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My APU had the alternator die last week so got my new alternator and installed now it wont start.

come to find out the fuel shutoff solenoid is not receiving an open signal. as i can take the shutoff solenoid out and the apu starts right up and runs.

What is all required for the shutoff solenoid to get open voltage?

so far i have checked
wires on alternator - 50amp Maxi-Fuse is good.
where apu shutoff switch used to be is bypassed so not a bad shutoff switch.
I have 1 Volt on RPM sensor above flywheel not running.
all fuses on computer board are good and the 3 or 4 on wires we all good.
i have also hooked up the shutoff solenoid to 12vs and it actuates fine.

Im unsure how to check if RPM sensor when running what to look for.

what else tells the fuel shutoff valve to open.

any help would be greatly appreciated.

Is your electric fuel pump working?

yes fuel pump works as i said pull out fuel shutoff solenoid and it runs all day long.

Chexk the 40 amp fuse in the brain box. That operates the pull side of the coil.

unless i missed it but all fuses in brain box were good but ill happily look again as im definately not infailable.

rechecked 40amp is good.

Can you trace fuel shutoff solenoid wires back to logic board terminals? Could be damaged wires when new alternator installation.

I don't know Evolution but usually this logic designed as
Ignition ON - fuel pump ON, shutoff solenoid ON
START - energize starter
RUN - flywheel sensor signal pulses are present, oil pressure signal good, coolant temperature good, might be some more signals monitoring.
If flywheel pulses present and any one sensor(oil, coolant temperature) fail immediately shutoff solenoid OFF, fuel pump OFF.

thats some info i needed so when i left out the shutoff solenoid it never contracted when the apu went to start so...rpm sensor probably good....not receiving an on signal period would be my problem if the logic board runs that way. so i guess its a wire hunting we will go in that little tin can.