inverting a signal

scoobie_wrc

scoobie_wrc

scoobie_wrc

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13 Aug 2005
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168
long story short,

ive put an instrument panel from a newer model into my car, everything works fine except the fuel guage.

when i fill it up it reads empty, when its empty it reads full, etc

so i need to invert the signel before it gets to the panel, so how do you invert a signal?

anyone help?
 
scoobie_wrc said:
the cars a mk3 fiesta, the clocks are from a mk5, there's only 1 wire for the guage m8 so cant swap :(
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Are you sure there is only one wire? It would need to complete a circuit for anything to happen. What about the dial itself, does it directly earth or anything like that?
 
Will probably be a reference for the guage, no doubt it will be a resistance type sendor, similar to how a scalextric car control works with a big coil of high resistance wire and the float in the tank controls how much of this coil the electrical current has to pass through.

You may have to find a way of reversing the Pos+ and ground inside the cluster
 
Yep you'll probably find the case of the gauge is the earth and the wore is the sender. I doubt it's a signal, just a variable resistor on a float inside the tank. It *might* work if you connect the case to the sender and attach the wire to the earth, but it's unlikely and you'll have to isolate them to stop them shorting out.

Sounds like on the later car the resistor in the tank was round the other way round so hence the gauge works backwards. I don't think there's anything you can do about that other than use the tank sender unit off the later car as you can't flip resistance like that without some pretty complicated electronics.

Edit: on second thoughts if you were to attach +12V from the ignition switch to the case of the gauge instead of an earth then it should work properly, but make sure it's VERY well insulated or fused.
 
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paradigm said:
Are you sure there is only one wire? It would need to complete a circuit for anything to happen. What about the dial itself, does it directly earth or anything like that?
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no, it'll just be a single wire with the guage creating a ground. The wire will run straight to the fuel tank and will get told what to do by the float.

Seems odd tho if the clocks came from another mk3 fiesta
 
[SKR]Phoenix said:
they didn't :p
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Dammit....that would be why then. MK5 clocks are one block connector. Its unlikely that you'll be able to sort this without an invertor of some type. You could get the felt tip pens out and alter the markings on the dials!! teehee
 
The other big problem... - the fuel gauge reads backwards on digital dials. All of them as far as I can tell. You've got two options on this - one is to fit the Mk5 tank sender, and maybe the tank as well depending on whether or not the sender fits the Mk3 tank - or grab yourself a 741IC (small chip, about 30p from ******) and a book on 741 circuits to invert the signal. I think swapping the tank will be easier for me, but it's your call!
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From http://www.djstalker.com/content/view/33/45/
 
andi said:
*snip*
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That would work I expect. A 741 is an operational amplifier, and will therefore require a power supply. As you're inverting the signal, I'd imagine something like a +12V and -12V supply would be necessary, my knowledge of cars is pretty limited so I've no idea if these are present already. The circuit will involve you connecting the -ve input (of the op-amp) to the output and the +ve input to ground.
 
Or just "ground" it with +12V instead of 0V. Should have the same effect as the gauge is only looking at a voltage relative to a fixed point. Give it a go before cracking out the soldering iron and Veroboard.
 
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