Fuel gauge reads empty despite replacing sending unit, gauge and wiring check

JLAILEY

MEMBER
1988 MAZDA B2600
2.6L
4 CYL
4WD
MANUAL
300,000 MILES

Hello,

I have recently replaced the fuel sending unit on my 1988 Mazda B2600. I did this for two reasons. The first to try and get my gas gauge working. The second was to remove the in tank fuel filter and fuel pump as the previous owner has hooked up a out of tank electric fuel pump leaving the in tank fuel pump in the tank but not wired.

After successfully replacing the sending unit and wiring it to the yellow wire as per the owners manual and grounding the sending unit the gas gauge still read empty. I checked the sending unit was working with using a ohm meter to see the resistance was changing as the float moved. I checked the ground as well. I also checked that there was voltage running through the yellow wire to the fuel sending unit which there was. I than took the gauge unit out of the dash and began to test the wires there. The gauge was receiving power from ignition I believe somewhere around 12V, the yellow wire to the fuel sending unit was reading about 6V, and I tested the ground going to the gauge with the negative terminal of the battery and there was continuity. I than tried cutting the yellow wire at the gauge and running my own wire directly to the sending unit which yielded no results. I tried a test I saw online taking a grounded wire and attaching it to the yellow wire to provide zero resistance than removing it to provide infinite resistance supposedly sending the fuel gauge needle from full to empty but had no results with this either. I than tried opening the gauge and moving the needle with my hand to see what it would do but whether it was upside or right side up it returned to fully empty. I went to the auto wrecker and got another gauge unit. I than retried all these tests with the same results.

On another note with the gauge unit out of the truck I tested the terminal to the fuel sending unit and the ignition terminal to see how much resistance there was and got around 57 ohms which was similar to the resistance from the terminals on the temperature gauge which is working correctly.

I am running out of ideas if you have any input that would be greatly appreciated. Sorry for the long message I wanted to be thorough.

Thanks,

Jeff

1 Reply

HMAC300

MECHANIC
48,601 POSTS

If you replaced all of that and it still doesn't work hard telling what it is unless the maybe the instrument cluster lost ground. The power for the yellow wire should come out of terminal 1 at dash and there seems to be a diode and resistor in the gauge, but don't know what it's values are. Either that or the battery to frame/body ground is no good those are the only things I can think of at the moment. Thee is a fuel module that has the yellow/blue wire that supplies power to the tach as well as gauge so maybe that has something to do with it. Pic shows how to diagnose gauge which you cold do with a variable resistor if you have one. Sending unit resistance should be3 ohms full, approx. 33 ohms half and 103-117 ohms empty. Maybe it's the wrong sending unit. Voltage should be 7 volts to operate gauge.

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