There is no trimmer pot or any other adjustment on the interface unit.

 

Looking at this site it has some good info on the trip computer

Jaguar XJ6 SIII trip computer details

The interface unit has 2 resistors soldered to terminals at the top left, this unit has what looks like 910k and 6.8k 5% tolerance resistor here. With the top mounting I would bet these change the fuel used pulses.

I'll go and see if I can pull my interface unit out.

 

I pulled my interface unit apart and ran some tests, I found reducing the value of R25 increased the output pulse rate (frequency). CH1 (yellow) is output and CH2 (blue) is the input.

Greg you could substitute R25 with a trim pot. If you post a pic of the PCB with the colour code of resistor R25 we can work out what value pot to use.

MY interface unit

 

Warrjon
Fantastic. I will geet to it as soon as my current wounded dog emergency is sorted. Silly girl managed to cut her chest badly and ATM I am sleeping downstairs to stop her biting out the stitches!
I take it Ch 1 and Ch 2 refer to the input connectors on the bottom? Assume nothing in the way of electronic knowledge my end.
Greg

 

I hope she mends quickly, I am in a similar pickle, I broke my wrist last week.

See Ch1 and Ch2 on the 2nd pic on the Oscilloscope.

I just jumpered a 420ohm resistor across R25 to lower the resistance, the frequency went up (increased pulse rate). This should increase the fuel used in the TC.

When you're ready let me know and we can modify your module.

 

I will not embarrass you by asking how you broke it! very sorry to hear of it though, how frustrating for you and us to have a 10 week layoff in the engine rebuild. I do hope it is not too painful.
Thanks indeed for your offer of help, and the time you are taking to find out about how the thing works and posting the photos. I'll post pic of it's insides shortly.
Greg

 

I have my heads 1/2 finished, I can't hold the grinder to finish the exh ports.

I'll need to modify my interface unit when I install the 6.7L so this is good practice.

 

There was a different interface for the 4.2 engine, might be useful to compare and see which one ends up giving a closer value for your 6.7.

 

The interface is the same,the PCB ID number is the same on my PCB and the one on the website you copied the info from, the ONLY difference are the frequency divider resistors.

All of circuit control resistors are the same as is the opamp.

 

This is getting interesting! I like the idea of a super precise trip computer!

The X308 one is basically bang on right (when I drive on petrol and use a full tank the average calculated is spot on...!) so an XJ-S can do that too!

 

Warrjon
Attached are a couple of photos of the unit and R25 colours, that looks to be quite differently coloured from your own unit. The photo colours are accurate, and in real life appear to be:
Yellow stripe
Orange stripe (slightly thinner band than the yellow one)
Two identical beigey/browny stripes both the same and thicker than the yellow one.
The barrel of the thing is dark red.


Daim and I for sure look forward to your advice.
Greg

 

R25 is 11k ohms 5% tolerance


Brown 1, Brown 1, orange x1000, gold 5%


All the resistors around R19, R25 R26 are different values to mine. Remove R25 (keep it so it can be put back) put a 20k pot in its place. Adjust the pot so it reads 11k ohms before you install it. Reducing the resistance increases the pulse rate and I would imagine increases the fuel used as the TC sees it.

This is a quick and dirty fix but I think it will work to trim the TC small amounts, if it does not I will do more indepth testing. I drew out the basic circuitry last week, and it looks to me the way this works is.
* Converts the injector pulse to a dc voltage.
* Converts the dc back to a pulse, this is called a Voltage Controlled Oscillator VCO, and can be tuned accurately to take into account component tolerance.

.

 

As I'm laid up I thought I would do a quick howto on replacing the resistor R25 with a trimpot. I have not tested this in my car, but it should work to trim the TC fuel usage.

This is a 20k pot as Greg’s R25 is 11k (my R25 is 3.9K so I would use a 10k pot). I'm more than happy to help identify resistor values for those who are unsure.

With a DVM on ohms across the 2 pins you are going to solder into the PCB set the pot to read the value of your resistor THIS MUST BE DONE BEFORE SOLDERING TO THE PCB, in this example I set it to 11k.

Remove R25 and solder the pot across R25 mounting pins. Temporarily mount the module in the car and adjust the pot until you get the TC fuel accurate. Once you are happy with the accuracy use silicone to fix the pot to the PCB so it does not interfere with the case, re-assemble and you’re done….

 

Warren
Thank you very much for taking the time to post a how to. Much needed. I have a couple of questions:


1) Setting the pot to 11 ohms: Please could you post a link to a suitable pot? Then I shall know exactly what to look for over this side


2) The left hand unused terminal on the blue pot: As this can be snipped off, how do I identify it please?


3) If I set the pot to 11 ohms initially, will this in theory give the same fuel reading on the trip as the original component? If so, which way do I turn the pot adjuster to make the trip record more fuel used: higher resistance or lower resistance?


As I am reading 12.5% too little fuel, would it be worth setting the pot a bit differently (depending upon your answer to Q3), say 1 ohm differently, to start with?


Thank you very much again for taking the trouble to do all this.
Greg

 

1) Set the pot to 11k ohms 11,000ohms

2) Either end pin can be snipped off, do not remove the middle terminal this is the wiper.

3) Yes if you set the pot to 11k ohms the fuel reading will be the same. Which way you turn the the adjuster will depend which way you install the pot. If you cut the left one off which is labeled 3 then CW will increase resistance.

You could try setting it to 10k and see how it works.

CT-94EW 20k Ohm | Copal Electronics 18-Turn Through Hole Cermet Trimmer Resistor with Pin Terminations, 20k? ±10% 0.5W ±100ppm/°C | Copal Electronics