[Sean's Jaguar XJS Tech Pages]
Jaguar Trip Computer Project

Last modified 2006 MAR 31 18:27:04 GMT

This is what I envision a prototype of the new trip computer might look like. Note the buttons are general purpose (their function being dynamically defined by the onscreen menu - unlike the OEM trip computer, where each button is single-function). If I can locate a good 4 or 5 position nav button (minimally, the points of the compass, plus optionally a select in the centre position), that may be located to the left of the display, and the buttons across the bottom will not exist. There may be more LEDs (for visual alert cues), although at the minimum, one LED would be standard for optional use by a car alarm.

Ever since I purchased my 1988 XJSC, the trip computer accuracy has been off. The fuel tracking seems fine, but the speed signal seems to be tweaked. If the TC can't track speed, then distance, average speed, and economy figures are all going to be inaccurate. That makes it virtually useless, though as I said, the fuel consumption (actual gallons) coincides with what I pump in, so it is useful. The clock works, the display, while difficult to read (a function of the components, not of some failure), works, and even though the speed signal isn't right, it seems pretty consistent in how much it is wrong (about 40% under).

Using a Fluke scopemetre to measure the signals, I've since determined that the +12V coming from the ECU isn't even near the voltage it should be. I haven't yet determined if the signal is correct at the ECU, or if by bypassing this signal and inputting a known good +12V (BTW, this signal is separate from the one providing the operating current for the TC), the TC might register correct figures.

In late SEP 2001, I took measurements from the (identical) trip computer in my 1985 XJ-S coupe, and considered making a small test apparatus with a dual 555 ic setup and a pair of switches. The idea here is to have two output signals from the box - one for fuel, and the other for speed. At one setting, these two would output constants that perhaps would equate to 10 lph and 100 kmph (use of metrics means that the same circuit should work on US and UK trip computers -- where the GALLON unit is different), and at another, they'd run 25 lph and 200 kmph (or whatever - the idea is to differ the readings). One could hook it up to the TC, reset the TC, then start the unit and let it run for 30 or 60 minutes. then turn the unit off and check the TC readings. If they're spot-on (or very nearly so), then it would demonstrate that the TC head unit is working, and if you have a reading problem, it must be in the signals coming from the speedo transducer or fuel management controller. Note that while I have not produced the test box (which can be constructed for circa US$10 in parts), the speed output signal should be capable of driving the dash speedo - not that I'd ever provide it to someone specifically for calibrating their speedo, but if you change your tyre size, this is something you'll need to fret with, and this sort of test box - if built with the correct parts and operating to tolerance - can provide you with a constant speed signal enabling you to adjust the speedo from a known value to a computed relative value. This would be fairly easy to do - do your math (say you've fit larger tyres to your car and need to increase the speedo reading by 2.5% - so when your original speedo setting claimed you were doing 200kmph, you're actually doing 205kmph) - then hook up the tester and set it to the 200kmph setting (one of the two speeds it should have) and adjust your speedo so that it reads 205kmph. You're done. If you require an instrument calibration certificate, you may have to have your speedo checked and set at a licensed shop (who would probably put your car on a rolling road tester), but for those that this doesn't apply to, the above procedure would probably work quite well.

Nevertheless, I'd been considering replacing the trip computer unit with a new one, of my own design. My idea is to replace the whole dash component with a new one having the same dimensions, but incorporating a backlit (blue) graphic LCD display, several buttons (in theory, not even as many as are currently used, because of how they'd be wired in), one or more LEDs, and a small speaker or piezo buzzer. The LEDs and speaker are optional - they're for future project use. At the heart of the unit would reside a PIC - programmable integrated circuit. In my searchings, I've found that there's a product which may best suit this project - the Tiny or BASIC Tiger stamp ICs produced by Wilke Technology. While I feel quite at home programming in assemblers, the nature of this dictates that future modifications should be something that anyone using it might be able to accomplish without having to become a software engineer. Because this PIC is programmed in a higher level language (their own variant of BASIC), it should be easier for people to pick up on. For this trip computer project however, I expect to do all the programming necessary to implement the functionality, so it isn't as if anyone will HAVE to learn how to program the device in order to build one of these - the code will be provided.

The Wilke product has direct I/O and serial support, right off of the interface pins of the device, which means a lot of complex external circuitry is unneeded. This is a GoodThing(tm), because it simplifies the circuit design and keeps the parts count down.

About the buttons - I mentioned that the new unit wouldn't necessarily require as many buttons as the original. This is because with the advanced display, we can use the buttons as navigation and menu selection, rather than as single-function switches. This plays a big role in future projects. Further, and while I've not yet purchased the development kit (about US$900 - a cost to be burdened by me to initiate development, whereas the actual processor cost is only US$60 in single-unit quantity), it should be possible to wire the buttons using parallel resistance (each button is in series with a resistor of a different specific value, but all buttons feed to one input on the IC), which would mean that all the buttons would utilise only ONE I/O line from the processor, leaving more lines open for other projects.

Initially, the trip computer and clock will be replaced by this project. Later, I hope to use this unit - display, switches, and logic unit - as a platform to which other projects can be added. Some of the initial projects I expect to implement include O2 readouts (a form of detecting what the engine fuel mixture is), temperature monitoring (separatley for the A and B banks, and with more accurracy than the dash gauge), digital tach (again, more accurracy than the dash gauge), door buzzer (to replace the obnoxious door alarm with something less annoying - and importantly - capable of being silenced), and a TH400 (GM) transmission "sport mode" kickdown option (as has been discussed on the Jag-lovers XJS list). I have a rather large list of other projects that could be implemented, including the possible replacement of the Cruise Control ECU (for the purpose of replacing a defective unit).

One of the beauties of this project is that a wide variety of suggested mods for the XJS normally require mounting of a switch someplace, or of a display, etc. Here, we have the basis for many such modification, without duplication of the components, and largely without having to find a place to cram something else.

While the display I expect to use will be graphic (capable of displaying images, not just characters), most of the initial projects that require any display functionality will be text only -- but having the graphic display means that when we do add something that normally would register best as a graphic (a temperature curve, tach, or even the O2 readings), the capability will be there, and can be implemented entirely within code -- no wiring changes whatsoever!

The LEDs and the speaker, which are new to the trip computer, can be used for alerting the driver to warning conditions (like temperature rising above the normal range, or O2 ranges beyond a threshold). Additionally, an LED could be used for your existing (or future) car alarm, or merely wired to the logic unit which could flash it when the ignition is off (giving the impression of an installed alarm).

Interested parties, especially those with electronics and PIC software development backgrounds are encouraged to contact me about participation in this project. Since I'm not expecting to produce this kit as a commercial venture, offers of financial support (in any amount) to defray the high cost of the development toolkit would be most welcome.

The sooner I purchase the development kit, the sooner this project starts becoming reality.

It is my plan to build a new housing for my own dash unit (which will be more of a clamshell design, allowing for easier - and complete - disassembly, along with a new facia panel, which I hope to veneer with wood. To this end, I expect to purchase a pair of the switch facias to either side of the trip computer from a Jaguar salvage house so that I can reface them all with matching wood grain (while retaining my originals in a box). My ultimate goal isn't just to have a working trip computer, and the foundation for future projects, but also to make one that looks more at home in the Jaguar than the original.

Since I cannot expect everyone to have the proper facilities to work with sheet metals (even though the housing isn't complex, it needs to be a snug fit), it is possible that we may use the cage (unmodified) from the existing trip computer to facilitate a no-hassles housing for the new display component. To do so requires popping the molex pins from their connectors, so that the harness can be slipped through the hole in the back of the cage (which is too small to permit passage of the connectors). Unless you elect to take this route, the original trip computer unit will simply be pulled from the dash and placed into a box for safekeeping, and will be unmarred by this project. If you do elect to use the OEM housing, proper disassembly will result in an undamaged and unmodified original unit, which can be returned to the housing using the reverse of the process.

While I haven't yet even produced my own housing (I'll get around to it when I have the development kit in hand and develop a better idea of component placement), I should be able to hand-produce a limited quantity of additional housings, which I'd pass along to interested parties for the cost of material and labor. The same will hold true of the display facia as well (although not necessarily with wood veneer, unless you're willing to bear the higher cost of that additional work).

Some of the various projects which have been conceptualized which could be handled by this new unit, listed in no particular order:

* Emergency brake engagement reminder (if the car is put into gear with the E-brake on, flash an attention LED and display a message reminding the driver that the e-brake is engaged, or play a digital recording, sound a buzzer, etc).
* LPG controller (or at least, be capable of monitoring LPG fuelling rates, or petrol start and switch to LPG for run).
* Speed interface (standard trip computer functionality).
* Fuel consumption (standard trip computer functionality).
* Clock (standard trip computer functionality). 24 hour display option.
* Timer Mode. Could work in conjuction with the speed input (and remember, distance is a function os time and speed, so it knows how far you've moved as well) to be able to report simple 0-60 or quarter mile type figures. Can't account for tire slip ("burning rubber"), so this isn't a precise measurement, but once the base functionality is there, adding this requires no additional wiring - it's merely code.
* The unit could have input for tyre profile so that it can compute speed correction for the speedo transducer between stock tyres and a tyre with a different rolling circumference. This will not adjust your instrument cluster speedo, but you have a comparative reference, and the TC will be computing fuel economy and distance correctly.
* The unit could have inputs for transmission and differential gearing. Coupled with an understanding of the engine RPM, the unit could provide an optimum transmission shift indicator for those who might fit a manual transmission to their XJ-S. Or, it might be able to control an electronic overdrive transmission (such as the electronic version of the 700R4).
* Marelli ignition failure warning. Either by utilising an inductive pickup coil on one plug from the A & B banks, or by monitoring catalytic temperatures. I'm more prone to go with the former - easier and more direct to implement, AND more likely to give advance warning before the cats become damaged.
* Left and right (V-12) O2 sensors. This would permit display of a "rich/lean" level for either bank - if both banks aren't running the same, you've got an indication there's a problem.
* Single bank running -- switching the injectors from either the A bank or the B bank to an off state thus running the engine on just 6 cylinders for economy. And since this unit is also the fuel monitor, it can automatically compensate for the fuel reading which it'd only be seeing from one bank (though BOTH would have to be tapped). There's even the possibility of putting the unit into an economy mode where it automatically kicks you into 6 cylinder operation under low load conditions, or when idling, etc - AND it could handle switching between the two banks periodically to balance wear.
* Tach (being digital, it should be more accurrate than the dash gauge). Coupled with the cruise control functionality mentioned below, it would be possible to set the engine to hold a 3K rev, so you can go check the timing/advance curve - while the Cruise control typically interfaces with the speed transudcer to determine what level it needs to be at, you could still use the bellows to drive the throttle while at a standstill. In theory.
* Progressive Nitrous controller.
* Could drive a garage door opener. You'd strip your garage door opener from it's oversize housing, probably also get to eliminate the battery, and install it into the remote project box -- the trip computer could actuate the open/close button just as if it were a triggered relay. Of course, this feature would only be enabled if the ignition were on (implying you have a key) and/or the alarm is deactivated, so it's better security than if you just had a garage door remote sitting in your console.
* Temperature (with possible control over an aux fan and use of aux thermocouples for RS vs LS or just underbonnet temp, as well as cabin temp). Note that an audible alarm could be sounded if the temp passes a threshold or if the differential between the two banks is more than some number of degrees. Also, such a display would be in actual temperature UNITS, versus the vague "C-N-H" of the factory instrument panel gauge.
* Trip computer faceplate could house an alarm LED (although I've mentioned that at least for 88+ models, Jaguar has a replacement lighter bezel that has an LED in it).
* Control the transmission kickdown (as per discussions in the Jag-lovers XJS forum some months ago) which would involve the throttle pot. Several curves could be programmed (as well as the function entirely being disabled), so that you can choose one that works to your taste.
* Window "auto up" and "auto down" feature - could sense an up/down or down/up action on a window switch and automatically drive a window relay for a configured amount of time in the selected direction (or you could instal microswitches in the doors so it could sense full open/close).
* Post-ignition timing for the window relays (thus allowing the windows to operate for say, 30 seconds after cutting the ignition - I don't like the idea of leaving the switches powered since if you leave the windows down just a bit, someone can lower the windows by depressing the button with a stick/cane/umbrella/etc).
* Intermittent wiper delay - truly programmable delay, not just "slow" and "fast".
* Improved trip computer functionality (logged trip legs instead of a simple master reset).
* Possible interfacing with a GPS unit to display position and heading information on the trip computer LCD display.
* Voltage/Charge gauge. Digital voltmetre, though with some additional circuitry, an ammetre could be installed.
* Digital display of oil pressure (remember, the sender is an electric signal).
* Automatic headlamp timeout when ignition is turned off.
* Replace the BZZZZZ door alarm with a more soothing "bong" tone.
* Could (esp with compass arrangement nav buttons) replace faulty mirror joysticks.
* Semi-security features (security through obscurity): ignition lockout, fuel pump interruption (could interrupt the fuel pump within 10 seconds of a button press, or whatever - a simple carjacking or anti-theft defence designed to put space between you and a violent criminal before they get screwed by the system).
* "Valet Mode" - along similar lines to the security mechanism, it should be possible to interfere with a signal to the ECU to keep the car from being revved -- now you can ensure the valet doesn't cause any ruckus with your precious kitty.
* The same PIC and black box could act as a host location for the Garner, Napoli, and Sawyers environmental mods - and providing the required switch(es) necessary to control them (all through multiplexing of the existing trip computer buttons).
* Relay/button for trunk solenoid release, if so (retro-)fitted.
* Could be interfaced with cruise circuitry to act as a less expensive replacement to the Cruise Control ECU. That'd be a lot more complex (and pose a safety issue) than any of the above projects, which mostly monitor the state of things (with the exception of the trans kickdown and possible security features). If I were ever to consider replacing the cruise control ECU with such a device, I'd do so long after the rest of the mods prove reliable.
* Interface with a Motorolla PMMA2001D 50G micromachined accelerometer. For what it is worth, you could have a G-Force readout. Technically would need two, so you could measure both lateral and horizontal acceleration. No, I don't have any need for this myself, but depending on the cost of the sensor, this could be a fun thing to have just for the heck of it.

Listing of a particular idea isn't to say that when I get around to building this unit, I will ever implement that myself, but the idea is that all of these are possibilities which are facilitated by the use of a unit such as this.

Many of these signals don't need to be monitored all the time (fuel pulses would, but temperature, O2, and other such readings could be polled in round robin fashion, as they won't change rapidly, and are only displayed for informational purposes). Even the clock, which will be an independant RTC unit, won't be processor speed dependant, and needs only to be read when that is displayed onscreen, or when being used to log an event (BTW - with the trip computer knowing travelled distance, it should be able to log events not only with timestamp, but also with a digital odometer reading, the initial value of which would be entered by the driver).

The choice to place it down near the Cruise Control ECU (which is behind the aux fuse panel) is both because the current trip computer box is much to small to work with for some of the other available SBC solutions, AND because in the new location, it'd be more central to many of the wires that other projects might tie into.

Something that came to me recently as a possible workaround to any power-on time delays would be to trigger the unit from standby to pre-ignition power-on via the door switches (which drive the courtesy lamp delay) - since the unit would normally do a controlled power up and power down using the or accessory voltage line, this would be an alternate signal (isolated by diodes to as to not energize other accessories) which could often be expected within a window of several minutes of ignition (no, it wouldn't take THAT long to initialize, and I'm hoping initialization will be a non-issue anyway).

Presently, the images on this page below are strictly of the OEM trip computer unit, accompanied by brief descriptions and some notes. As the project progresses, additional images and data will be presented here.

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Front panel of the removed trip computer.

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View of rear of trip computer. To open the housing, you have to unthread the two small hex nuts on the back, remove the screws on either side, and unscrew something from the bottom near the rear (which anchors to the transistor heat sink visible in the upper left corner of the front PCB view). Then the faceplate can be pulled away (straight forward, as it has a pair of shafts which protrude straight out the back). The PCB can be brought out fairly easily (if it doesn't almost fall out, try gently feeding the wires into the housing, which should displace the board).

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Closeup of the connectors. By convention, I refer to the one with the pins (the actual conductors) as the male connector, which is shown here on the left, with the red/brown/black leads.

I'm working on obtaining a small quantity of these connectors for the trip computer project, so that the original trip computer can be removed and stored intact (for those like me who don't like to butcher parts). This way, if you decide to try the project (for its other benefits), but actually have a WORKING trip computer, you won't be doing something that can't easily be undone.

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These two images show the size of the connectors.

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Got it?

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This should give you some idea of how cramped it is where the trip computer is situated - the unit is less than two inches deep - the trip computer isn't the size of a stereo, even though the faceplate is about the same dimensions. The PCB assembly inside is folded in half.

Our replacement Trip computer project will be housing a display and control buttons up where the trip computer originally was installed, but the new logic will actually be located down near the cruise control ECU (above the passenger footwell). Although a trip-computer only replacement would be possible to house entirely in the original casing, the potential for this same project to act as a platform for a variety of other improvements led to the decision to relocate the logic unit - the additional wiring and support circuitry necessary for some other projects would be a pain to route behind the centre dash area.

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The front side of the PCB assembly. Lots of components. You can say goodbye to this complexity with the new trip computer, which consists of a few switches, an integrated backlit LCD graphic display panel, and a self-contained logic unit. Very little support circuitry will be necessary for the new unit.

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Backside of PCB assembly. Nothing special, though it should be noted that when the PCB is folded over, some pins MIGHT contact. I wonder why they didn't place a sheet of mylar in between the PCBs (or perhaps, one is in there originally - this unit being displayed is a salvaged unit).

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IC2, IC3, and IC6 apparently are units which fail (as evidenced by the fact that the backside of this PCB has these components resoldered). Note that this is a scrapped nonop trip computer graciously provided by Walter Petermann (THE guy to go to if you want your OEM Jaguar electronics repaired).


Sean B. Straw
Post Box 751224
Petaluma, CA 94975-1224 USA

EMail to: Sean.Straw+Jaguar@mail.professional.org

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