I fitted a switch to bypass the thermostat on the electric
fan. Very carefully wired and all connections done with
insulated crimps; thoroughly checked and tested. After a few
test runs I was happy but on the last run I came home and
parked leaving the engine running. Then what happened is I
hear the fuel pump stop ticking, then the engine cuts out
and I notice ammeter showing full discharge. Next I see
smoke coming up from behind the instrument panel.
After isolating the suspect switch, all seemed well but
obviously a cable had got rather hot !! My theory is that
the switch had an internal failure, shorting from live to
the earth connection to the switch for the ‘on’ light in the
switch. So this needs sorting out but two questions for
electrics experts please.
why didn’t the 50A fuse (A3) blow? Seems like the short
to earth was passing enough current to start charring the
insulation on a cable but not enough to blow a 50A fuse.
2 which wires do I need to replace to be sure I’ve dealt
with all that are likely to have overheated? The electric
fan was fed from fuse A3 which feeds all the switched
auxiliary items. Working back from fuse A3:
A3 fuse gets power from the ignition switch
Ignition switch gets power from a connection on the
lighting switch
lighting switch connects to the ammeter
upstream of the ammeter I can’t work out what happens but
presumably at this point the cables are designed for heavy
current and should be OK.
What I’m hoping is that all these wires are individual wires
and not part of a wiring loom, so can be easily replaced.
Does anyone have a diagram that shows which wires are
bundled inside a loom and which are separate?
Next I’ll have to remove the instrument panel and have a
good look but I can already see a charred wire in the
vicinity of the speedo.
Thanks
Paul–
bramshill
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In reply to a message from bramshill sent Wed 25 May 2011:
Hello Paul,
I don’t know if your 50 amp fuse is a Lucas or American
rating. As with other things, the Brits did things just a
bit different from us in rating their fuses and a 50 amp
Lucas cannot be replaced with a 50 amp American fuse. The
Lucas number is the current that will make the fuse blow,
or the the rupture value, whereas American fuses are rated
for the current they can carry continuously.
Some years ago Buss published a cross reference: For an
English type 50 amp fuse the Buss replacement is an AGC 30.
I’ve seen some recommendations that the 50 amp Lucas should
be replaced with a 25 amp Buss, and both the 30 and 25 amp
Lucas fuses should be replaced with 15 amp American fuses.
In reply to a message from p8099 sent Thu 26 May 2011:
Thanks Monte. The fuse in question is a replacement, sourced
from a UK auto electrical supplier. It carries no
identification othet than the rating. I’ll have a think
about what you say though.
I still need some advice to figure out the route that the 12
v supply to fuse A3 takes and especially if it goes through
the loom. If it doesn’t then that makes it easier to
identify and replace any heat-damaged cables.
A diagram showing what cables are in the loom would be
really useful if someone could point me to such a thing.
Regards
Paul–
The original message included these comments:
I don’t know if your 50 amp fuse is a Lucas or American
rating. As with other things, the Brits did things just a
In reply to a message from bramshill sent Wed 25 May 2011:
Paul,
Was the fan running when the engine died? Permanent magnet DC
motors will act as a generator when you remove the supply voltage
such that while the fan spins down it feeds current back into the
circuit. This energy is enough to drive the ignition which results
in the car starting itself again just after you turn it off
Depending on how things are connected you can end up not being able
to turn off the car.
Not sure if this is relevant to your case, but if your
thermostat is a fan specific one it may have a diode built in to
stop the run on, and your bypass switch will have disabled the
action of the diode at turnoff. No big deal, all it needs is a
(nother) diode at the motor - polarity depends on the earthing
scheme of the car.
Andrew–
The original message included these comments:
I fitted a switch to bypass the thermostat on the electric
fan. Very carefully wired and all connections done with
insulated crimps; thoroughly checked and tested. After a few
In reply to a message from JagWaugh sent Sat 28 May 2011:
Andrew
The fan was not running when the engine died so I don’t
think your point is relevant to this actual event but it
does make me wonder about the way I fitted a bypass switch
for the fan. The thermostat is a fan specific one so should
have the diode protection you mentioned. But my switch will
not provide the same protection. If I turned off the
ignition whilst the bypass switch was set to ‘on’ then it
would be possible for the fan motor to feed back into the
ignition circuit. If this happened then surely setting the
bypass switch to ‘off’ would interrupt the current flow? But
needs thinking about …
Paul–
The original message included these comments:
Paul,
Was the fan running when the engine died? Permanent magnet DC
In a message dated 5/27/2011 05:44 Eastern Daylight Time, mail@bcl.eu.com
writes:
I still need some advice to figure out the route that the 12
v supply to fuse A3 takes and especially if it goes through
the loom
Hi Paul,
Yeah, almost all wiring is in some sort of loom. The July 1957 Lucas
drawing I have shows POS. GRD. Mine is NEG. GRD. but as far as the wiring
route, it won’t matter.
Start at the BAT. Whichever one ISN"T GRD., the #6 or #8 wire connects at
the starter solenoid top lug, also connected on that some lug are 2, #10
brown wires, (the bottom lug goes to the starter,) one goes directly to the
ammeter, right side, from the back & the 2nd connects to fuse block SF6,
term A-1 or"B", fuse # 5. Off the other side of fuse #5, (2) BR/GR wires going
to the interior light. sw. & cigar lighter.
Fuse # 6, post A-3, which is activated from the ignition sw. via a WHITE
wire also connects 2 WHITE to fuel pump & coil from post A-3, through post
A-4 which then feed fuel gauge, WW, reverse lamp, stop lamp, horns, choke
sol. & flasher sw.
The ign. sw. is fed by a BROWN/BLUE wire from the other side of the
ammeter through term # 3 of the lighting sw.
I’ll try & copy the diagram & send it along if you need it.
In reply to a message from bramshill sent Sat 28 May 2011:
Paul,
Yep, that would work, although you should still install a diode
to snub the spark when you actuate the switch, otherwise the switch
will burn out fairly quickly. Any time that the override switch
carries the load of the fan on its own it will be subject to
sparking as you open the circuit.
In a perfect world thermostats would have connections for an
external momentary pushbutton to invert the current state of the
Tstat output, and the Tstat would revert to its normal operation
when the ignition was switched off.
OTOH, presumably Thermostat designers are of the opinion that
the whole point of fitting a thermostat is to eliminate the need
for human intervention.
Andrew–
The original message included these comments:
ignition circuit. If this happened then surely setting the
bypass switch to ‘off’ would interrupt the current flow? But
needs thinking about …
Thanks Otto, that’s very helpful. I have the wiring diagram
that’s in the owners handbook but even if it’s enlarged it’s
quite difficult to figure out. If your wiring diagram is
something better then I’d appreciate a copy please.
I’ve had another good look behind the instrument panel, with
good lighting and a mirror. I can still only see one wire
that’s heat damaged but can’t see what it connects to/from.
I also noticed a fuse behind the panel, in the vicinity of
the tacho. What’s this I think? It looks old enough to be
original and is properly mounted, not just an in-line fuse
that someone’s added. Also a few wires that aren’t connected
to anything …
Next move must be to remove - with some trepidation - the
instrument panel and have a proper look behind it. I hope
there’s enough slack in the speedo/tacho cables and the
oil/water gauge pipes, that you can pull the panel forward
enough to disconnect them; it seems impossible to get at
them from underneath with the panel still in place.
Any further advice from yourself or anyone else is most welcome.
Paul–
The original message included these comments:
I’ll try & copy the diagram & send it along if you need it.
Regards, Otto M.
Otto
Thanks so much for your advice and the diagrams you sent me.
With this help I have been able to solve the problem and
engineer a fix.
A few minutes spent searching the archives gave me good info
on removing the instrument panel to get at the cables.
Another very quick search solved the silly problem of how to
first remove some of the switch knobs. You have to think
carefully about what keywords to search on and for those of
us on the east of the pond, we have to remember that most
contributions come from the other side where folks don’t
always speak quite the same language as us. But all this is
just in the nature of information retrieval - it’s not
something for total idiots.
So generally, I find the XK forum really really useful; I
highly value the advice that is so freely offered; the
archives are a gold mine of information; and if I want
something to make me smile I look out for posts from certain
people … (you know who you are, old fellas)
Anyone else share my views?
Cheers
Paul–
The original message included these comments:
I’ll try & copy the diagram & send it along if you need it.
Paul;
Now you ought not talk down about them folks over in Miss-Sippie… Not
everyone can have the smarts us Texicans do… We just put up with them
boys… Bless their pea-pickin’ hearts…
Charles #677556.----- Original Message -----
From: “bramshill”
we have to remember that most
contributions come from the other side where folks don’t
always speak quite the same language as us.
In reply to a message from Bishop13 sent Thu 2 Jun 2011:
I agree that there is a slight difference in language
between us Yanks and you Brits. I have, however, found a
greater difference amongst ourselves. It has even gone so
far as to have the local vernacular, from certain areas of
this country, sub-titled on Reality television shows, so the
rest of the viewers can understand them.
Joel–
ex jag, '66 E-type S1 4.2, '56 XK140dhc, '97 XJ-6
Denison, TX, United States
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