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Fuel gauge testing

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Old 07-12-2017, 09:10 AM
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Fuel gauge testing

Yes, you all should be able to tell I'm working out a few bugs. I have another one. How do I test my fuel gauge and my sending unit?

The gauge is brand new from Macs. The sending unit was included in the used '56 tank I got second hand. All is now installed but the gauge doesn't work. When I turn on the ignition it pegs at full and stays there. It goes back to empty when I turn the key off but it takes a while. I know for a fact I only have a few gallons in the tank.

I recall a thread I saw some time ago that showed the process but I can't find it.

Thoughts?
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Old 07-12-2017, 09:49 AM
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Old 07-12-2017, 10:11 AM
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That helps! I'll go through the check list and report back.
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Old 07-15-2017, 10:34 PM
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I am also having a problem with a new fuel gauge and sending unit from MidFifty. From my testing efforts, if your needle is pegged at full, the sending unit wire might being grounded out. To fully test, need to read the ohm readings coming from the sending unit.

My problem is that the gauge was always reading half. I got another new gauge and this one is always reading quarter tank (after I filled the tank). My sending unit was tested before going in and seemed to be ok. I'm still not sure what the problem is. I have an extra gauge and sending unit to hopefully figure it out. If I wanted a non-working gauge, I would have avoided all the time and expense and just left the original one in it!
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Old 07-15-2017, 11:21 PM
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Originally Posted by BTclassics View Post
I am also having a problem with a new fuel gauge and sending unit from MidFifty. From my testing efforts, if your needle is pegged at full, the sending unit wire might being grounded out. To fully test, need to read the ohm readings coming from the sending unit.

My problem is that the gauge was always reading half. I got another new gauge and this one is always reading quarter tank (after I filled the tank). My sending unit was tested before going in and seemed to be ok. I'm still not sure what the problem is. I have an extra gauge and sending unit to hopefully figure it out. If I wanted a non-working gauge, I would have avoided all the time and expense and just left the original one in it!
I got a new sending unit from LMC Truck. It didn't seem to be working. But I took it out and tested with an Ohm meter. The resistance was changing when the float was moved up and down.

So, I figured it just wasn't calibrated correctly. I bent the wire with the float on it way down, almost vertical. Now, if there's ANY fuel in there it lifts up. When I have 7 gallons in there now it reads about 1/10 of a tank, but at least now I can tell when I need to get gas.

I think these things just aren't made correctly, as many others have had similar problems. I do know it matters if you're running 6 volt (I am) or 12 volt.
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Old 07-16-2017, 09:55 PM
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I'm running 12V. I wouldn't mind if the calibration was slightly off. But with all new parts and it reads half full or quarter tank (depending on gauge) all the time. Very frustrating!
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Old 07-16-2017, 11:01 PM
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I'm running 12V. I wouldn't mind if the calibration was slightly off. But with all new parts and it reads half full or quarter tank (depending on gauge) all the time. Very frustrating!
If you're running a 6 volt sender with 12 volt it probably won't work.... and vice versa. Voltage is related to current (I) and resistance (R) via V = I*R. I don't know what the gauge is actually reading, but based on the equation the resistance OR the current would have to be 2x the value for a 12V system vs 6V system and therefore the gauge would not read correctly (if at all) if mismatching 6/12 sender.
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Old 07-16-2017, 11:05 PM
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Both the sender and gauge were supposed to be 12V. Is there any way to tell on the reproduction gauge if it is 12V or 6V?
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Old 07-17-2017, 07:17 AM
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Hi

I recently had exactly the same issue. A brand new fuel sender, the original (6v) gauge and I have converted the electrics to 12v. If you bought the voltage reducers that are for sale on macs/lmc then you have to create little adapter wires as the reducer needs to wire the wrong way round. I'll try and dig out some pictures of the back of my dash if it would help you but the stud on the reducer needs to go to the stud on the gauge and then you have two female connections between the sender wire and the reducer.

I think the senders aren't specific for 12v or 6v systems but you need to ensure the gauge only sees a maximum 6v and, as the sender comes after the gauge (at least in my system) the sender also just sees 6v.

Hope this helps? Let me know if it's a bit messy as I've written it in a bit of a rush
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Old 07-17-2017, 10:10 AM
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Quote:
Originally Posted by o7i View Post
...
I recently had exactly the same issue. A brand new fuel sender, the original (6v) gauge and I have converted the electrics to 12v. If you bought the voltage reducers that are for sale on macs/lmc then you have to create little adapter wires as the reducer needs to wire the wrong way round. ...
Not sure what you mean here. The Hot (+) from the reducer goes to the Hot (formerly [-]) post on the gauges, which is a metal strip connecting all the Hot sides of the gauges. The sender wire stays on the same post as before.
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Old 07-18-2017, 07:10 AM
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I was told a few times that you needed a reducer per gauge. As the metal strips connect all the gauges together, the only place I could put the 3 reducers was on the sender side of the gauge, which means a bit of difficulty in the wiring. Of course I could be completely wrong but it seems to be working
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Old 07-18-2017, 08:14 PM
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Do you need to use a reducer if you use the 12V kit from MidFifty?



(Sorry for hijacking. How did you make out Calvin?)
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Old 07-21-2017, 07:19 PM
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I decided to do a bench test with just the gauge, the sending unit and a battery. Hooked it all up and the gauge reads "full" until the sending unit lever is pushed all the way down (empty) and the gauge reads around "3/4". Since the bench test isn't working correctly, I'm think I should try a reducer.

What type of reducer are people using for the fuel gauge?
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Old 07-21-2017, 08:20 PM
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Quote:
Originally Posted by o7i View Post
I was told a few times that you needed a reducer per gauge. As the metal strips connect all the gauges together, the only place I could put the 3 reducers was on the sender side of the gauge, which means a bit of difficulty in the wiring. Of course I could be completely wrong but it seems to be working
I'm surprised that works. If you are using the Runtz regulators, or the LM7806's, yes you need one per gauge. Usually the metal strips are removed and the reg's put in the power supply to each gauge. With the reg in the sender, it seems like the regulator would be trying to regulate a voltage already reduced by the resistance in the dash gauge. In other words, it might make the gauge read the same no matter what.
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Old 07-22-2017, 08:39 PM
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I'm lost. I guess I will start a new thread as it is getting confusing between 6v and 12v systems. Hopefully Calvin has his working!
Thanks.
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