Diesel Engine Oil Pressure during Startup

[Beltzington]

How much oil pressure would you see during cranking with no fuel? Reason I am asking is to determine how I need to wire a low oil pressure shut off on an old school JD 80HP diesel with no oil pressure gauge.The shutoff switch has open contacts when the pressure is below 10psi and they close above that pressure. If cranking the engine would quickly reach 10psi I could simply put the switch in series with the fuel pump solenoid. If not I will need to wire a momentary contact switch that would need to be held during startup until the engine started and pressure was stable. Am I missing something with this idea?

 

[Jerry/MT]

Generally speaking, it takes a few seconds for the oil pressure to rise so by putting a switch in series with the fuel solenoid, your fuel solenoid would not be energized when you tried to start until the oil pressure came up high enough to close the circuit. Without the fuel solenoid energized, how could you start the engine?

I suppose you could put a jumper across the oil pressure switch for starting and then remove it to have "loss of oil pressure " protection for running operation or a by pass circuit on the starter switch that bypasses the switch on starting and when the start cycle is over and the key switch is in the run position, the oil pressure switch could be in the circuit on the run side of the key switch. Personally I would not add any of this complication to an all ready simple system.

If you have an oil pressure tapping to put a pressure switch in, why not put a mechanical gauge in and read oil pressure directly? I had a JD435D that had a Bourdon type gauge installed ahead of the oil filter and it could be seen from the seat. A simple mechanical oil pressure gauge could be configured to attach to the dash using Nyloflo tubing and I believe these are readily available.

 

[Katahdin]

I think you missed something. Wiring the low oil pressure switch to offline the fuel pump may not shut down the engine, that pump's main purpose is to prime the injection pump. Once the injection pump is primed and cylinders are firing, the engine will suck fuel until the tank is dry.

 

[Beltzington]

Thanks for your replies. I did not mention this was a wood chipper engine not a vehicle. Gauge wouldn't save the motor as the operator will be feeding limbs into the chute and couldn't see the gauge. Also, I am sure fuel solenoid will shutoff the engine with voltage removed as that is how I have been doing it during this rebuild project.

My thought is if cranking would bring the oil psi up quickly enough fuel solenoid would be energized and engine would start. My main concern would be overheating the starter if extending cranking would be needed. So how much cranking would be needed to reach 10psi? I guess I can buy a cheap Oil Pressure gauge and test this theory.

 

[Soundguy]

murphy switch gauges do this. they cut a line at low oil pressure, and have a momentary bypass while starting.

to see if your idea using the 10 psi switch would work, plub in a oil pressure gauge and see how much pressure you get cranking... simple observational test. if it takes excessively long to build pressure, you are getting wear and tear onthe system needlessly. if it comes up fast.. then you are fine.

 

[Samuellm]

All the stationary engines I have used have a small button that you depress when starting that bypasses the low oil cut off this would be much easier than trying to crack it over on a cold day to get it up.

 

[MHarryE]

Use the bypass switch solution. There is no guarantee on how long it will take to build pressure. Think of the start after an oil change - on my equipment always a longer time for the light to go out. Have seen the bypass switch installation on your type installation.

 

[Beltzington]

murphy switch gauges do this. they cut a line at low oil pressure, and have a momentary bypass while starting.

Thank you Soundguy - the murphy gauges where just what I was looking for, both pressure and temp cut-offs available for about the same price I almost spent on Steward Warner gauges.

 

[Soundguy]

we use murphy switch gauges for un-atended things like pumps and generators at work. they work good.

 

[Beltzington]

I think you missed something. Wiring the low oil pressure switch to offline the fuel pump may not shut down the engine, that pump's main purpose is to prime the injection pump. Once the injection pump is primed and cylinders are firing, the engine will suck fuel until the tank is dry.

I misspoke, the solenoid I was referring too is on the injector pump not the fuel pump so your observation is accurate.