpose-built for speed tracking with a high refresh rate, maybe it'll be better.

I second the

There are very few tunnels in my area. And I can generally maintain my speed with the Tachometer.

Plus, I won't have to worry about odometer correction for rallys, and I can put in a small switch to turn off the speedo for track events.

Please tell me you'll use a better crimper for the final connections...

--DD

Interesting, but does that mean the trip meter is disabled?

Dave... You know me better than that. This is just a test rig.





The odometer and the trip meter worked great when I ran the calibration sequence.

I have been discussing this for a while with Tim... I am patiently awaiting your tests.

I notice that VDO sells their own GPS sending unit. I was thinking of going that way since it's all one brand, but if this works for you I will go that way.

Good Luck!

I will sell you the VDO unit I bought if you want it. But it won't work with a 911 speedo. It will send the signal, but it pegs the speedo at anything over 20 MPH.

And there is no way to calibrate it.

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.

This is really neat. I've been using the "Dashboard" screen on my Garmin for years and don't put speedo drive gears in my transaxles any more. I hadn't considered an Autometer speedo to match the tach. Hmmm.

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.

Nice Clay can you please define "later model 911?"

Could you hook it up to a HUD?

Mine came from a 76 911. But the easy way is to look at the face of the speedometer. It will have a reset button at the bottom, and under the VDO logo it will say "Electronic".




I suppose you could. But I was not looking for that.

No thanks. But thank you for the warning.

my 914 came with this when I bought it.
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It is a GPS speedometer and works pretty good. Of course there is the delay upon start up and it doesn't work in the tunnels.

I've outrun the GPS speedo app on my iPhone.

But in reality, that's probably just a software programming issue with the timing of it pinging the satellite(s). I mean, my car is quick, but it shouldn't be THAT noticeable.

I ran around all day today with it in my truck. It worked fine. No delay when comparing it with the truck's stock speedometer.

Unlike your phone, this was designed to be a speedometer sender. So the update rate is very high, it doesn't have a delay.

GPS doesn't work like that. The receiver is passive, it doesn't "ping" the satellites, it's just listening to them talk.

The app may have some silly restrictions in it about how high a speed it will display; might be something as simple as UI layout, or some stupid law about encouraging speeding like the one that prevents the roadside "this is your speed" display from showing more than ~15mph over.

Thanks you just saved me a lot of time...

Just an update on this project.

I got it all hooked up and in the car. I had to move the antenna to the top of the windshield hoop, but otherwise it works great. It took some time to get the calibration right, and now both the speedometer and the odometer are accurate. The speedometer is calibrated to within 1mph of the cable driven speedometer, and both of them are accurate when compared to the GPS speedometer app on my phone. The update rate is so good that the needles on both speedometers move in synchronization when accelerating and decelerating.

The only thing left is to remove the speedometer cable, the angle drive, and the speedometer drive in the transmission. I am going to put a plug in place of the drive so there will be no more oil leaks.

Now the weather needs to warm up a bit.

Nice work!