Fuel Gauge

[Daddiojoe]

I've got troubles with my fuel gauge. I put in ten bucks and it only shows a quarter of a tank.

But seriously folks, it's pretty erratic on what shows up and what doesn't. Sometimes I can fill it up and it will show half or three quarters, and then go down drastically. Very strange.

Today I had the seat out to get to the sending unit, couldn't really get the unit out esaily--removed keeper ring, but the rest didn't want to come.

So I checked it with my little tester light and hooking it up to the terminal and ground with the key in an "on" postion, I kept getting a blinking light. Is that because of the gas sloshing around? I removed both blinkers to make sure it wasn't just something cross wired.

Then I removed the gauge, cleaned it up, etc. The gauge will go to full with 12 volts pumped through it if I hook it up directly to the battery.

Can't really tell if things are fixed, because at this point I'm not sure how much gas is actually in the truck. I'm saving up for a full tank.

Do you all think I should be looking at the sending unit as the problem?

Thanks,

Joe

[JimE]

Joe I think that guage works off an intermittment 5 volt pulse same as the temp guage. Which means you should have a blinking light. There is a voltage limiter built into the fuel guage that makes all this happen. Jim

[Jeffc]

Jim is correct about the pulse.
I would be looking a sending unit frist
for your problem.
If you got the ring off the sender, you just need to
pry the rest up, be gental, they tend to stick a little
if they never have been pulled out of the tank.
Also, there should be a grounding strap that runs between the sender fuel tube and the fuel line.

Once the sender is out you need to check it with the
OMS setting. Should give a full swing as you move the
arm, areas that drop to 0 oms are bad spots on the little fingers in the sender that the arm rides agianst.
It may also be the float is bad. A float that is partly filled with fuel will tend to read low.

[65D100]

I have a keen interest in this thread since my fuel gauge does not register at all. Just haven't gotten around to checking into it yet. I am going to check grounds first because this seems to be 99% of the trouble on my truck however just to educate me:.......(A trained electronic technician by the way)

Where on earth is a regulated and timed 5V logic pulse coming from on a 40 year old truck

Kevin

[65D100]

Or is 0V - 12V actually coming from the sender (Depending upon the float position and resulting potentiometer resistance) and being knocked down through a resistor in the gauge pod, then timed by a resistive capacitive network resulting in Old School 5V Logic

This has to be it unless I missed the electronic module with the 555 timer integrated circuit within which wasn't developed until the 70's.......

[Jeffc]

Very old tech. The voltage limiter built into the gauge is a common heat sink resitor that pulses the voltage to hold it under 12 volts. Since these gauges respond slowly it was really not a issue for them. The reason for the 6 volt power supply is because that was what was used in the 1950s gauges and know one wanted to spend the bucks to
upgrade the systems being made at the time.
Cars of these period also used the system, most of them had the limiter(regulator) plugged into the rear of the gauge cluster instead of built into the gauge.

Here is a link to a updated type that can be made.
http://members.aol.com/moparnuts/modify.htm

[65D100]

Wow, that is old tech. That has to be a fairly hefty resistor to drop 6 volts or so at that current.

There also still needs to be a capacitor somewhere....maybe in the guage itself, or the Sending unit in order to create the pulse.

Thanks for the info.

Kevin

[GMAN69]

i had simular problems with my 69 the unit in the tank was bad one contact inside reistat housing broke off from wear i got a universal replacement unit from jc whitney and it took ALOT OF ENGINEERING was difficult but not impossible for me it was my only option money you know
good luck