The Stovebolt Site Forums The Shop Area The Electrical Bay AD Truck Fuel Gauge Troublshooting ?

Hi Guys, I bought a 53 1/2 ton this last fall and have since totally rewired the truck using an American Autowire kit. Before I rewired the truck I could only get the gas gauge to read 1/3 of a tank when it was full. Since the rewire I cannot get it to read at all. Here are the trouble shooting steps I've gone through.
(1) Confirmed I have a good ground between tank sender and body of truck. (2) When I hook up an ohm meter to sender unit it reads 1 ohm at empty and 30 ohms at full. (3) I have good continuitee on the wire between the F side of the gauge and sender. (4) I have 12 volts at the + side of the gauge when the ignition switch is selected on. Every indication to me is that I need a new gauge but want to exhast all possibilities before I go and order one. Any suggestions?

oldkoot,

Great troubleshooting. My only comment would be, have you checked the gauge 'only' by turning the key ON and grounding, then removing the sending unit wire at the tank. With key ON and wire grounded to a known good ground, the gauge should swing to EMPTY. Pulling the wire from ground should bring the gauge to FULL.

Your voltage reading indicates you haven't opted for a voltage reducer before the fuel gauge, or you have purchased a gauge advertised as 12 volt compatible. The gauge itself must be grounded well in the gauge cluster. Fiddle around some more and see what develops.

Stuart

I don't remember so I will ask...was the truck 6v originally? If so is it the original tank and sending unit? If it is you will need a voltage reducer before the sending unit.

The sending unit is a variable ground. You do not need a reducer between the sender and the gauge. You will need to reduce the voltage to the gauge if it is a 6V gauge.

The sending unit in my 46 was 6v and would not work without a voltage reducer. I also know this was the same for others who were having the same problems and then fixed it by using a reducer. Maybe the sending unit in the later models is different.

My case was 12v gauges and truck upgraded to 12v.

zipper,

The man from Texas is agreeing with you...the voltage reducer goes ahead of the gauge. Your post might lead the unsuspecting to think it could work installed between gauge and sender.

Stuart

The gauge on my truck is a 12 volt unit. I'll have to break intothe bake of the gauge cluster to see if it is grounded.

Not if you have 12v gauges. I do have a voltage reducer between the gauges and sending unit. My whole truck is converted 12v with ezwire harness. The only 6v left is the heater and the sending unit. The gauge would not work without putting a voltage reducer (the small one from ACT) between the gauges and sending unit. Search back posts and you will find others that had the same issue and fixed the same way.

zipper,

This sure is a new one on me! I don't remember seeing previous posts mentioning this, but if you did it and it worked, then 'it works'!

Stuart

Maybe it was a fluke for me and a few others but it did work. The sending unit was still the 6v and new aftermarket gauges were 12v. Without a reducer the gauge would constantly read just over E no matter what. But once the reducer was put in place it works perfectly fine. I came to the conclusion that it was a voltage issue when trying to figure it out and tried a reducer.

Anyway. If that is not the proper way to get it to work and it is only masking an underlying problem then I am open to suggestions. But like I said it works well now.

Maybe zipper's sender is the "newer range" sender?

6v (and I think some 12v) are 0-30 ohms

later 12v are 0-90 ohms (I do not know when this changed)

Fuel gauges and senders are magic to me - I have followed trouble shooting instructions and have successfully traced several problems. The instructions in the Shop Manual have also been helpful.

OK...just to update on this subject. Someone here mentioned that the use of voltage reducers may make your gauge read full and empty but be very "off" for the rest. I can confirm this now that I have been driving for a little bit and find that the empty is a bit off as well. dont ask how I now know...

So I took out the original sender in my 46 tank ( I am not sure if it is original or its origins) and modified the one that came with my dolphin gauges so I would actually fit in the tank and the arm would work properly. I am now happy to say that I have an accurate working fuel gauge.

Some of you that understand the reducers and ohms etc may be able to get the gauges to read close enough for comfort but I prefer not to use the gas can again.

That's why Cletis said that a reducer between the gauge and the sending unit wouldn't work. It would reqire a complex logic circuit to make 30 Ohm stuff work accurately with 90 Ohm stuff on the other end.