Found out how to test a Hall Effect sending unit like the one we have in our Humvees, like to the you tube video is below. When trying the tests the guy describes I found that our sensors do not have an AC output like he describes, ours is DC. My sending unit showed 2.9 volts while the truck was running and only on the female leg, the male leg showed only 0.08.
I also found that with the plug form the sender disconnected from the harness\diag plug my dam tach worked! Just for grins I shut the truck down, reconnected the plug and fired her back up and found that the tach did not work, just did the little jump thing when I turned the key. I then shut it down and disconnected the plug. Started it back up and the tach worked again! Will see if it now works consistently or not.
Since the tach doesn't seem to work when the plug is connected I went a head and checked the harness\diag plug side for voltage and found that on both legs with the engine on or off it had 3.9 volts. I'm wondering if there is a short or something else wrong in the harness which is screwing up the tach signal.
Hope this info helps someone with their mysterious tach issues.
If it starts to have issues again I think I will clean up the mount for the sensor since it must be using the connection to the motor for ground.
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https://youtu.be/N5khjYulTLQ