[xj-s] Trip Computer

XJ-S
21

The second hand interfaces go for around $80 AUD, since I’m not 100%
convinced it is the interface unit yet, I don’t want to spend the money yet,
especially as I could replace all the components on the board for about
$10… I also need to find out whether the Australian interface is the same
as the UK interface.

I assume US gallons are smaller than Imperial Gallons (Why do you Yanks
alway have to be different from the rest of the world) so for my trip
computer telling me I’m using 80+ Gallons would make it over 100 US Gallons
per hour which is not possible. The TC has a metric/imperial switch and to
measure gallon, I had it in imperial mode.

I will disconnect the wire between the interface unit and the ECU and try it
again. Should it read the maximum fuel output or should it read nothing at
all. If it reads maximum fuel output, the interface is working properly and
the fault must lie within the ECU, or the connection between the ECU and the
interface unit.

If the fault is in the ECU, I will not even bother fixing it, as the ECU
works fine as far as the engine goes and gives good fuel consumption (better
than most on the list) and I don’t want to upset it.

Iain Burgess
85 XJ-S UK Spec
Brisbane Australia-----Original Message-----
From: channell@cvc.net [mailto:channell@cvc.net]
Sent: Friday, 24 August 2001 13:31
To: Iain Burgess
Subject: RE: [xj-s] Trip Computer

At 07:52 AM 08/24/2001 +1000, you wrote:

I tested the trip computer as you recommended - With the ignition on but
not
started, I get 1.4 Gallons in 1 minute reading which is around 84 Gal/hour.
Driving all the way home, I got 24.5G in 17 minutes which gives me
86.7Gal/Hour (the 17 minutes is not exact).

Had you disconnected the wire between the ECU and the interface unit?

I will re-check the wiring between the ECU and the sender unit, but it
looks
like I have a lot more “theoretical fuel” usage than the 18.5 Gal/hour.
Would mine being a UK spec car make that much difference.

It should have been higher than my XJ6 readings. The data numbers say full
duty cycle flow of 6 XJ6 injectors is 20.5 gallons per hour and 12 XJS
injectors is 34.5 gallons per hour. Those are US measures.

I am not familiar with UK spec equipment so I have a couple of
questions…and my cat isn’t here right now.

Is the interface unit the original? What is normal function of your
computer? Do you have a switch which allows metric display? Could you be
measuring liters?

The ECU is obviously not giving out a continuos duty cycle to the
injectors,
so I am back to suspecting the sender unit.

Sounds like you are on the trail of it. The sending unit seems to me to be
the culprit.

Does the square wave frequency get faster with more fuel used or slower, if
I am using maximum fuel, would it be that the square wave is just
continually “on” thus fooling the Trip computer into thinking it is
infinitely high frequency hence lots fuel being used. If that is the case,
which one of the components in the sender unit is likely to produce this
effect?

The “base” frequency we found was the maximum frequency the interface would
produce with a constant voltage input. The injector pulses provide
interrupted voltage. Thus the constant voltage generates the maximum
frequency the interface produces…Or alternatively stated the full duty
cycle frequency. Thus if the interface is sending a signal with a high
frequency it will as you speculate fool the trip computer into incrementing
more fuel units.

The information I have says each 5.3 injector delivers 18.96 lbs/hr of fuel
at 300KPal. When corrected for Jag fuel pressure of 36 PSI they should
deliver 17.25 Lbs/hr. I don’t have the conversion to imperial gallons
handy but you should be able to grab it. Convert lbs/hr to gallons
per hour then multiply by 12 to get an estimate of the fuel measurement
you are looking for.

My guess is the problem is in the interface rather than the trip
computer. I would be looking there.

By the way interface units are not that expensive here. Have you checked
the price in your end of the world?

Wes

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22

At 02:05 PM 08/24/2001 +1000, you wrote:

The second hand interfaces go for around $80 AUD, since I’m not 100%
convinced it is the interface unit yet, I don’t want to spend the money yet,
especially as I could replace all the components on the board for about
$10… I also need to find out whether the Australian interface is the same
as the UK interface.

Ouch! A lot more than we picked one up for destructive testing.

I assume US gallons are smaller than Imperial Gallons (Why do you Yanks
alway have to be different from the rest of the world) so for my trip
computer telling me I’m using 80+ Gallons would make it over 100 US Gallons
per hour which is not possible. The TC has a metric/imperial switch and to
measure gallon, I had it in imperial mode.

I dunno’ we are a strange breed. Imperial gallons x 1.201= US
gallons. Liters x .22= Imperial Gallons. If you retest in metric mode and
get readings which convert back to your test readings in imperial mode I’d
say the trip computer is probably functioning properly.

Since your test was with the Y/G wire plugged in what you noted was what
the interface signal generator changed the injector pulses to while in
operation. My bet is on the signal generator in the interface unit.

I will disconnect the wire between the interface unit and the ECU and try it
again. Should it read the maximum fuel output or should it read nothing at
all.

In my XJ6 the Y/G wire exits the interface harness and disappears behind
the side valence in the boot. Lifting out the valence reveals a bullet
connector connecting the wire to one which enters the ECU harness
bundle…Unplug the bullet connector.

Thew trip computer should read approximately the max fuel flow at full duty
cycle. I estimate approximately 28.7 Imp Gallons. (34.5 US gal/1.201)

If it reads maximum fuel output, the interface is working properly and
the fault must lie within the ECU, or the connection between the ECU and the
interface unit.

If you get correct numbers in the trip computer from that test then the
interface is operating and the ECU injector pulse is getting converted
properly and the trip computer is incrementing properly. If MPG is off, it
will be in the trip computer. If fuel consumed is off, it is probably
either in the connection to the ECU or in the interface itself. And, you
were right the non-destructive test is to substitute another interface unit.

The ECU does nothing with the interface other than sent the injector pulse
to it. Inside the ECU the injector pulse terminal is jumpered to the
interface unit terminal. Injector pulses are created by completing a
ground. If the vehicle is running properly, its almost impossible for your
problem to be in the ECU.

I’m sorry I don’t know anything about electronics or I would be able to
tell you what to look for inside the interface unit. From what you’ve
identified I’d still bet on the signal generation circuit.

Wes

23

At 08:48 2001-08-24 +1000, Iain Burgess wrote:

My distance reading on the TC is very accurate (exactly what is shown on the
speedo that is).

Lucky you. This is what is bodged on my 1988, and I need to get off of my
duff and investigate it.

Does the fuel input pulse to the sender unit, or is it a voltage that varies
which the sender unit then changes to a series of pulses whose frequency
then varies?

I took some measurements using my DSO, and the fuel is pulses of 0V - 10V
(about 10.5 peak). There is NO SIGNAL when the ignition is turned on but
the engine is not running. The Trip Computer may be internally generating
some clock and bleeding that to the fuel counter circuit, but when
measuring the actual signal, I’m simply not seeing any fuel pulse unless
the ignition is actually running.

Could this be an indication of something wrong with someone’s ECU - a fuel
consumption reading when the ECU is on, but the motor isn’t running? Hmm.

For the fuel signal, between 900 rpm and 2900 rpm (estimated - I need to
take fresh readings with a digital rpm gauge attached), the positive duty
cycle went from 53.87% to 56.6% (stationary, warm, and probably not exactly
reproduceable). Pulse durations varied between cycles (i.e. not every fuel
pulse was exactly the same width - nor were they on a gradually declining
or increasing cycle - and my DSO is 20MHz, so this is WELL within it’s
rez). When counting or timing the fuel pulses, remember that these are
BATCH fire not sequential. The ~900 rpm readings time out to about 808 rpm
if you take the (small) sampling of pulses and multiply their duration
properly. Similar operations on the ~2900 rpm pulses worked out to 2285
rpm (I may have slipped the accelerator – I was standing at the door, with
the metre and a laptop on the hood, and my foot stuck on the accelerator,
after I’d registered an estimate of the rpms from the tach).

There are two signals from the speed interface unit – one on the green
wire which splits off to the speedo (RIGHT AT THE CONNECTOR TO THE TC), and
which predictably was zilch while the car was stationary. The yellow wire
however, appears to be an ignition pulse (when the ignition is
running). I suspect it is simply noise in the electrical system on a
signal which may go high during motion. I need to get the DSO battery
charged so I can do a rolling test.

You can see the orange (fuel) and yellow signals (speed, but not speedo) at
different (approximate) rpms:
http://jaguar.professional.org/TripComputer/signals/

I’ll try to get some more accurrate (and rolling) readings over the
weekend, when I have some assistance.

That would test the Trip computer unit, and then I would know if the sender
unit was not working, or the TC unit.

If your analog speedo works, the sender unit should be good - it sends
that signal too. The YELLOW wire though, is used only by the Trip Computer.

— http://jaguar.professional.org/
Sean Straw '88 Jaguar XJSC 5.3L V12 (LHD) ‘Black Cat’ 63K
Marin County, California '85 Jaguar XJS 5.3L V12 (LHD) ‘Bad Kitty’ 210K
'69 Buick GranSport 455 V8 324K

24

I think the trip computer head (dash part) is the same between the XJ6 and
XJS/XJ12? Someone will correct me if I am wrong. If so, I have a spare trip
computer.
What I need is the 12 Cylinder interface unit. I am trying to put the trip
comp in my XJ12. It has a clock now.

Burton------------------------------

Date: Thu, 23 Aug 2001 05:50:10 -0700 (PDT)
From: paul garnett mrcobra11@excite.com
Subject: Re: [xj-s] Trip Computer

Hi,Iam new to the list and enjoy it vevy much.If some one can help me,I have
a 87 xjs and need a trip computer can enyone help,Paul
On Wed, 22 Aug 2001 16:22:26 -0700, sean.straw+Jaguar@mail.professional.org wrote:

Get free email and a permanent address at http://www.amexmail.com/?A=1

25

Some time ago I had a problem with my trip computer showing all 8’s.
After trying the suggestions I received on the list I asked my trusted
friend and club member Len Olson who suggested I disconnect the battery
for a while and treat the computer just like a regular computer. Well
success, after disconnecting the batter for 10 minutes, I reconnected it
and with the ignition on, the clock at least came back to life. Now I
am going to fill the tank and check the other functions.

Len W
68 E
86 XJS
88 H&E XJS

26

At 17:31 2002-01-04 -0700, Len Wheeler wrote:

suggested I disconnect the battery
for a while and treat the computer just like a regular computer. Well
success, after disconnecting the batter for 10 minutes, I reconnected it
and with the ignition on, the clock at least came back to life.

Alternatley, you could have just removed the trip computer FUSE.

(For those of us with alarms, disconnecting the battery isn’t a preferred
method to reset devices.)

— http://jaguar.professional.org/
Sean Straw '88 Jaguar XJSC 5.3L V12 (LHD) ‘Black Cat’ 63K
Marin County, California '85 Jaguar XJS 5.3L V12 (LHD) ‘Bad Kitty’ 210K
'69 Buick GranSport 455 V8 324K

27

Can anyone advise, My '89 3.6 never had the trip computer only the clock. I found a 2nd
hand one but its not operating quite right, time functions are OK and its showing the
total fuel used as it should but Im not getting the distance info so of course theres no
mpg readout.
Is there some interface missing or do you think that the computer is faulty. The only
thing I can see in the parts catalogue releting to speed interface is the cruise control
which I dont have, maybe the trip computer needs it to be fitted?
BTW this is the 8 wire blue multi-pin plug type which also displays ecu fault codes, not
quite the same as the v12 type and there isnt a fuel interface due to the ecu type.
Many thanks for any ideas,
Alan

28

Alan

Check the Lumps page FAQs for a write up of making a trip computer work
with a GM TPI. In the write up there is a complete explanation of all the
parts involved.

Wes Channell

At 04:09 PM 04/01/2002 +0100, you wrote:>Can anyone advise, My '89 3.6 never had the trip computer only the clock.

I found a 2nd
hand one but its not operating quite right, time functions are OK and its
showing the
total fuel used as it should but Im not getting the distance info so of
course theres no
mpg readout.
Is there some interface missing or do you think that the computer is
faulty. The only
thing I can see in the parts catalogue releting to speed interface is the
cruise control
which I dont have, maybe the trip computer needs it to be fitted?
BTW this is the 8 wire blue multi-pin plug type which also displays ecu
fault codes, not
quite the same as the v12 type and there isnt a fuel interface due to the
ecu type.
Many thanks for any ideas,
Alan

29

Ever since I acquired my '88 H&E convert. (c.9months),the trip
computer registered Time and Distance but nothing relating to fuel
flow- Ave/inst MPG). I ignored it for more pressing repairs, maint,
etc. Yesterday, it suddely began working for about an hour giving
MPGs at the touch of the control and showing how really thirsty the
V12 is when going uphill @75. Then it suddenly stopped working
again, reading 99 for the mpg settings. Could it be as simple as a
bad connection to some fuel flow sensor? or a bad ground? Any
ideas? BTW, where does the computer get its fuel data from and
where is it hidden?
Mr. Wizard–
Mr. Wizard '88H&E
Bridgewater, NJ 08807, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

// please trim quoted text to context only

30

Mr. Wizard wrote:
Could it be as simple as a bad connection … ?

Yes. Start with this:
Gently ease the T/C straight out from the dash; disconnect the two multi-pin
connectors; clean the male and female connections with 4-600 grit emory
cloth or sandpaper (male) and a 1/8" round jeweler’s type file (female),
spray with electrical solvent; re-connect and turn ignition to power. See if
you get read-outs for all settings.
Periodically, I ‘lose’ my T/C and twice have been told by Jag mechs that
“your T/C is bad/fried/kaput”.
The above has worked for me, but it may be just the re-booting. Worth a try.
Good luck,
although I don’t suppose Wizards need much luck.
Jay Gilmour
'85 XJ6 vdp US; '91 XJ-S cv US
Mercer Island, WA, USA

// please trim quoted text to context only

31

I took apart my trip computer once did nothing to it but reassemble it
when I was done. More of the options worked after that.

Between the ECU and trip computer, I am under the impression that many of
their problems stem from bad solder joints.

I don’t know if these things just go bad after a while because-Jaguar
used some poor materials, or if the inherent design is tough on the
componments and connections (the way a 6V system is tougher on a given set
of wires than a 12V).

I have an 84’. The buttons have worked at times, seems to have
gotten-probablility wise-less affective-like it deteriorated.
Now, some work and others don’t

-Rob-

Ever since I acquired my '88 H&E convert. (c.9months),the trip
computer registered Time and Distance but nothing relating to fuel
flow- Ave/inst MPG).

// please trim quoted text to context only

32

(snip)
computer registered Time and Distance but nothing relating to fuel
flow- Ave/inst MPG).
BTW, where does the computer get its fuel data from and
where is it hidden?
Mr. Wizard '88H&E
(snip)

For the fuel usage data, the trip computer gets information from the fuel
injection ECU circuit (as per the wiring schematic in my service manual for
1979 to 1988-1/2 XJ-S).

Tex Terry, II
86 XJ-S Coupe V12 GM400 DANA 2.88
Franklin, PA USA

// please trim quoted text to context onlyFrom: Mr. Wizard gtortarolo@yahoo.com

33

Is it possible to take the clock out of a 95 XJS and retofit the
trip computer from an older XJS, or would it be more trouble than
it’s worth?

Thanks

Lee Murray–
LeRoy Murray
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

// please trim quoted text to context only

34

Good question! I have a computer and all the other bits from breaking a 1989
XJ-S. I was wondering how difficult it would be to transplant them into
another XJ-S of that era, which left the factory with just a clock.

Is the wiring loom standard?
What are the other elements of the system, beyond the computer? Sensors
presumably? I don’t recall noticing any components which looked different to
those on the other car.
Any other considerations?

Regards
George>From: lmurray@citlink.net

To: xj-s@jag-lovers.org
Subject: [xj-s] Trip Computer
Date: Mon, 3 Feb 2003 06:58:35 +0100

Is it possible to take the clock out of a 95 XJS and retofit the
trip computer from an older XJS, or would it be more trouble than
it’s worth?

Thanks

Lee Murray

Surf together with new Shared Browsing
http://join.msn.com/?page=features/browse&pgmarket=en-gb&XAPID=74&DI=1059

// please trim quoted text to context only

35

In reply to a message from George Bray sent Mon 3 Feb 2003:

The trip computer requires inputs from the speedometer and the fuel flow sensor. Don’t know if the
harnesses for the clock have those inputs.

Richard Wilder
Sparks, NV
95 XJS 6.0L Cvt.–
The original message included these comments:

What are the other elements of the system, beyond the computer? Sensors
presumably? I don’t recall noticing any components which looked different to
those on the other car.

–
Richard Wilder
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

// please trim quoted text to context only

36

The trip computer requires inputs from the speedometer and the fuel flow
sensor. Don’t know if the harnesses for the clock have those >inputs.

Thanks. I guess the vehicle has both sensors already fitted, for feeding the
ECU, so retro-fitting a dashboard computer might be even easier than I’d
anticipated before your message.

Regards
George>Richard Wilder r-wilder@charter.net wrote:

Surf together with new Shared Browsing
http://join.msn.com/?page=features/browse&pgmarket=en-gb&XAPID=74&DI=1059

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37

In reply to a message from George Bray sent Thu 6 Feb 2003:

Ive done it

it was as simple as taking the clock out and plugging the trip into
place…job done :-)–
XJS Monaco http://www.geocities.com/jagzmanv8/Jaguar_Monaco.
Norfolk, United Kingdom
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

// please trim quoted text to context only

38

I ran out of gas with the '91 today. What a butt head, but it isn’t
all my fault. I got the car up and runing with the new heater hoses
and was driving it back to NYC. The gas was low,a nd Iw as a few
miles from a place to fill up. I only panick when the low fuel
warning light comes on, but I rarely get this far. Has anyone seen
the Seinfeld epsiode when Kramer take the car for a test drive?

Anyway, I’m on the NYS Thruway and poof, the car slows and dies.
Here I am a college educated artist, own a half dozen cars, couple
of homes, keep stuff inorder, and bam! I am embarrased, but I’m
going to continue.

On an outside chance I was thinking it may be the ignintion
amplifier, but the '91 doesn’t have one, DOH! I get a service call
and get 2 gallons of gas at $24.45 each. I get to the staion and
fill 'er up. I reset my computer and, guess what, it gives me weird
numbers for Fuel, Average, Current, Distance(.6, 1.2, 0, .3
whatever)…I wonder if the sender unit from the gas tank is
going? Does the sender also affect the low fuel light? Can a sender

Sorry I let us Jaguar owners down folks. It was a sad sight to see
the car on the side of the road. But at least I didn’t let out my
shorts on the aerial!from my '85 parts car work with a '91.

Charles
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

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39

Funny story Charles,

Everyone thinks that silly mistakes like that can only happen
to someone else!

I laughed out loud at the implication that such an embarrassment
shouldn’t occur to a college educated artist, or a guy who owns lots of
stuff

I don’t know that either of these qualities makes one immune to mistakes.

Richard Drozdowski
1992 XJS (quarter tank…I think)

I ran out of gas with the '91 today. What a butt head, …

Anyway, I’m on the NYS Thruway and poof, the car slows and dies.
Here I am a college educated artist, own a half dozen cars, couple
of homes, keep stuff inorder, and bam! I am embarrased, but I’m
going to continue.

…- –
Charles

// please trim quoted text to context only

40

In reply to a message from Richard Drozdowski sent Tue 6 Apr 2004:

Richard,

I guess the point of that was the absurdity of educating oneself
through college, or university, only to become an artist. Not that
being an artist is not worthy of an education, just funny how if
one posesses a modicum of talent, higher education is somewhat
redundent, I think! Who knows, certainly not I. I’m happy to get
through life creating stuff.

The good news is, my auto club will accept the invoice and I will
be reimbursed fully. I still feel bad that others saw a Jaguar
stalled, althought I bet most couldn’t even recognize an XJ-S as a
Jagaur, so maybe I didn’t let us down afterall!

Charles
keeping it 1/2 full–
The original message included these comments:

I laughed out loud at the implication that such an embarrassment
shouldn’t occur to a college educated artist, or a guy who owns lots of
stuff
I don’t know that either of these qualities makes one immune to mistakes.
Richard Drozdowski

–
Charles
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–

// please trim quoted text to context only