Fuel guage problem

I hav e a 1992 Jersey boat with twin 6V53 Detroits and have been experiencing a fuel guage problem lately. I have separate fuel tanks for each engine, but only one fuel guage. A toggle switch flips to the port to read port tank and to starboard to read starboard tank. As of now the guage just twitches a fraction from empty on either position of the toggle. I replaced the toggle switch and that was not it. I checked voltage at the guage and do have about 6- 8 volts there. I was not going to put 12 volts to the guage as it probably should not have 12 volts to it. What I can not comprehend was when I last filled up both tanks the guage read fine for both tanks.
Ran the boat a few days ago and the guage just is twitching agin by the empty mark. I know there has to be over 300 gallons in the boat but it will not read. I did check the grounds on both tanks and clean them up and that did not work. As a last resort I am thinking it must be the guage being intermittent. I am asuming that the guage must match up with the 2 sending units brand wise. I would appreciate any help regarding this problem.

Need a little help understanding what you mean by "6-8 volts there".

Most every fuel gauge I've touched in the past 30-odd-years has a +12VDC source. If that isn't present, most won't work at all.

The quick test for the gauge is to remove the sender wire and ensure it has power (12 VDC supply). The gauge should sit at empty. short the SENDER terminal to ground and it should go FULL. (A few imported gauges will be 'backwards'.) If you wanna check 1/2, use a 110 ohm resistor between GND and the SENDER terminals.

You can check the sender units with an ohm meter. If full, they should be about 35 ohms between the sender wire and ground on the sender unit. If that is OK, do the same test, up at the gauge. That'll let you test the wiring in between. Repeat for the other side, too.

You don't have to match brands but match the standard used by the gauge and the sender. Most are 240-33 ohms (empty-full). This is typically referred to as the "US Standard".

Thanks for your expertise. The first time it would not read and just twitched I probably was 1/2 full on both tanks. So acording to what you have said, I am assuming that resistance from the tank sending units drop the voltage at the guage as the fuel gets lower in the tanks. Thus we should read twelve volts with the sender wire taken off the tanks, and if tanks are 1/2 full it could read 6 volts at the guage if I read you correctly. That being the case it appears that the guage has gone bad because I do have the six volts at the guage wire. The last time I filled up the guage read full, but as the fuel was used the guage would not read again. So it looks like with full tanks we have 12 volts at the guage and it makes the guage work. But as the voltage drops by using fuel, the guage does not respond.

makomark said:

Need a little help understanding what you mean by "6-8 volts there".

Most every fuel gauge I've touched in the past 30-odd-years has a +12VDC source. If that isn't present, most won't work at all.

The quick test for the gauge is to remove the sender wire and ensure it has power (12 VDC supply). The gauge should sit at empty. short the SENDER terminal to ground and it should go FULL. (A few imported gauges will be 'backwards'.) If you wanna check 1/2, use a 110 ohm resistor between GND and the SENDER terminals.

You can check the sender units with an ohm meter. If full, they should be about 35 ohms between the sender wire and ground on the sender unit. If that is OK, do the same test, up at the gauge. That'll let you test the wiring in between. Repeat for the other side, too.

You don't have to match brands but match the standard used by the gauge and the sender. Most are 240-33 ohms (empty-full). This is typically referred to as the "US Standard".

Click to expand...

ED:

I think you've interpreted what I said without accounting for the rest of the circuitry in the gauge.

Its been a long time since I've experimented with a gauge so here's what I remember. The +12VDC going to the gauge is the supply. Since it makes a commplete electric circuit, it needs a ground path - not only back to the battery but the sending unit too. Most of them work as current sources and by sensing the change in the sender's resistance, they show the units the gauge is calibrated in.

In other words, It is very possible to mmeasure a fixed voltage on the S terminal of the gauge. If you want to test the GAUGE, supply its power (Key ON) and with the sender wire disconnected, clip a variable resistor (500 ohms is plenty) between S and GND and twist the resistor. If the gauge tracks, the gauge is fine.

Similar thing can be done on a float driven sending unit. If the sending unit's output, measured with an ohm meter, varies with float position, then it is functioning. All that would be let is the interconnects. (Usually most builders assume a ground connection, thru an indirect path. Unfortuantely, it usually degrades as the boat ages.)