gas gauge calibration

I bought a universal sunpro gas gauge for the Jalopy which has a rear mounted 15 gal gas tank. When the gauge reads 3/4, the tank is empty. Is there anyway I can calibrate the gauge? I don't have 100% faith in the sending unit either. I have another sending unit that I could try hooking up to it and see what it does.

 

M38.. Not knowing a whole bunch on this vehicle but was it 24V? Is the sender original? Did the sunpro come with any resistance values for the sender?

 

No, the tank and sender are from a CJ5. It just has an M38 body on a 46 2A chassis. Everything is 12V.

 

I carry half a yardstick to accurately measure my fuel. That an a 5 gallon Jerry can as a reserve tan,.

 

Does the sending unit have full range of motion? Do the resistances(ohms) of each unit match up?

 

I don't have a sunpro sender to take the resistance readings from to compare with the Stewart Warner. However the Sunpro manufacturer states their gauges work only with their senders which is ordered separately.

 

Have you checked out Vern's writings on this subject?

http://www.vernco.com/sparks/id697.htm

 

Well, that's dumb. If it is a universal gauge, then shouldn't it work with all senders?

Yes, it has full range of motion. I don't know how to check the ohm readings though.

 

Not necessarily. Universal means will fit just about anything, doesn't mean it has to work with everything.

 

You state that the gauge reads 3/4 full when the tank is empty. So will the gauge read empty if the tank is full? If so then reversing the swing on the float arm may solve the problem. Getting full and accurate adjustment may be a problem tho.

 

When the tank is full, the needle is way past the full line.

 

If you want to tinker with this, I would get a small variable resistor from like Radio Shack and put it in series with the sender wire. Cut the sender wire and connect the resister between the two wires. The resistor value (I am not sure of) from 0 to 100 ohms may have enough resistance to work. The wattage can be in the neighborhood of 1/4 watt. Not sure of price but should be relatively cheap.

 

As I recall, there are two main types of fuel gauges. The GM-type gauges expect a resistance from 0 to 90 ohms from the sending unit. The Ford/Stewart-Warner gauges expect 8-12 ohms to 73 ohms. If you use the Ford gauges with a GM sender, or vice versa, the gauge will move but it won't be accurate.

The gauge you bought will indicate what resistance it expects from the sender. Use your meter and measure the range of the sender. Compare that to the specification for the gauge.

If you look at the Summit web site, it lists the following types of gauges
* 240 ohms empty, 33 ohms full
* 0 ohms empty, 90 ohms full
* 73 ohms empty, 10 ohms full
* 0 ohms empty, 30 ohms full
* Programmable
* 16 ohms empty, 158 ohms full
* 10 ohms empty, 180 ohms full
* 0 to 280 ohms adjustable
* 65 ohms empty, 10 ohms full

None of these will be accurate with the wrong sender.

Your meter is your friend. Measure the range of the sender and buy a gauge to match. Or buy a sender to match the gauge.

 

Right now the tank is pretty close to full. I'll see what the resistance is later on today.

 

Sounds like the same problem I had, Postal Jeep tank & sender with Willys replacement gauge were a mix and different OHMS. Matched a gauge to the 8-73 ohm sender and all is well. Vernco does have a good report on this as well.

 

Measure the resistance from the tank sender to ground. Siphon the tank dry (put the gas in another car, or in cans if you have enough). Measure it again. Then fill it up and check your first measurement.

 

Or drive it until it runs out.

 

I kinda like that idea, Is there more resistance when the sender reads full or empty? IIRC, the resister on the sender was wider at the full position and narrower at the empty position. I guess I'll play around with my other sender. Thinking out loud.

 

A simple answer to that question. If grounding the sender wire (zero resistance) makes the gauge go full scale then adding resistance brings it down, right.

 

Correct. I checked my spare sender. So, I'm going to try adding a variable resister to the circuit like you suggested earlier.