12 volt fuel gauge issues

[wc56daveyboy]

I know the fuel gauge system electrics have been discussed in detail many times on other areas of this site, but my issue I cannot find in searches

First off, I have a 43 command car with stock 12 volt system. Trying to get a working fuel guage on this truck

Instead of trying to Mickey Mouse around with the
Existing unknown condition gauge and sender, I simply bought a new set from a supplier.

Supposed to be the cats meow of 12 volt gauges he writes!

So I purchased the set, 12 volt specific gauge and new sending unit.

I have power on the wire from the battery (ignition) meaning power to the IGN terminal on the gauge.
I checked to be sure that the dedicated wire from
the GA terminal on the gauge to the sending unit was intact using an ohm meter.

I’ve sanded down the area around the gauge opening in the dash to be clean metal

So I’ve done all the right stuff, correct?

I even have a separate ground wire from the sender unit body (screw) to the chassis

Here’s what’s happening:

I turn on the ignition switch, nothing happens on the gauge. No movement.

I turn off the ignition switch and I get a little “jump” at the needle on the gauge.

So what am I seeing here?

A bad ground?
Did I mix up wires?

What does that little “jump” when turning off the switch indicate?

[Ron D]

I don't know about Command Cars or what gauge and sender you have, but I also have an aftermarket 12V gauge and sender on my M38. I also run ground wires to the sender and gauge bodies. It still doesn't read consistently (a little erratic, but when it does read it's accurate to how much gas is in the tank) --- so I'm still chasing a poor connection somewhere or crapola aftermarket junk parts (still sorting it out).

But one thing I did learn that will check only the gauge function, to see that it'll do a full needle sweep, is to touch a grounded wire to the sender wire connection on the tank. The needle goes instantly and steady to full every time, which indicates to me there's nothing wrong with my gauge.
Which then leads me to suspect a weak or funky sliding contact switch on the sender arm (inside the tank) is my problem...

Hope this might help. Good luck!

[Ron D]

I guess I should add that my aftermarket gauge and sender are so aftermarket that I can't get or find any specs on it's sender resistance. I'm guessing it's what seems to be the more common 30-ohm setup, where the sliding resistor in the tank is at one end of its travel (30 ohms) and reading empty, and at the other end of its travel its 0 ohms (ground) and reading full.

I've toy'ed with the idea of going to Radio Shack and buying 10, 20, and 30 ohm resistors to selectively insert into the sender wire line to see if the gauge reads 2/3rds full, 1/3rds full, and empty, respectively. If it did, steady and without any erratic bouncing, I believe it would prove the defective sender. At least then I'd know and could take my chances buying another aftermarket POS. Resistors used to be 10 cents each.

There are other resistance setups for gauges and senders, maybe you know or can find out yours. Good luck!

P.S. I think the little "jump" you see on the needle when turning the ignition switch on-off is normal.

[YLG80]

Hi Davey,
I have no WC56 but I guess that the fuel gauge system is very similar to the jeep one.

Usually when you have no reading on the gauge, that means that there is a grounding defect on the sender side.
Here is a little wiring diagram of the gauge together with the sender.

You should use a multimeter in ohmmeter to measure the resistance to ground on both sides and clean / remove paint and rust from the body on both side. The resistance should be nearly zero ohm.

Good luck.
Yves

[wc56daveyboy]

Thanks guys. I’m glad you posted. Reminds me to post my update.

Today got it working! Yipeee!

Actually, it took most of yesterday and
part of today to get this figured out.

Becuse I’m no electrician, the process of elimination was used. Ya know, replace one wire with another, power switched to sender lead, etc

After hanging upside down under the dash switching out wires, as well as crawling around from front to back, I decided to bench test the pair.

Cannot get simpler than that.

After a bench setup proved that the pair was fine and it all worked great, time to put the setup in The truck.

In a nut shell, two things:

1- Found out the new harness had a short somewhere between the firewall junction block and the tank. That is the wire from sender to gauge (GA) connection.
That caused me hours of grief!!

It was perfectly fine from
Junction block back to the gauge though

I wasn’t tearing the entire harness apart to single out that one wire, so simply bypassed it with a strand from the junction block back to the tank
Mounted sender.

2- I needed a dedicated ground from the sender to chassis

3- needed a dedicated ground wire from back of gauge to chassis.

In the case of #2 and #3, BOTH were required. Eliminating either one caused the system to not work at all

So.. that’s my update!!

I’m seeing that a ground to chassis was required on both items and I learned, never trust any of your wires no matter how new!

[wc56daveyboy]

I know I wrote this:

I checked to be sure that the dedicated wire from
the GA terminal on the gauge to the sending unit was intact using an ohm meter.

Now that’s where it was very weird, right? Caused me to chase my tail for a long while.

All i can think of is I read the ohm
Meter wrong or it was an intermittent grounding in that wire

[YLG80]

I know I wrote this:

I checked to be sure that the dedicated wire from
the GA terminal on the gauge to the sending unit was intact using an ohm meter.

Now that’s where it was very weird, right? Caused me to chase my tail for a long while.

All i can think of is I read the ohm
Meter wrong or it was an intermittent grounding in that wire

Congratulations, you have fixed it!
Very good year 2022 start!

And do not forget to switch your multimeter back to volts to avoid problems when you will use it later to read volts.
Yves

[wc56daveyboy]

Yes good feeling and a great 2022 start!

Now I have a working guage.. but how accurate!!!

Instead of worrying if I run out of fuel becuse I dont know the amount in the tank, I can worry about running out of fuel becuse the gauge sender is not telling the truth!!


Ha!!

[Ron D]

Instead of trying to Mickey Mouse around with the existing unknown condition gauge and sender, I simply bought a new set from a supplier.
Supposed to be the cats meow of 12 volt gauges he writes!
So I purchased the set, 12 volt specific gauge and new sending unit.

Can you tell us where you bought the cats meow set?
I might need to see if they have one for the M38.
Thanks!

[wc56daveyboy]

I got the set from brent mullins.

He sells the so called “best of the best” repro guage out there. Claims he uses it in his own jeep.
Is it perfect? Does it look stock? Not really but close enough for me.

I’ve seen the sane gauge sold by Dan Sharon jeep in Arkansas too, but apparently he is out of the gauge.

I also bought the sending unit from Brent for a Willy’s jeep to go with it.

I know, it’s a Willy’s jeep sender and I’m using it in a dodge 3/4 ton tank

No biggee..

The 5 mounting holes are universal

I needed to “modify” the arm slightly by hand bending it to get a more accurate reading

But it works.

I actually installed one of these jeep sending units on my dodge wc12 years ago and it is still going strong

[Ron D]

Thank you!