The Stovebolt Site Forums The Shop Area The Electrical Bay Fuel Gauge Help

Trying to figure out why my 54 Chevy truck original
in-dash fuel gauge does not work. Needle sits on E and never moves. Truck was converted to 12V prior to my ownership and the previous owner has an after market fuel gauge attached under the dash, which works fine.
Also the truck appears to have the original sending unit, but not positive. It reads 140 OHMS at 1/4 full. I purchased an after market 30 OHM sending unit that I bench tested and it works fine.
I tried connecting the current in tank sending unit to the in-dash fuel gauge, using a Runtz 12V to 6V reducer, and still needle sat on E. Tried same with the new 30 OHM sending unit and nothing still. At this point I figured someone had possibly hooked up 12V to the in-dash gauge without a reducer and toasted the gauge. I pulled the in-dash gauge and inspected it and it looks good but hard to tell. Nothing looks burned and all the little wires are intact. Is there a way to bench test the gauge out of the dash?
Thanks,
Jim

Sure.

How to Bench Test Your Fuel Gauge System (47-55 AD)

1) A 30 ohms Sending Unit is correct for this system. Attach a 12v power supply to your 12volt to 6volt converter. Runtz or a 6.8 volt Zener Diode are fine.

2) Connect your Ground wire to the outside of the gauge cluster.

RESULT: Indication of FULL TANK

3) Connect the terminal post of the Sender to the Sending Unit terminal on the gauge.

4) Connect a ground wire from the Sender to the Gauge Cluster.

RESULT: Indication Depending on where the Float is. All the way up is full/30 ohms/1.6 volts. All the way down is empty/0 Ohms/0 Volts.

Lots more info here near the bottom: http://devestechnet.com/Home/Native12VUpgrade

Most people get hung up because they do not make sure they have good grounds all around. A good idea if you decide to get it working and the gauge is bad is to get the NATIVE 12 volt replacement available from the vendors. That way no reduction is necessary. They are about $60.

Someone before you may have killed the original gauge with 12 volts, so rather than fix it they added the under dash gauge...if truck could talk!!!

Mike B

Thanks, I'll try the bench test tomorrow and let you know what happens.
Jim

Did DADS50 post a link to nice video of a "bench test"?

When you got the 140 ohm reading did you disconnect the sender wire at the tank to take the reading at the sender?
...test...


...bench test...

...livingroom test...

Tested the gauge as Deve suggested and got no movement on the needle. Still sat at E. Tested my in-tank sender as DADS50 suggested and it reads 140 Ohms (st the sender and disconnected)
at 1/4 full. Seems odd since the aftermarket gauge under the dash still works but now I'm not sure it is very accurate. Bottom line it appears the gauge is most likely shot, but not too sure what is going on with the sender. Thanks everyone for the quick responses and suggestions.
Jim

PO likely replaced the sender to match the under-dash gauge - you really need to check the sender out of the tank

Bill

Dads50 makes a good point about removing the terminal wire from the sending unit before testing its Ohm value. Like Bill states, it is entirely possible for the aftermarket gauge to require a much higher ohm sender. Chances are, like Mike said, someone probably smoked the coil putting 12v on it. It only smokes for a second and when it's over, you cannot tell from appearance that it's toast. Don't ask me how I know. To eliminate this issue forever, just get one like this:

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

That one has a white needle, there are red needle ones available to. Point being it's 12 volts natively with no runtz or diode required. Available from all of the truck vendors, so shop around.

Stopped by my local speedometer repair shop and they fixed and calibrated the fuel gauge free of charge. The small wire leading from the power side winding to the post terminal had either broken or fried apart. I missed it when I was looking at it. I'll test tomorrow and post the the results.
Jim

Gauge back in along with new 30 Ohm sending unit. Works great! Thanks again everyone for your input.
Jim

That's a case of beer for the guys at the speedometer shop! Congratulations on getting it sorted out.

Congratulations

What is the name of the speedometer shop - give them credit here.

Yeah, I don't have a speedometer shop within 1000 miles (probably) so they may get some business that way.

Chuck at North Hollywood Speedometer in Los Angeles gets the credit.
Jim

so what happens to the 6v fuel guage when you put 12V on it ?? mine sets on e until i turn the switch on then it pops all the way to full.

Most people report that a 6v gauge gets "fried" if/when 12v is run through it.

Some people have reported that nothing happened (a minority report).

Chances are you have some sort of reducer on the gauge if it doesn't smoke right away. The operation you mention is the way a proper gauge works until you connect the sender.

That symptom indicates improperly grounded sending unit (unless, of course, your tank is actually full). Attach a temporary jumper wire from the top of the sending unit and the battery ground post. The gauge should then read what is in the tank.
Carl