The Stovebolt Site Forums The Shop Area The Electrical Bay fuel guage voltage reducer

My 51 has been converted to 12v by the PO. The fuel guage has worked fine but to be safe I put a voltage reducer on it from Classic Parts. It read 3/4 full before the reducer was installed but now it only reads 1/2 full.I believe it is the 6v guage. I know it is about 3/4 full. (24 ohm at the sender} What might be wrong or is this drop normal?

Thanks, Howard

It's not normal. I wonder if the previous owner already put a 12v fuel gauge in and you are trying to reduce something that doesn't need reducing. Or, there is already a reducer in the 6v gauge line somewhere you haven't found yet. The ohm value is about right for 3/4 of a tank.

The Guage has two polls and returns to 0 with the key off so I believe it is original. The PO put a new decal over all the guages.
I have gone over every inch of the truck wiring due to poor wiring practices by the PO and found no other reducer. I checked the voltage in and out of the reducer. 14v in and 7v out.( my tester may be a little off but still it is half. I have run new dedicated grounds. The only thing that doesn't make sense is when I disconnect the sender wire the guage goes to E not to F. I have cleaned the ground contacts inside the cluster. The guage must be good being that it shows 3/4 on 12v. The ohm reading across the resister is about 45. Any other thoughts?

Thanks for your response.
Howard

Checked again. The guage DOES go to F when the sender is disconnected.
Howard

If I understand you correctly, it works fine without the reducer you just put in, but with the reducer in, it reads off right?

The 12v replacement gauges look exactly like the 6 volt ones. I am saying it's possible he put a 12v replacement gauge in which wouldn't be noticeable because they look the same. If he replaced decals, you REALLY wouldn't be able to tell. So, you are reducing a gauge that doesn't need reducing. That would be my guess.

If you put 12 volts on that gauge and it was not properly wired (or it was a 6v gauge), you would see a small puff of smoke and nothing would work. It is not possible to get ANY indication other than DEAD if you applied 12v to a 6v gauge without reducing. It's a 12v gauge?

http://www.classicparts.com/1949-53...2-Volt/productinfo/24-700D/#.VtsJKZwrKhc

Right. The needle has the hump in it also indicating an original guage. I bought the temp and oil guage as repros. They have a flat needle.
Although I agree with your thinking, all signs point to an original guage.Some posts from others on this subject say they have been running a 6v guage on 12v for years. Quite confusing.
Howard

I happen to know first hand from previous stupidity that it takes about 1 second to fry the 6 volt gauge. Maybe less. Its immediate and very painful. Sounds like you are good to go!

Try this and let me know what you get...
If you have a 6v gauge, there is about 1.6 volts between the center terminal on the sender and ground. If you have a 12v gauge, there is about 2.8-3.2 volts. (these are full tank numbers, so reduce them by however much is in your tank).

I have the reducer out and everything hooked back up. With the key on and the guage reading about 3/4 -full, the voltage reading between the sender terminal on the tank and the ground is 3.2v. If I have done this correctly you are saying it is a 12v guage? (tank is probably 3/4 to almost full)
My sender only has one terminal and the ground.
Howard

Yup. If it were a reduced gauge, the voltage would be half of that. We hear all the time about ohm values and that is a smarter way to check with ignition off since we do not want residual sparks near the tank, but what people forget is that those wires need a low voltage 'sent' through them in order for the sender to communicate with the gauge. It is why good grounds and good clean wiring is important.

Thanks for all your time and help. I did not know about the voltage variance. It seems pretty clear it is 12v. The PO said the "restoration" was done about 15 years ago. I guess if it were 6v it would have been toast long ago.
Howard

You cannot engineer a low fuel warning system without knowing everything there is to know about this system down to the millivolt (both DC and ripple). Happy to help!

I would remove the sender from the tank, hook up the wires, and move the float from top to bottom to verify if the gauge is reading correctly with or without the reducer.
Currently you are assuming that you actually have 3/4 of a tank of gas. That might not be the case.
Carl

I filled the tank a couple months ago and have only driven 20-30 miles so it is almost full. I checked the resistance again with a new battery in the tester and it reads 26 ohms so all that checks. I had the sender out a few months ago and it worked fine.
Thanks for your suggestion.
Howard