Ok, here's how the controller (that I have) works:  They use all the gates
as simple inverters.  Two gates form a multivibrator that puts out a square
wave at about 100 Hz.  This is coupled through two small caps to a voltage
divider with ground at one end and the third gate input at the
other.  Without the square wave this gate input goes to ground, and the
output high; with it the output reproduces the square wave.  It is
diode-coupled into another voltage divider from B+ to ground, with a big
(47u) cap parallel with the high side and the divider driving the last gate
which feeds the base of the output transistor.  This transistor is
open-collector with a big fat zener in the ground leg to keep it above two
volts.

The diode-coupled square wave keeps pulling the divider output toward
ground, buffered by the big cap.  Takes it say five seconds to pull it down
enough to flip the other gate and open up the transistor.  That's the
normal steady-state condition of the circuit.

The sender terminal is between the two caps that couple the multivibrator
output into the rest of the circuit.  Tie that to any constant voltage
(such as ground through the half-inch of coolant btw the sender pins) and
pouf goes the square wave.  This lets gate three input go low, output high,
it quits pulling down the divider which floats up, trips gate four and
shuts off the transistor.  Voila, or as my cellist aunt used to say, Viola...

Clever Huns...very high-impedance, as you need for it to work at coolant
conductivities.  The main voltage divider is composed of a 1.2Mohm and a
10Mohm resistor, the couplers are 1000pf (? 103Z).

Why do I think Toyota would have done it with a custom chip and a diode?

david

David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"