The way I interpreted the question he is talking about the 3 senders on the thermostat housing. (only 85s) have this, an 83 SD would have only one sender on the thermostat and a vacuum switch for the egr.

AFAIK the 3 sensors are for the egr control computer, the aux fan control, and blower fan cut in.

**Edit, the 300D's might only have 2 sendors on the theromstat housing.

The other posters are correct about the overboost sensor infront of the banjo.

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1984 Euro 300SDC, (4spd standard)
1986 Toyota Landcruiser Diesel HJ60 5spd X2

Gone but not forgotten (some sold, some stripped)
1983 300 SD, 1985 300 SD, 1983 240D, 1986 300 SDL, 1985 300 SDL, 1983 300 D, 1984 300 D, 1985 300SD, 1987 300 SDL, 1983 300 SD, 1985 300 TD Euro, 1983 380SEC, 1990 300 D, 1987 300D, 1982 300D, 1982 300D, 1994 E420, 1987 300 TD, 1987 300 D, 1984 300 D