[xj] Trip Computer Troubleshooting

XJ
1

Hi guys,

My trip computer hasn’t been working for quite some time. I
figured that I had a blown fuse somewhere in the 12v
constant line, but I never wanted to tear apart the interior
to look for the inline fuse. I finally did the deed today
and sure enough there was a blown fuse inline.

I replaced the fuse and checked the voltage at the trip
computer – the Constant (brown to purple to brown) wire
reads 12v, the switched ignition (green wire to fuse 4) also
shows voltage. The panel still is not illuminating with
ignition on and the car running.

I noticed the red wire (for dimming lights?) has no voltage
with either ignition setting – I followed the small harness
that runs from the trip computer and noticed that the Red
(dimming wire) turns into a blue/red wire which according to
the wiring diagram goes to the sidelamps. This is where it
gets interesting.

The wire harness ends in a little 4 pin plug – but there is
no plug attaching to it! I looked all around, but there is
no ‘‘other end’’ to this plug.

I also noticed that one of the pins is 12v hot – so where
ever this thing is going, it was to energize something.

However, since that other part of the harness is missing,
can I run another 12v ignition jumper wire to fuse 4, or
another available hot wire? I think my trip computer is
alive… just not showing anything because the panel lights
wire has become disconnected (probably when I lumped my
car…)

Any ideas? Or any ideas where this secondary harness goes?
Perhaps I can track it down in the mess of wires up in
there.

Thanks!–
Tyler Werrin, 1984 Jaguar XJ6 Series 3 w/ 1994 Camaro t56
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

===================================================
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

To remove yourself from this list, go to http://www.jag-lovers.org/cgi-bin/majordomo.

// please trim quoted text to context only

2

In reply to a message from tylerwerrin sent Mon 24 Oct 2011:

Never have had a problem with mine so obviously I am not a
source of info. I did run across this site some time ago and
bookmarked it for the day that I might need it.

http://www.backglass.org/duncan/jag/ifu/

Give it a read, maybe it will help.–
The original message included these comments:

My trip computer hasn’t been working for quite some time. I
figured that I had a blown fuse somewhere in the 12v
constant line, but I never wanted to tear apart the interior
to look for the inline fuse. I finally did the deed today
and sure enough there was a blown fuse inline.
I replaced the fuse and checked the voltage at the trip
computer – the Constant (brown to purple to brown) wire
reads 12v, the switched ignition (green wire to fuse 4) also
shows voltage. The panel still is not illuminating with
ignition on and the car running.

–
Gordon S. Tompkins ('73 XJ6, '85 XJ6 & '03 S-Type)
Springville, CA, United States
–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: Jag-lovers Forums - Jag-lovers

To remove yourself from this list, go to Jag-lovers Forums - Jag-lovers.

// please trim quoted text to context only

3

In reply to a message from GSTompkins sent Mon 24 Oct 2011:

Thanks for the link – I actually was using it before,
trying to troubleshoot. The wire in question is the Red
wire, noted as the panel lights wire – my guess is that my
computer is alive and well, but because the LED’s aren’t lit
I can’t see a thing.

Would it be possible to just run a jumper wire from the red
wire (quicksplice) to an available 12v ignition hot source?
I’m not sure how the circuit of the red wire works – it
looks like it connects to the sidelamps somehow – if that’s
the case, I could trace them, the problem being the plug
terminates (looks like it is missing whatever connected to
it beforehand…)–
Tyler Werrin, 1984 Jaguar XJ6 Series 3 w/ 1994 Camaro t56
–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

To remove yourself from this list, go to http://www.jag-lovers.org/cgi-bin/majordomo.

// please trim quoted text to context only

4

tylerwerrin wrote:

Hi guys,

My trip computer hasn’t been working for quite some time. I
figured that I had a blown fuse somewhere in the 12v
constant line, but I never wanted to tear apart the interior
to look for the inline fuse. I finally did the deed today
and sure enough there was a blown fuse inline.

I replaced the fuse and checked the voltage at the trip
computer – the Constant (brown to purple to brown) wire
reads 12v, the switched ignition (green wire to fuse 4) also
shows voltage. The panel still is not illuminating with
ignition on and the car running.

There is no need to run the car if the dispaly is not lit, Tyler - ign
‘on’ is enough to check display…

There are three sources of power;

a. Iinline fuse brown to purple wire, giving ‘stand-by’ power at all
times to kep computer memory ‘alive’ - also possibly connected to fuse
#16 (2 A)
b. ‘Main’ power, running the computer - green from computer to ign
‘on’, which should give display, connected at fuse #4…
c. ‘Lights on’, blue/red connected to anything that is lit with the
light switch out of ‘off’ - it is unpowered unless lights are on. The
sole function is to dim display with when darkness require a dimmer
display…

*With the computer out to access wiring; connect green a +12V source -
the display should come on. This is the only thing that turns on the
display - the others do not. If there is still no display; ‘we have a
problem’…:slight_smile:

With the display ‘on show’ the nominal first test is to set the clock,
then remove power from green - normally done by simply turning the ign
‘off’. (Or in this case disconnecting green from power). Wait a bit,
then repower the green; the display should come on again - and should
show the time set, with the addition of the time disconnected (the clock
is ticking).

If the time reads ‘00’; you have lost standby power - add power to
purple as well, and repeat test…

These two connections are essential to computer functions, the third,
‘lights on’ is an add-on with no computer functions - but can also be
tested by jumping of course.

Remedy connections as appropriate; while the original connections is
‘best’; connecting the computer wiring to power points giving power as
required will also work. Point here is that the original car wiring
might be bastardised, making the original wiring ‘discoloured’, but at
the computer itself it should be unchanged. Then you can test the
computer on the road, to verify its proper functions…

And of you cannot get display with green powered at the computer; you
most likely have a dud computer…

Frank
xj6 85 Sov Europe (UK/NZ)>I noticed the red wire (for dimming lights?) has no voltage

with either ignition setting – I followed the small harness
that runs from the trip computer and noticed that the Red
(dimming wire) turns into a blue/red wire which according to
the wiring diagram goes to the sidelamps. This is where it
gets interesting.

The wire harness ends in a little 4 pin plug – but there is
no plug attaching to it! I looked all around, but there is
no ‘‘other end’’ to this plug.

I also noticed that one of the pins is 12v hot – so where
ever this thing is going, it was to energize something.

However, since that other part of the harness is missing,
can I run another 12v ignition jumper wire to fuse 4, or
another available hot wire? I think my trip computer is
alive… just not showing anything because the panel lights
wire has become disconnected (probably when I lumped my
car…)

Any ideas? Or any ideas where this secondary harness goes?
Perhaps I can track it down in the mess of wires up in
there.

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

To remove yourself from this list, go to Jag-lovers Forums - Jag-lovers.

// please trim quoted text to context only

5

In reply to a message from Frank Andersen sent Mon 24 Oct 2011:

Hi Frank,

Thanks for the info – perhaps it is also a ground issue? I
checked the diagram on captain jaguar’s cathouse, and it says
the ground point is behind the steering wheel – do you know
exactly where it grounds to? I may have inadvertently disabled
the ground when I removed the annoying key-in buzzer…–
Tyler Werrin, 1984 Jaguar XJ6 Series 3 w/ 1994 Camaro t56
–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

To remove yourself from this list, go to http://www.jag-lovers.org/cgi-bin/majordomo.

// please trim quoted text to context only

6

tylerwerrin wrote:

In reply to a message from Frank Andersen sent Mon 24 Oct 2011:

Hi Frank,

Thanks for the info – perhaps it is also a ground issue? I
checked the diagram on captain jaguar’s cathouse, and it says
the ground point is behind the steering wheel – do you know
exactly where it grounds to?

No, Tyler - others might…

…however; any ground will do. I don’t remember if there is a separate
wire; if it is it is black at the computer. And you can ground it at
any chassis metal for testing…or permanent…

Frank
xj6 85 Sov Europe (UK/NZ)> I may have inadvertently disabled

the ground when I removed the annoying key-in buzzer…

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

To remove yourself from this list, go to Jag-lovers Forums - Jag-lovers.

// please trim quoted text to context only

7

In reply to a message from Frank Andersen sent Tue 25 Oct 2011:

Fixed – well sort of.

I tested all the leads – everything ‘‘made sense’’. Popped in
another trip computer – wah la, works fine. Turns out the
computer decided to konk out for some reason or another. i’m
going to open it up and see if there is an internal fuse that
burnt out.

Now onto the radio wiring!–
Tyler Werrin, 1984 Jaguar XJ6 Series 3 w/ 1994 Camaro t56
–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

To remove yourself from this list, go to http://www.jag-lovers.org/cgi-bin/majordomo.

// please trim quoted text to context only

8

tylerwerrin wrote:

In reply to a message from Frank Andersen sent Tue 25 Oct 2011:

Fixed – well sort of.

I tested all the leads – everything ‘‘made sense’’. Popped in
another trip computer – wah la, works fine. Turns out the
computer decided to konk out for some reason or another. i’m
going to open it up and see if there is an internal fuse that
burnt out.

Probably the display unit has failed, Tyler - it is the usual fault for
‘no display’, there are no internal fuses…

Doubt of wiring combined with a failed unit takes some time to unravel -
well sorted!
Let’s hope the radio is more accommodating…:slight_smile:

Frank
xj6 85 Sov Europe (UK/NZ)>Now onto the radio wiring!

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

To remove yourself from this list, go to Jag-lovers Forums - Jag-lovers.

// please trim quoted text to context only