Fuel Sender unit - 12v and 24v - does it matter?

Submitted: Monday, Mar 14, 2005 at 23:49
ThreadID: 21228 Views:4677 Replies:4 FollowUps:3
This Thread has been Archived
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

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Reply By: Wok - Tuesday, Mar 15, 2005 at 00:04

Tuesday, Mar 15, 2005 at 00:04
Tim,

You are right, its just a variable resistor, unfortunately the value of the pot assembly appears to be model/vehicle specific. I went through this when I fitted an aux tank & picked up a sender from the wreckers......wrong value!...reads high on empty. It appears there is a series resistor with the wiper which matches the model.Hopefully you have better luck.

cheers
AnswerID: 102461

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: 102463

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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FollowupID: 360176

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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FollowupID: 360183

Reply By: Big Woody - Tuesday, Mar 15, 2005 at 07:51

Tuesday, Mar 15, 2005 at 07:51
G'day Tim,

I have a 24 volt Nissan Safari (Patrol) import and removed the 65 litre fuel tank to fit an aftermarket long range fuel tank from the wreckers. I did not have to change the fuel sender unit because as a previous writer has said, it is just a variable resistor designed for low voltage and both 12V and 24V can be considered low voltage. You should have no problems at all.

Regards,
Brett
AnswerID: 102475

Follow Up By: Tim HJ61 - Tuesday, Mar 15, 2005 at 10:17

Tuesday, Mar 15, 2005 at 10:17
Thanks Big Woody a perfect answer - I'll keep a Caramel Freddo in my console for when we meet up. :-)

Wok deserves one too for his input too.

As for Rosco, well he'll get two, because my resistence is low.... :-)

Regards
Tim
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FollowupID: 360192

Reply By: Member - Phil G (SA) - Tuesday, Mar 15, 2005 at 21:43

Tuesday, Mar 15, 2005 at 21:43
Tim,

I don't think you'll have a problem.

But.....the fuel gauge on the 61 series is neither 12 or 24 volt. Its actually about 7 volts and is regulated by a bimetallic regulator inside the fuel gauge. I'd expect this to be the same with both 12 and 24 volts.

Best of luck. If any hassles, then you can always add a resistor in series in the sender wiring.

Cheers
Phil
2002 HDJ79R
AnswerID: 102583

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