Does anyone here have a good understanding of fuel level sending units? My sending unit, along with my Smiths gauge has always read backwards. When the tank is full, the gauge reads E. When the tank's empty, the gauge reads F. It's not a big deal, I tell people it's a British thing. E stands for "Entirely full," F is for "Fuel needed." I thought perhaps it was simply a polarity issue. After reversing the positive (+) and negative (-) leads, I realized it was a bit more complicated. I now understand sending units use to resistance to send their signal to the gauge.
I was fine with the backward reading gauge, but lately the gauge has become wonky and unreliable. So, I'm going to replace the sending unit and might as well get one that reads correctly. The problem is I don't know whether I need one that uses high resistance to signal a full tank, or low resistance to indicate full. Is there anyone here that would know?
To complicate matters, I have a fuel cell with a neoprene bladder that requires a sending unit with an enclosed float (see pic). I'm sure that will limit my choices. But, let's start with the resistance issue first. Thanks!
As the float raises, the resistance lowers. Proof of this, if you ground the wire at the float (no resistance) the gauge reads full- or should.
It depends a bit on the fuel gauge as well. Very early gauges required high resistance for full and low resistance for empty. Those were not used for very long before being changed to the more common method Dave described - low resistance for full and high resistance for empty. The typical Mini gauge seems to want about 90 ohms for full and 270 ohms for empty.
My Smiths gauge is about 8 years old, so I'm guessing it's not one of the early units. So, low resistance for empty, high resistance for full. Got it! Thanks guys!
Caution! Just saying "low resistance for full and high resistance for empty" does not mean you will get it right or even close. There are several different ranges that meet that definition. Pick the wrong one and your fuel gauge will be very inaccurate. You need to find the nearest match to the 90-270 range.
Yeah, well I've found the Mini fuel gauges are only accurate in two places - empty and full! ;D
Picking the right range sender is more about getting those two ends reasonably accurate.
Quote from: MPlayle on March 01, 2019, 10:20:01 AM
Caution! Just saying "low resistance for full and high resistance for empty" does not mean you will get it right or even close. There are several different ranges that meet that definition. Pick the wrong one and your fuel gauge will be very inaccurate. You need to find the nearest match to the 90-270 range.
Thanks MPlayle. I'll be shopping for a sending unit in the 90-270 range. Will advise.