Gas Gauge Problem

Gas Gauge Problem/RepairTri Five Chevy1955, 1956, 1957 Classic Chevy

  -Alan Arnell

I go to start up my my 1957 Chevy Model 150, 2-door Sedan at the beginning of a short trip to a Dallas Area Classic Chevy Club monthly event.  I learned through driver's education and a life of driving classic cars to scan my gauges.  I see that the water temperature in at 180 degrees Fahrenheit. I have 13 volts of charge from the alternator.  The oil pressure is steady at 40 pound per

square inch.  What! The gas gauge is registering empty!  Then, I remembered that I had taken the Hell Bitch 57 Chevy out for a shake down run to the gas station last night for a tank fillup.  Now what?

My first thought was that the gauge sending unit is bad.  The ‘57 very well may have the original sending unit in place.  Who knows. The Hell Bitch is 50 plus years old and I am the 16th owner.  Then I mused that I have a full tank of gas.  I said it before and I’ll say it again, why do you always have gas tank problems when you have a full tank of gas?  That is as much thought that I put into my car’s gas gauge problem that day.  I just used the odometer as a gas gauge.  I know I can go about 150 miles and not have to worry about running out of gas, so that is what I did for about 6 months.  

I have a good excuse, I’m lazy.  Besides that, every time I think of fixing the gauge and or sending unit I have a full tank of gas.  Bad cycle!

Finally, I get around to fixing the gauge.  in the beginning, I was told my problem could be just a bad

float on the sending unit.  The float is like a float in a toilet tank, made of copper and hollow.  The float may spring a leak and fill with gas, therefore alway settling on the bottom of the tank due to the weight of the gas filling the float.

The float-floats on top of the gasoline in the tank.  As the fluid level goes up and down the float, stays at the highest level of the gasoline.  The float is connected by a lever to a resistor.  As the float moves up and down as the amount of gas changes the resistor changes its amount of resistance.  The whole fuel gauge system is on a 30 ohm circuit.   Therefore, if you just run power to the gauge in the dashboard by bypassing the sending unit the unrestricted flow of electricity will make the gauge read full.  So, when the float goes down in the tank it changes raises the resistance and the gauge reads that and lowers the gas gauge.

It takes the pressures of one volt of electricity to move one amp of current through one ohm of resistance, which is basically Ohm’s Law.

I decide that the float very well may be bad, but the sending unit’s resistor may be bad as well. Before, I fixed the gauge problem, I made a preemptive strike and bought a complete sending unit from Danchuk.  

While I was waiting for my part to arrive, I decided to check to see if the gas gauge was functioning correctly.  

Here are the steps I took to see if it was okay:

To Check Gas Gauge: Take wire off of tank sender, leave it in the air not touching anything. Turn key on, gauge should go to past full. Touch sender wire to frame {full ground}, turn key on, gauge should read empty. If wiring and tank ground is good then sender is not working. If you suspect the wiring this check should repeated by removing wires at gauge.

The gauge checked out as functioning correctly.

I next usied a Volt/Ohm meter checked to see if my wire from the gauge to the sending unit was okay.  To complete this check I unconnected both ends of the wire. That is, I disconnected the brown sending wire from the gauge and the sending unit.  I use to have a long jump wire on a reel to use to check circuits that are long distances apart.  However, I was not paying attention and hooked the reel of jump wire to a live circuit and shorted circuited the wire melting it into an unusable piece of copper that went into the trash bend.  

Necessity is the mother of invention.  I took an extension cord I use for my weed wacker and attached two short jump wires equipped with alligator clips at each end. That monstrosity hooked up, I make a complete circuit with my ohm meter in the circuit.  I shot a small amount of electricity from the meter through the circuit and the meter showed almost no resistance. That test told me the wire was okay.   Zero resistance would mean the wire was bad.

The UPS man rang my doorbell and I had my my sending unit.

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To replace the sending unit on a Tri Five Chevy is not all that bad of project.  I disconnected the negative side of the battery.  I then jacked up the car and placed it on jack stands.   I grabbed a fin and gave the car a good shake to check to see if the car would fall of the jack stands.

The gas tank does not have to be taken out of the car to replace the sending unit.  The unit is put in the tank through an access hole at the front of the tank.  I made sure the tank was empty, so when the hole is opened the tank will not leak gasoline.  

I disconnected the brown wire to the sending unit.  Next, is to the removal of the screws that hold the sending unit in place.  That done, the sending unit just pulls out.  Upon inspection of the old unit the float was empty of fuel, however the sending unit itself looked corroded and just nasty.  The unit was not original. The old cork gasket fell off the tank.

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To put the new sending unit in I put a new cork gasket in place.  Inserted the sending unit into the tank, and screwed it all back in place.  I removed the compression connector that was being used and soldered a new connector in its place.  Then I hooked up the brown sending unit wire.

With the ground wire connected to the battery, I tested the gauge.  Of course I had a empty tank, but I noticed just a little movement off of the empty mark.  That was good enough for me.  With a smile on my face I drove to the gas station to put just 5 gallons of fuel in the tank.  I did not want a full tank

incase I had to take the new unit out again.  The gauge registered a half a tank.  All is right in the world again, except.

I completely fill the tank.  I see a small problem.  Even with a full tank of gas the gauge still will not go all the way to the full mark..  

I have learned to live with the gauge not registering completely full.  I think I may have to put a ground wire on the tank.  But, I am told by two of the most knowledgeable guys in our club it is not the ground.  They believe  the float is hitting on top of the tank and does not have full range of movement.  They suggest, I take the sending unit out and custom bend the lever for maximum travel.  

Maybe someday I’ll get around to doing that fix.  In the meantime I just go on and continue to enjoy my Texas Classic Chevy Experience.

Guide to:

Troubleshooting a Gas Gauge: FUEL GAUGE Checklist with Ignition 'ON'. Gauge pegs itself to the right, above full, some possibilities are: Brown wire is off back of gauge. Poor connection at underdash to tail lamp harness plug. Brown wire is broken between gauge and sending unit. Brown wire is off of the sending unit. Bad tank to body ground. Sending unit is bad. Gauge pegs itself below empty, some possibilities are: Brown and pink wires are swapped on back of gauge ('55 & '56) Brown and tan wires are swapped on back of gauge ('57) Gauge reads 'E' all the time, some possibilities are: Brown sender wire is shorted to ground. Sending unit is 'stuck' or defective. Gauge won't budge when the ignition is turned on, some possibilities are: Pink wire is off of gauge or switch is defective ('55-'56). Tan wire is off of gauge or switch is defective ('57). Gauge is bad. Gauge 'bumps' but will not read at 'E' or above, it may be: Gauge is bad.

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