The V8 Owners Forum -

Hi
I'm wiring the Lucas system into my p5 and don't have the resistor wire that goes between the single spade connector at the airflow meter and ignition coil, for pulse to trigger ECU.
Taken from the injection post.....
In engine bay there will be a single black/white wire usually with the airflow meter loom, this has an inline resistor MAKE SURE THIS IS FITTED, this then goes to the negative side coil for efi ignition trigger to tell the ecu engine is running and to fire the injectors,
Two black wires with ring tags on, these MUST GO TO A VERY GOOD EARTH !!!!!!!!!!!!!!! (usually the back of the passengers side cylinder head)
I don't know colour length or resistance value etc...any help appreciated

Thanks Chris

What you seem to be describing is the ignition pulse current limiting resistor which is different from the tune resistor. On a 14CUX this is, I believe a 6K8 resistor although I don't know the wattage. There may also be a similar resistor in a branch to the tachometer feed.

Sounds like youve been here before...you know where I can get it, is it just a mini loom ?
Is that 6800 ohm value resistance?
Thanks Clyde

Half of my wiring loom is missing too, there's no ignition supply to coil...I can make one up.

Clyde Archibald wrote: Sat Aug 15, 2026 1:16 pm Sounds like youve been here before...you know where I can get it, is it just a mini loom ?
Is that 6800 ohm value resistance?
Thanks Clyde

The standard resistor (at least on my loom) was a rectangular package around 10mm square, 5mm thick with male spade connectors on each side. You should be able to use any 6K8 resistor as long as it is of high enough power. This resistor will have fairly high voltage across it. I have seen values greater than 200v on my 'scope.

Clyde Archibald wrote: Sat Aug 15, 2026 7:52 pm Half of my wiring loom is missing too, there's no ignition supply to coil...I can make one up.

I'm a bit confused by this surely your coil supply would have been in existence prior to fitting EFi? (I'm assuming your car was originally carb' given the age) The EFi loom (I'm assuming Hotwire) should be pretty much a self contained loom with only a few connections to your car's wiring for things like fuel pump, A/C and of course the ignition coil.
Are we talking at crossed purposes?

Hi
I've looked at this car since January..
Thanks for the information, just to confirm this is the Lucas flapper...4CU system I'm fitting.....To standard su equipped car.
The ignition wiring I've found out was an option at factory where the ignition feed to coil from ignition switch was armoured, this section is missing...I will replace it no problem..I spent some time trying to find the coil wiring in the engine bay to no avail.

Ah, 4CU not 14CUX, I'm not too familiar with that system but I believe the 6k8 resistor is common from previous conversations I've had.

Hi
Thanks for your responses...every bit of info is wisdom.

This is what my tune resistor looked like……different colour as I did not have cats, but that was CUX ecu
https://powersperformance.co.uk/store/s ... istor-cat/

You could buy something like this and change the resistor to the correct value!
I also thought I may try a variable resistor so could change the tune by turning a switch…..never did it though

Excellent thanks for the link, were getting closer to a resolution.
Yes I'm tempted to get one made up from resistors and wiring.
The adjustable idea sounds viable.
Cheers
Chris

Ian Anderson wrote: Wed Aug 19, 2026 10:19 pm This is what my tune resistor looked like……different colour as I did not have cats, but that was CUX ecu
https://powersperformance.co.uk/store/s ... istor-cat/

You could buy something like this and change the resistor to the correct value!
I also thought I may try a variable resistor so could change the tune by turning a switch…..never did it though

You could just get each of the values and wire them to a rotary/dial switch to switch between tunes, no so hit and miss as a variable res'/potentiometer.

I don't think there's much/anything to be gained though, as long as you have a map specific to your car why would you want an alternative?
Perhaps if you were running different fuels?

In my case the engine was modded to a radical amount by John Eales with a JE102 cam……huge overlap!

It ran extremely well and pulled like a train 2000 to 6800 where it cut the spark.

Problem was on the road where the box ratios put the revs 1200 to 1800 and in these reeves it was a bucking bronco with overfuelling and non fuelling which ended up being the regress rate at around 1.5 seconds.

Did try a few tune resistors but the problem always remained.

So drove in a lower gear and made a bit more noise to keep it above the kangaroo rev range.

Thanks for the responses, is there any more input from you like-minded folks....?
I'm not far away from getting it started
Cheers
Chris