I have implemented several frequency input based speedometers in PV750's using a dedicated output from various transmission ECM's. At one time, it worked seamlessly for two different generations of transmission controls:

Older generations (J1587 only) of these transmission ECMs provide a TTL output (.5V-3.5V).
Newer generations (J1939) provide a square wave referenced to Vbatt (.5V-22V).

Suddenly, speedometers will not work on the older TTL controls. They will sometimes read only in reverse and appear to hang up in FWD. They don't track ground speed accurately. This is true for the last three of five systems delivered. A standalone VDO programmable speedometer works perfectly in the same vehicle. The speedometer continues to work with newer generation controls. Older PV750's continue to work on both generations of controls.

The samples are not averaged. Nothing has changed in this portion of my config. I'm using 2.3.11175. Was there a hardware revision to the frequency input section of the PV750?

I sent your question on to someone that should be able to answer.

The first thing I notice is that you list the "older generation" TTL output to be 0.5V - 3.5V, whereas the frequency input of the PV750 is rated from 3.6V to 120VAC (2Hz - 10kHz). I do know that the frequency input will function below 3.6V, but obviously it is not rated below that and will be unreliable. Based on this, I am more than a bit surprised that it would work at all.

As far as I'm aware, the last update to the PV750 power supply PCB was in 2009. I am checking with sustaining engineering to see if there have been any component substitutions on that board.

Thanks Jake/Scott. So the board is essentially not TTL compatible.

Eric Carmen called me today and we discussed the circuit and some workarounds since it has proven too insensitive.

In the PV750 IOM 00-02-0686 I find this:

Inputs (3) 0-4 VDC analog inputs. (1) input configurable to support measurement
frequencies from 2 Hz-10 kHz.

Is there another document with the voltage details?