Fuel Sender unit - 12v and 24v - does it matter?
Submitted: Monday, Mar 14, 2005 at 23:49
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Tim HJ61
One for the electrically informed.
My question is whether a fuel sender unit is voltage dependent. i.e. will it work on a 24v system if it has come from a 12v system?
My guess is that it will be fine as the sender unit appears to be a chunky variable resistor, which in my limited electronics experience are not linked to a critical voltage – unlike a lamp which has a relatively tight voltage tolerance.
The scenario is that I have an HJ61
grey import running 24v electrics.
I have installed a second hand Opposite Lock Aux fuel tank, which contains a fuel sender from an FJ60 – I am assuming this came from a 12v vehicle.
I want to connect the aux tank sender unit to the main dash fuel gauge via a switch the same as normal aux tanks are set up.
As a
test before I installed everything, I disconnected the plug from the main tank fuel sender and plugged in the aux tank sender unit. The dash fuel gauge appeared to function as it should – that is showing full, half full and empty when the sender unit arm was in the corresponding positions. The sender unit did not get hot or blow any fuses etc! so I am figuring it will be okay to mount permanently like this.
When I tested the voltage at the lead to the fuel gauge, it was very low, almost zero – i.e not 24v or 12v, so this confirmed my theory about the sender unit being okay across 24v and 12v vehicles.
Does anyone else have an opinion on whether my assumptions are reasonable and this will work okay? Or alternatives if necessary?
Thanks
Tim
Reply By: Rosco - Bris. - Tuesday, Mar 15, 2005 at 00:07
Tuesday, Mar 15, 2005 at 00:07
Tim
I probably am less leccie savvy than you, but my guess is that I'd disagree.
A resistance is a resistance is a resistance, not a variable regardless of applied voltage or current.
V=iR ... if R remains constant if would appear to matter whether it's in a 24V or 12V system.
Assume 12 ohms resistance .... i=V/R so 24/12=2 Amps in 24V system, but equally 12/12=1 Amp in 12V system.
Does this mean you'll get twice as much fuel in the same tank .... ;-D
Cheers
AnswerID:
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Follow Up By: Tim HJ61 - Tuesday, Mar 15, 2005 at 00:23
Tuesday, Mar 15, 2005 at 00:23
You might have just hit on something Rosco - best way I've heard to decrease fuel consumption! :-)
Seriously though, the resistance actually DOES change with the height of the float in the tank. There is a wire wind setup inside the sender unit and the float arm travels across it, much the same as the old hand controllers from Scalextic sets - oops, just got the memories going...
Somehow mysteriously this changes the resistance of the unit which is reflected by the guage on the dash. The second lead on the sender goes to earth.
Fortunately it seems from Wok's response, that when I tested the aux sender on the wiring to the main tank sender, the gauge reacted as it should - full equalled full on the gauge, empty equalled empty etc.
Thanks
Tim
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Follow Up By: Rosco - Bris. - Tuesday, Mar 15, 2005 at 07:49
Tuesday, Mar 15, 2005 at 07:49
Correct Tim
The resistance changes across the coil, however at any given point it is constant, hence my above hypothesis should still hold true. But apparently it doesn't .... did say my knowledge was as dangerously low as a fuel tank on empty..... ;-))
Cheers
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