Considerations in wiring power to a prelube pump with a control circuit

The power to operate the prelube pump can come from the starting battery or from some other auxiliary power source.

Manual Prelube Wiring

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 or until pressure starts to read on the oil pressure gage and then starts the engine. An electric relay should be used in such a circuit, as shown in the diagram below.

9608SimplePrelube1 (4K)

Automatic Prelube Wiring

In automating the engine prelube, there needs to be some measurable condition that indicates that the prelube function is complete.

Presure as an indicator

One possibility is to check the oil pressure. This is the fastest way to safely start the engine. If the oil pressure is above zero, then you know oil is present and you can start the engine.

Some prelube systems are too small to significantly raise the oil pressure in the engine, especially if the engine has been run recently and the oil is still hot and thin. While these pumps may fill the passages with oil they cannot raise much pressure within the engine because it may reach a point where it is draining back to the sump at the same rate that the pump is pumping it into he engine. Therefore, unless the prelube pump is on the larger end, checking oil pressure as a condition to start the engine is usually not sufficient by its self.

Note: 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.

Time as an indicator

Another measurable condition is time. One could run the prelube for, say 15 seconds, and then start the engine. The actual time would depend upon the pump, its plumbing, and the engine. There are several options here for determining the appropriate amount of time for a given engine.

  1. Prelube for the maximum time it could possibly take. 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 reasonable to conclude that a warm engine with hot oil will take about the same amount of time to fill the passages. A few seconds may be added as a margin. Advantage: the time to start is shorter and the drain on the battery from the prelube pump is less. Disadvantages: it does not take into account any variations in starting condition so certain situations may require longer prelube, such as when the engine oil has been changed.
  3. One might temporarily remove an oil port at the furthest point possible from the prelube input 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 an example for wiring a prelube pump using a timer.

9607TimedPrelube1 (6K)

Pressure and Time used Together

A third option would be to combine the two in order to optimize the prelube duration further by using both a pressure switch and a timer. If the oil pressure comes up quickly the prelube cycle can end and both start time and battery drain are minimized. If the pressure switch is not triggered the time switch finally kick in to end the cycle and start the engine anyway to prevent the scenario where the oil is hot and the prelube pump is never able to measurably raise pressure.


When all is said and done, the length of the prelube cycle does not need to be perfect in order to be effective. If the prelube cycle is shorter than ideal, the oil passages will still have gained substantial additional oil. As the engine starts, the passages will fill much faster than if the engine where started with all the oil still in the pan. There will always be some benefit. Any prelube at all is better than starting an engine dry.

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