Determing correct Ohm vs Volts for fuel gauge senders

4x4M38 · Post by **4x4M38** » Thu Mar 24, 2016 7:02 am

FYI,
According to the troubleshooting downloads info for the M38 senders they are 0 ohms empty and 29.5-31.5 ohms full.

GM products up to 1964 used fuel senders that were 0-30 ohms.

Here is the Tanks, Inc. universal sender for 0-30 ohms:
http://www.tanksinc.com/index.cfm/page/ ... cat159.htm

Here is Classic Instruments' sender for 0-30 ohms:
http://shop.classicinstruments.com/fuel-sending-units

The trick I have found is finding a 0-30 ohm gauge for 12 volts that looks right.

Both vendors sell senders in several resistance ranges.

So, I agree with Wes that if you are at 12 volts it would be easier to just buy an aftermarket gauge you like and match its required resistance with one of the universal units above.

There are some earlier used gauges out there that look right and even have the light windows cut into the case, specifically old John Deere tractor gauges. Just be aware that many of those old tractors and other vehicles were positive ground. Trailing up information like that is a challenge, as it is not usually tacked into the descriptions on Fleabay.

Take care,

Post by **wesk** » Thu Mar 24, 2016 8:29 am

The reaction of 0-30 OHMs with 12 volts applied is not the same reaction of 0-30 Ohms with 24 volts applied. The needles are designed to be driven a certain distance by the applied voltage trickling thru the resistor.

Let's not try re-inventing the wheel here and KIS keep it simple.

4x4M38 · Post by **4x4M38** » Thu Mar 24, 2016 9:07 am

Hi Wes,
I understand what you are saying.

What I have found in some sales literature shows using the same
particular sender for 24 volts as 12 volts, just using a resistor
between the ammeter and gauge. That tells me the sender
doesn't care, or some individual senders don't care.

That doesn't square with your statement above, which I've said
makes sense to me.

Wonder if we can get an EE to help with this.

Take care,

RonD2 · Post by **RonD2** » Thu Mar 24, 2016 10:12 am

Ohm's Law says that current (amps) is equal to voltage divided by resistance.

If resistance stays constant and volts double (from 12 to 24), then amps also double.

I'm by no means an EE, just the beneficiary of one of Uncle Sam's avionics schools many moons ago, with a few shocking experiences along the way......

4x4M38 · Post by **4x4M38** » Thu Mar 24, 2016 10:34 am

Hey Ron! Yep, get that.

I understand if you have a 24 volt system and want to use a 12 volt gauge/sender that you can use the proper dropping resistor as recommended by the vendor and that should work.

We know that in our case because the first M38's used 6 volt gauges with a 90 ohm resistor between the supply and the back of the gauge.

What I'm questioning is going the other way, namely using 24 volt components on a 12 volt system, specifically as the folks up top asking about the 24 volt sender. The gauge I get. Won't work.

What I've found to muddle the waters are pages in the Stewart Warner catalogs showing 12 and 24 volt electrical gauges (temperature in this case) that use the same sender.

24 volt gauge 84002 100-240 degF uses sender 280EC.

12 volt gauges 82374, 82389, 82777, 82778, 82730 and 82740 all use sender 280EC as well.

In the case of 12 volt gauges used in 24 volt service there is a specific reference to installing a resistor. Nothing in the literature is mentioned about a resistor being necessary with the above universal sender being used in both 12 volt (and 32 volt) systems.

See my confusion?

Take care,

RonD2 · Post by **RonD2** » Thu Mar 24, 2016 10:52 am

Hi Brian,
I'm not a gauge expert either.......maybe a call to Stewart-Warner tech support might answer the sending unit question?

A variation of Ohm's Law says that power (watts) is equal to current times voltage. If current doubles then so does power (watts), meaning components have to be rated (in watts) to handle it.

I'm guessing the sender you mention is rated to work with a wide range of voltage, current, and power, and so long as the rest of the components in the circuit (wire, gauge, resistor if any, etc) are rated the same then nothing real bad (like smoke) will happen. Getting the needle on the gauge to deflect to the right number on its scale is another matter.

I think using a mated pair (gauge and sender) as sold by reputable vendors is the only way to fly.

4x4M38 · Post by **4x4M38** » Thu Mar 24, 2016 11:05 am

Hey Ron,
I'm reluctant to try and talk to human tech support anymore, especially on 70 year old equipment, and particularly if one is asking a question that cannot be answered by looking at the tech pages on line.

As has been suggested here many times, if only I could see if the person on the other end had gray hair.......

So taking this the other way....
"Ohm's Law says that current (amps) is equal to voltage divided by resistance.

If resistance stays constant and volts double (from 12 to 24), then amps also double. "

Going from 24 volts to 12 volts with an OEM sender should offer the same resistance, but drop the current. What happens in the gauge is another matter, and the 64 dollar question.

Take care,

RonD2 · Post by **RonD2** » Thu Mar 24, 2016 11:13 am

No need to mention the 70 year old jeep!

We just want to know the current, voltage, and wattage ratings of the sender. I haven't looked, but would think all that should be available from their web site. Maybe not.

4x4M38 · Post by **4x4M38** » Thu Mar 24, 2016 11:23 am

Hi Ron,
I have not seen any of those specs on any of the vendors pages except for resistance and maybe voltage. One would think 24 volt components would handle 12 volts, but maybe not.

Skip, Bado, do you guys already have 12 volt fuel gauges? If so, let us know what they are and maybe the masses here can help you find proper senders.

In all of my digging I have not found 12 volt senders with Douglas or Packard connectors. Other than that if we know your gauge specs we should be able to help match up with what you need. I know Summit Racing and Speedway both carry aftermarket senders, including Tanks, Inc., universal senders.

Take care,

Post by **wesk** » Thu Mar 24, 2016 11:50 am

There. We are back to 8 posts ago! KIS Keep it simple. Buy a matched aftermarket 12 volt set and enjoy life.

For the super inquizative, refuse to stop searching folks just take this info to a electrical troubleshooting board:

Stock arrangement

1-operating resistance 0-30 Ohms.
2- 0 = empty
3 - 30 = Full
4- Input voltage 24-28
5- Measure internal resistance of the original 24 V indicator.

Desired modified arrangement

1- Retain original gauge
2- Use 12 V as input power

or

1-Retain original gauge & sender.
2- Use 12 V as input power.

Let us know which electrical wizard board gave you which answer.

4x4M38 · Post by **4x4M38** » Thu Mar 24, 2016 12:57 pm

I thought that was the purpose of the Technical Knowledge Base board Wes.

To ask questions and get answers about Willys M jeeps.

Post by **wesk** » Thu Mar 24, 2016 1:33 pm

Then if you wish to keep the discussion about a Technical subject on the tech board it is always nice to the original poster's problem for you to start a new discussion thread for the technical aspects of the poster's issue. At this point Skipanellin & badojeep's original question is lost under OHM's law equations.

RonD2 · Post by **RonD2** » Thu Mar 24, 2016 2:59 pm

Seeing as I obviously instigated the offense, I offer my apologies to anyone offended, especially the Ohms Lawyers. Being your average dumb jar-head, I suppose sooner or maybe later I'll figure out the fine points of what forum things belong in --- kind of like tip-toeing through a minefield on some days.......

I was going to think that looking for (wanting?) a fuel tank sending unit might belong in the "wanted" forum but I guess I better not.

Dangit! Did I say that out loud?

Seriously, there was no intent by me to obscure the original post. My sincere apology.

Btw, I do see them available at the usual favorably reviewed reputable suppliers frequently mentioned elsewhere.

Post by **wesk** » Thu Mar 24, 2016 3:52 pm

There's no offense here or apologies needed. We all need to be cognizant of what the post is about and address that only. When a new thought drifts far from the original post put that new thought in a new post with a title that makes it obvious what the new though is about.

RonD2 · Post by **RonD2** » Thu Mar 24, 2016 4:52 pm