Just had the thought that when the "handbrake temperature gauge" get too hot you were probably working pretty hard in 5th gear. I have spoken to people that have put an oil temp gauge in to the LT77 gearbox (disco 1) and found that there was a significant reduction in oil temp just by driving at the same speed but in 4th gear. So you could use it as a sign to change down. This gear is 1:1 so uses no gears. (except transfer case. In pratice i have seen that it is better to tow a heavy load using 4th not 5th. Don't know about the Td5 gearbox though.
WR.
84' 120" ute - 3.9 isuzu.
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