SOME DATA ON FAIL-SAFE OIL PRESSURE PROCEDURES

September/October 1978
Morgan Davis 
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11 Brookside Drive, Wilbraham, Massachusetts 01095

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FOR COIL AND IGNITION

Switch must be 'open' (off position) for starting. Turn 'on' for oil fait protection. Should oil pressure fail, the 'idiot light' sending unit will ground out the ignition current going to the points and spark will cease at once. Since ign. is grounded out through the coil's load, the wires will not get hot and/or overload the circuit. Maximum current draw is the same as when points are closed: about 2 amps.

FOR MAGNETO IGNITION

Here the safety feature functions like a remote control magneto kill switch-which it is, since the oil light switch grounds out when there is no pressure. To start the SPST switch is in the 'open' (off) position - so that the oil switch won't ground out the mag. After the startup, close the switch for Ioss-of-oil-pressure protection.

You can save wire and time by using a single-contact style oil sending switch such as used on Chrysler products. Slant six and most V-8 engines. These switches are closed when no pressure exists-thus supplying a ground for the bulb; they open with pressure, eliminating the ground.

Simply wire the switch up with a single pole-single throw (SPST) switch in the line as per the diagrams enclosed. It will work for either coil and battery or mag. ignition. It simply grounds out the coil or mag in the event of oil pressure loss.

I do wish to point out that most oil pressure light sending switches open (break contact) at about 15 PSIG and close (make contact) at about 7 PSIG. So if the engine in question runs at less than 20 PSIG, then the system cannot work. Many older engines use a pump-splash system to wit: Chevy 4 cylinder, Chevy 6 cylinder, 1930 to 1953, model A Ford, Dodge fours 1914-1928 etc. which never exceed 15 PSIG under normal running (some Chev. engines will, but don't count on it).

Also, be sure your pressure sending switch is for a light - the units used with gauges are of the variable resistance type and will not work for this application.

A high temperature cut-off can be improvised by using a 'hot light' single contact switch instead of the OK unit switch-or in conjunction with it. These units are normally open and close at about 250° F. If both switches are used, wire them in parallel; then either low oil pressure or high engine temperature will kill the ignition.

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