take a gander http://ds-restoration.blogspot.com/2019/08/ducellier-4155b-distributor-strip-down.html

the pressure switch would stop the a/c pump from destroying its self if no refrigeration liquid ,assuming petrol

when you are grounding do it by a temporary wire connected to the battery and check the earths on the instrument cluster as they will affect the stabiliser and gauges

I believe the voltage output is 10 volts to gauges but is pulsed to keep it stable at 10 volt along the lines of the old flasher relays a bimetal strip heat up break the circuit cools down and makes the circuit you can get electronic units to replace this item . you may have to set your DMM...

as Datatek look like a casting fault to me and a warranty issue. As most of the equipment used by JLR is European branded and the majority manufactured in China who are renowned for short cutting processes like there Indian owner if it was due to damage the cracks would be sharp and have a...

assuming the square type have you connected the black/red and a good earth to the two side tabs for the warning lamp

hella hazard switch

fuses F22,F9 on BECM but if righthand is working unlikely and as Datatek said

relays

what make is the used tacho you are using

Are you referring to the starter relay or the anti-theft relay

try checking header connection on H289- 3 you may have disturbed it when chopping wiring

glad you fixed it :) you have to compensate for the lead resistance when on ohms scale and really you cant beat an old Avo mark 8 analogue meter that calibrated in my opinion and of course remove any voltage source

what year is it as the harness changes from 2000 to 2003

does the charge warning light work, main harness to engine harness connection ok .if the charge bulbs faulty no excitation same with bad harness connection unless it has a resistor and diode fitted to the loom ,looks like the voltages you are reading are the battery if all above ok then...

have a gander https://www.landyzone.co.uk/land-rover/rev-counter-for-4-cylinder-petrol.386612/ not sure I covered diesel

not strictly defender (P38) but land rover use same items on various models

don't try to short cut the bonding process , had a rear roof spoiler fitted by dealer for my son's birthday two week later it split and one piece left the car at motorway speeds never to be seen again

in my blind spot ,looks like they are made of gold judging by the greedy price's

check coolant connector for clean contacts