After some searching I have determined that a 78 f20 4x4 sending unit should have a resistance value of about 10-70. Is this correct? I have a tank that has no reading at all. I pulled it for other reasons, but while it is out I want to fix the sender. The float was cracked so I thought I was good to go. When I put my digital multimeter on it I got a reading of 35-95. I have to think that it was just a float problem, but I do not want to fry the CVR or fuel gauge if it is not the correct value. Is it possible I need to do some conversion from the reading on the meter to the actual ohm value? LIke the analog stuff?

Thanks, Todd

 

Typically about 10 ohms for a full tank and 70 ohms for an empty tank is what you're looking for, so the information you've found seems reasonable. Typically the resistance will go up with age, which explains the shifted reading you're seeing. A sending unit that ranges between the numbers you have measured will not fry your fuel gauge, however it will read low. In other words, it may not reach Full, and will likely rest on Empty for a while before the fuel tank really is empty.

I don't understand your question about conversion. The sending unit is a rheostat which is a fancy name for variable resistor. Whatever resistance you see with your meter when you probe the two terminals is the resistance of the rheostat, assuming you're in the correct range if your multimeter does not auto-range. This resistance will change as you move the float up and down, because the movable part of the rheostat (called the wiper) is changing the effective resistance of the rheostat by varying how much of the rheostat coil is used. Does this answer your question? If not, please restate your question.

There are two things that commonly go wrong with sending units. First, the solder holding the brass float together cracks, which lets the float fill up with gas so that it sinks to the bottom. This results in a false Empty reading. The second problem is that the crossmember to which the sending unit grounds often shifts from the electrical system's true ground. This again results in a false Empty reading. The reason this happens is because the loss of ground is effectively an open circuit, which the fuel gauge interprets as a very large resistance. Current must flow through the rheostat inside the sending unit for the needle on the gauge to move. The more current flows, the farther toward Full the needle will go. This is why the lower of the two numbers is for Full. This is also why grounding out the sending unit lead will peg the needle past Full, because the gauge sees a short circuit.

Just an FYI to anyone who is interested, this principle is not true for newer trucks.

 

I guess my next question to you is this. Would you put it back in your rig and run it? You answered my question about the digital/analog deal. Autorange is the key and it sounds like mine does! And yes the ground on cross member needs to be reworked on mine as well!

Thanks

 

If I'm reading your first post correctly, you are stating the float is cracked. If that is the case then I would definitely replace it.

In case I didn't read that correctly and you're asking if those resistance numbers are acceptable, I'd tend to say no. With those numbers, your fuel gauge will probably only read about 3/4 of the way when it's full, and it will rest on E for a deceptively long time. However, fuel gauges are unreliable and inaccurate by nature. As such, I can't gaurantee that a new sending unit would be all that much more accurate. They're pricey, so it's something to put some thought into. Personally if it were my truck, since I already had the tank out, I'd want a new one just because there is likely to be at least some improvement in accuracy. If you do go this route, buy a high quality replacement, not a cheap knockoff.

A final item to consider is the condition of the "sock" at the end of the sending unit. If it's clogged, varnished or very dirty, it should be serviced. I believe you can replace the sock separately without replacing the entire sending unit, but a new sending unit would come with a new sock too.

 

10-70 ohms is the factory specs. Any other will give false readings.

I was happy when both of my original units still read 10-70 ohms.

When you put the tank back in and hook the unit up; turn the ignition on and see where the gauge rests. This will tell you the position when the tank is really empty. Then put 5 gallons of gas in. Then turn it on again and see where the gauge rests. This will tell you you have five gallons left of gas. After arming yourself with that information, you will know for sure how much gas is left!

 

COAZ9202B .. Fuel Tank Sending Unit Float / Available from Ford

D1AZ9A011A .. Fuel Tank Sending Unit Plastic Meshed Filter Screen aka sock / Available from Ford.

COAF9276A .. Fuel Tank Sending Unit Gasket / Available from Ford.

The fuel sending units themselves are all obsolete.

 

No kidding. I was disappointed when nobody had the midship tank sending unit left for my ol' brownie.