Yet Another fuel gauge question

[daddyo23]

I know this may be getting old, but, it's keeping me up at night (morning). I've had trouble getting an accurate reading from my gauge. I bought a new sender, ran new wires and nothing seemed to work. This morning after work I went to the local Old Plymouth guy to see what we could figure out. We found that both wires going to the sender had power. That does not seem right. Being a thermostatic (two wire) sender and gauge, could the blades in the gauge be shorting out, causing both wires to have power? Should I try a new gauge? Any help will be avidly read and appreciated. I have to try to go to bed now and forget about this for awhile. Las thing I need is a nightmare of a huge fuel sender chasing me down the street:eek: Thanks, Wayne P.

[Young Ed]

Wayne I'm thinking the both having power thing sounds right. Hopefully someone else will chime in with a more scientific answer. If you need a gauge I've got a few spares.

[Norm's Coupe]

I believe the way it's supposed to work is. One wire goes from the gauge sending unit to power it. Then the gauge activates, sending power back to the gauge in the dash through the second wire. The ground is supposed to come from the tank bolted to the car, and also the gauge is grounded by mounting it to the dash. The car body being the ground for both. So.........try running a jumper wire from a good clean source on the frame or bumper to the sender to see if that helps. If not, look at the connections at the dash gauge. Those connections are supposed to have a star washer under them to provide for a better ground.

In addition, make sure you have the wires connected from the dash to the sender on the right post in the sender. If you reverse those wires you will not get a good reading either.

[TodFitch]

I believe the way it's supposed to work is. One wire goes from the gauge sending unit to power it. Then the gauge activates' date=' sending power back to the gauge in the dash through the second wire. The ground is supposed to come from the tank bolted to the car, and also the gauge is grounded by mounting it to the dash. The car body being the ground for both. So.........try running a jumper wire from a good clean source on the frame or bumper to the sender to see if that helps. If not, look at the connections at the dash gauge. Those connections are supposed to have a star washer under them to provide for a better ground.

In addition, make sure you have the wires connected from the dash to the sender on the right post in the sender. If you reverse those wires you will not get a good reading either.[/quote']

I don't have this style gauge on my car so my information is basically from reading the manuals...

It is my understanding that power comes from the dash unit over both wires. In the sending unit the two wires are connected through a resister that has a float driven wiper to ground. The position of the float/wiper sets the relative current between the two wires to ground. In the dash unit the two wires work heating elements on bi-metal levers the relative strength of which set the position of the needle.

So basically current though one wire tries to pull the needle to empty, current through the other tries to pull the needle to full. No ground means it is totally confused. A bad wire means it is only working on half the information it needs.

[Norm's Coupe]

Like I said, I wasn't sure how the power flowed in both wires. But........I do know two things. If you hook the wires up backwards at the sender, the sender will not read correctly. Also, as Tod and I both mentioned. Having a good ground is also important for it to work at all.

[alan32433]

If the ground is not the issue, you probably need to calibrate your gauge, the part in the dash. In saying this, I assume your gauge is similar (most likely is) to my 48 DeSoto. I was aware the gauge had to be calibrated so I did mine while the gauge cluster was out. I will try and explain how I did mine.

You need to measure the depth of your tank, and set your sending unit up on your work bench. I made a jig that limited the float movement to duplicate full up and full down just like if it were installed in the tank. Next step is to build jumper wires with alligator clips or terminals to attach the gauge to the sending unit and your 6V battery. Use your car wiring schematic to duplicate the circuit.

The gauge has two resistor coils wrapped around a thermostatic spring which can be viewed when the gauge is out of the cluster. The sending unit provides varying amounts of current to either of the resistor coils (wrapped around the thermostatic spring) depending on the float position which of course is related to fuel level. Your gauge then responds to these currents by moving the fuel needle. To calibrate your gauge, you will actually move the thermostatic spring and resistor coils (as a unit). You don't bend them, they are actually hinged to facilitate adjustment. Be very careful in moving them.

Start with your float all the way down simulating an empty tank. Give the gauge time to react to the heat generated upon the thermostatic spring. It's designed this way to prevent rapid movement of the gauge needle when fuel sloshes around in the tank. Once you have given it time to heat, gently slide the hinged spring and coil to achieve an empty reading on the gauge and do it slowly, again to allow time for the circuit to heat.

Now raise the float to simulate a full fuel tank. Allow time for the springs to react to the heat, then adjust the other hinged spring to achieve a full reading.

Now for the fun part, each spring adjustment will have some effect on the opposite spring. Moving one will alter the adjustment of the other. It will take several adjustments at the empty level followed by adjustments to the full level before you are fully calibrated.

In the end, setting the float position to mid level will show half a tank and so forth and should be linear from empty to full just like you want.

When you reinstall the gauge in the cluster, be very carefull. It's easy to knock your adjustment out of calibration again.

One of the most important steps is to set your sending unit up to exactly simulate the movement it will experience when installed in the tank.

Last thing, for your battery, use your auto battery. Don't use the small 6V lantern battery or a battery charger. The adjustments require a very stable voltage source, otherwise your adjustments will be in error when you reinstall in the car.

Hope this helps.

[daddyo23]

Wayne I'm thinking the both having power thing sounds right. Hopefully someone else will chime in with a more scientific answer. If you need a gauge I've got a few spares.

Thanks for all the advice. I am going to try a better ground tomorrow. Ed, I'll let you know before the meeting if I need a gauge. Thanks again.

[TodFitch]

Great post!

I'd like to have this in the repair section on my web site http://www.ply33.com/Repair/ would you mind? Would you like to touch it up for use as a stand alone article without the context of being in this thread?

Thanks!

If the ground is not the issue, you probably need to calibrate your gauge, the part in the dash. In saying this, I assume your gauge is similar (most likely is) to my 48 DeSoto. I was aware the gauge had to be calibrated so I did mine while the gauge cluster was out. I will try and explain how I did mine.

You need to measure the depth of your tank, and set your sending unit up on your work bench. I made a jig that limited the float movement to duplicate full up and full down just like if it were installed in the tank. Next step is to build jumper wires with alligator clips or terminals to attach the gauge to the sending unit and your 6V battery. Use your car wiring schematic to duplicate the circuit.

The gauge has two resistor coils wrapped around a thermostatic spring which can be viewed when the gauge is out of the cluster. The sending unit provides varying amounts of current to either of the resistor coils (wrapped around the thermostatic spring) depending on the float position which of course is related to fuel level. Your gauge then responds to these currents by moving the fuel needle. To calibrate your gauge, you will actually move the thermostatic spring and resistor coils (as a unit). You don't bend them, they are actually hinged to facilitate adjustment. Be very careful in moving them.

Start with your float all the way down simulating an empty tank. Give the gauge time to react to the heat generated upon the thermostatic spring. It's designed this way to prevent rapid movement of the gauge needle when fuel sloshes around in the tank. Once you have given it time to heat, gently slide the hinged spring and coil to achieve an empty reading on the gauge and do it slowly, again to allow time for the circuit to heat.

Now raise the float to simulate a full fuel tank. Allow time for the springs to react to the heat, then adjust the other hinged spring to achieve a full reading.

Now for the fun part, each spring adjustment will have some effect on the opposite spring. Moving one will alter the adjustment of the other. It will take several adjustments at the empty level followed by adjustments to the full level before you are fully calibrated.

In the end, setting the float position to mid level will show half a tank and so forth and should be linear from empty to full just like you want.

When you reinstall the gauge in the cluster, be very carefull. It's easy to knock your adjustment out of calibration again.

One of the most important steps is to set your sending unit up to exactly simulate the movement it will experience when installed in the tank.

Last thing, for your battery, use your auto battery. Don't use the small 6V lantern battery or a battery charger. The adjustments require a very stable voltage source, otherwise your adjustments will be in error when you reinstall in the car.

Hope this helps.

[alan32433]

Great post!

I'd like to have this in the repair section on my web site http://www.ply33.com/Repair/ would you mind? Would you like to touch it up for use as a stand alone article without the context of being in this thread?

Thanks!

Tod, you are welcome to use the post on your web site. Feel free to add to it if you need to. I have an extra fuel gauge I can take pictures of. It might help people if the adjustment points are highlighted in the pictures.

Alan

[daddyo23]

Like I said' date=' I wasn't sure how the power flowed in both wires. But........I do know two things. If you hook the wires up backwards at the sender, the sender will not read correctly. Also, as Tod and I both mentioned. Having a good ground is also important for it to work at all.[/quote']

You pegged it, Norm. I had a ground on there but it wasn't very good. I ran a new ground to the frame and the gauge is reading now. It may be reading a bit high, but, we'll find out on the next trip. Thanks for the help and the good info on calibrating the gauge, Alan.

[Norm's Coupe]

Glad you got it working Wayne. I think that's the problem the great majority of people have with their fuel gauge. Most of the time it's just a poor ground. Those who have not cleaned and painted their cars have rust and corrosion that causes the problem. Then those who have painted their cars don't get a good ground because of the paint at the connection breaking a good ground. You need good clean bare metal for the best ground.