FMTV change in speedometer models, types?

[tennmogger]

The speedometer in my 1994 M-1078 [A0] has always been way off on speed and Odo reading. Working behind the dash on another project I decided to swap out the speedometer.

The old speedo is not the same as a 1996 model. The original speedo is not programmable like the replacement is. There has been good coverage of how to program the 'newer type' but nothing can be found on the old type, or even the fact the speedos changed at some time.

The old one has a 4 pin socket built in to the housing. The new one has a pigtail. The connectors are the same and the new speedo plugs in to the dash wiring harness just fine. Maybe backwards compatable?!?

My plan is to plug it in and try it unless someone knows of a reason not to.

The remaining question is how the old speedo is calibrated. I remember a reference to a _frequency converter_ in the schematics. Possibly for driving the non-programmable speedometer? If so, that might keep the programmable speedo from working.

Any info out there?

 

[tennmogger]

Well the swap worked fine. The new type speedometer can directly plug and play with the older type. Road speed reads correctly compared to GPS. WooHoo.

Picture of 'old type' is attached. There are lots of pictures of the new type with the programming DIP switches in other threads and in the manual.

Bonus question: Is an XXX year old TWINKIE still eatable? Found it in a drawer. If I never show up here again the answer is NO!