fuel tank sender unit for an R4
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    Default fuel tank sender unit for an R4

    Hi all,
    I have an unexpected problem with the R4 I am restoring. The car has been converted from the original 6volt set up to 12volts - well almost! There are still a few items that have not, such as the wiper motor,the heater fan motor and the tank sender unit. I am having trouble finding any of these items. The sender unit is, for obvious reasons, the most pressing item. I've tracked down a 12 volt unit at Franzose, but the info on them seems to suggest that the tanks are different and a 12 volt unit may not fit the 1963 R4 tank I have. Has anyone else come across the same issue, and is there a 12v sender unit I could use, or a work around? I am facing the complication of setting up an entirely different tank and sender unit, with all the hassle that the fitting of a different tank and supply pipes. HELP!

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    Ron

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    1000+ Posts schlitzaugen's Avatar
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    No need to worry.

    If you're somewhat literate with electrics and have a basic multimeter you can convert your existing unit to 12V easily.

    Your sender unit is just a resistor that is using some sort of special wire (easily found in shops) to give a varying signal to the in dash meter.

    Measure that resistor (highest value is what you need). Pull the wire out, measure its length. Divide resistance by length and you have the constant of that wire. You need to go buy a wire with twice that constant (or close to) and replace the wire you pulled out.

    You can buy resistance wire from Jaycar, Altronics, RS components and most likely many others I don't know about. Have a search for nichrome wire.

    You're welcome.

    A nice bottle of red should do it.
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    Thanks Schlitzaugen,
    I'll give it go when the end of year dust settles. Much appreciated as I am desperate to get this little car going - after I deal with all its strange issues.
    Ron

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    Alternatively you could get a 12v to 6v converter (Jaycar used to sell kits) and feed the fuel sender the 6v it wants.
    If you convert a 6v car to 12v I think you would have to change both the sender and the gauge, wouldn't you??

    If you have already changed the gauge to a 12v one, then yes you will need a 12v sender.

    Sorry I have no knowledge of R4s but do know a bit about electronics.
    Last edited by simca1100; 24th December 2019 at 11:03 AM. Reason: spelink

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    1000+ Posts schlitzaugen's Avatar
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    The dial is most likely an ammeter, so I think it will work fine as is.
    Last edited by schlitzaugen; 24th December 2019 at 02:56 PM.
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

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    Hi Simca1100,
    A visit to Jaycar has been on my to-do list for a while -this episode will bring it on sooner, but I am holding off as every day brings forth a new electrics issues. I had been lulled into thinking more 12 volt stuff had been done, but not so. On top of the problems I have already mentioned I am also aware most, but not all globes have been replaced- I wonder if the interior light will work?

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    1000+ Posts alan moore's Avatar
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    Quote Originally Posted by schlitzaugen View Post
    The dial is most likely an ammeter, so I think it will work fine as is.
    I would expect the fuel gauge would be a 6V voltmeter and that at full the sender takes the negative side to ground giving a full scale deflection on the fuel gauge. The resistance builds In the sender as the tank empties dropping more voltage across it, reducing the deflection on the gauge. It may be possible to measure the full resistance of the sender and put a resistor of the same value in series between the gauge and the sender to allow the gauge to work reasonably on 12V. The wattage of the resistor would not have to be huge but might be worth putting a 5 Watt one in.

    I cannot guarantee the outcome will be successful, but the experiment should be easy and inexpensive, and might even work or give you lead on how to progress.
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    You may want to have a read of these links, it may answer some of your questions.

    Making a 6v to 12v Conversion Easy!

    Converting From a 6 to 12 Volt Electrical System
    Regards Col

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    1000+ Posts schlitzaugen's Avatar
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    Quote Originally Posted by alan moore View Post
    I would expect the fuel gauge would be a 6V voltmeter and that at full the sender takes the negative side to ground giving a full scale deflection on the fuel gauge. The resistance builds In the sender as the tank empties dropping more voltage across it, reducing the deflection on the gauge. It may be possible to measure the full resistance of the sender and put a resistor of the same value in series between the gauge and the sender to allow the gauge to work reasonably on 12V. The wattage of the resistor would not have to be huge but might be worth putting a 5 Watt one in.

    I cannot guarantee the outcome will be successful, but the experiment should be easy and inexpensive, and might even work or give you lead on how to progress.
    I thought about that as well and wrote an entire post with the same suggestion, but if that is the case, the resistor solution will mean the meter might lose one end or the other of the scale depending on circuit design and/or where you place the "ballast" resistor in circuit. I deleted these details as I thought it unlikely based on my experience with similar vintage cars (it's more complicated to build for the manufacturer).

    But, if that is the case (gauge is voltmeter) my suggestion would be to leave everything as is and just bring the voltage down to 6V for this circuit using a linear voltage regulator IC, which is cheaper even than the nichrome wire:

    https://au.rs-online.com/web/c/semic...ons=4294574133

    I would wire that in such a way that the ignition switch disconnects the IC otherwise you will have a (small) current drain even with the ignition off (I guess this is simple enough because the fuel level gauge only comes on with the ignition anyway). 2.2A should be plenty for the fuel level circuit. Heck, I reckon you could power the entire dashboard with one of those.
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

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    That's interesting because a friend of mine, who seems to know about these things also, suggested that as a possible answer to keeping my windshield wiper with it's 6 volt motor. He also suggested that another, cheaper, inline regulator, was to splice in a 6volt light globe along the feed line to bring down the voltage . Don't know if I'd be game enough to try it for the fuel line tho'. I did find another , slightly better 6volt sender unit in my box of spares.

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    Regards Col

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    Yeah, the 6V inline globe is a pikie sort of solution. That will peg the current draw and you don't want that. An IC regulator as above is practically invisible by the rest of the circuit.

    For the wiper motor however you might need more than 2.2A, I would look at something in the region of 10-15A.
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

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    Thanks Col and Schlitzauger. I'll make a start when the current heatwave pisses off Not much fun outside at the moment. I'll keep you posted when I make a start.

    Ron

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