XJ6 Series 3 Injector harness

Hi one and all, a bit of help needed.
I am making a new Injector harness and have found a mod to the coolant sensor plug. There is a small variable pot measuring 700 ohms, installed across the 2 wires at the plug of the sensor. In my reckoning this would be in parallel to the sensor. I guess it’s slightly altering the voltage going to the computer. The car runs very rich on start up and slightly rich when warmed up so maybe this mod was trying to overcome this ? Is this a typical mod. Any help would be greatly appreciated. Cheers Peter

PS. When I try to upload a pic it loads as a file. Not sure how that works.

Hard to see in the pic but if the orange wires on the variable pot go to each pin then it is in parallel. That means reducing the resistance on the pot reduces the resistance the ECU sees, making it think the engine is hotter. That is opposite the usual modification as the systems generally move leaner with age. US Series 3 XJ6 were usually too lean for best running by the time the motor was fully broken in with several thousand miles and they would benefit from a resister in series with the coolant temp sensor, typically 50 ohms or so.
With that mod and still being too rich, there is something way off here.

Thanks Greg. I am in the early stages of this restore and haven’t even driven the car yet. Have replaced the coolant sensor, new plugs and leads and dizzy cap. The TT switch seems to be working as if I disconnect it the motor is slow firing up does that sound right. Have replaced AFM but not started since. It also had the capacitor mod done.
When I’m next ready to fire up I will see how it runs and go from there.

Also from what you say that 700 ohms across the sensor is way too high. I might try disconnecting it first up. Peter

Not a typical mod.

Over fueling has a number of likely, common causes:


When ordinary trouble shooting yields no culprit the over fueling issue is usually corrected by applying “the cap mod”



Thanks for your comments Doug. I do have a bit of eliminating to do. Cheers. Peter

The mod completely defeats the purpose of the original CTS, Peter - the reason for installation remains unknown…

The CTS’s sole purpose is to induce the ECU to run the mixture fat with the engine cold (high resistance, Kohms) - gradually leaning out mixture as the engine the resistance variations heats up (around 300 ohms when hot)

This is a necessity for any engine, and ECU is calibrated to a spec temp/resistance curve to be correctly fuelling. The resistor will mislead the ECU as to engine temp; in series causing general mixture fattening - in parallel general leaning…

As the ECU is deliberately designed to deliver the assessed correct mixture for the temp given, presumably decided by Jaguar experimentation - the purpose if the resistor installation is odd…

One possibility is that a PO installed it to compensate’ for some other fault - another to ‘improve’ Jaguar groundwork; POs do strange things…

Generally; mixture is, legitimately, adjusted by either the AFM bypass screw or, outside its range, by adjusting the AFM internally. Given a well functioning ECU; no other fiddling will do the job properly and should not be attempted…

The first step is to check the resistance/temp curve on the bare CTS to verify if it works to spec. If it does not; change it and the run the engine to see what happens. And, of course, if CTS is to spec; remove the resistor and see what happens.

The resulting engine behaviour may reveal something - it is perfectly normal for the engine to run fat when cold - and slightly fat when hot…

As an aside; the only reliable way to set mixture is by measuring CO with the engine fully hot - CO2 sensor, if fitted, disconnected…

xj6 85 Sov Europe (UK/NZ)

Thanks for your reply Frank. There is Interesting and sensible info there. I will remove the mod and see how she runs. It’s a learning curve thats for sure. Cheers. Peter