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Rewiring an old jigger from scratch

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6 years 1 week ago #192106 by Dave_64
Hi Guys,
making up a wiring loom for the Karrier Bantam rebuild and looking at both the old mangy harness (removed) and the workshop manual, can follow the wiring pretty well except for the fuel sender to gauge wiring.
Original wiring had only the one single wire from the tank sender unit up to the gauge via the ignition switch.
Fuel tank has been replaced with a larger ( possibly Mazda E series) type which has two wires from the sender.
Hook up an ohm meter and connect to both these, tested the resistance and seems to cover a range from about 29 ohm to 130 ohm, so assume that the sender is still OK, this was on the bench with the sender disconnected from the tank itself. Reinstall the unit in the tank and test again (unit has a rubber seal to insulate it from the tank) thereby NOT earthing out on the chassis.
So far so good. Replacing the original gauges with VDO units and about the only thing I know is that the gauge/senders have to be a matched pair to read correctly.
Without getting TOO technical, have heard that by installing a variable resistor (potentiometer??) between the sender and the gauge, should be able to get it to read alright.
If you know how much fuel is in the tank at any given time, (check with wooden dowell), should also confirm readings?
Trouble is, ONE of the wires coming from the sender goes to the gauge, would have thought that the other would have to go to earth somewhere to complete the circuit??
Anyone chuck up any ideas?
Thanks, Dave

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6 years 1 week ago #192107 by Bluey60
Hi Dave
What holds the sender in the tank screws or a ring thingy because that would earth it and the other wire might be low fuel warning light just a thought

Bluey

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6 years 1 week ago - 6 years 1 week ago #192108 by Lang
Dave

I am doing up a 1941 Dodge and have been driven mad with fuel gauges.

The most common type is single wire with power running through the gauge then running through the resistance of the float rheostat to be earthed through the case and fuel tank.

The 41 Dodge (NOT the 42 or later) had two wires that were live pulling two little wound magnets left or right, according to the Resistance from the float rheostat, inside the the gauge to move the needle. The earth was through the gauge case to the instrument panel.

Your modern sender just sounds like the old single wire but with the second wire to be connected to a proper earth instead of relying on a dodgy circuit through the fuel tank to the chassis. If the fuel tank was plastic like many are it was essential to make it work. If you did your test on the two wires getting a normal change of resistance as you moved the float it must be an in-wire and an earth-wire.

As Bluey said the second wire might be just an extra to run the warning light making your unit certainly a standard old earthed through the case single wire unit for gauge operation. I doubt it because the light circuit would not run through the range but just turn on or off at a single point in the float movement.

I converted the Dodge to 12v from 6v and just put in a resistor (available on Ebay, from Hotrod suppliers or any Jaycar shop for a couple of dollars) in the gauge supply wire to bring the voltage down to the original 1941 6v gauge requirement.

You can get little converters to do 12-24, 24-12 and even 24-6 for a few dollars. The float unit does not care what voltage but the gauge does.

I have found in the past that the cheap "universal" fuel tank senders work just fine on many different type and make of older vehicle so there must be some sort of manufacturing standard in there somewhere. Give it a go unmodified first then mess with variable rheostats etc if it is wildly out

Lang
Last edit: 6 years 1 week ago by Lang.

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6 years 1 week ago - 6 years 1 week ago #192109 by Dave_64
Bluey60/Lang,
thanks for the prompt replies.
Wasn't considering the possibility of one of the sending units being a low fuel level warning light, but could possibly be it.
Sending unit is held in with a lock ring type affair and whilst it's easy to get at, will rip it out again and check my readings next coupla days.
If I use the ohm meter on one output terminal at a time and the other to the body of the sender, should give better indication, as long as I insulate myself from the test probes.
Looked up the VDO charts for the 52mm gauges, plenty of different styles, but seem to come in a few different ranges, 0--90 ohm---10--180 ohm, even 330--30 or thereabouts.
Get a better idea by re-checking. Don't know what the original fuel tank sending unit values are as I don't have a wiring diagram, bloke who have me the tank seems to be pretty sure it came from an E series Mazda diesel, around the early eighties.
Already converted everything from 12V POS-Earth to 12V NEG-earth, so would have thought it would be relatively simple.
As you mentioned, Lang, the sending unit doesn't work on voltage, but on straight resistance, so as long as the gauge voltage matches the truck voltage, should be sweet.
Bloke who suggested putting a potentiometer in the system was saying that once I get the correct reading, remove it and put a fixed resistor in it's place.
Thanks again.
Dave
Last edit: 6 years 1 week ago by Dave_64.

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6 years 6 days ago - 6 years 6 days ago #192114 by Mrsmackpaul
I think the pot will only work if the resistance has to be increased if the resistance is already to high it would make the gauge read even further out I would think

And even then if the resistance has to be increased wouldnt it then not be right the other way as the reading would be to high
For example if it is zero when full and 15 ohms when on empty if you had to 5 ohms in to make the gauge read empty, wouldnt it read only a 1/3 full when full because it has 5 ohms from the pot already there ??

Paul

Your better to die trying than live on your knees begging
Last edit: 6 years 6 days ago by Mrsmackpaul.
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6 years 6 days ago #192115 by Lang
Paul

I think you are right.

The fuel gauge is set to swing over a certain distance with a certain resistance. If you put in a float unit with a bigger range than standard the needle would be jammed against E or F half the time and only work in its constructed range. We have all seen gauges that never go over half or run out when showing quarter full, usually caused by bad connections.

Lang
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6 years 6 days ago #192129 by asw120
To match most any gauge to most any sender, you could make a Wheatstone Bridge. I have long forgotten how to do it, but some variant thereof, should be able to work. Will at least get the grey matter going.
The gauge would go in the middle and the sender would be one of the four resistors. Possibly the gauge could be one of the four resistors.

Would take some working out, but should work. Google is all I can really suggest for this one. This would allow you to use the original gauge, if you wanted.

Jarrod.


“I offer my opponents a bargain: if they will stop telling lies about us, I will stop telling the truth about them”

― Adlai E. Stevenson II
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6 years 5 days ago #192149 by Morris
All I know about electricity is that it is powered by smoke. If it goes wrong, smoke comes out!

I may be wrong but I was told a Wheatstone bridge was a rectifier as used in battery chargers to change AC to DC. The one I saw made was simply four resistors with the tags all pointing in the same direction, the tags from one to another were then twisted together so that it formed a four-pointed star. (if that makes sense) It was described to me that the current could only flow in one direction.

I have my shoulder to the wheel,
my nose to the grindstone,
I've put my best foot forward,
I've put my back into it,
I'm gritting my teeth,

Now I find I can't do any work in this position!

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6 years 5 days ago - 6 years 5 days ago #192150 by Lang
Jarrod and Morris

Poor old Dave hasn't got many years left in the world and we would hate to have him found in his workshop on an early departure with slashed wrists trying to figure out a Nobel Electrical Prize solution to checking the fuel in his tank. Richard Hawking is not answering his phone for advice.

Lang
Last edit: 6 years 5 days ago by Lang.

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6 years 5 days ago #192153 by PDU
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