Fuel Gauge?

[Jobless]

So I just finished the new dash in the '72 Ford and the new Autometer fuel gauge still only registers 3/4 tank when the tank is in fact full.

Both factory tanks do the same thing which leeds me to believe that it may be the electrical switch that transfers the signal from the tank to the gauge? Or could both sending units have the some problem?

Thanks
Brad

[FORDification]

From the Fuel Tank Sending Unit Tech page:

"If you have dual tanks and a non-operative or sporadic fuel gauge, and grounding the sending unit wire on each tank pegs the gauge, you probably don't have problems with both sending units. Take a look at your fuel tank selector switch as a culprit. Test the switch for continuity. You might find you'll need to carefully disassemble and clean the switch internally, and lubricate it with dielectric grease."

[Jobless]

Sweet...thanks

[Jobless]

Yep...just grounded the orange wire on the sending unit and it pegs the gauge at full.

I'll pull the switch out after work and clean it up.

I take it that it that you can disassemble these things with regular tools?

[bluef250]

Carefully, very carefully, bend tabs back using a punch. Use a gentle touch with the hammer. Examine the assembly (guts) as they come out. Don't lose any parts as there are 2 steel bearings at opposite corners that will want to run away and hide. Clean all contacts, then carefully reassemble using dielectric grease. Make sure tabs are bent and base plate is fixed. Test for continuity. If the assembly refuses to work, used switches are available occasionally on eBay from $1 to $10.

[68F250]

Be gentle when bending the tabs out, only bend them out enough until the guts spill out. If you bend them out too far and then try to bend them back in again, they'll crack off and then you're hosed.

You could first try spraying contact cleaner into the cracks and then switch it back and forth a bunch of times, nothing to loose.

[Jobless]

Ok...so I allready grounded the orange wire at the fuel sender on the main tank and the gauge went to "full"

and I just finished cleaning the internal and external connections on/in the fuel guage switch.

I still have the same problem, both tanks register 3/4 when they are full

So it must be that both sending units are bad?

When I disconnected the orange wire from the sending unit I measured the voltage from the orange wire to ground to be about 4.8V dc is this correct or should it be exactly 5V dc?

[FORDification]

I doubt they're both bad, but the only way to check is to remove them and check them out with an ohmmeter. Depending on what you find, you might be able to just bend the rods a bit to adjust the level...takes a little trial-and-error to get it right, though.

I know you said you cleaned the selector switch, but did you test it for continuity?

[Jobless]

I doubt they're both bad, but the only way to check is to remove them and check them out with an ohmmeter. Depending on what you find, you might be able to just bend the rods a bit to adjust the level...takes a little trial-and-error to get it right, though.

I know you said you cleaned the selector switch, but did you test it for continuity?

I checked the switch after I cleaned it. I was measuring less than an ohm between contacts.

If it's the sending units that are bad, especially the auxillary tank I'll probably just live with the error like I have been.

[FORDification]

If your auxiliary tank is the in-frame type tank like I'm using, there are a couple sending units up on E-bay right now. They're kinda hard to find, so....

The only difference between these two is the direction the line takes coming out of the sending unit....either will work, but one might take some minor reworking of lines, depending on which one you have.

http://cgi.ebay.com/ebaymotors/ws/eBayI ... 4523952675

http://cgi.ebay.com/ebaymotors/ws/eBayI ... 4523953239

[Jobless]

Fixed it.

It seems as though what should have been 12V (or battery voltage) going to the gauges was in fact about 10 - 11.5V. I connected the new dash assembly to switched 12V and now the fuel guage seems to be working fine.

Not quite sure why that one wire (light purple or violet) has less than battery voltage on it though It was one of the original wires going to the factory dash gauge assembly that was tied into the ignition switch.

[68F250]

Ahh, that's the problem! That wire is a resistance wire (8-9 ohms) which feeds the constant voltage regulator for the stock gauges. They started putting in that wire in '70, lucky you.

It was dropping voltage depending on what the load you had on it.

[Jobless]

Ahh, that's the problem! That wire is a resistance wire (8-9 ohms) which feeds the constant voltage regulator for the stock gauges. They started putting in that wire in '70, lucky you.

It was dropping voltage depending on what the load you had on it.

What's funny is that the stock fuel gauge was having the same issue when the dash was still being supplied by that "resistance" wire.

[68F250]

You're not alone, I've heard of many '70 - '72 owners complain about the same thing.

That resistor wire was incorporated when the instrument cluster switched to the flexible circuit board, it's the same deal with the cars and vans. The older cars, trucks and vans with the hard-wired gauges don't have it.

All I can guess is it's a current limit safety feature?

[QC]

The instrument cluster voltage regulator is adjustable, and the gas gauge is the indicator. If the gas gauge reads low, all the gauges are reading low.
You need to fill the gas tank and turn the small screw on the regulator until the gas gauge reads full. The voltage regulator grounds thru its base, so if you take it out to adjust it you need to ground it or put it back in to see if the adjustment is correct. An old Ford mechanic taught me this.