Fuel Sending Unit

I started looking around for a sending unit for my underseat tank. Surprisingly, I didn't have a whole lot of luck.

Soooo, I remembered that the P.O. of the tank (M38WILLYS) said that the sending unit worked fine. It's rusty as heck, all except the "contact" part of the unit, which is as shiney as new. I'm thinking that maybe it does really work.

Is there an easy way to test it before installing it in to the Jeep? I don't have any type of voltage meter, but was wondering if I could hook it up to a light bulb or something, and (theoretically) the bulb would get dim and bright with movement of the float?

Possible?
Crazy?

 

Hmmm - I didn't realize they were that cheap.... I suppose I might just do that.

One more question: How do I wire this thing? There's only one post on the sending unit. I'm guessing a wire comes from the power source to this post, and another wire will leave the source and go to the gauge (in this case, the red probe on the multi-meter). What about ground? Just ground the entire sending unit? Touch the negative probe of the multi-meter to the sending unit itself? Watch for a change in voltage as I move the float up and down?

 

One wire from the guage to the post on the sending unit. That's it. If you have a steel tank, it should be grounded. If it's plastic, just run a ground to metal from one of the set screws.

Looking for a change in resistance. Ohm meter.

 

Gotcha - Thanks Bob!!!

One more quick question while we're at it.

On the back of the guage, I'll run power *into* it with a wire from a keyed ignition source - correct? I assume power goes into one post and the sender hooks up to a different post?

<looking for Binky's site with a diagram of the back of the speedometer>


Edit: OH NO!!! Stan Day's site is not longer in operation... Anyone have a diagram of the back of the speedometer?

Edit #2: WHEW, nevermind, I found it!

 

Struck out again - looks like Stan has two separate guages for fuel and temp, rather than the gauges in the speedometer like I have.

Anyone got a schematic of the "all-in-one" style?

 

No power to the fuel tank...don't want any sparking in it

 

That makes perfect sense.

Good thing I asked!!!

 

Well, sort of. There has to be some current flowing through the sending unit to ground, otherwise there's nothing for the gauge to measure. The sending unit is definitely not connected to a 12V source though. It'd be interesting to measure the voltage at the sending unit with a full tank of gas.

 

I thought that the sending unit just varied the resistance and the gauge read this resistance and translated it into movement of the needle?

 

what i did . hooked up a test light to the sending unit where the sending unit wire connects. if there is juice going to the sending unit from the gauge the test light will blink slowly. if the test light does blink and the gauges does not work try grounding the tank with a wire. if there is no blinking light at the tank then the problem is at the gauge. remember that the fuel and temp gauges work together from the same power source. hope this helps....

one other thing my gauge did not work because of lack of ground at the tank once i ground the tank with a ground wire no problems

 

Very good info. Thanks guys! I'll try to mess with it this weekend...

I found a schematic at www.monsterslayer.com/jeep/Gauges.htm

I'm hoping this is the same as the earlier cluster.

 

Yeah, but without current, there's no voltage, and nothing to measure. The gauge is a galvanometer, and measures changes in voltage.

Consider the following circuit:

12V-----R1-----x------R2--------GND

The voltage at "x" will depend on the value of the resistances, R1 and R2. Let's say that R1 and R2 are each 1000 ohms. Then the current flowing through the circuit will be 12V/2000ohms = 0.006A or 6 milliamps. The voltage at "x" will be 6 volts. Let R2 be a variable resistor. If we change it to 200 ohms, then the current will be 12V/1200ohms = 0.01, 10 milliamps. The voltage at "x" is now 0.01 amps * 200 ohms = 2 volts. Thus, as the resistance of R2 goes up/down, the current across R2 goes down/up and the voltage drop across R2 changes by ohms law, V=IR.

This is how a voltage divider works, and it's the only way that you can measure a change in resistance.

 

THUD

 

Damn. His head exploded.

 

thank you i needed a reason for a few going to the refer now

 

If you put R1 and R2 in parallel, you can adjust the total current by varying R2 but the voltage drop across R1 and R2 stays constant. See... no equations!

 

Okay, I'm feeling light-headed again....

<swaying>

 

Stay away from the light Timmy, there trying to suck you in to geekdom!!