Id use the jumper cable to provide a "insignificant " resistance. Path due to its large cross section area.. Using a meter lead adds on average 0.2 ohms. Don't believe me, go measure your meter leads one day. That's why there's a zero adjust or "rel" function on multimeter, to remove this lead resistance from the measurement

 

What do you all think to this, is this my problem?

The answer is yes, this was my problem!!!

I will go and have a blast about in the fields tomorrow to check but the 4x4 light now stays on with everything switched on.
Just out of curiosity, i did the pin 16 test again
Berore After
Engine off
Heat lights off 11.7v 12.4v
Heat lights on 10.4v 12.2v

Engine running
Head lights off 13.9v 14.4v
Head lights on 12.8v 14.3v

Thank you all for your help, couldn't have done it without your input.

I'm a mechanical engineer by trade, now a welder and fabricator. I know my way arounds spanners but electrical is witchcraft to me, and this just proves it! who would of guessed a stupid bit of rust would stop a 4x4 system to stop working!

 

Bad earths/grounds can and do cause all kinds of strange problems.

 

Where is that ground bolt located?

 

it was right behind the negative post on the battery going into the firewall.
The wire was no more then about 3" long

 

Then you probably have other ground wires either in the same condition or possibly removed / disconnected. Engine to frame or firewall, at a minimum. That one wire you found is way too light in gauge to support all of your grounding needs.

Revisit the main leads to the from the battery and out, wire condition and connections for corrosion too.

 

Then you probably have other ground wires either in the same condition, diconnected or missing. Engine to frame or firewall, at a minimum. That one wire you found is way too light in gauge to support all of your grounding needs.

Revisit the main leads to the from the battery and out, wire condition and connections for corrosion too.

This has now been put on my list of things to do.
There was a bigger earth cable that went off behind the engine to the other side of the firewall so this wasn't the only one, i can get a new one made up at work if you think its needs to be bigger? what gauge do you think?

 

No need to go bigger, just check the integrity of the OE ones at this juncture.

 

i misunderstood your last comment, i thought you were saying that one was too light but now i re-read it i think you meant if the others were compromised then that one would be doing all the work and therefore not b big enough

 

The bigger cable normally goes to the engine where the starter motor bolts on and is primarily for cranking current.

 

I put my meter probe on the bolt head this morning when I noticed the 2v’s and I had to scratch the bolt head then with my probe to get a reading then after people on here saying that’s a problem I had been thinking about it all day while I was out so it was the first thing I checked when I got home.

 

Well done, good find, thats one if the main "body" grounds. As everyone else has said, if that ones bad, the rest probably are. I can see a lot of cleaning and checking in your future, but success at the end of it