Gas Gauge Revisted

[moparjack44]

I checked the archives, but could not find the answer.

On my '48 DeSoto, gas gauge not working/reading properly. I have spending all my resources on the gauge, now I am moving to the sending unit at the tank.

1. Do I have to drop the tank to get to the sending unit, or is there a removable plate in the trunk floor that accesses the sending unit? There is in my buddies '49 Merc.

2. I 'ass'ume the sending unit in the tank will have two wires, since the gauge has three. The ones on the gauge reads 1, 2, & ignition. Are the wires at the tank numbered?

I'm really grabbing for straws here, as I really want my gauge to work. I am tired of always having to write down the mileage everytime I fill up with gas. Thanks for any help.

Jack

[RobertKB]

Yes, there is an access plate in the trunk for the sending unit. It should be two wire. Don't believe they are numbered. Just remember where they go. Disconnect the battery before removing the wires and certainly before reattaching to avoid any possibility of a spark. I had to replace both the gauge and the sending unit before I got my gauge to work. Tried one and then the other and no luck until I replaced both. Both replacements were used Mopar items but I know out there somewhere you can buy a new sending unit. Maybe someone else who has bought one can chime in.

[55 Fargo]

Yes, there is an access plate in the trunk for the sending unit. It should be two wire. Don't believe they are numbered. Just remember where they go. Disconnect the battery before removing the wires and certainly before reattaching to avoid any possibility of a spark. I had to replace both the gauge and the sending unit before I got my gauge to work. Tried one and then the other and no luck until I replaced both. Both replacements were used Mopar items but I know out there somewhere you can buy a new sending unit. Maybe someone else who has bought one can chime in.

Both Roberts and Burnbombs sell brand new 2 wire sender units, about $85.00 each.

Whats with this 3 wire thing, there is 3 contacts on the gauge, but open the access plate in the trunk and you should discover the 2 wires on the sender.

Try cleaning up your connections first, then see what happens.

What shape is this gas tank in? Should it be removed , cleaned, and reinstalled, with a working sender, and known to work gauge.

Let us know whats happening, will try and walk you through this.........Fred

[55 Fargo]

I checked the archives, but could not find the answer.

On my '48 DeSoto, gas gauge not working/reading properly. I have spending all my resources on the gauge, now I am moving to the sending unit at the tank.

1. Do I have to drop the tank to get to the sending unit, or is there a removable plate in the trunk floor that accesses the sending unit? There is in my buddies '49 Merc.

2. I 'ass'ume the sending unit in the tank will have two wires, since the gauge has three. The ones on the gauge reads 1, 2, & ignition. Are the wires at the tank numbered?

I'm really grabbing for straws here, as I really want my gauge to work. I am tired of always having to write down the mileage everytime I fill up with gas. Thanks for any help.

Jack

Remove the access plate and see how many wires arew hooked up to the sender, probably 2.

Make sure you have good wires from gauge to sender, with good connections.

If all fails, pull sender and gauge, bench test, one wire for pwer, ground both sender and gauge, use jumper wires betwen gauge and sending unit. Then manualy move the sender float, see what your gauge is doing. Make sure your wires from the gauge to the sender are in the correct positon............Fred

[55 Fargo]

I checked the archives, but could not find the answer.

On my '48 DeSoto, gas gauge not working/reading properly. I have spending all my resources on the gauge, now I am moving to the sending unit at the tank.

1. Do I have to drop the tank to get to the sending unit, or is there a removable plate in the trunk floor that accesses the sending unit? There is in my buddies '49 Merc.

2. I 'ass'ume the sending unit in the tank will have two wires, since the gauge has three. The ones on the gauge reads 1, 2, & ignition. Are the wires at the tank numbered?

I'm really grabbing for straws here, as I really want my gauge to work. I am tired of always having to write down the mileage everytime I fill up with gas. Thanks for any help.

Jack

Hi Jack, was looking at a doc I have saved, on a 2 wire set-up, if #1 wire from gauge is grounded, it will display on the gauge 1/2 full when the gas tank is actually full and it will display 1/4 tank, when you your gas tank is half full.

Are you experiencing these symptoms?

Another thing, try and keep this topic if you can on this thread, until the problem is resolved. This allows are viewers continuity on this thread from beginning to a successful solution.

I can send this 2 wire fuel gauge service page if you can PM me your email address........Fred

[Gary Manes]

If you do not find a problem with the sending unit, check out the shop manual which has a section on "re-calibrating" the gauge itself.

I have a problem with the gauge itself in my 1948 New Yorker coupe, but just have not had the patience to do the re-calibrating. It gets pretty involved.

I should not say "I" do not have the patience because it is really a medical condtion that keeps me from digging in like I use to.

Good Luck!

[greg g]

Usually the problem is a corroded ground. Run a jumper wire from one of the sending unit mounting screws to a good clean ground. Usually to one of hte screws retaining the access cover will do. That will take care of most problems in 90% of the cases. Alos make sure there is no inadvertant ground at the gauge end. My defroster tube fell off the difuser and fell across one of the posts on the posts and grounded it through the reinforcing wire. This screwed up the gauge reading.

[55 Fargo]

with the access cover off in the trunk you can unhook the wires from the dash gauge, and the tank unit. Run 2 new temporary wires (out the door and into the trunk) to test things out.

That'll work............

[moparjack44]

Thanks everyone. You have given me some good suggestions to work with. I will get in the garage tomorrow and see what happens. I will post results, good or bad.

Thanks,

Jack

[grey beard]

Back last fall when I had my own tank down for cleaning, I set up my gauge on the bench with a new JCW sending unit and tried to make the circuit work likee it should. Would you believe that I spent four days playing with this simple circuit before it finally hit me - I was duplicating the book schematic for the gauge circuit WIHOUT the body ground, which is NOT shown in the schematic. Once I ran a jumper fom the gauge base to the sender base, things worked just like the good Lord intended. Dumb me . . . .

[moparjack44]

Do you happen to know the part number of that JCW sending unit, just in case my needs to be replaced?

Jack

[TodFitch]

Do you happen to know the part number of that JCW sending unit, just in case my needs to be replaced?

Jack

I don't think that JCW has the two-wire sender. If you have a 49 or later with a one-wire sender then you can adapt the JCW sender:

http://www.ply33.com/Repair/fuelsender.html

[55 Fargo]

Do you happen to know the part number of that JCW sending unit, just in case my needs to be replaced?

Jack

Sorry Jack JCW does not sell the required 2 wire sender, they only have a 1 wire universal unit that you can buy anywhere, for $15.00.

These will work on cars 1949 or 1950 and newer.

You will need to open the wallet for a 2 wire unit from Roberts or Burnbombs for around $85.00.

You cannot make a 1 wire sender work with a 2 wire gauge either. the only way would be to use a uinversal gas gauge and sender , wired with a 12 volt system.........Fred

[moparjack44]

I'm back from the garage, and no good luck. The DeSoto does not have a plate that gives access to the sending unit. It just has two wires running thru a gromet in the trunk floor . Guess if I'm gonna work on the sending unit, I will have to drop the tank.

Thanks everybody for the info and responses of help.

I will return !!

Jack

[55 Fargo]

I'm back from the garage, and no good luck. The DeSoto does not have a plate that gives access to the sending unit. It just has two wires running thru a gromet in the trunk floor . Guess if I'm gonna work on the sending unit, I will have to drop the tank.

Thanks everybody for the info and responses of help.

I will return !!

Jack

That is very strange Jack, no access hole, I know if it were me I would be making one for future repairs/inspections.

How many wires run through the grommet BTW?

Actually all is not bad, you can clean or have the tank cleaned when you drop it out, plus you can inspect the sheet metal under there..............Fred

[Frank Elder]

Maybe the PO replaced the trunk pan and did it HIS way, more work for you unfortunately.

[moparjack44]

A friend is restoring a 48 Dodge 4dr, and I checked his trunk, and it had the access panel. I wanted the location, so I could just cut an access thru the carpeted trunk. I eventually ended up taking the carpet out, but still nothing.

It had two (2) wires running thru the gromet. I had already taken the tank out several years ago, cleaned and coated. I promise, when I drop the tank, I will cut an access panel for future times.

Thanks everyone, but may be few weeks before I get back on this particular item. My new speedo cable should be here next week, so thats next. I took out the old one, cleaned and greased, still had same issues.

Jack

[moparjack44]

I am back, but not much better off. I bought new tank sending unit from Burnbombs (other was toast). hooked it up, gauge would only read 1/4 when tank was full. Tried 3 different gauges, all the same.

HOWEVER ,while on a road trip yesterday, full tank, gauge reading 1/4, I realized the further I drove the higher the gauge reading was. It's reading backward. Showed 3/4 tank by the time I reached my destination. I filled the tank back up, gauge fell back to 1/4:confused: .

Jack

[DonaldSmith]

Is the sender (and gauge) two-wire (plus any ground wire)? The DeSoto two-wire gauge compares the resistance each side of the sliding contact at the sender, and makes the gauge needle conform. It sounds like your wires are reversed.

[moparjack44]

Gauge and sender are both two wire. Gauge, two wire + ignition.

Jackie

[Norm's Coupe]

Check the hook up on the sender and the gauge. On the back of the gauge it will have a letter stamped by each post. One post will say ignition and the other Alt. Sounds like you have your wires backwards. Or.....you have a poor ground someplace.

If everything is hooked up correctly, and it still doesn't work correctly run NEW jumper wires. Disconnect the wires from the gauge and the sender. Then run new wires from the gauge to the sender. Temporarily the wires can be run out the window to the trunk. If the gauge then works correctly, the problem is a broken or bad wire between the gauge and sender. So...then hook up new wires and run them to the trunk inside the car.

[moparjack44]

Did run new wires.

As well as I remember, the gauge has 3 post, labeled Ignition, 1 & 2. Checked with a rod shop, they said on the 'old' cars, did not make any difference order wires were hooked up on the gauge to the 1 & 2 post, so long as the Ignition was correct?

Didn't sound right to me, but I accepted his word.

I have a lot of cash & labor in this project, and I am at the same place as when I started:( .

Jackie

[Young Ed]

I believe you have to have the same wire hooked to #1 at both ends and same with #2. Doesn't sound right that you can reverse them.

[moparjack44]

Ed,

All I need to do to check it out, is to reverse the wires on the gague? I'm giving it a try today.

Jackie

[Norm's Coupe]

I believe you have to have the same wire hooked to #1 at both ends and same with #2. Doesn't sound right that you can reverse them.

Ed,

Take it from me, you can do that. Been there, done that. If you do that, the tank will read empty when it's supposed to be full. Then a partial tank will show roughly around 1/4 tank or so.