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martx-5
06-05-2008, 06:58 PM
I was reading this article about calibrating the fuel gauge. (https://mgaguru.com/mgtech/electric/fg_01.htm) It's for an MGA, and it says that the tank sending unit goes from zero to about 70 ohms when full. The procedure is easy enough, but when I measure the resistance in the new sender I got from Moss, "Full" is about 110 ohms. Is this correct for the TR3 sending unit??

Edit: Actually, I don't really care that the ohm reading is 110, as I will just set-up my gauge to correspond with that. But, this weekend, I need to calibrate a friends TR3 gauge, and want to be sure of what the OE reading should be.

TR3driver
06-05-2008, 07:21 PM
Art, I'd have to measure one to be sure; but ISTR mine was about 100 ohms "full". I just assumed that was wear in the sender, but maybe the TR3 sender is different than the MGA.

dklawson
06-05-2008, 07:35 PM
I suspect the 70 ohm value was determined by measuring a few MGA sending units, not from published specifications. It wouldn't surprise me to see the top end go from 80 to 100 Ohms for full. The occasional +/- 20 Ohm variation on the full condition wouldn't surprise me.

0-90 Ohms was one of the industry standard ranges (not just to Smiths) prior to fitting the voltage stabilizer to gauge systems. That's the range I tell people to look for when they're working on their pre-stabilizer fuel sending units.

If you follow the instruction on the MGA Guru site, you'll be able to match your new sender to your existing gauge. The instructions are pretty simple and if you haven't already installed the sender you can do this comfortably while sitting at your work bench.

Adrio
06-05-2008, 09:00 PM
I looked in my lab book and back in 1998 I measured the OEM sender on my TR3. Then I calibrated the gauge to it. This is what I measured on the sender:
Empty = 48.5 OHM
1/4 tank = ~54 OHM (my notes say this one was hard to measure)
1/2 tank = 82.2 OHM
3/4 tank 103 OHM
Full tank = 129 OHM

I hope that helps.

Adrio

martx-5
06-06-2008, 05:58 AM
Thanks for the info guys, It looks like 90-100 omhs is more in the ballpark for the TR3 sender.