Trouble shooting Fuel Gauge

[erikhillard]

Morning all! My fuel gauge as never went all the up past 3/4 and recently hasn't been rising at all. I replaced the sending unit but nothing seems to have changed.

When I touch the black wire to the nut that the red wire attaches to as in the below photo, the gauge goes all the way up. Any thoughts?

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[Mach428]

Sounds like a bad ground to me. You can remove the sending unit and connect the wires, move the actuator and see if the gauge responds.

[erikhillard]

Thanks. I guess I'm not clearly understanding how this works. If I touch the black (which I believe is ground) to the sending unit center bolt, gauge goes up to full. Wouldn't it not go up if the ground was bad?

If I touch the black ground wire to the sending unit mounting bolts and/or the plate, gauge has no response.

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[Mach428]

The positive voltage is supplied to the gauge whenever the key is on. The gauge gets its ground through variable resistance from the sending unit. Since the gauge goes FULl whenever you ground the sending unit wire, it tells me that the gauge and wiring (including the ground wire) is operational. The problem is in the sending unit with an open circuit. This is why I suggested removing the sending unit; connect the wires and move the arm up & down. I bet the gauge won't move

[DuckRyder]

Did the new sender have a new float?

Is the rubber boot still on the float?

I agree sounds like time to pull the sending unit and do some trouble shooting because you’ve effectively eliminated everything else by grounding the sender wire.

[erikhillard]

Thanks, guys. Pretty sure the new sender came with a float. Slammed with work for the next week plus, so I'll yank it when I'm back do more testing. Thank you so much for your guidance.

[tsherry]

I recently discovered that ethanol fuel ate my original brass float in my '69 Galaxie. I wasn't particularly happy to find that out....

[colnago]

I recently discovered that ethanol fuel ate my original brass float in my '69 Galaxie. I wasn't particularly happy to find that out....

How so? I can't see ethanol eating brass. If it did, it would kill sending units, carb floats, anything brass in line with the fuel. My original float had a pin-prick hole in it, so it would only register when I first fueled up. I replaced it somewhere along the line.

Any pics of your Galaxie?

Joseph

[tsherry]

>>How so? I can't see ethanol eating brass.>>

I have no idea. I've had it since 1989 or so. The tank had been drained and cleaned probably five years ago, the gauge functioned. Filled with ethanol gas, regularly drove it. Two years ago, gauge quit, figured bad sending unit. Last winter, pulled the sending unit, found that there was no longer a float on the arm. Drained and pulled the tank. Flushed it. NO float unit. Had some remains of the rubber end cap, but only about 30% of that--it was eroded from all sides to the center. No bits and pieces of brass or solder, no rust, nothing.

Just **gone**. I no longer run ethanol in most of the vehicles, but ethanol-free premium in all the small engines and anything that runs a carburetor. The insides of my older carburetors, including the rubber diaphragms and the aluminum itself, were being eaten by the alcohol. The accelerator diaphragms had no rubber left on them, and looked like screen doors. Newer projects that had non ethanol only don't have any of the pitting and corrosion of the float bowls or rubber parts. So, they may say that yellow metals are not impacted by ethanol. I'm not buying it.

A not great photo of the Galaxie is attached. 351W C-6, 2bbl, and new Edelbrock heads. In addition to the tank work, also did springs on all four corners last winter.

[colnago]

Wow. I would have never guessed. That is too weird.

Very nice Galaxie! Thanks for sharing.

Joseph

[erikhillard]

Think I may have fixed the sending unit. Need to go fill the tank up to be sure!

One wire connecting to the spring part of the sending unit seemed to have short in the connection. I think the short was caused by having to bend the wire to get it into the tank. I snipped it off, cleaned off some insulation, and sautered it back on the plate at a 90 degree different angle and the gauge seems to be working now. Now, outside of the tank the sending unit goes up to the common 3/4 when full but its never went to full on the gauge since I've owned the truck.

Will fill up the tank later and see if it goes up to "full" !

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[erikhillard]

Yep! SUCCESS!

Cutting and reconnecting that wire on the sending unit has brought me back to 3/4 when the tank is full. So next I guess I'm going to pull the gauges and try that gauge adjustment screw.

[erikhillard]

Well, seems I spoke too soon. Gauge is dropping to empty with about 5 gallons of use. Goes back up to 3/4 when topped off but the gauge is clearly inaccurate as its dropping way too quickly.

I ordered another new sending unit. This time from Dennis Carpenter instead of LMC in hopes of slightly better build quality. Probably the same exact thing but figured it was worth ordering from a different vendor. No difference in price.

[rabbithunter2005]

Been where your at on my 67 f250 I used the crappy lmc sending unit same thing said empty with about 5 gallons of gas. I ended up having to bend the arm on the sending unit to get it to read empty with about 2 gallons left which im ok with. However it reads full till it gets to half a tank now and drops fast from there. Either way the important part is i don't run out of gas without warning. Now on my 69 f100 my gage only went to 3\4 tank when full just like yours. I found out there is this little voltage regulator on the back of the instrument cluster that wears out over time. Its a small rectangular box with 2 wires going to it I believe. I had an extra cluster from a junkyard trip and I took the regulator from that one and put it on my 69 and solved the problem. Also found out the temp gage was doing the same thing when I did that swap so now im running right on the edge of hot with a 195 thermostat so im gonna drop it to a 185 just to be safe. Hope this helps ya out.