The reason I mentioned the grounds is because you may have caused a ground loop. Look it up on Wikipedia. it's preferrable to a common ground for electronic rathrr than different one.

As Pete suggests, you have probably caused electromagnetic radation which is not being picked up by the HE sensor, but by the wiring connected to it.

Solotions can include,
re routing wires
Twisted pairs
Checking grounds and making them common.
Shielding.
Its thought better to avoid creating the interference, rather than shielding from it.

Resister type spark plugs are also preferable when using electronics in cars.

As I think of anything else ive read about, I will post it.

I asked the same question on the Speedy forum with regard to building my loom, and was given a list. Some of which I've posted as I remember it.

I'll see if I can find it.

Roger - what are resister-type spark plugs, please?

Because of interference with radio and TV braodcasting signals, sparkplugs had to be changed in order to reduce the spikes which were heard on the radio and TV . The simple solution was to add a restistor inside the sparkplug. Most types since many decades have these resistors inside.

What he said.

Sorry ..quote from a tv program.

There's a resistor built into the plug itself, designed to cut out electrical interference for electronic circuits. As @edce suggests, all modern cars with an ecu use them. The electromagnetic environment under the bonnet, is very hostile for electronics.

I have a list somewhere for do's and don'ts.

What does NGK R stand for?
resistor spark plugs
A: NGK "R" or resistor spark plugs use a 5k ohm ceramic resistor in the spark plug to suppress ignition noise generated during sparking. NGK strongly recommends using resistor spark plugs in any vehicle that uses on-board computer systems to monitor or control engine performance.

In my search for the cause of the irratic firing with the new ignition module, I noticed that the flasher unit (LED flasher) started to flash. Even when the indicator handle was still in neutral position. Opening the LED circuits did not change the ignition behaviour, it did not function properly in all cases.
So my guess is that some of the pulse voltage, normally going to the coils over the sparkplug, disappears via the flasher. So I have to fix a condenser (between the + and - connection on the flasher) to take away the unwanted ignition pulse over the flasher .
Any idea which type of condenser I best would apply ?

Still sounds like a ground issue to me.
Have you ever experienced when your brake light flashes like an indicator when you get a bad ground
Cost nothing to investigate

Will check that too Roger V !

+1 Still sounds like a ground issue to me.

Brake lights function correct ! No flashing at all !