Fuel sender resistance and gauge readings

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Fuel sender resistance and gauge readings

February 13, 2011, 10:48:54 PM

So, I'm thinking about picking up a 4-1 gauge that includes a fuel indicator. The resistance for it is said to be 240 E/ 33 F.

According to Eric on Coolcats, the 83 Sender would be 75 E / 10 F.

Anyone happen to know what that would look like if they were used together? Should I probably just go get a universal sender?

Let me know, and thanks in advance.

Fuel sender resistance and gauge readings

Reply #1 – February 14, 2011, 05:37:23 AM

Here's what I figure. The sender in the car would make the gauge read full until you were above 33ohms, or roughly 1/2 tank. (75-10=65 65/2 = 32 1/2 this is the first time the gauge would move.) The gauge would then drop to between 1/2- 3/4 full indication when you sputter to a stop on an empty tank. (240-33=207, 207-75=132, 132/207~.64) The true empty would be at 64 percent of full on the new gauge.

Fuel sender resistance and gauge readings

Reply #2 – February 14, 2011, 07:15:13 AM

Quote from: flylear45;352889

Here's what I figure. The sender in the car would make the gauge read full until you were above 33ohms, or roughly 1/2 tank. (75-10=65 65/2 = 32 1/2 this is the first time the gauge would move.) The gauge would then drop to between 1/2- 3/4 full indication when you sputter to a stop on an empty tank. (240-33=207, 207-75=132, 132/207~.64) The true empty would be at 64 percent of full on the new gauge.

Ahh, there we go. Thanks, I was having a hard time "getting" that one last night. Looks like the gauges come with a universal sender, but It doesn't look like too much fun to get it in there with the current fuel pickup.

Fuel sender resistance and gauge readings

Reply #3 – February 16, 2011, 01:59:28 AM

The ratio on the replacement sender you have specs on is about right.

10x3=30ohms
75x3=235ohms

having said this, i saved a big bunch of bs explaination on how to get it dead on when its going to read right anyway.

your kinda lucky the ratio is right though,
the big question is how the tripple resistivity agrees with the resistance of the fuel guage itself since its actually a voltage divider ckt.
looking now at the shop manual.

the guage is 10 to 15ohms, the power to the guage comes in on a 8ohm resistance wire from the ign sw and up to the IVR where the voltage is decreased.

seems reasonable to use the sender you found.

Fuel sender resistance and gauge readings

Reply #4 – February 16, 2011, 07:30:57 AM

Well, the gauge I'm going with is 240-33, so it sounds like if I can triple the stock sender's resistance, it will work perfect...Hmm.

Fuel sender resistance and gauge readings

Reply #5 – February 16, 2011, 08:37:33 PM

Yes,, its not really the resistance but the ratio of low vs Hi.

the multiplier seems close enough if not dead on.

id like to see other opinions though, this isnt somethign you wanna dive into without a second opinion.
Im not perfect but, based on what i see, id do it.

Fuel sender resistance and gauge readings

Reply #6 – February 16, 2011, 08:50:35 PM

I don't know how to make that work, personally. If you add a resistor to make one reading right, it will only be right at that value. Let's say you pick a resistor to add to make it read right at full. OK, that's 33-10=23 Ohms. So, you add in a resistor and now it reads right at a full tank. When you get to empty it would be 240-23,=217. In other words you would have run out of fuel near 1/2 tank again.

I'm no electrical genius, believe me. I could be wrong, but I don't think it is quite as simple as a resistor IMHO.

Fuel sender resistance and gauge readings

Reply #7 – February 16, 2011, 11:00:48 PM

hmm,,
thinking twice, I may eat my words.

so if your sender is a two wire,
solder an 80ohm resistor across both leads.

This will get you to 23ohms at full and 60ohms at empty.

23ohms isnt so great and is out of tolerance per the shop manual, not sure how you feel about that.,,
60ohms is acceptable for an E reading per the shop manual.
ie- the needle may not go all the way over to the "F" but should reach the full mark.
E-will be pretty dead on the money.

now i feel better.

your resistor first three color bands should be "brown""black""black"

as an FYI,,
I just noticed the MarkVII are reversed like the 2Oth starwars clusters.
about 140ohms at full and about 15 at empty.

Fuel sender resistance and gauge readings

Reply #8 – February 16, 2011, 11:20:41 PM

The only problem I can see with that is;

Stock sender/gauge only needs 60-75Ohms of resistance for E, but the aftermarket gauge I will be installing needs 240Ohms of resistance to read E.

So, somehow we need to get a stock sender to "transmit" a 240 Ohm E signal. And a 33 Ohm F signal.

Somehow I don't think this is going to work, but the alternative(Using the aftermarket sender) Doesn't seem very enjoyable either...

Interestingly enough, I may have found a thread with schematics for what I want, but I don't think they will work with my all-in-one gauge. So, I think I'll just graft the new sender to the old pickup tube in the tank...Unless you guys come up with a better solution.

Thanks for your assistance!

http://forum.allaboutcircuits.com/showthread.php?t=11342

Fuel sender resistance and gauge readings

Reply #9 – February 17, 2011, 09:03:54 PM

wtf are you talking about???????????,,,i just found your solution,, even spelled it out.
basic electroincs.,,but not a very basic math formula if you dont know how parallel resistors are calculated.

bond an 80ohm resistor to the two wires coming off the new sender.
that will put the variable resistor in parallel with the 80ohm resistor thus converting it to the more correct resistivity.
your total your high and low resistance will now be 23ohms and 60ohm respectively.

Fuel sender resistance and gauge readings

Reply #10 – February 18, 2011, 06:24:43 AM

Sounds like a solution to me. Good work jcassity.

Let us know how it works out!

On second thought, that makes the new sender work for an OEM gauge, not what the OP wants.

Fuel sender resistance and gauge readings

Reply #11 – February 18, 2011, 12:48:56 PM

Quote from: jcassity;353205

wtf are you talking about???????????,,,i just found your solution,, even spelled it out.
basic electroincs.,,but not a very basic math formula if you dont know how parallel resistors are calculated.

bond an 80ohm resistor to the two wires coming off the new sender.
that will put the variable resistor in parallel with the 80ohm resistor thus converting it to the more correct resistivity.
your total your high and low resistance will now be 23ohms and 60ohm respectively.

Sorry Jcassity, I guess I'm failing to see how 60 Ohms on E = the 240 Ohms on E that I need for the gauge.

I get that the full reading will be correct, but it sounds like the tank would be empty at "3/4 full" on the gauge

I think I see what the issue is though, you think I'm changing to a new sender and using the old gauge,

when actually I'm changing to a new gauge and I want to use the old sender.

Fuel sender resistance and gauge readings

Reply #12 – February 18, 2011, 02:41:26 PM

I was just thinking series resistance, jcassity is using a parallel resistor.

Here's what he wants you to try. Instead of putting the resistor in line with one of the wires, he wants you to put the 80 Ohm resistor ACROSS the two wires. (think H)

Different laws apply, then. That's why he says he's not going to take you through the equation and math, just tell you simply what to do.

Fuel sender resistance and gauge readings

Reply #13 – February 19, 2011, 02:13:26 AM

Quote from: jrad235;353239

Sorry Jcassity, I guess I'm failing to see how 60 Ohms on E = the 240 Ohms on E that I need for the gauge.
I get that the full reading will be correct, but it sounds like the tank would be empty at "3/4 full" on the gauge

see below, how i arrived at the numbers to answer the above question.

Fuel sender resistance and gauge readings

Reply #14 – February 19, 2011, 02:26:17 AM

When you said "the resistance for ""it"" "
I took that to mean it needs a sender with that resistivity and you were going to install a new sender.

Leme regroup.....
do the manuf specs indicate "the guage requires" 33-240 ohms to read right?
do the manuf specs indicate the coil resistance of the guage?

do you have a spec sheet or data sheet from the manuf?

simply put,, does the guage "NEED" to "Receive" 33-240ohms to function correctly?
or..
is this a simple range of resistance it will function within?
if my last statement is true then my initial gut feeling may have been correct. Your guage may not go "past full" but will read full while empty should be dead on the money.

Its back to the internal design of the guage from here forward.
Once we understand the guage specs, resistor(s) can be positioned up there to compensate for the disparity.