Fuel Sender

[Loren L.]

What's the quick and easy way to test this fuel sender while it's out of the tank? Thanks.

[Duke W.]

Re: Fuel Sender

This is applicable to seventies GM cars, and I think it's the same for sixties vintage. The guage is basically a potentiometer. Total resistance is 80 ohms, so you should meassure this between power and ground.

The reading between the center tap and ground should smoothly cycle between zero and 80 ohms as you rotate the float back and forth between its limits of travel. As you do this you should see very smooth needle movement with an analog ohmmeter.

If the needle jiggles it could mean some deposts have built up, and these can be removed by taking the cover off the wire wind and cleaning the wiper and wire with industrial strength (wooden) Q-tips soaked in acetone.

Duke

[Ted L.]

Re: Fuel Sender

Sounds like the ohmeter is doing the same thing as the fuel guage, measuring resistance. So really, the fuel guage could be used if you don't have an ohmeter, right?

Ted

[Joe C.]

Right

That's how I checked mine. Used the gauge, sender, 12v.power source, and a few jumper wires. Bench tested for smooth operation with full range.

Joe

[Loren L.]

Thanks, guys. NM *NM*

[Clem Z.]

Re: Fuel Sender

you can use the fuel gage but if you fuel gage is not operating properly you still need a V/O meter to see if it is the gague or the sender that is defective.

[Duke W.]

Actually...

I believe the fuel guage acts as a d'Arsonval movement current meter. The current between the center tap of the tank gage and ground varies with the position of the float. An ohmeter also acts as an ammeter when checking resistance. That's why they have a battery. Using Ohms law I = V/R, the resistance is inversely proportional to current.

If any EEs out their find my humble ME explanation incorrect or inadequate, please correct me.

Duke

[Tom R.]

Re: Fuel Sender

My fuel gauge (78)is slow to respond after the tank is filled or on long trips. I've assumed its the sending unit "and its starting to go." Could it be as simply as removing, testing and cleaning as described here?

[Joe C.]

Re: Actually...

Duke:

Right. I gave the simplified answer to Loren's question, as the original post did not specify a bad fuel gauge. In my case, I had a known good gauge. Its rotational transducer, or D'Arsonval movement, registered a linear response to the current flow through the moving coil, which could be expressed as:

T=B(2Nl)iR

where:

T=rotational torque

B=magnetix flux density

N=number of turns in the coil

l=conductor length normal to the magnetic field

r=distance from pivot to conductors normal to the field

Under steady state conditions, the restoring torque of the two spiral springs were just equal to T, above, and thus the response was directly proportional to the current, i, delivered from the fuel sender. I therefore knew that any inconsistency in the gauge's movement would be due to an anomalous current flow from the sender.

Joe