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Gas Gauge Help


31model70
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My 1935 Packard super eight has a combination gas/oil gauge that doesn't function properly.  Before disconnecting any of the wiring, the gauge always read 1/4 full regardless of whether the gas tank was full or near empty.  There is a push button switch under the dash which when pushed, connects a circuit to a float sending unit in the oil pan to show the level of oil in the engine.  When the button is released, the gauge reading goes back to the normal state of reading the gas tank level.  When the button is pressed to read the oil level, the needle quickly jumps to the full position.  While I always keep the engine oil full, I would think the needle would gradually go from the empty to the full position more or less gradually rather than immediately pegging at full like there is a dead short somewhere.  With the oil pan removed for engine inspection when I first got the car, I tested the resistance on the oil float unit and found it to give the expected variable readings as the float was moved up and down so that sending unit seems to be working normally.

 

The problem seems to be the gauge itself rather than with the fuel sending unit or wiring because when the ignition is first turned on and with the wire to the sending unit disconnected, the needle sometimes immediately pegs at the full mark.  This does not always happen as more frequently, the needle does not move at all.  At these times, when I gently tap the face of the gauge, the needle jumps to the full mark.

 

There is continuity in the wire from the gauge to the sender.  I have 6.01 Volts going into the gauge but only 2.73 V coming out of the gauge on the wire going to the sending unit in the tank.  There is 2.71 V in the wire at the sending unit when it is connected to the gauge so it appears the  wiring is OK.  Is 6 V into the fuel gauge and only 2.7 V out normal?  Testing the resistance of the sending unit still mounted in the tank only gives a variable reading of about 0.05 mv with the tank almost full so the sending unit may also be questionable.

 

If it is determined I have a bad gauge, I would be content with installing an after market fuel gauge but do not know the resistance rating of the sending unit in order to order a compatible gauge.  The unit is marked AO, made in US, Model C.  It would be very helpful if someone could give me the correct resistance rating of this sender and also help me determine how to be sure if I have a bad gauge or sending unit.  Thanks in advance to all who can assist with my problem.

 

Glen

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Correction.  The gas sending unit is a single wire circuit using the tank as a ground.  Today, I took another resistance reading on the sender using a better ground to the car body and got a reading of 05.8 ohms steady with the tank almost full.  Coud this reading indicate the sender is working properly?

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I was hoping someone would jump in here with specific Packard Super Eight knowledge, but since no one has, I'll throw a few ideas out there.

 

16 hours ago, 31model70 said:

There is continuity in the wire from the gauge to the sender.

Good to have that potential problem out of the way.

 

16 hours ago, 31model70 said:

I have 6.01 Volts going into the gauge but only 2.73 V coming out of the gauge on the wire going to the sending unit in the tank.  There is 2.71 V in the wire at the sending unit when it is connected to the gauge so it appears the  wiring is OK.  Is 6 V into the fuel gauge and only 2.7 V out normal?

I don't know, but on most gas gauge systems the voltage on the wire to the tank isn't something you would know or care about.

16 hours ago, 31model70 said:

The unit is marked AO, made in US, Model C.

If that turns out to really be AC, it will work on the same principle as a GM gas gauge, and I have a lot more ideas.

16 hours ago, 31model70 said:

If it is determined I have a bad gauge, I would be content with installing an after market fuel gauge but do not know the resistance rating of the sending unit in order to order a compatible gauge. 

That is going to have to come from a service manual, or from someone who just knows. If it happens to use the same resistance range as some common GM, Ford, or Mopar gauge, or an aftermarket standard like Stewart Warner or VDO, then maybe. Even then it would most likely be a 12 volt gauge. I would bet against this happening to work out on a Packard, but until you know what the resistance is, it's hard to tell.

 

Most electric gauges will swing all the way one way or the other with the wire to the sender disconnected. For instance an AC (General Motors) gauge swings high with the wire disconnected, and low with the wire grounded. The range on those is 0-30 ohms at the sender, 0 being empty and 30 being full. Some other brands of gauge may go the other way. Some types might not use 0 as an end point. Using 0 as an endpoint was in my opinion a really bad idea.

 

 

 

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With the key on and the Guage pegging to full, that is reading as an open wire from the Guage to the sending unit. Also, be sure the tank and sender has a good ground. We make a ground harness for all our cars because the tank straps and lower frame are often dirty/rusty. 

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Bloo & edinmass,

 

Thanks for your input on my gas gauge problem.

 

Looking at the sending unit cover, it is worn, rusty and hard to read, meaning it could very well be AC instead of the AO I originally thought.

 

Per the suggestion, I installed a temporary ground wire from the body of the sending unit to a clean and rust-free body bolt and things changed.  With the ignition off, the gas needle sits a little below the empty line.  After turning on the key, it now jumps to directly even with the empty line and wiggles a little before stopping there.  This is with an almost full tank of gas.  When switching to the oil level mode, the needle now moves quickly to the full line rather than the immediate pegging to full it was doing before.  Does this mean the gauge is working properly and the problem is with the sending unit?  How do I test to be sure?

 

Even with the separate tank ground wire, I now can't get any resistance readings on the sending unit.  I think the readings I got before were probably false because of the lack of a proper ground to the tank.

 

If it is felt the gauge is OK, should I remove the sender and bench test it at this point?  With the holidays coming up and the car needed for a family wedding the first week in January, I am hesitant to fool with the sender if it needs to be repaired, at least until the second week of January.  Is there anything I should be doing in the meantime?

 

Any thoughts on this?

 

Glen

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  • 4 weeks later...

With the holidays, I didn't have much time to work on this problem.  However, the gas gauge is now working properly.  When I removed the sending unit from the tank, I noticed a blackened stripe on the bottom of the cork float which indicated the float had been stuck to the bottom of the gas tank.  Since the car was stored for some four years before I bought it, I think the gas in the tank evaporated and the cork stuck to the bottom.  

 

I replaced the float with a plastic one from Summit Racing and the gauge is now working as it should.  This info may be valuable to those with similar gauge problems with cars that have been stored for extended periods.

 

Thanks to those who responded.

 

Glen

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