shortens the "bulb check" time when the ignition is first turned on.
U1B and U1C form a classic multivibrator oscillator. The output is sent to the inputs of U1D.
The input impedance of the CMOS gate is extremely high, so the 100K resistance of R5 does not materially
affect the signal to pin 12. It's there to protect U1's internal clamp diodes from improper signals picked up from the sensor input.
When the coolant level is OK, the oscillator signal is shunted away from pin 12
through R2 and C1 to ground. If the resistance to ground at the sensor is less than about 8500 ohms,
then the oscillator signal is weak at pin 12 and the output of the gate stays high. U1C stops pulling
down and allows C4 to charge up through R3, R4 and D2. The output of U1A goes low, turning the transistor and lamp off.
If the coolant level is low, the oscillator signal is present at both inputs,
and U1D's output is an inverted version of the oscillator signal. The pulsing output of U1D
discharges C4 by grounding it through R4, R6 and D3. Once the voltage at the inputs of U1A drops enough,
the output goes high, turning on Q1 and the scariest lamp on the dash.