Speedometer output for Ford Speedo

[fairmont50]

Has anyone been able to get the speedo output to work with a ford speedometer ('98 mustang specifically)? I have not been able to get this to work. When I drive the speedo directly from a function generator everything works fine but the msgpio output does not even make the needle wiggle. During this test, I was driving the vss input to the msgpio with the function generator so I do not believe that it is a noise issue. The pull up resistor is installed and I have tried both 5v and 12v for the hot side of it. My trans is a 4l80e so the vss sensor is set to 31 teeth, the rear gear is 3.08 and the tire diameter is 25.5 inches. The pulses per mile on the ford speedo is supposed to be 8000. Based on these numbers, the output frequency ends up being almost exactly 1/10 of the input vss frequency. Not sure if this is pertinent to the problem or not. I have looked at the output on a scope (sound card scope since my real scope drowned), and the output from the msgpio looks reasonable although the pulse width varies a bit depending on speed.

Thanks,

Chris

[fairmont50]

Also, according to the schematic, the speedometer is normally driven directly from a vr sensor in the transmission. It drives the speedo, the cruise control and an engine control computer input. I have driven the speedometer from the function generator with a sine wave and a square wave with no problem at all.

Thanks,

Chris

[fairmont50]

Just an FYI,
I ended up using a speedometer box made by Dakota Digital (SGI-5) to resolve this issue. It is outputting the 8000 ppm sine wave that is required by the speedometer and also sending a signal to the gpio. Not sure of the ppm on this but I had to change the wheel tooth count to 49 to get the tuner studio speedometer to agree with the dash speedometer. Could probably have just run them in parallel and may try that in the future.

Chris