TR2/3/3A TR3 Fuel Gauge Calibration

[martx-5]

I was reading this article about calibrating the fuel gauge. 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]

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]

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]

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]

Thanks for the info guys, It looks like 90-100 omhs is more in the ballpark for the TR3 sender.

 

[trbby2]

I'm puzzled by the above conversation. My 1965 TR4A fuel gauge reads between 1/2 and 3/4 full when the tank itself is full. My troubleshooting research describes the gauge as a current sensing device, where higher current creates more deflection of the spring holding the indicator needle. The gauge has regulated 10 volts on one side and the current is highest when the other side is at ground. It was recommended that shorting the connector at the sending unit to ground would either send the needle all the way up, or a continuity problem would cause it to read low. I did that and found it to be true and my continuity is good, thus a FULL indication. That means the resistance of the sending unit when the tank is full should be close to zero. Resistance goes up as the fuel level goes down, so current goes down and the gauge indication goes down. I have a new sending unit from Moss Motors, pn 763-020. I tested the resistance of the sending unit with the tank about 3/4 full and found 47 ohms. The gauge read 1/2 full. I then placed a potentiometer in parallel with the sending unit and adjusted it to 120 ohms, which allowed the needle to increase to a reading of 3/4. (Putting the resistances in parallel causes the net resistance to be lower, so higher current and a higher reading.) I can use this strategy to calibrate the full tank reading to be FULL. However, I don't yet know how an empty tank would show on the gauge when FULL is reading correctly. It would show higher than without the parallel resistance in place, but I don't know how much yet. I'm a bit reluctant to test this until I run out of gas if the EMPTY reading is too high! So, I'm curious to learn if anyone out there has had a similar experience and, if so, how they ended up.

 

[CJD]

The TR3 sender runs from 70 ohms to 0 ohms. This is original senders, as repro parts are far from original, and as noted above, can be as much as 110ohms at full.

The fuel gage has 2 coils, with hair-thin wires. Calibration is done by loosening the coils from the gage back plate and carefully moving them closer or farther from the steel balance on the inside of the indicator needle. When I was adjusting mine, I used the article mentioned for the MGA gage, but noted the following:

1) The "E" side is best set by bending the needle to line up when the input from the sender is 0 ohm. This is because there really is no adjustment for the "E" needle position.

2) The "F" adjustment is mostly done using the "F" side coil adjustment. It is so sensitive that just starting to tighten or loosen the nut is enough to make a 1/4 indication difference.

3) The "E" side coil adjustment is primarily for the mid range fine tuning. Same here...very sensitive!!

4) never loosen the adjusting nuts more than one turn! This will allow the alignment tab for the coil to come out of the slot...and then the wires will break when you try to tighten the nut!!

I adjusted my gage to be empty at 0 ohms and full at 70 ohms...and then tweaked to get the 1/2 at 35 ohms. When done, I started with a dead empty tank and added 1 gallon at a time, taking note of the reading on the gage. In the end, I can tell you within 1 gallon how many are in the tank, and it has not changed in 7 years since I calibrated the gage.

 

[jfarris]

If I recall correctly, the TR4 and TR3 fuel senders/gauges operate on a different electrical principal.
I found out when I tried to replace a TR3 sending unit with a TR4 unit.
I finally sent my TR3 duo to Nissonger for re-calibration - can't do that anymore, but one of the usual shops can do it for you.

 

[TRopic6]

Yes, the TR2/3 gauge used a 'balance bridge' set-up (Wheatstone Bridge?) which jumps to the reading as soon as it is switched on. From TR4 onward, the gauge used a thermal set-up where a ~10 volt average input flowed through a resistive wire inside the gauge. The heating deflected the needle and the current was metered by the resistance at the sender. It was somewhat delayed in response, but gave a pretty stable reading. The TR2/3 needle would move around in sync with the float as you went around corners, bumps, etc.

Jeff

 

[RJS]

Hi all

Interested. I have measured gallons vs gauge readings too but, have not measured ohms. Not home til this weekend to reference my notes but can tell you my gauge drops from full to 3/4 almost immediately. Then very slow to go to 1/4. Plus, at empty, it appears there is still 2 gallons reserve in the tank.

Bob

 

[CJD]

By fiddling, I actually go mine to start coming off full at 1 gallon used...9 gal at 3/4, 7 gal at 1/2, 3gal at 1/4, and it will actually read 1 gallon left in the tank! Well, fiddling is probably an understatement. It took an anul weekend.

 

[RJS]

Hi Again,

To be clear, I am a 66" TR4A which reported has 14 U.S. gallons tank capacity. When I am on 1/4 on the gauge, she typically takes 8.0 gallons to fill - give or take. Mind you, I do not top it right up so, expect I could squeeze another 1 or 1.5 gallons in the remaining space at the very top of the tank.

If I try a bit of math, that means at 1/4, it takes 9.5 gallons to "full". Which means there was 4.5 gallons remaining. If I deduct 1 gallon for "reserve" (even modern cars hold 1 to 2 gallons "reserve" in the tank even when the gauge reads "E") that gives 3.5 gallons (being exactly 25% of 14 gallons).

Bob

 

[mezy]

All i can remember from growing up with these cars in the UK most fuel gauges were not that acurate,
under normal driving conditions backwards and forwards to work it would seem like it was taking for ever to get from full to half a tank, perhaps four days.
but then the bottom half of the gauge would drop more rapidly.

 

[CJD]

On pretty much every car I ever owned, the gages were slewed to show full for several gallons, but then show movement more accurately for the end of the tank. I assume it was deemed better to read the end of the tank, since many drivers (like my son who still regularly runs out of gas) push the last gallon. It drives me nuts, as most cars only have less than a third tank when they show 1/2, so it makes planning on long trips difficult. An extreme is my big diesel RV, where at 1/2 tank indication, it has already used 50 of it's 75 gallons!

I only mentioned my TR2, as it shows you CAN get the gage to read perfectly, with a little work. And I set it over 7 years ago, so it stays consistent.