Aftermarket Gauges Not Accurate

[Clunker]

I recently bought a 1970 F250, and one of the previous owners installed aftermarket gauges that don't seem to be very accurate. The speedometer seems about right, and the oil pressure looks good, but the fuel gauge shows barely above 1/2 when the tank is full, and the voltage gauge only shows 11V while the engine is running when my meter shows 14V. Are these gauges just junk? Is it possible that they just have a different impedance than the stock gauges? I would think that the fuel gauge would either read correctly or not work at all if the tank sending unit is bad, but the voltage gauge should definitely work regardless of which vehicle it's installed in. Any suggestions would be greatly appreciated.

[Calfdemon]

I dont know about the voltage gauge, but it is possible that he bought the wrong fuel gauge. The proper fuel gauge for our trucks need to a resistance range of 8-12 ohms full and 70-73 ohms empty. It is possible that he installed a fuel gauge with a different resistance rating and did not know any better.

[flyboy2610]

Assuming he used the original fuel gauge sending unit in the truck, it could be the sending unit.
Here's how to clean it up and test it:
http://www.fordification.com/tech/fuel- ... -units.htm

[Clunker]

Thanks for the info. I'll check the sending unit first. It looks like it has never been replaced and could probably use a cleaning even if it's still working. Unfortunately, if it is an impedance problem with the fuel gauge, then I will probably have to replace it. I could always add a resistor if it reads too high, but it's reading too low.

As a side note, I noticed that one of the previous owners installed an electrical toggle to switch between the two gas tanks even though the truck still has the valve by the driver's door. Is this toggle stock or aftermarket?

[Calfdemon]

On trucks with 2 tanks, there were 2 switches that were stock. The valve switch on the floor by the drivers seat switches the actual gas flow to the carb from one tank to the other. The 2nd switch is a toggle on a metal tab under the dash on the ignition side (stock location). This toggle just switches between the sending units so that the gas gauge will read whichever tank you are on. So when switching gas tanks, you need to turn the floor valve to switch the flow, and then hit the dash toggle so the gas gauge is reading the right tank.

If he added anything other than those 2, it is aftermarket.

[colnago]

What flyboy said. Take out the fuel sending unit (easy to do), and take an ohm-meter to it. Then check it against the link that flyboy gave. The arm might need some bending, too. I have some generic aftermarket, so it needed some tweaking before its range was semi-accurate. It also has some dead spots in it, so some days it works, and some days it doesn't ...

Joseph

[Clunker]

Thanks for the info guys. I pulled the sending unit today and tested it. 8.5 ohms full and about 79 ohms empty. That says to me that the sending unit is working properly. I checked the tank level with a dip stick and it was about 3/4 full, but the gauge is reading 1/4 tank. The voltage to the sending unit was 4.8V, so I think that all the wiring up to the sending unit is in order. I'm wondering if the aftermarket gauge is designed for a 12V system instead of 5V used in the old Fords. I found the original receipt and instructions for the gauge and it is designed to read full at 10 ohms and empty at 73 ohms, but it doesn't say what the voltage should be. Any thoughts?

[Clunker]

***Update***

Bad News! After a successful rewiring of all of my gauges and getting a full 12 volts to all of them, and after tracking down the impedance issue with the sending unit wiring, my fuel gauge still is reading half of what it should be. It is the AutoMeter Sport-Comp model 3515, which was designed for the uneven impedance of the stock sending unit. I have the stock sending unit, the correct gauge, the correct voltage, and correct impedance reading AT THE GAUGE for the level of fuel in the tank (about 3/4 tank reading about 17 ohms). If the gauge is getting exactly all the right electrical parameters, can it possibly be anything else other than a faulty gauge? I'm at my wit's end, so any help would be greatly appreciated.

[Calfdemon]

And you are sure that the float on your sending unit is good? If it even has a slight hole and can fill with gas, it will not float all the way and you will never show full.

If the float is good, then you are at the point where you might look to replace your sending unit first (cheaper I believe) and then if that does not fix it, you might replace your fuel gauge. It sounds like you have done everything to ensure that it is hooked up properly and getting the signals it needs. So something is just not working properly and it is not a hook up issue.

My first step would be to replace the float in your sending unit, then the sending unit itself, and lastly the gauge if you cannot get the problem fixed.

[Clunker]

Thanks cafldemon. I thought about the float too, but if the float was bad, then I wouldn't be getting the correct impedance for the amount of fuel in the tank. If all I did was bypassed the sending unit completely and just put a resistor between the ground and receiving terminals on the back of the gauge rated at 17 ohms, the gauge should tell me that the tank is about 3/4-full. Actually, that might not be a bad test.

[Clunker]

***Update***

Found the problem. Sometime between my rewiring last weekend and testing it again today, the original factory toggle switch decided to bite the dust. Replaced the switch and the fuel gauge went up to above 1/2-tank. I'm not sure why I was getting 17 ohms with the power off, and then getting something completely different with the power on, but replacing the switch seems to have fixed the problem. Now all I have to do is adjust the sending unit arm.