Determing correct Ohm vs Volts for fuel gauge senders

[RonD2]

And thank you Wes. I'm not alone in appreciating what you do here. It must be like herding cats.......

[4x4M38]

Ron,
Can't let you walk the plank alone. In the process of addressing
the specific question of a 12 volt sender I opened a can of worms.
I wrongly assumed it was germaine, but understand Wes'
point.

I appreciate Wes moving the general discussion to its own
post. Hopefully we can continue to run it down as it has
been asked numerous times.

[RonD2]

Brian,
Appreciate that! Wes very cordially let us off the short end of the plank. I was just <badly> trying to add that it seems to me that with all the clear schematics available to us on these M38 jeeps and the vehicle being so versatile --- that basic electricity rules like Ohms Law (the relationship of volts, amps, resistance, and watts) might help better understand how things work. That, and a multi-meter, helps me anyway. I learned a long time ago that guessing or experimenting with electricity usually doesn't end well. Obey the Law, or else......

Lately I find myself thinking it'd be cheaper in the long run if I could just pay travel and per diem for a "tech assist visit" from guys like Wes or G740, or many others in this forum <including you> to come to SC for a week or so to jerk the knots out of my 1st ever jeep rookie rope....... there I go again......that's a topic for a different forum.....

[DC]

Thanks for the post. I have been asking these questions of myself, am a rookie as well, so glad I joined up and can follow. Please post your final outcome. I have an M38a1 that was converted to 12v, and can't afford to bring it fully back, so will stick with the conversion. My gauges are also 24v, so the same delima. I did test the light switch with 12v on the bench, and it works fine. I put together a 12 v power supply so I can test before I ruin the wiring. Anyway, thanks.

[wesk]

DC, Keep in mind that the built in circuit breaker in your light switch will not give your light system the same protection it enjoyed under 24 volts. With 12 volts and the wattage/resistance of the 12 volt lamps you have changed the load a great deal. Plan on adding circuit protection in the form of separate fuses or circuit breakers for the various lighting circuits or at least a new properly rated fuse or circuit breaker in wire # 15.

The military went to 24V for several reasons. It meant lighter electric motors and wires. But it also meant increased voltage (read pressure here) to make things turn faster). This was bound to be the case just like the upgrade from 6 to 12 V over the early years. Trouble is as voltage goes up load or amperage/current flow is reduced. Now when you take this backwards and reduce voltage to 12 V then you are increasing the load or amperage/current flow which then requires heavier wires and higher amp rated fuses and circuit breakers. If you are using the military harness then you are in luck a bit as the Army used a much heavier gauge wire then needed. But with circuit protection devices you will have to calculate the new current draw / amperage/ load and select the correct size fuse.

[4x4M38]

DC,
I sent you a PM regarding the light switch and other subjects.

[wesk]

PM's really don't help share information with the group which is why most folks came to this forum to start with!

[4x4M38]

As this was beginning to wander off volts and ohms for gauges
and senders and I was replying to DC with other electrical information
I sent him a pm trying to avoid the last dust up Wes.

[wesk]

Then why not just tell him here that your going to open a new thread on the light switch? At this point the discussion is still in the 24v to 12 V area.

[DC]

Thanks both of you for your reply, I can see this site will be extremely helpful in my project. Wes, I planned on installing a modern fuse box and running my circuits through that. The old harness has been farmerized much, and will need to be upgraded anyway. Also, thanks for using water terms as well. "Pressure". I have always thought of electricity as water flow, makes it much simpler for a fre fighter like me to understand. Like I say, wish I could afford to go back to 24v, but out of my budget at this point. If I find some solutions on gauges since that was the original post, I'll try to post that as well.

[southpw]

This is the combo Ijust bought. VDO 301-015k
hoping it does the job.

[DC]

Thought this could be a KIS option for the original fuel gauge question. I have found a step-up converter that will convert 12v to 24v, usually in various amp ranges. If you know the amperage input for the fuel gauge, then a step-up converter could conceivably give you the 24v you need to run the gauge. I think I saw a 2 amp converter for around $10, and a 10 amp for around $15.
If someone measures that amperage coming to the gauge with 24 volts, that would be good to know.

[85jeepcj]

Keep in mind that your gas gauge is actually a voltmeter. When 24 volts are applied to a stock 24 volt gauge the meter will read full.

As the fuel level lowers the sender increases resistance, the voltage is decreased causing the gauge reading to drop.

You will not be able to make the stock 24 volt gauge work correctly unless you apply 24 volts,

No tank sender resister will make it work. You would need a stepup power supply, 12v-24

When the tank is full of gas the resistance would be zero. When the tank is empty you would see the highest resistance.

[4x4M38]

85cj,
Not to beat a dead horse but the military sender (and maybe others) work the other way. 0 ohms is empty, 30 ohms is full.

[wesk]

http://willysmjeeps.com/downloads/MS500 ... 20TANK.pdf