The rad is in fairly bad condition, so I will replace this. But, there was an air lock. Auto bleeding did not help - I had to remove the bleed screw,slowly fill up and squeeze the hoses.
The pressure cap was also no good. Replaced with one from Ebay.
The overheating on the move seems to be better.
But, the fan is still not working - on either speed.
I have eliminated the resistor by shorting the cables.
I took the top off the large black relay, and manually closed the switch with a piece of card. This triggers the high speed fan.
Is it likely to be a relay problem (probably not, as it appears low and high speed are triggered by different relays, and it is unlikely both have failed.
With the relay disconnected, what voltage should be present on the terminals? I get 12v across the left and right terminals, which drops to 0v when the temperature gets high enough to trigger the fan. The other two terminals are at 0v.
I presume that the terminal with voltage is the input, and it is negative switched?