Left tank sending unit confusing me

The entire time I’ve owned my 76 XJ12C, about 14 years, the left tank only reads about 1/2 when full. Recently, I was changing all the fuel hoses aft of the rear axle so while the tanks were dry, I pulled out the sending units.
With the left tank sending unit in my hand and connected to the wiring harness, ignition on, I moved the float to full and the gauge indicated full.
With both sending units on the bench, I measured the ohms at similar float levels and they were very close.
The right hand sending unit has a part number still on a sticker and reads: C27739.
With the left tank sending unit installed and the drain plug out of the tank, I used a stick, light and slowly lifted the float arm all the way to the top of the tank. It does not hit any tubing inside the tank.
I also took the float to my swimming pool and verified the black neoprene(?) float does have the buoyancy to lift the float arm.

But, when i filled up the tank, it was again only reading 1/2. When driving on this tank, it does seem to drop proportionally for the miles driven, meaning it doesn’t stay at 1/2 for many miles and then start going down.

What the heck?

Thanks, Charles in Dallas.

Hey! I just noticed on SNG Barrett website that C27739 is actually listed as the left tank sending unit and mine is in the right tank. The difference, I believe, is the arm for the float faces the opposite direction.
So I wonder if gas in the left tank moves the arm a little differently than me pushing with a stick and it is hanging up inside tank?

What model year is your car and what market was it originally sold in? Sending units were different part numbers in different markets. I see no reason to expect a sending designed for the left tank to work properly in the right tank.


'76 XJ12C. Originally sold in New York. A U.S. Fed car.

When filling, do you stop when the pump shuts off? If so, that’s the problem. My Series 2 does this too. The newer the pump the worse it is.

Modern gas fill nozzles are SO sensitive that the merest suggestion of the tiniest drop of escaping liquid trips them OFF. Part of the problem is the Anti-Theft contraption in the neck of the tank. It causes pre-full splashing that, again, trips the nozzle off and you have maybe a little over half a tank.

I have found ONE station where I can fill both tanks FULL without dribbling gas in at a snail’s pace. I go there when I want a FULL tank NOW for whatever reason. Otherwise I just run with not-quite-full tanks.

I confirmed in my hard copy of the Jaguar XJ6 Series Two Parts Catalogue, RTC9097C of January 1977, that the right fuel sender is C27738 and the left fuel sender is C27739. Due to the design of the tanks I see no reason to expect a sending unit designed for use in the left tank to work properly in the right tank due to the shape of the tank and some of the internal obstructions. You will need a right tank sender (C27738) for your right tank and a left tank sender (C27739) for your left tank. Otherwise you are expecting a component to work in an application that it was not designed for.


Did you actually check how much fuel got into the tank, Charles? As said by others, sensitivity of the pump may be the issue - if the tank is actually half full it would explain regular level drop…

If the float is stuck, level would of course only start to drop when level reaches half tank - so it’s all a bit odd if you indeed have a full tank.

The float rods are bent differently for ‘right’ and ‘left’ unit to follow the shape of the tank - so you have the incorrect unit. Since the problem has been there throughout your ownership; a PO is guilty.

Your assessment of the ‘stick’ method is likely correct, the float does hang up, it hasn’t a great buoyancy (have you ‘rattled’ the float to check if it is empty?) - though it doesn’t explain everything.

You could source the proper unit, but as the rods are mirror images of each other; you may get away with bending the rod using the ‘right’ rod as a template - or maybe twisting it…?

xj6 85 Sov Europe (UK/NZ)

It is not a fuel level problem. I’ve owned the car for 14 years and put on 20k plus miles. As you know with the V12, that’s a lot of fuel, ha!
Unfortunately, I need to pump the gas I just put into the tanks back out, at least down to the level where I can pull out the sending units and swap them.
As you know from the very first post on this thread, I just filled up the tanks. And yes, it was adding 8 gallons to the left tank which I know had a couple or more gallons contained because I poured it in after reinstalling the drain plug cap and the sending unit.
The right tank has more than 1/2 as I’ve driven about 50 miles since filling up - My method for years has been fill up both tanks & reset the trip odometer. Drive 100 miles, change to other tank, drive 100 miles and then fill both back up. (even though the right hand tank has always indicated a fairly accurate fuel level).

The floats are the black neoprene (?) material, thus not hollow. No, they don’t rattle as if full of fuel. As I previously wrote, it has a lot of buoyancy capability as I tested it in the swimming pool - the float “really wanted to keep the arm out of the water”.

I’ll chime back in after swapping sending units and filling both tanks back up.

Thanks for your input!

By the way - anyone who is a JCNA member - your May/June issue has an article I wrote about this car.

Yesterday evening, I swapped the sending units for my fuel tanks. THey are now in the appropriate tank. Prior to installing them, I tested the movement while connected to the harness and observed the gauge.
They both moved the needle as expected.

I filled up both tanks, and the left tank with the left tank sending unit installed, reads full. The right tank with the right side sending unit reads about 1/8…What the heck.?
Only $130 at Moss Motors.
I ordered a replacement unit.

Will report back again.