Speedometer Compatibility

[sharper]

I was wondering if there are compatibility issues with different year speedometers. I have a 915 transmission (in my 914) that I'm guessing is from 1987/1988 based on the style of impulse sender (not the puck looking one). I wired it up to a speedometer that has a 1977 date on it and got nothing. I know the sensor is working as I checked it before doing the wiring. There are two wires from the sensor so I ran the green one to the 31A terminal and the brown to the negative (-) one. The red power from other gauges is on the positive and nothing on the A terminal. Wondering if I have a bad speedometer or if it just isn't compatible with my setup.

[Flat6pac]

The 87/88 is a G50 sender that seals into the trans.
The 915 takes the puck type after 1975 mechanical drive
Bruce

[sharper]

Originally Posted by Flat6pac

The 87/88 is a G50 sender that seals into the trans.
The 915 takes the puck type after 1975 mechanical drive
Bruce

Yes, that is how mine is. My understanding is that it still works by picking up the magnets as they spin by. So the question is would a speedometer from a 1977 be compatible with it.

[spuggy]

I've used various speedos - '77 911, 930, 3.2 - with a couple of different 915s ('77, '86) and most recently the 3.2 speedo with a G50; apart from calibration, all been plug'n'play for me.

Perhaps try borrowing a speedo that's known to work? Poke the sensor output with an oscilloscope? Feed a square wave generator into the speedo input?

All the sensors have the same number of magnets; it's my understanding the sensors all generate a (lol, sort-of) square wave, which is what all the electronic speedos want to see. Caveat is that the year/model speedo may be calibrated for different (eg the original) wheel/tire sizes that don't match the rolling diameter of low-profile 17's (or whatever you're running). The Yellow Dog speedo recalibrator works great for me to get them reading dead nuts accurate.

The swaps all Just Worked for me, as far as moving the needle is concerned. Unless/until they don't (the 15V caps in the speedo really aren't fond of 18V voltage spikes from a bad VR, and the instrument can act flakey for a year or two afterwards before dying suddenly/entirely).