The Stovebolt Site Forums The Shop Area The Electrical Bay Possible fix for a 90 ohm sender to a 30 ohm gauge

I recently put in a new 20 gallon RCI fuel Cell with a 0-90 ohm sender in my '57 Burb. I took the "0-90 ohms" as a workable range for all gauges falling in that ohm range. Dummy me, that would be too easy huh? Well, in trying to locate a sender that would work, I stumbled across a possible fix for the old "90 ohm sender to a 30 ohm gas gauge dilemma". I came across this post http://www.hotrodders.com/forum/gm-fuel-sending-unit-conversion-30-ohm-106069.html where a guy claims that you can put a 47 ohm resistor or a 50 Ohm potentiometer (which apparently is adjustable) in line from your sender to the gauge. It sounds like this will correct the ohms allowing your fuel gauge to read the level of fuel correctly. If you use the 50 Ohm potentiometer, you can adjust the gauge to read empty with several gallons of fuel in your tank which gives you the ability to set your reserve amount so you won't run out of gas. Has anyone ever heard of this or tried this? I haven't yet, but will as soon as I can locate the parts. I'll let you know how it works. Bongo

Carefull there, the trick is to have the pot or resistor in parallel not "inline". There is another link in the article you linked to that defines that. You want the resistance to go from the sender terminal on the meter to meter case for ground or from the terminal on the sender to ground. Since our Stovebolts have the meter mounted in the dash the metal back of the dash is sufficient. The quick way to set this up and check it, without working onder the dash, would be to put the resistance on sender terminal on the gas tank and to ground.

Larry

So this is possible huh? Thanks for your reply and pointing out the grounding of it. Bongo

If my memory is correct the formula for calculating the value of resistors in parallel is the sum of the inverses equals the inverse of the sum. That sounds like double-talk.

Or to caluculate to get the require conversion value it would be the inverse of the target minus the inverse of the fixed value equals the inverse of the required conversion value.

I don't know if that makes sense, I'm finding this difficult to put into text...

1/30 - 1/90 = 1/(needed value)

or 1/(1/30 - 1/90) = needed value

My math says it needs to be 45 ohms, which is not a standard value, but 47 ohms is and would get you real close. Using a 50 ohm variable resister would allow you to adjust until it was perfect.

It should work, they say it works. It been about 45 years since I had my one and only electronics class in Direct current Theory. Meter movements are a little strange, but I don't remember the particulars. Remember to tie the two end terminals together on the pot if that's what you get. Here's a couple of examples I found on that auction site. The first doesn't have enough wattage, it's only a 1/2 watt, but it has 25 turns from min ti max. With that many turns you don't necessarily have to have a 50 ohm pot you go a little larger say 100 ohm max.

(Ebay link deleted)

This next one is closer at 1.5 watts, also notice that it could be used for volume control on speakers if you were having problems balancing them. When used on speakers then you can put them inline, because your adding resistance. These are a little pricey, especially since they are coming out of China. It might be due to close tolerance.

(Ebay link deleted)

Hope that helps, and doesn't confuse.

Larry

I found the exact POT that is called for in the blog. It is on this web site http://www.smcelectronics.com/pot.htm and is the eleventh one down on the page. I ordered two today at $4.50. I thought it was allot better than a thirty dollar sender, plus shipping and I get a reserve POT and in the tank. I'm assuming the darker terminal in the middle is for the ground, is this correct?

NO, the center terminal is your resistance adjustment, aka center tap. Run a wire from the center terminal, to either outside terminal, then to the sender terminal on meter or gas tank. The other terminal will then go to ground, back of dash or gas tank ground, where ever you decide to mount it. Make sure all exposed wires or terminals are covered, heat shrink, tape, potted in epoxy, what ever when you are done.

That POT should work fine.

The center terminal or center tap is usually a little piece of metal or brush material (the same as used in an old generator) that rides on the resistance material, carbon, wire wound, etc.. If you've ever had your light switch out and looked at the light dimmer rheostat, aka POT, same thing.

Larry

Please let us know how this works out. In theory, it will work.......sorta. The "pot" should be used to set the guage to read full when the tank is full. After that, the only times your guage is going to be accurate is when the tank is full, or completely empty. Anything in between, especially between full and 1/2 full, your guage is going to be off. (example: at 3/4 tank, your guage will be reading 1/2) However, the closer the tank gets to being empty, the closer the guage readings will be. I think whoever said you can adjust the pot to give you some reserve is all wet, but then, I've been wrong before. But, all that being said this is a cheap fix that should at least keep you from running out of gas. The only way I know to keep all readings accurate would be to adapt a 30 ohm sender to your tank.

Good luck, and keep us posted.