Fuel Gauge Help

[1947coupe]

I have a few questions about the fuel gauge on my 1948 Dodge Coupe.

I'm assuming it was 6v originally, should the fuel gauge be supplied with 6v or some other voltage? I know a lot of later cars used voltage limiters to drop gauge power to about 5v but not sure if anything was used on 6v cars.

The original sending unit only had one wire connection but the replacement I got has two. Is it possible to wire this sender to my gauge? I forgot to check the resistance of the sender before I fitted it but did take some readings the other day. With a couple of gallons of gas in the tank terminal  one had a reading of 8 ohms between it and ground and terminal two had a reading of 23 ohms between it and ground. The resistance between them was 30 ohms. After adding 4.5 gals terminal one had a reading of 23 ohms between it and ground and terminal two had a reading of 13 ohms between it and ground. The reading between the two terminals was still 30 ohms. What resistance scale is the gauge designed for?

 

Thanks for any help

[Plymouthy Adams]

if you have a single wire originally, the car would be very late production and for sure a first gen 49.   The two wire system is 30 ohms across the terminals and the wiper will split this to ground in the manner you described and is normal.  What you have to have mounted in the instrument cluster is the two wire gauge to work with this sender however this gauge will have three wires, +6 in and two leads usually ,marked with letters so that you must route A-A and B-B as marked on gauge and sender.  Note also internal to the gauge is voltage regulator even for 6 volt application.  Check you instrument cluster to ascertain what gauge is mounted, the sender will then need to match that.  If your gauge is but two wires.....you have the later system and will need the later style sender that operates on the single wire to ground through the resistor and wiper. (wiper is the float lever and is the source to ground)

Edited February 6, 2022 by Plymouthy Adams

[1947coupe]

Looks like the later style. So, 6v to the SW terminal, get a 1 wire sending unit and connect the GA terminal to it?

What should the empty and full resistances be on a one wire sending unit?

[Plymouthy Adams]

that is not normally specified in the book but I have a note to the effect 70 ohm is empty and approx. 10 is full..you do not want to run less than the 10 ohm on the average as you do not want to over drive the gauge needle into the upper stops....it should be pointing at the full line....when empty the reistance is higher thus lower voltage to move the needle it will point to E at the 70 ohms...of course if this is higher it will only drop lower but is NOT driven low....just weighted to go low...so if this falls below the E mark...nothing is damaged by design as they say.  I think you can find the correct sender in the 10-73 range  or 10-90 may do the job, the critical part is the 10 so not to overdrive the gauge electrically.  These are on e-bay daily in generic.....will not even speculate on actual direct replacement cost and availability. 

 

just a friendly link....buy what and where you wish...........

 

1950 PLYMOUTH WAGON ONLY FUEL SENDING UNIT GASOLINE FUEL SENDER GAUGE | eBay

[TodFitch]

I believe that the 1950s single wire sending units used a 10 to 78 ohm range with 10 ohms being full.

 

This might help: https://www.ply33.com/Repair/fuelsender.html

[Plymouthy Adams]

yes, evident in the link I provided and as yours listed as generic 10-78.....but for some reason the very note written in my book states 70...could have just been a number Bullwinkle pulled out of his hat, for sure he never pulled out a rabbit...?  Other sending units listed on line swears they are correct at 10-73 and others say 0-90  I do not recommend the '0' as that will drive the needle to the stop..  Tanks-Inc list 10-73.....as the higher resistance is empty and no driving voltage to a stop....this is the end you can fudge a few ohms.

[1947coupe]

Thanks for your help, I’ll look for a new sending unit.

[harmony]

I'm hoping I'm not hijacking this thread, and I apologize if I am.  But speaking of fuel gauges, and since I have the attention of guys who sound knowledgeable about the gauge, when a bought the car a few years ago the gauge was fairly accurate.  I would stop for gas at about 1/4 full and filler her up and the needle would go to full.  But lately the needle only drops to just below half when it needs gas again.  I've gotten used to setting my odometer at ooooo when I fill up and at around 100 miles, I fill it up again and it takes around 15 gallons or so.  So I have maybe a couple gallons left in the tank.

I thought I read somewhere that there is a way of adjusting (calibrating)the gauge? Plus I'm curious why it has gotten lazy so to say?

[maok]

2 minutes ago, harmony said:

I'm hoping I'm not hijacking this thread, and I apologize if I am.  But speaking of fuel gauges, and since I have the attention of guys who sound knowledgeable about the gauge, when a bought the car a few years ago the gauge was fairly accurate.  I would stop for gas at about 1/4 full and filler her up and the needle would go to full.  But lately the needle only drops to just below half when it needs gas again.  I've gotten used to setting my odometer at ooooo when I fill up and at around 100 miles, I fill it up again and it takes around 15 gallons or so.  So I have maybe a couple gallons left in the tank.

I thought I read somewhere that there is a way of adjusting (calibrating)the gauge? Plus I'm curious why it has gotten lazy so to say?

Best to check the sender first.

[harmony]

2 minutes ago, maok said:

Best to check the sender first.

You mean with an ohm meter?  If so what am I looking for?

[TodFitch]

23 minutes ago, harmony said:

You mean with an ohm meter?  If so what am I looking for?

If I recall the factory service manual trouble shooting for the two wire fuel gauge system, readings only going half way indicates a bad wire between the sender and the dash unit. Which one and how to confirm would require me to go look at that manual. I assume you have a copy of the factory service manual. . . If not it would be a good investment for you.

[harmony]

5 minutes ago, TodFitch said:

If I recall the factory service manual trouble shooting for the two wire fuel gauge system, readings only going half way indicates a bad wire between the sender and the dash unit. Which one and how to confirm would require me to go look at that manual. I assume you have a copy of the factory service manual. . . If not it would be a good investment for you.

Thanks, I'll read up on it.

[Plymouthy Adams]

I think in the past this page has been scanned by someone and posted in a thread.  The book as Tod says is very informative on the testing of the unit.  You may start with a search.  Use:  +fuel +guage +schematic

 

the two wire is designed to pull the needle both toward and full and empty at the same time thus the rock solid dampened non flicking needle.  This is why the wiper divides the approx 30 ohms as it takes both legs to ground at the same time, this is how you get say 23 to post A and 7 to post B and this divide will add to 30 wherever the wiper is positioned across the resistor if clean contact and intact resistor winding.  IF you have an open in the resistor only one leg will read as the wiper is on that side of the open and when the wiper drops to the other side of the open, then the other leg picks up and the first drops out, you will not add up to 30 as only one leg is going to ground in this situation.  It is important both wires A and B having continuity gauge to sender, clean connections A and B and most important of all the wiper must not be intermittent to ground and you ground must be good ie: clean and tight.  I mention wiper must not be intermittent.  This is often an issue as the wiper transfers the resistor divide to the case of the sender by a pivoting contact lever inside a small ball indent in the housing...if this is rusted up and not making constant contact...expect the gauge to drop out completely as the path to ground is lost here.  Other key important thing here is that wires A and B should never be shorted to ground as it travels from the gauge to the sender.  This will lead to what ever leg is grounding to be in full strength pulling action of the needle.  Really not that hard....No shorts, no opens and good connections, good resistor, good wiper contact to ground...an added ground wire form sender to another point on the body is recommended due to the age of these components and possible compromised metal to metal bonding.