Feeling a Little Low?

BL finally figured out that their engines didn't last long running in "air cooled" mode, and added a warning light to indicate a low coolant condition. Instead of a magnetic float switch in the cooling system tank, a fully solid state design using a stainless steel probe was employed. It wasn't made by Lucas, but it seems to break anyway.

		Since cooling system corrosion is a big enough problem without introducing lots of extra mischief-causing electrons, the sensor system uses narrow pulses of current to do its job while minimizing side effects. Of course, it would be quite un-Leyland like to solve one problem without creating another. So, we have a nicely ironic situation where the sensor that was added to detect leaks can cause one. The smooth stainless probe is just pushed into a grommet installed in the side of the reserve tank, with nothing to hold it other than friction and wishful thinking. When the grommet ages and stiffens it can act as a nice pressure relief valve. The electronics for the system are housed in a plastic tube mounted with a spring clip to the lower dash rail just to the left of the steering column. The tube is glued shut, but a little work with a sharp blade will pop it apart. The circuit is pretty simple and doesn't use any custom parts. D1 blocks negative spikes coming from the electrical system, and Zener diode D4 keeps the CMOS integrated circuit U1 from getting blown up by positive ones. C1 is the power supply filter and C2, for lack of a better description, does some pulse shaping, and

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.