Alternator driven tachometer mystery problem

[erasedhammer]

My tachometer (style that mounts next to the w/s wiper motor) was working after my 200amp upgrade and after my A2 engine harness upgrade, but now is not working after my A2 body harness upgrade.

A/c ring terminal is hooked up to the tachometer a/c tap. The other cable is plugged into the original harness A/c wire.
Power wire for the tach is plugged into the 24v cable for the fuel gauge. The other wire on the tach harness is plugged into the fuel gauge itself.
Tach grounding wire is secured down onto the fuel gauge grounding stud with the fuel gauge ground wire.

There is 24v power on the end of the original fuel gauge power wire, and the fuel gauge works.

I check voltage accross pins A and C as well as A and D on the tachometer plug. When the truck is running I get 12v and 5v for A & C, A &D.
All the other pins show no voltage when compared with pin A.

It's not the tach gauge itself cause I plugged in another one I had laying around and it didn't work either.

It used to work... What am I missing?
Only things that have changed is the body harness and the three wires that plug into the dash. Rest is constant

 

[doghead]

Looks like you had issues before.

http://www.steelsoldiers.com/showth...tor-driven-tachometer-crapping-out&highlight=

http://www.steelsoldiers.com/showthread.php?156888-Tach-not-Working&highlight=

http://www.steelsoldiers.com/showthread.php?156744-Humvee-tachometer-wiring&highlight=

 

[erasedhammer]

Looks like you had issues before.

http://www.steelsoldiers.com/showth...tor-driven-tachometer-crapping-out&highlight=

http://www.steelsoldiers.com/showthread.php?156888-Tach-not-Working&highlight=

http://www.steelsoldiers.com/showthread.php?156744-Humvee-tachometer-wiring&highlight=

Yep I've had my share of problems with the thing, but it did work (for the most part) before my body harness upgrade. I'm just looking for some input on what the possible culprits are.

 

[erasedhammer]

Problem is still persistent

I have the 24 power hooked into the fuel gauge.
I have tried hooking the ground wire up to every possible ground stud I could find on the dash and nothing works.
This problem doesnt make any sense as there was simply a body harness change and nothing else then it stopped working.

Help

 

[BLK HMMWV]

Have you tried the technical manual?

 

[erasedhammer]

Have you tried the technical manual?

I've checked the wire diagram that came with the tach kit, but other than that I didn't really find any thing in the TMs

 

[erasedhammer]

Okay so it's fixed. But there was some serious black magic going on in there.

I measured 24v coming from the input power wire that originally goes into the fuel gauge. Then measured the +24v pin on the circular connector that plugs into the tach, and amazing it was 2v!!! I had to check twice on that one, couldn't imagine how 24v was turning into 2v through one single wire.

I then unplugged the wires from the fuel gauge wire and plugged them into the voltmeter gauge wires.
Measured voltage at the circular connector that plugs into the tachometer and boom, 24v. Plugged the tach back in and everything works.

For the life of me I cannot imagine how 24v gets turned into 2 volts through the harness...

 

[Wire Fox]

Bad connection = high resistance = loss of power. You're going to want to see if the wires are corroding at those connectors, a wire is cracked and operating intermittently, or the connector itself is corroded or gunked.

 

[erasedhammer]

Bad connection = high resistance = loss of power. You're going to want to see if the wires are corroding at those connectors, a wire is cracked and operating intermittently, or the connector itself is corroded or gunked.

I'm guessing that the drop in voltage was due to my fuel gauge problem, it was reading half when the tank was full. Just replaced it a couple minutes ago and now it all works fine.