I replaced my fuel sending unit in my 64, I had to opt for a 65 unit since the 64 was not offered in 3/8". Now my gauge reads full all the time, I was reading in 64 the ohm rating was 0 for empty and 45 ohm for full (some reason they were unique from the rest). The sender for a 65 and later years is 0 for empty and 90 ohm for full. Is there some sort of resistor I can install in the hot line to basically reduce my ohm in half for the fuel sending unit? Surely there is a way to make this work. Please chime in!

You need to put a resistor in parellel ( one end to ground, the other end to the sender wire ) with the sender. The ideal value to get 45 ohms total would be 90 ohms. I got that value by using the formula of the product divided by the sum ( 90 x 90 / 90 + 90 = , 8100 / 180 = 45 ohms. ) The closest easily found resistor is 100 ohms
( http://www.radioshack.com/product/index.jsp?productId=2062295 ). Those resistors have a 5% tolerance ( 95 to 105 ohms ) so your final ohm reading would be between 46.2 and 48.5 ohms.

So one end of the resistor would be installed in the positive (hot line) and the other end of the resistor would be attached to a ground? It would not install "in line" of the hot wire? I was reading several other posts which suggested (from experience) to use a 39 ohm 10 watt resistor...I know nothing about ohms.

One end of the resistor would be connected to the sender wire ( tan color on my '70 ) which I believe you refer to as the hot line. The other end would connect to ground. If you connected a 39 ohm resistor in line ( called "in series" ) the resistance would increase to ( 90 + 39 ) 129 ohms. I did a search but didn't find any posts refering to a 39 ohm resistor. However, I did a power calculation and the one watt resistor ( in the link above ) would be at its limit. It would be better to use a 10 watt resistor as shown here , http://www.radioshack.com/product/index.jsp?productId=2062293
You can see how the gauge will react before cutting, splicing, etc. Look for the intermediate wire harness to rear lamp wire harness connector in the trunk. It should be on the left side. Leave it connected and find the sender wire ( tan ? ). Push one end of the resistor into the back of the connector so it makes contact with the sender wire terminal. Ground the other end of the resistor. The gas gauge needle should move down depending on how much gas is in the tank.

I have had several opinions on this, I went and got a 39 ohm and a 47 ohm just to tinker with it. When I installed the 47 ohm it dropped the gauge to about a half tank plus a little, when I installed the 39 ohm it dropped it to half a tank less a little. There should be between a quarter and half tank of fuel in it...I think the 39 is close but according to testing the 2 resistors I am thinking around a 25 to 30 ohm is going to work out best. I have not tried a 100 ohm yet but it seems to me with testing the others and since the lower ohm dropped the gauge more then the 100 ohm would work opposite. Maybe the math is backwards?

When dealing with only 2 "resistors" in a circuit (#1 being the sender, #2 being the added resistor) the math/theory/calculations is easy.

In series they simply add - #1 + #2 = total
In parallel there's a little math - 1/Rt = 1/R1 + 1/R2 or in your case 1/Rt = 1/90 + 1/47.... which is 33.33 ohms. Using the 39ohm resistor in parallel would be 28 ohms.

John, thanks but I dont understand all the math (been too long since school) so what would be you opinion running in parallel? If the sender is 90 ohm and the gauge is 45 ohm (both full)...according to what I tested with the 2 resistors the 39 ohm was still not low enough, thats what has me tricked. I am assuming also the resistor will provide the same resistance across the 0 ohm empty to 90 ohm full from the sender, is that correct?

Doh! Just rattling this around in my head I realized something. There isn't a way to make this work. Because there is a fixed value resistor in the circuit, the gauge will NEVER zero out - regardless of series or parallel connection - it will always "see" some percentage of the fixed resistor.

You'll have to get a '64 sender, or a '65 gauge. The '65 guage should be a bolt-in for an idiot light dash.

(Gotta go chase electrons for pay now.... (work). I'll check back later.)

The sending unit must not be grounded correctly.
The original post states the gauge reads full all the time.
If zero ohms is empty on both units,it should have no trouble
reading empty.
The problem would be in reading full at half a tank.
If using the 0 to 90 ohm sender on a 0 to 45 ohm gauge it would
show more fuel than you have, until your way too close empty.
As the fuel level drops the gauge should read empty, both senders
being the same value at empty, 0 ohms.
Adding a resistor in the circuit will never allow the gauge to see 0
and always show some fuel.
In short, a resistor will not work in this instance.

Go to RadioShack, buy a 47 ohm resistor and a 33 ohm resistor. Wire them in series. One leg of the 47 ohm resistor gets connected to one leg of the 33 ohm resistor. Take the free leg of the 47 ohm resistor and splice that into the wire that connects into the back of the gas guage (not the power wire!!), the wire (tan?) that is from the sending unit. Take the free leg of the 39 ohm resistor and connect that to a clean metal ground.

You have now paralleled 80 ohms of resistance with the gas tank sending unit's variable 90 ohm resistor.

Your gauge wants to see a 45 ohm sender. You have a 90 ohm sender.

This is what your guage needs to see: 45 ohms=Full * 22.5 ohms= half * 11.25 ohms= quarter tank * 0 ohms (ground)=empty

Since your 90 ohm sender is in parallel with 80 ohms of resistance, it doesn't look like a 90 ohm sender anymore.

80 ohms in parallel with 90 ohms "full" = 42.3 ohms FULL

80 ohms in parallel with 45 ohms "half" = 28.8 ohms HALF

80 ohms in parallel with 22.5 ohms "quarter"= 17.5 ohms QUARTER

80 ohms in parallel with 0 ohms "empty" = 0 ohms EMPTY

Notice the numbers are off, reading higher than they should, so it will fib to you indicating more gas than you have, but should be CLOSE enough.

Try it!!

u referred to the 39 ohm resistor towards the end of your paragraph... so r u saying to leave the 39 ohm I have in now or was it a typo?

u referred to the 39 ohm resistor towards the end of your paragraph... so r u saying to leave the 39 ohm I have in now or was it a typo?

Oops, typo. Get a 33 ohm and a 47 ohm resistor

Go to RadioShack, buy a 47 ohm resistor and a 33 ohm resistor. Wire them in series. One leg of the 47 ohm resistor gets connected to one leg of the 33 ohm resistor. Take the free leg of the 47 ohm resistor and splice that into the wire that connects into the back of the gas guage (not the power wire!!), the wire (tan?) that is from the sending unit. Take the free leg of the 39 ohm resistor and connect that to a clean metal ground.

You have now paralleled 80 ohms of resistance with the gas tank sending unit's variable 90 ohm resistor.

Your gauge wants to see a 45 ohm sender. You have a 90 ohm sender.

This is what your guage needs to see: 45 ohms=Full * 22.5 ohms= half * 11.25 ohms= quarter tank * 0 ohms (ground)=empty

Since your 90 ohm sender is in parallel with 80 ohms of resistance, it doesn't look like a 90 ohm sender anymore.

80 ohms in parallel with 90 ohms "full" = 42.3 ohms FULL

80 ohms in parallel with 45 ohms "half" = 28.8 ohms HALF

80 ohms in parallel with 22.5 ohms "quarter"= 17.5 ohms QUARTER

80 ohms in parallel with 0 ohms "empty" = 0 ohms EMPTY

Notice the numbers are off, reading higher than they should, so it will fib to you indicating more gas than you have, but should be CLOSE enough.

Try it!!

Coppertop's math is correct, there is no perfect fix for your problem if you keep the 64 gauge and 65 sender, but his suggestion should get you in the ballpark. The gauge will keep you from running out of fuel, but the needle won't be linear as the tank level changes.

Here's my solution....

Buy a '64 sender and have a 3/8 or whatever size pickup tube brazed onto it. Done, no more resistor math and no new gauge needed. The resistor stuff is just a bandaid for what should be there in the first place.

Just an observation, aren't the early GM senders ('64 and earlier) a 0-30 ohm unit? Saw your reference to "a 45 ohm unit".

Have tried several things just to get a somewhat accurate reading, the thing that is wierd...there is approx. 1/4 tank of fuel in the tank currently. Installing a single 39 ohm reads 1/2 tank, installing a single 47 ohm resistor reads 1/2 tank plus about an 1/8th, adding them both in parallel shows 3/4 tank... is this not completely backwards from what all the math shows? Shouldn't more resistance in adding ohms lower the fuel level on the gauge? I had a person to email me with there own math and they showed approx. a 25 ohm should be the closest. I understand easiest thing to do is get matching gauges/unit but I just want to know when I am gonna run out of gas, I may put 2 tanks a year in it so its not that great of a deal to be dead on. Also per Daves question,,,the sending unit is a 45 ohm and not a 30...correct? If it is a 30 then I am sure the math would change

I found this with google. These guys don't even make a 45 ohm sender. (My '33 uses a 30 ohm unit and pretty sure they didn't change till '65.
http://www.egauges.com/vdo_send.asp?Sender=Classic_30ohms

Here's a source for a '64 Chevelle only with 3/8" line

http://www.bobschevelleparts.com/fuel-tank-sending-units/

Well this is news to me....I was under the impression that '64s used a 0-45 ohm sender?!

If it's 0-30? Can someone definitely confirm? I'll redo the math.....

Well this is news to me....I was under the impression that '64s used a 0-45 ohm sender?!

If it's 0-30? Can someone definitely confirm? I'll redo the math.....

I don't REALLY know either Joe. I was just surfing around looking for 45ohm senders and couldn't find any. I knew that my old hotrod was 0-30 and got to looking at Impalas, Novas, etc. They were 0-30 up through '64. They just had different configurations for tank depth and shape. I haven't found a '64 Chevelle sender description that "lists" the resistance spec. And on top of all this, my ISP SUCKS and goes to dialup speed without warning........... gotta love Sprint Broadband.

Did find that YearOne sells a "Rick's Hot Rod Shop" '64 ONLY A-body stainless tank for efi that "includes 0-30 ohm sending unit".

could u do the math on if it is a 30 ohm and I will test it and see what it shows, its a cheap try the resistors are only about .75 each... seems to me from testing so far that about a 25 ohm is what is gonna work out,,just wondering if the math can confirm

Okay, here's the re-work for a 0-30 ohm sender system. Assuming your '64 guage wants to see 0-30 ohms, here's what you do to your 90 ohm sender in the vehicle.

Okay, the gauage wants to "see":

30 ohms FULL TANK

15 ohms HALF TANK

7.5 ohms QUARTER TANK

0 ohms (ground) EMPTY TANK


Sooo....Go to Radio Shack and obtain a 33 ohm resistor, you probably have one already as it sounds.... Connect one leg to a good clean metal ground, connect the other leg to the tan wire that is the sender wire that leads to the back of the gauge in the dash.



Now you've paralleled 33 ohms of resistance with a 90 ohms variable resistor (the sender inside the tank).


24.1 ohms FULL TANK

19 ohms HALF TANK

13.3 ohms QUARTER TANK

0 ohms (ground) EMPTY TANK



Try it!!

Did you ever try it again, how did it go?

I'm pretty sure '64 is 0-45 ohms while '65 and up are 0-90 ohms.

Tried it again with that ohm setting, I think its a 45 ohm gauge. I am just gonna deal with it, it is pretty accurate on the first go round u suggested..it just drops off fast after a half tank