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I installed a voltage reducer for my gas gauge on my 48 3100. Obviously I converted to 12 volt but I have a question...

If I check the voltage at the gas gauge I am reading around 8 volts, is this OK or is this normal for the voltage reducer? It is supposed to be 6 volts but I understand that these things may not be precise and was wondering if I am good to go? I installed the one from "vintage auto garage" and it hooks right up to the back of the gauge. (looks like the runtz type)

Thanks

The running voltage of the 6 volt system is +/- 8 volts.

Just like the running voltage of the 12 volt system is +/-14 volts.

scenarioL113 My 51 3600 has been 12 volt since 82 with no resistor,gas gauge really flies up to attention but has worked that whoole time. However is best to have it I think.

A couple of things about the fuel gauge:
One, it will work at 12 volts and there are reasons to think it may be ok to do that for a long time. Your fuel gauge cares more about current than voltage. I've not heard of a fuel gauge failing from being run on 12 volts, but I do know things can change inside that gauge. (see two, below)

Two, operating it at 12 volts may alter the permanent magnet in the gauge. Please see the image below and the light blue pointer. Look closely and you'll be able to see an hourglass-shaped piece of thin metal which pivots along with the needle. Doubling the voltage going through the 2 electromagnet coils you see (one copper colored and one black) will increase the electromagnetic field present in your gauge and this means the permanent magnet (the hourglass-shaped piece of metal) will consequently have its magnetic field increased. When that occurs, the gauge will now reflect a fuel level lower than true fuel level if a lower voltage is then applied to the gauge. Any permanent magnet subjected to a magnetic field which is stronger than that permanent magnet's natural field will have its magnetic field increased. What I just said can be summed up as: you can go through this door, but you can't go back. Once the permanent magnetic field is increased, the gauge will forever read differently, especially if the voltage to it is reduced.

How long does the change in magnetism take? Opinions differ. There are too many variables in the case of this particular gauge to pin this down and no studies have been done to my knowledge. But when it does occur ("it" meaning the change in magnetism of your permanent magnet), your gauge will not work correctly unless it continues to receive the higher voltage. If later subjected to a lower voltage (example you changed from 6 v to 12 v and now you want to use a voltage reducer), the gauge may read all fuel levels as being lower than they actually are.

Bottom line in my opinion: if you've been running your fuel gauge at 12 volts, just keep doing so. Lowering that voltage now will possibly make your gauge inaccurate. If you're converting from 6 volts to 12 volts, then I'd use a voltage converter, but I'd get one that's adjustable. Then, I'd fill my fuel tank, connect the converter and adjust it until the fuel gauge reads F. At that point, other readings should be as accurate as the AD gauge can be (and it can do a very decent job if you have a good sender unit and an adjustable circuit). And the neat thing is if your fuel gauge ever starts reading too high or too low...you can adjust it as needed. If you need more info on an adjustable converter, please send a PM or an email to jon_goodman@yahoo.com The one I make can be seen below.

As showkey indicates, you are good to go. No problem.

I have a different issue with mine. It sounds like Frank's question has been answered so I will ask mine. I converted to 12v but I went ahead and replaced my gauge with a 12v gas gauge. Here is my problem When I have a full tank it buries the needle to the right past the F. After I run the truck enough to get to about 3/4 tank to needle seems to read correctly after that. So between 1/2 and 3/4 tanks is when the needle drops from buried to the right to working correctly. I have never seen the needle actually just drop while driving. It just randomly is working at some point when I start it.

I also have a new 12v sending unit.

Is there a chance that the float is hanging up when filled, then after the fuel drops enough, the float gets "unstuck"? Maybe vibration or a bump in the road helping it out? Just a thought. Unless there is an issue with the new sending unit? You can pull the sending out and check with an ohm meter to see if it reads in all positions. And while your at it, look for other issues like hangups, and stuck movement of the float in the full position. Not to mention tank clearance....

I would not worry about it. There are tolerances involved. Both mechanical and electrical. Gauge needle calibration, float arm calibration. sloshing, whatever. The bottom line is that is not an issue of running out of gas. It does settle to read semi accurate. You would have to remove the sender and the gauge and set it all up on the bench to calibrate. There would be a chance to make it worse or damage something. The system was never super accurate to begin with.
Senders are not voltage specific. They can be used on 6v or 12v. The gauges are specific.

Other folks have their needle peg and stay pegged. This is a wiring/grounding/shorting issue. Yours works and does not stay pegged, so seems not a wire/ground issue.

Dave it sounds to me like an issue with the sending unit and that can be fixed. Please remember a couple of things. 1...your sender is a resistance affair. When the float is all the way up (tank full), the sender must register 30 ohms. 30 ohms of resistance to ground to be specific. And when the float is all the way down (tank empty) sender must register 0 ohms (another way of saying no resistance at all or straight to ground). 2...your fuel gauge cares more about current than voltage, but since you've already bought a 12 volt gauge, let me concentrate on that for the time being. That new gauge...was it specifically made for the AD truck or was it an add-on generic type gauge? And that new sender...was it a 0 to 30 ohm sender or could it have been a 0 to 90 ohm sender? GM changed to 90 ohm tank sending units in 1965 and those won't work on the AD gauges. Almost all of the add-on gauges you buy will be 0 to 90 ohms and then when you wander way off into the weeds, Ford and others used some different ohm ratings. The AD gauges look for 30 ohms of resistance to display F on the gauge. More resistance will cause them to read over-full. Let's start there. Do you own a handheld electrical tester? If so, unhook the wire at the top of your sender with a full tank, set the handheld tester to the ohm reading setting (will look like a little horseshoe) and set the selector as low as possible (could be 200 ohms or 400 ohms...something like that). Then connect one lead to the connection on the top of the sender and one lead to ground. That will get us started...

Gauge was made specific for my model truck. I think but have not confirmed yet that my sending unit is 30 ohms. I do have a tester and will check it. Thanks Jon