Considerations in wiring power to a prelube pump

A prelube pump is operated prior to cranking the engine. Its purpose is to fill the oil passages with oil prior to startup. The power to operate the prelube pump comes from the starting battery or from some other auxiliary power source.

The simplest wiring system consists of a pushbutton switch to operate the prelube pump. The operator pushes the switch for the appropriate length of time and then starts the engine. Most prelube pumps draw too much current for an ordinary pushbutton switch, so an electric relay is usually added to the circuit, as shown in the diagram below.




In automating the engine prelube, there needs to be some measurable condition that indicates that the prelube function is complete. One possibility is to check the oil pressure. This is the fastest way to safely start the engine. If the oil pressure is not zero, then you can start the engine. But, most prelube pumps are too small to significantly raise the oil pressure in an engine of any size, especially if the engine has been run recently and the oil is still hot. While these pumps may fill the passages with oil they cannot raise much pressure within the engine because of the many places the oil must go, such as spraying cooling oil to the underside of the pistons, etc. Therefore, unless a large prelube pump is used, checking oil pressure as a condition to start the engine is problematic.

If an oil pressure switch is used to signal prelube completion, make sure that the prelube circuit is disabled after the engine starts. This is to avoid the situation where the oil pressure gets low enough to restart the prelube pump, such as when a hot, well used engine is running at idle. At this point the prelube pump could cycle endlessly as it adds enough oil to bring the oil pressure up only to be shut down again.

Another measurable condition is time. One could run the prelube for, say 5 seconds, and then start the engine. The actual time would depend upon the pump, its plumbing, and the engine. There are several options here.

1. Prelube for longer than anyone would think is enough. Advantage, for sure the passages have oil in them. Disadvantages, longer time until start, and extra drain on the battery.
2. Find out experimentally how long it takes for the oil passages to have sufficient oil. Prelubing a cold engine will probably cause some pressure increase when the oil passages become full. At that point an oil pressure gauge may indicate a little pressure or the sound of the pump may change a little. It is a reasonable guess that a warm engine with hot oil will take about the same time to fill the passages. Advantage, the time to start is shorter and the drain on the battery from the prelube pump is less. Disadvantages, it is just a guess and certain situations may require longer prelube, such as when the engine oil has been changed.
3. One might also remove an oil port at the furthest point possible from the oil filter and see how long it takes for the oil to start flowing out. Advantages and disadvantages, the same as #2 above.
4. If the engine has a glow plug cycle, the prelube pump circuit can be connected there to provide a prelube cycle the same length as the glow plug cycle. Advantage, easy to do and requires little extra circuitry. Disadvantage, the glow plug duration may not be optimal for prelubing.

The diagram below shows one example for wiring a prelube pump using a timer.



When all is said and done, how accurate does the length prelube cycle need to be? What happens when the prelube cycle is too short? One must realize that any prelube at all is better than none. So, if the prelube time is a bit short, the oil passages will still have gained additional oil. As the engine starts, the passages will fill faster than if the prelube was not done at all. There will always be some benefit.