hey guys
i have an autometer electric speedometer in a 69 camaro with 4 speed m21
my problem is that the mph does not go over 20mph i know im going way faster than that!
i did a little test by taking the sending unit off the trans and hooking a cordless screwdriver to it and pulling the trigger allowing the sending unit to spin. all with ignition on, went forward and reverse with screwdriver, and it still does not read over 20mph
any ideas?
thanks for any
steven

Use the AC Volts function of the DMM and place the leads across the output of the speed sensor/sending unit. Spin the unit up and the output voltage should vary in voltage with speed, its proportional, speed up, volts up.

If unit flattens out, sending unit is bad. If above test works for sending unit, then gauge is bad, providing all connections are soldered/shrink sleeved. I think the cable/wire harness between the sending unit and gauge are usually shielded, meaning a pair of wires wrapped/encased by either a wire braid or foil. If this is the case, then ground the shield at ONE end of the wire harness, preferedly the gauge end.

i have two sending units
a three wire hook up one for ground another for sending unit and the third for ignition 12 volts. this one did not make the needle move at all.

the other is a two wire. one for ground and the other for sender.
this is on the car now and makes the eedle move only up to 20mph

the three wire is an autometer 5291 8000 pulse
the two wire covans classic sent with the guage package 16000 pulse

And they are probably sealed units, they're broke.

All they do is supposedly, open and close a switch, passing 12 volts or not. If one were to look at the output with an oscilliscope, the waveform would be shaped like a square wave. By turning the shaft faster, would lessen the time between the polarity changes.

Maybe the 2-wire has a "slow action" switch and can only handle a max speed of 20 mph.

I had the same issue with a new speedo that I installed in my old Shelby Cobra. After a brief investigation I found that the needle was pushed too far down on the pin and was causing the needle to hit/stop on one of the small phillips screws that hold the pin assembly(?) to the gauge face (does any of this make sense?). If it is a new speedo unit you might want to check this and see if the 20mph mark lines up with any screws or other possible obstructions.

To fix the problem in my car, I simply cracked the gauge open and pulled the needle a little higher up on the pin.

how would you open an autometer speedometer without breaking it?

I'm not exactly sure how the Autometer gauges are constructed, so I'm not sure how to get the speedomenter apart, but I think that Autometer manufactures the gauges that Carroll Shelby uses. On my speedometer, if I remember correctly, I was able to gently un-snap the chrome bezel ring from the gauge housing. Once the bezel is removed you can lift the glass and gain access to the needle. When you are done, the bezel can be gently "snapped" back into place.

However, one word of advice, I would make certain that you have narrowed the problem down to the needle before you go cracking into your brand new speedometer.

mine is the programmable one, do i need to program it os do they come pre programmed from the factory?

ahh forget it i just ordered a mechanical one