XK Fuel Tank Sending Units
Fuel Tank - Sending Units
11 Oct 96, from Cleo Bay:
When I was rebuilding my XK140 and E-type, both of the
metal floats on the gas tank sending units were leaky. The electrical parts
were fine. I first tried on the XK140 a new reproduction sending unit.
It leaked. Nothing seems worse than gas on a tailpipe (except for the possible
fire!) I could only keep the thing from leaking about 3 months at a time
and it never gave warning before it started. I got rather tired of this!
I finally decided to try and fix the original float without soldering.
(I was afraid of things going boom). Anyway, I had some leftover gas tank
sealer which was used to coat the floats once I had the fluid out of the
floats. It worked! The sealer has been keeping gasoline out of the float
as well as keeping air in. The original unit body does not leak and has
not for over four years. I have used this repair method on three other
cars, all of which have not reported any problems.
As an after the fact note on the reproduction sending
unit, I did manage to get the one on my crankcase oil pan on my XK120 to
seal up properly. I tightened up the electrical fitting and sealed the
small access cover with no. 1 Permatex. The reproduction units are definitely
11 Feb 97, from Dick White:
The 150 fuel gauge sender has two functions - 1) to provide
a variable resistance for the fuel gauge to indicate fuel level and 2)
close a switch to light the fuel low light. There are only two wires are
attached to the sender - one for the gauge and one for the light. The circuit
for both functions is completed through chassis ground (not a good method
in my opinion). Inside the sender is a copper wound coil providing the
resistance. There are two wipers, one on each side of the coil, which are
moved, in unison, by the float. The wipers are electrically and mechanically
tied to the float. They are grounded by the bushing to the case of the
unit. When the tank is full the wipers are at their highest resistance
position (lowest physical position) providing approx. 100 ohms resistance.
As the fuel level drops the wipers move closer to the 'ground' end of the
coil lowering the resistance. As the wipers approach the top, one of the
wipers strikes a tab which lifts it off the coil and completes the electrical
circuit for the low fuel light.
My sender was showing an intermittent open as it traveled
from top to bottom. I opened it up expecting to find crud on the coil and
a need for cleaning. This wasn't the case. The electrical contact between
the wipers and coil was good but the electrical connection between the
wipers and the case was intermittent. I fixed it by applying WD-40 to the
bushing and working the float lever up and down. While apart I also cleaned
the wipers and coil with Radio Shack Tuner Cleaner/Lubricant and cleaned
the tab which functions as switch for the light with a small burnishing
21 Oct 97, from Don McClellan:
Hi Dick... another trick I have used dozens of times (auto
restoration-electrical) is to use some extra flexible wire (fine strands)
between the case of the sender unit and the float arm or shaft. If the
case is aluminum (ie Jags), a connector terminal and machine screw can
be used to secure the wire to the case, or if it's steel, just solder it.
Then the wire should be bent in a couple curly-q's(to provide flexibility)
and soldered to the float arm (near the shaft). Works great and prevents
intermittent connections to ground PERMANENTLY! Regards, Don McLellan
Hi All, I have removed the gas tank sending unit to replace
gasket. I have a very slow weep. Anyway, while I have it off I want to
clean up the tapped holes which attach the unit to the tank. The only problem
is I cannot find a tap in my tap and die set which matches the thread of
the set screws which secure the sending unit to the tank. They appear to
be a non standard size. Does anyone know what size they are? - Thanks,
Neville: Spending a few minutes with the Parts Book reveals
that the tank "element unit" securing screws are Part No. AS.303/4H.
This number shows up in the Standard Parts section under "Setscrews"
as having a "3 BA" Thread. It also indicates that there were
only two other "cheese head" screws that same size used elsewhere
in the car (8 total). The alloy cars used 20, 1/4" countersunk "3
BA" thread screws on the "windscreen tapping plate"? I hope
someone else on the list can translate "3 BA" into a more recognizable
tap size for us. Good luck, Dick Cavicke
Neville, There are six screws holding on the sender unit.
The original thread is not available in the US, I believe it is 8x36x1/4".
I use a stainless steel fillister head 8x32x1/4" as a substitute.
All you have to do is run a 8x32 tap into the holes, and they will accept
the new screws. If you can't get SS at the local hardware store, you might
be able to get brass, both will allow you to take things apart later if
you ever need to. - Regards, Wray Schelin
I believe the screws are 5-40 (#5 screws with 40 threads
per inch). I never could find a flat head brass replacement, but did find
an allen head screw which fit correctly. You may be able to find a correct
tap at an industrial supply house, like (if you're in the US) Enco, Grainger,
or McMaster Carr. - Good Luck - Jag150
Assuming that they are 3BA (and I have not checked this),
3BA is 34.84 tpi and 0.1630" diameter (talk about standardisation!!).
This happens to be a thread pitch of 0.73mm and diameter of 4.01mm, a standard
4mm screw has a pitch of 0.7mm, so a standard 4mm tap should do more good
than harm. I got these numbers from Newnes Engineer's Reference Book 1951.
Check the screw pitch against your 4mm tap before you put it in. - regards,
Wray, Neville, One of the exasperating or amusing "charms"
of the Jaguars from the transition period is the several different thread
systems we must contend with. My FHC sports at least four, and possibly
five or more if you get pedantic about it. They include: Whitworth, BA,
Unified, Lucas' own spiral shapes, and some of the odd choices made by
respectively Girling, Lockheed, Trico and Skinners Union. Although most
can be easily bought from the UK, for the single car owner it is rarely
worth it nor is the need frequent enough to stock all of the taps and dies
for each of the variations, so sometimes a compromise is indeed warranted.
However, forcing an 8-32 into a 34 pitch hole is to me too much of a "beat
it to fit, paint it to match" approach. If you do not want to buy
a modest set of the most common taps, a 4MM thread comes much closer to
fill the 3BA thread form over the length of engagement than a -32 and it
might even provide a more durable seal. - Regards, Klaus Nielsen
Klaus, No forcing required, the diameters are the same(.160"),
only the threads are insignificantly different. I think there's confusion
here, with the four smaller screws that hold on the small cover that is
on the sender unit. I think Neville asked about the sender retainer screws,
not the cover screws, which are something completely different. - Regards,
Wray, I agree that forcing the slight (32 to 34+) difference
will not make one whit of near term difference; but I just cannot bring
myself to do it when better ways are at all possible. After all, we are
talking about repairing/restoring rapidly diminishing resources and both
BA tools and spare screws and nuts are still abundantly available from
England. Just one guy's view......... - Still, best regards Klaus Nielsen
I have found the threads to be very close if not the same
as 6-32 on the several XK tanks I have done... - Lee Eggers
Thanks to all wo have responded to my question.I have
a 8nc -32 and 6nc-32 tap in my set, and as stated before, neither of them
are correct. The 6--32 is a rattle fit.. After starting this discussion
I got to admit that I do not think I have serious damage to the threads,
the bigger problem is an accumulation of dirt in the bottom of the hole.
I suspect the screws are bottoming out against the slug of dirt before
pulling up on the sending unit. A can of WD40 and a needle may be all I
need. - Thanks again Neville Laing