M1078 A1 tachometer diagram

[Xengineguy]

Did some more experimenting today, tapped in to the crank sensor wiring . Did get a frequency reading, however the tach didn’t like it. Went back to the ac on the alternator. Works as long as the dash light are on. Will check out the ecu pins when I get more time.
Might be the best way.

 

[coachgeo]

Did some more experimenting today, tapped in to the crank sensor wiring . Did get a frequency reading, however the tach didn’t like it. Went back to the ac on the alternator. Works as long as the dash light are on. Will check out the ecu pins when I get more time.
Might be the best way.

maybe a capacitor(?) smooth out the signal?

 

[GeneralDisorder]

maybe a capacitor(?) smooth out the signal?

A capacitor would tend to delete the signal by smoothing the AC into closer to a DC signal. Fundamentally the AC signal from the alternator VR is not designed to be a tach signal and the fact that it sort-of works is basically a coincidence - it's designed to send alternator RPM To the LBCD and to be sent/received by Niehoff equipment to matching Niehoff equipment - not a tach gauge.

As much of a pain as it will be, the ECM produces a tach signal for just this purpose and it's output will be perfect and uniform. It requires only to pin a few ECM pins and run a few wires and you have the actual correct data.

 

[Ronmar]

What General said, wouldn't really effect freq, just suppress/average the signal amp;itude(turn AC into DC). Way back when, it was common to pull the tachometer signal from the alternator, but those signal points were designed for that. As mentioned, Neihoff intended this output for a specific purpose, so i think it is just luck that it worked at all

i am not sure what bell housing was used to interface the 3126 to the Allison, i wonder if it has the port for a tach sensor counting ring gear teeth like on the A0/3116 trucks?