[xj] Air con fuse blowing

Could someone who has their thinking cap on help with this?

I got the V12 car out of its lockup this evening and when I turned
on the fan switch (engined warmed up) there was no response.

It didn’t take long to establish that fuse no 6, which on this
vehicle is marked as being ‘AIR CONd RELAY & CLUTCH’ and is rated
at 15 amps, had blown. I replaced it and the fans started but then
it promptly blew again. A little investigation shows that the
resistance to earth on the equipment side of the fuse is 35 ohms
with the function switch off, 55 at low, and something else in
between at auto. These resistances are only visible on the right
hand side of the holder clip. However, when I measured the current
draw through the fuse path it shows a whacking great 16 amps. Not
quite a dead short but quite a low resistance.

I can’t quite reconcile these two measurements - 35 ohms shouldn’t
(and doesn’t) cause the fuse to blow. By my reckoning the current
draw should be about 0.4 amps. But nor should 55 ohms - the
current should be even less, ~0.25 amps.

The schematic is a bit confusing in my copy of the S57 diagram.
The feed from the fuse should to the on/off switch and then either
to the vac control for the recirc flap or (when mode switch is on)
to the water temp switch and the amplifier. Could one of those
last two be the source of the current draw? If so, which?

Any thoughts on what to check next would be most welcome.

I am a bit reluctant to tear into the 6 cylinder car for
comparisons as at the moment its air con is working and we are
having some unseasonably warm days.–
al mclean '93 XJS 4.0 - '84 4.2 Daimler - '84 DD6
Telford, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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I’m a bit brain dead tonight, Al, so the best I can come up with at the
moment is to unplug the A/C compressor (easiest to access) and see if
anything changes. If not, you can try unplugging anything else on the
circuit and/or isolating the branches of the harness.

Cheers
Doug Dwyer
Longview Washington USA
1995 XJRFrom: “almcl” al.mclean@blueyonder.co.uk

Could someone who has their thinking cap on help with this?

However, when I measured the current
draw through the fuse path it shows a whacking great 16 amps. Not
quite a dead short but quite a low resistance.

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In reply to a message from Doug Dwyer sent Mon 24 May 2010:

Thanks, Doug.

I should have mentioned that, as the system is uncharged at the
moment, the feed to the compressor clutch is disconnected. I did
wonder if this was shorting but it doesn’t appear to be.

After I’d posted I tried unplugging the thermostatic switch on the
inlet water pipe. This made no difference to the current draw. I
then tried the amplifier harness plug(on the rhs.) This too, made
no difference. I think this leaves the servo assembly, the mode
switch or the wiring harness itself. Plus that weird component
with a diode and resistor marked as being ‘in harness’.

I’ll go with unplugging some of these items tonight.–
The original message included these comments:

anything changes. If not, you can try unplugging anything else on the
circuit and/or isolating the branches of the harness.


al mclean '93 XJS 4.0 - '84 4.2 Daimler - '84 DD6
Telford, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

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The archives and FAQ will answer many queries on the XJ series…
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almcl wrote:

Could someone who has their thinking cap on help with this?

I got the V12 car out of its lockup this evening and when I turned
on the fan switch (engined warmed up) there was no response.

It didn’t take long to establish that fuse no 6, which on this
vehicle is marked as being ‘AIR CONd RELAY & CLUTCH’ and is rated
at 15 amps, had blown. I replaced it and the fans started but then
it promptly blew again. A little investigation shows that the
resistance to earth on the equipment side of the fuse is 35 ohms
with the function switch off, 55 at low, and something else in
between at auto. These resistances are only visible on the right
hand side of the holder clip. However, when I measured the current
draw through the fuse path it shows a whacking great 16 amps. Not
quite a dead short but quite a low resistance.

I can’t quite reconcile these two measurements - 35 ohms shouldn’t
(and doesn’t) cause the fuse to blow. By my reckoning the current
draw should be about 0.4 amps. But nor should 55 ohms - the
current should be even less, ~0.25 amps.

The 15A fuse protects circuits directly associated with the AC
amplifier, Al; only the compressor clutch is a ‘serious’ power user -
the rest are principally electronics, relay currents and servo drive…

But importantly; such resistance measurements as taken on connected
circuitry are really invalid - the various connections to all and sundry
will give meaningless resistance readings. In this case; unpowered the
system will show overall resistance of connected ‘items’ - but powered
up these connections are changed. Indeed, some items may show resistance
to the low powered meter probe - but be overwhelmed by the full fury
from the battery…

Like the compressor; it is not directly connected to the function switch

  • it is under some indirect control from the amplifier. And
    disconnecting the compressor clutch will not eliminate a short somwhere
    in the wire to the compressor - as an example, not to accuse it. But
    that long wire through the compartment to the clutch is vulnerable…

Finding the shorting circuit will be diificult - disconnections must be
made for meaningful ohming, and the result must be subject to some
scrutiny. As the compressor clutch wire is disconnect one may start
there - if it shows short measured at the clutch end, it’s almost
certainly ground shorted.

This is another mere example - a grounded wire really require
disconnections at both ends for a sure-safe ohm result. One may assume
that no legitimate user under amp’s control can draw 16A and be intact.
Obviosly you can easily disconnect the amp connection and the
amp-to-servo connection in turn - the result may narrow the range of
suspects…?

Subsequent disconnections may likewise reveal something - adding; a
touch of luck and inspiration is now required…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)>The schematic is a bit confusing in my copy of the S57 diagram.

The feed from the fuse should to the on/off switch and then either
to the vac control for the recirc flap or (when mode switch is on)
to the water temp switch and the amplifier. Could one of those
last two be the source of the current draw? If so, which?

Any thoughts on what to check next would be most welcome.

I am a bit reluctant to tear into the 6 cylinder car for
comparisons as at the moment its air con is working and we are
having some unseasonably warm days.

===================================================
The archives and FAQ will answer many queries on the XJ series…
FAQs: http://www.jag-lovers.org/xjlovers/xjfaq/index.html
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In reply to a message from Frank Andersen sent Tue 25 May 2010:

Subsequent disconnections may likewise reveal something - adding;
a touch of luck and inspiration is now required…:slight_smile:

Isn’t it just?

I found the approx area of the problem. Somewhere on the
white/blue wire that leads from the in-car temp sensor to the
ambient sensor (where the devil is that? - thought it was in the
lhs inlet duct but haven’t located it so far) there must have been
a short.

Disconnecting the white/blue at the spade connector on the
thermistor dropped the current from 16 amps to less than 1.
However, my attempts to follow the wire through the harness removed
the fault and I now can’t get it back. Wonderful, job done, you
may say, but I’d really like to know where and what the problem
is/was and so have delayed reassembling the glove box, underscuttle
panels, cheek panels, air ducts &c until I have had a chance to
look around a bit more…–
al mclean '93 XJS 4.0 - '84 4.2 Daimler - '84 DD6
Telford, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

===================================================
The archives and FAQ will answer many queries on the XJ series…
FAQs: http://www.jag-lovers.org/xjlovers/xjfaq/index.html
Archives: http://www.jag-lovers.org/lists/search.html

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