There has been a lot of discussion on both websites about fuel gauges reading an 1/8 or so low when the tank is full. Some Don Rush people measurements indicate that OEM sending units are 60-70 ohms full but aftermarket are 80 ohms. Since many sending units are now aftermarket, this higher resistance would lower the current and cause the gauge to read low. Someone suggested that a parallel resistance circuit added would be able to fix the problem. It would be great if one of the resistors in this parallel circuit were adjustable. I’m more mechanical than electrical but I know there are people with electrical design background that could create this parallel circuit and help a lot of us out. Any takers?
Maximum resistance is not at the full point but rater at the empty point. If you ground the feed wire at the tank (0 ohms), it will peg the meter. If you unplug it (infinite resistance) it will show empty.
Do not leave the gauge pegged it can burn up the internal winding. The resistance is non linear so 1/2 resistance doesn’t equal 1/2 full.
10ohms should be just above full.
12 ohms is full
15 ohms is about 7/8
26 is about 1/2
50 ohms is about 1/8
68 should be empty
73 is just below E
150 or more is pegged empty.
Individual gauges will vary.
What is happening is that the total resistance in the system is a little high. You could measure this at the gauge behind the dash and see what the gauge is being presented with. Cleaning up the all of the connections so that you have nice clean low resistance connections everywhere is a good start. In particular don’t forget the sender is grounded to tank through the flange and the tank is grounded to the body through the mounting bolts. A jumper wire from the sender to a good body ground will show you if the ground path is the problem.
The gauges are internally adjustable. This is not for the faint of heart and you can easily break the very small wires so take great care an eliminate the other easy stuff first. You have to remove the gauge from the dash and then remove the rear cover from the gauge. You will see a small lever that adjusts the tension on the clock spring that controls the needle position. You can move the lever and adjust the gauge reading.
Personally I think knowing when you are out of gas is more important than getting it to read full so that is the point that I would recommend getting right, and remember both points move at the same time and the action is non linear.
One can also adjust the output of the constant voltage regulator. On the case itself it usually a small post embedded in epoxy. Break the epoxy off and rotate the post (I forget which way), but it changes the stop point of one position of the bimetallic strip, and thus the cycle time between 12 and 0 V. When I do this, I ensure I have a 1/4 tank of gas and adjust the fuel gauge to read 1/4 full. The CVR output affects the oil pressure and water temperature gauges as well, so you’ll have to adjust your thinking to a new “set point” for normal operations.
That’s another good idea but I personally would not do it for any situation other than having just the fuel gauge relying on CVR voltage for its operation. It would be at the expense of other, properly functioning gauge systems for the sake of making up for a fuel sender that does not work properly.
Wow - These have been great discussions and information for my post on both websites. They should provide info for anyone with the problem to resolve it. The only thing I don’t think was mentioned directly: The fuel gauge could be put on its own adjustable voltage regulator and the rest of the gauges left on the existing regulator.
Increasing the voltage to the gauge will get the needle to the full mark, but at the expense of reading too high at the low end. I’d rather have more fuel in the tank than what the gauge shows than less! But if you want to go with the voltage-increase route to find a happy medium, I say increase the voltage from the CVR as was suggested above, but put a variable resistor on the plus side of each gauge. This will allow you to vary the voltage to each gauge independently. Bob can help you with the maths to figure out what size variable resistor to use. (I would just keep sticking different values in and see which ones worked and which ones smoked )