Thanks all for helping...truck nearly up and running... While I was underneath tightening the alternator I came across what looks like some sort of sensor. Its in the middle of the photo. It has a black plastic block on it and looks like it should have a wire attached...any ideas?? Also is the alternator oil cooled?? Ta Nik
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That is your oil pressure switch and should have a wire going to it which usually runs in the same loom as the alternator wires.

No the alternator isnt oil cooled it has a vacuum pump on the back of it which is oil lubricated

Cheers Alan...tracked down the unattached yellow wire nearby....put a new terminal on it and voila,the oil light works...I was expecting it to have been deliberately removed and probably the dash bulb too the disguise a low oil pressure problem but no, all fine! luck of the Irish...dogdged another bullit !!! TaNik

Given the location next to pipe clamps, it's entirely possible that it got knocked from a previous owner/mechanic who clipped it and didn't notice. What's with all the dampness?

dampness....just changed the oil filter and had the rad off...coolant dripping onto the alternator . It does look a bit messy but I think a good hot run will sort it! The rocket cover was leaking also....just an excuse really