I view the TCs simply as an “80s vintage gizmo” and was oft surprised at their accuracy. Nowadays I’m more surprised that any of them are still working at all. I have a slew of dead ones in a box somewhere!
Does solid state circuity ‘degrade’? We know, for example, how solder joints can break over the decades, causing harder malfunctions. But do transistors, resistors, and such, slowly loose optimal function, which would then create less accurate TC readings… even if the unit is still fundamentally operational?
Very interesting. You confirmed my understanding as to how the computer got fuel use data.
Decades ago, I installed an after market trip computer on my IHC Scout II. NO ECU or EFU!!! A basic two barrell carb. The kit had a simple flow meter which I installed in the fuel line just after the filter and before the carb.
Pulse can be modified. In the USA, Dakota Digital makes various devices for that purpose. One is in my lump. Modifies the pulses taken from the transmission
for the speedo… A bunch of dip switches for tuning…
If one injector is clogged; 8,2% less fuel is actually used - but the computer read-out will read as per ECU injector operation. The same applies if some, or all, injectors are partially restricted - less fuel is ‘computed’ than the ‘actual’ fill.
The engine will of course have less power, but particularly with the V12 the power loss or possible misfiring is not readily discernible…
On the contrary Frank, if one injector is clogged, or partially restricted, then less fuel is used, therefore more fuel is computed than the actual consumption.
The problem is that the trip computer is calculating in a “blind” mathematical way:
Amount of Fuel per Pulse at 3bar per unit of Time x unit of Time the injector is open x Vacuum percentage factor = amount of Fuel per Pulse
[Number of injection pulses x amount of Fuel per Pulse] per Km travelled = Lt/Km
A very small variation in any of these factors would result in a much bigger error considering the very large amount of pulses per litre of fuel or per distance travelled.
A higher FPR pressure would indeed result in higher consumption than actual readings, as well as leaking injectors.
I wonder how accurate their theoretical model is as to the most optimal fuelling of the engine.
I also wonder if one used a adjustable FPR and brought the real consumption values to the same as the theoretical ones, i.e. what the trip computer says they are, if it would have a benefit on the performance, and most important, at the fuel consumption.
Burt then again, we have 0² sensors…
@ Frank: the calulation avgr. consumtion reading is calcultated mathematically correct based on distance and the fuel metered by trip computer. As the metering is wrong (60 liters instaed of the actual 66 liters consumed) averag mpg or L/km calculation ist wrong by the same amount.
The issue is simply that the calculation of fuel consumed is wrong. The car actually uses 10% more than calculated by TC.
The car is equipped with catalytic konverters and Lamdaprobes. As the lamba regulation is fine it is not running too rich - this is correctd by ECU and Lamda probe anyway. If the fuel pressure would have been too high and for that more fuel would have entered via injectors than it would be with a lower pressure than the car would run too rich -> this would immeditatey lead to a correction (shorteing of pulse) via ECU. Or am I thinking the wrong way?
So my guess is that the “translation” of pulses and time to the trip computer is just 10% out.
By the way: I alway fill both tanks when comparing to the calculated metering so a crossfeed would make no difference. No Fuel leaks present as well. For that I would exclude that idea.
So the only option seem to be to change the interface as nobody (including me ) did come up with an idea how to adjust the part.
As your reading is spot on (XJ6 and XJ12 shall make no difference , as it is jut the interface that is different between the two) it must be possible to get a match. Will look for a different Interface now.
**
You’re dead right, Aristides - I should not work nights…
The ECU ideally varies injector pulse width to maintain fuel air mixture at the ideal Lambda=1, 14,7 grams (oz) air to 1 gram (oz) fuel. Less fuel; loss of power - too much fuel is left unburnt with no additional power…
A lot of experimenting went into the ECU to suit it the xk and the V12 engines. Not perfectly; analogue inputs, and the overall engine characteristics means that preventing too lean mixture in all situation requires the engines to be set up slightly fat…
The V12 uses manifold vacuum, along with the throttle pot, to ‘read’ how much air is entering the cylinders. The xk, in contrast, use the AFM to measure the amount of air entering the manifold. To refine pulses; the ECU use the rapidity of input data - sort of 'acceleration pump function used with older carbs. The fuel pressure regulator uses manifold vacuum to maintain (approximately) constant pressure differences between the fuel rail and manifold - otherwise more fuel would be injected with low manifold pressure. But in any case; it removes this factor from ECU computations…
How how fuel data is transmitted to the cruise computer, retaining the pulse width data, displayed as precise fuel display, I don’t know. The interface unit must retain the information without distortion - but the number of pulses is not in itself enough.
As an aside; one factor relevant to the V12 is that the ECU uses data from one bank to run both. If there is any anomalies between the two banks; it may influence the fuelling readings…?
**
In addition, Peter; do check the vacuum connection to the pressure regulator(s) - measure hose vacuum with a gauge. If this vacuum is lost; the fuel pressure will be too high relative to the manifold vacuum - and more fuel is used than the trip computer shows…
Quick-check; with the fuel pump, or engine running, disconnect the pressure regulator’s vacuum hose. If fuel leaks out of the vacuum spigot; the pressure regulator is defective - and the leak will feed the engine directly. Increasing the reading anomaly further…
Deeper test; measure fuel pressure with and without the engine running. With the engine idling, the fuel pressure will be some 4 to 6 psi lower…
I don’t see anyone here referring to the excellent article written a while back by a guy named Duncan, user name Frobozz, about the operation of the trip computer installed in Series 3 XJ6. I think you can find it with a search in this forum for “Trip computer unmasked”. This bit is copied from there:
“For fuel usage, the trip computer simply counts pulses fed to it by the Interface Unit, which is mounted next to the ECU in the trunk. The Interface Unit takes pulses from the ECU and converts them into a format that the trip computer can use. The ECU pulses are simply an exact copy of the pulses being sent to the injectors. The wider the ECU pulses (i.e. the more fuel being injected per pulse), the more pulses are generated by the Interface Unit. It’s a pulse-width-to-number-of-pulses converter. The rate is approximately 1 output pulse per 3.3ms of input pulse time, with a floor of 1ms subtracted off the input pulse time (i.e. you get one output pulse for a 4.3ms input pulse, and two output pulses for a 7.6ms input pulse, and so on.) The input pulses are assumed to be coming in constantly, at about one every 75ms at idle, and more frequently at higher RPMs. The output pulse rate varies smoothly with the input pulse width and rate; it does not only change at intervals like I may have implied above when giving some specific numbers. *
*It’s safe to assume that the different Interface Units in cars with different engines simply alter this timing/pulse ratio, since the trip computers are all the same. If the trip computer is expecting a certain number of pulses per gallon, the Interface Unit has to be designed so that the width and number of injection pulses that deliver a gallon of fuel in that engine, generate that number of pulses on the Interface Unit output pin. *
*The following specific numbers and features were based on playing with a 1985 era trip computer. I have already seen that one older trip computer behaved slightly differently (a few more pulses needed per gallon, different behavior of the trailing zeros and decimal point, etc.) so don’t fret if you have a different era trip computer and it does not behave exactly like this. It’s similar, at least. *
*When the trip computer receives 4800 pulses, it will add .1 US gallon to the fuel used meter. It will do this through .9 gallons, but then it only takes an additional 4060 pulses to get to 1 even gallon. This is apparently how they decided to make up the stacking error from dividing out all these decimal numbers on an integer processor. So that’s 47260 pulses per gallon. *
*Similarly for Liters it takes 1269 pulses per .1L but only 12493 pulses for a full Liter because of the short “make-up” count from .9 to 1. *
*And again for Imperial Gallons, it’s 5777 pulses for .1 I.G. but 56890 pulses for a full I.G. *
**None of these numbers work out precisely against the official conversion rates between units. Imperial Gallons seem to be the farthest off though we’re still only talking a hundred pulses out of nearly 57,000… or about a tenth of one percent. For the other units its much closer. I guess since the trip computer only gauges amounts to the nearest tenth of a unit anyway, it just doesn’t matter.”
He also at some point repaired a malfunctioning interface unit. If the issue is simple an incorrect determination of fuel consumption based on the displayed fuel used rather than mileage computed or unit of measure, the interface unit could be the culprit.
Niles
thank you for quoting. I thought this info was known already. It was this article that prompted me to ask if anybody knows how to adjust/modify the interface unit so the reading will become more accurate.
So far I did not find any info if adjustment/mod is possible and how to do.
I just got a used second Interface unit and will swap it as soon I find time. But my guess is, with so many reporting a similar deviation of 10% that this may be kind of build in error that will stay the same after the swap. Will take a while until I get some results as I’m quite busy in the moment, but will report back my findings once done.
Woudl be great if anybody comes up an idea how to adjust the interface unit.
By the way:
Fuel resssure regualtors I did check, both are working. While theoretically because of too high fuel pressure too much fuel could enter during a pulse, such creating a mismatch of fuel acutally injected during the period of a pulse and the setting what theoretically should be consumed during a pulse, in the end this will come to the same result: the interface need to be adjusted to compensate for, ad the pressure regulators can’t be adjusted and shall not be adjusted at all if working.
As outlined in Niles’ post, it’s not adjustable - nor should adjustments be necessary. His reference to ‘repaired a malfunctioning interface unit’ is interesting - and could be pursued?
That interface units should have some sort of ‘built in error’ is not credible - some sort of ‘common’ fault is a more likely. Given that all ‘our’ ECUs and interface units works the same way; only different fuel pressure can vary the amount of fuel actually injected with each injector pulse…
Did you actually measure fuel rail pressure, which is adjustable on the V12, according to specified procedure - and verified vacuum connections?
There is another oddity; in my manuals, the V12 fuel pressure is specified to between 28,5 and 30,8 psi while other numbers have been presented. Indeed, pressure regulators vary for different markets - though eventual pressure variations is not mentioned…
If V12 ECU operates with different pulse width from the xk; the two interface units are not interchangeable - nor if the engines use different injectors and pressure. Any interface unit mix-up will cause trip computer misreads - provided the trip computers are otherwise identical…
Anyway; Niles’ write-up confirms that when everything is to spec, the trip computers are very precise - as mine is…
And then there is the additional issue that a higher (or lower) fuel pressure will not only effect the amount of fuel flow calculated from the pulse duration for the trip computer, but perhaps more importantly the amount of fuel introduced to the engine. If a high or low fuel pressure is delivering more or less fuel than the ECU determines is necessary, the engine will run rich or lean. That’s not what we would like. Circling back, it seems possible to use an erroneous fuel consumption display as a diagnostic for a fuel pressure anomaly and a poorly running engine. My car is a XJ6; I’ve never worried about fuel pressure and neither checked the accuracy of the trip computer. I think I will do so before too long. It will be interesting to verify the fuel totalization with a fuel pressure reading.
Hi Niles,
a too high or too low fuel pressure will enter more (or less) into the combustion chamber as originally calculated. That assumption is correct. But wit a catalytic konverter and lamda probe ECU would get a signal to bee too rich/too lean and will alter its pulses accordingly. So mixture will stay as calculated in practice. As the amount of fuel per puls/duration is a fixed asset to base the calculation on and not the acutal amount injected this may result in such a deviation. With the pressure regualtor no longer adjustable for HE engine any adjustment need to be done at the interface or in the trip computer. (@ Frank:All TC are identical, differences between 6 and 12 cylinders are handled by the interface, so correct the interfaces are not interchangable between 6&12). This interface is “translating” pulse and duration to a value the TC is using for fuel consumption calculation. For that reason I did ask for means to adjust the interface. On a second thought adjusting the TC could be an option as well But if I lood at the pictures in Duncans article I can’t identifiy something that will give an adjustment opportunity.
I think that if the FPR has a slight different pressure from what the trip computer is basing its calculations, the displayed result will be analogically erroneous.
The engine will work fine though because the O² sensors will trim the duration accordingly to achieve stoichiometric combustion, but the calculation will still be wrong.
I still believe that testing with an adjustable FPR might give some clues.
**
That diagnostic is one of my points with the trip computer, Niles…
Properly working, the trip computer is precise enough to give a fair idea if the engine is fueled to spec. Of course, too ‘lean’ will reveal itself by engine performance - but both the xk and the V12 is fairly tolerant of excessive fueling, up to a point…
I would first check a different trip computer and then look at the fuel pressure - a faulty regulator is unlikely to work but besides tire size only a reliably intermittent contact would work. The system is digital, an error hard to explain.
David
**
Couple of things, Peter - the range of the of the Lambda system’s influence on the ECU is uncertain. Secondly; the Lambda disengages (open loop) when the gas pedal is moved - only at steady cruising is it effective. In addition; the Lambda only starts up when the O2 sensor is heated to 400C - no Lambda during warm-up.
So whether the V12 fuel pressure regulators are adjustable or not; with excessive actual fuel consumption unexplained - checking fuel pressure is still relevant. It is an obvious suspect…
Due to the smaller V12 cylinders; it requires some 20% less fuel per injection than the xk engine. Basically, excluding different type of injectors; either the fuel pressure can be lowered by 20% - or the injector pulse width narrowed by a like amount.
The original V12 set-up may have used the xk ECU logic for generating injector pulse width - and just lowered the fuel pressure. The use of digital selection of pulse width may have benn a later development, possibly in conjunction with the HE introduction? Allowing for a different solution, with a change in pulse width? However, this raises the question of when the trip-computer was added; if it was available throughout - the possibility of different interface units exists.
That the trip computer fault is 10% rather than 20% is just one of those things which makes life interesting…
The ECU is there to meter correct fueling for the engine - the trip computer is an add-on. As far as I know, there is no adjustments on the trip computer or the interface unit - they must be considered correct or faulty?
The ECU is considered a fairly reliable piece of kit - and most likely your engine is fed properly. Which of course can be verified with an exhaust gas analyser - or at least a CO tester. Living with a faulty trip computer is tolerable - faulty fueling is not…
IMO, the trip computer can be useful as a warning device. That is, if thru experience you’ve established ‘typical’ fuel economy readings and then suddenly get different readings, it merits investigation. Of course, without the TC we’d use our odometers, pencil, and paper to the same effect.
I wouldn’t consider using the TC as a diagnostic device. Diagnosing V12 engine management faults is trick enough as it is !
Interesting discussion. I’ve never considered the TC as anything but a novelty item, personally. But, then again, there a lot of things that I’m nonchalant about
Cheers
DD
) did come up with an idea how to adjust the part.