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| Residual Oil Pressure | Rate Topic |
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| Posted: 12-05-2024 08:08 pm |
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1st Post |
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JH12947 Member
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I am installing a oil pressure switch to the circuit that runs my fuel pump, with the idea that when there is insufficient oil pressure (<9psi) power will be interrupted to the pump. I have this set-up on another car and it works well, shutting the pump off immediately when I turn off the engine. On the JH however, when I shut the engine off, the oil pressure takes a long time to fall and the pump stays on. I haven't let it run long enough to see how long it would actually take to shut off. (The car is wired with an aftermarket harness and the pump circuit is energized in the "run" position). Curious as to what others may have noticed about the time for oil pressure to be gone when the car is shut off. I found threads discussing the oil pressure relief and am wondering if in addition to help building pressure, this may play a role in allow the pressure to fall and if something may be wrong there. Thoughts?
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| Posted: 12-06-2024 02:34 pm |
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2nd Post |
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noomg Member
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I'm not even sure what's going on here or what you're trying to accomplish. When you switch off the ignition the fuel pump keeps running? The pump should shut off immediately when the ignition is switched off. If the pump stays on it's been incorrectly wired. The oil pressure takes a long time to fall? So what. With the engine off that's a no harm, no foul. When I shut off the motor I never look at the gauges so I don't know if that's normal or not. When oil is added to the motor it takes a lot longer for the oil to reach the pan than in most engines maybe that has something to do with. Short of making sure the fuel pump is correctly wired I'm not sure what advantage will be gained by installing an oil pressure switch. But as always your car, your choice.
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| Posted: 12-06-2024 03:44 pm |
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3rd Post |
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JH12947 Member
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The oil pressure switch is a fairly common safety item that ensures that if the car is in accident where the engine stops running but the ignition is switched on fuel will not continue to be dumped into the engine bay.
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| Posted: 12-06-2024 09:32 pm |
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4th Post |
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vnavaret Member 
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JH12947 wrote: I am installing a oil pressure switch to the circuit that runs my fuel pump, with the idea that when there is insufficient oil pressure (<9psi) power will be interrupted to the pump. I have this set-up on another car and it works well, shutting the pump off immediately when I turn off the engine. On the JH however, when I shut the engine off, the oil pressure takes a long time to fall and the pump stays on. I haven't let it run long enough to see how long it would actually take to shut off. (The car is wired with an aftermarket harness and the pump circuit is energized in the "run" position). My guess is that you are back feeding power to the fuel pump through some subtle loop back path in the wiring or using the wrong source for the power feed. The oil pressure will normally fall to near zero (~5 PSI) in just a few seconds. What does your oil pressure gauge say is happening when you turn off the ignition - i.e. how fast does it fall? What viscosity oil are you running? High vis oil will certainly aggravate this issue. Are you running an oil pressure switch (on/off) or a sender? How many terminals are there on the oil pressure switch (assuming you installed one). What pressure is required to close the OP switch? If it is very low (1 PSI? <shrug>) the pump may run for quite a while. Does your oil filter have an anti drain back valve in it? This will prevent the oil pressure from falling rapidly since oil is not free to move backwards through the filter when the pump stops moving. My GUESS is that you are feeding power through the OP switch to the pump using the battery (unswitched +12v - usually a brown wire) as your power source. This would be wrong. You must draw your OP switch power from a solid white wire (i.e. SWITCHED +12v) and you must reroute the current power lead at the fuel pump from the pump to the hot side of the starter (not the solenoid, the starter motor). This will do three things: 1. Your pump will only operate when there is oil pressure OR the engine is cranking. 2. Turning off the ignition will immediately turn off the fuel pump no matter what the OP switch is doing. 3. If oil pressure drops, the pump will shut off. With your current setup, in an accident that stalls the engine you will pump fuel all over the scene of the accident unless you remember to turn off the ignition - potentially causing a fire. You want the pump to shut off in this scenario since you may not be thinking clearly. The downside to this is that the first start of the day may require a few extra seconds of cranking since the fuel pump may need to refill the float bowls as fuel can evaporate from the bowls overnight. Vance Last edited on 12-06-2024 09:34 pm by vnavaret |
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| Posted: 12-06-2024 10:10 pm |
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5th Post |
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JH12947 Member
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Vance - Thanks for your reply. I am fairly confident that I have the wiring correct since I have the exact same setup in another car and it functions as intended but in the case of my JH when the engine is shut off the pressure settles down to about 10 psi per the gauge, and with the ignition still on this allows the pump to run. The switch triggers at either 7 or 9 psi depending on which source I found is correct. To answer your questions,
The OP switch is an on-off signal with 3 terminals, however the NC terminal is only used during startup The pump power is running through the switch from a switched source, not constant 12V Corey Last edited on 12-06-2024 10:13 pm by JH12947 |
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| Posted: 12-06-2024 10:44 pm |
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6th Post |
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Tom Bradley Member
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Interesting idea. The oil pressure on my JH takes about 15 seconds or so to drop below 10 psi when I shut off the engine. The oil flow down from the cylinder head is pretty slow, probably to keep the pressure up. So what you are seeing sounds about right. My question is how you expect to be able to start the engine after it has been sitting for awhile. When mine has been sitting I have to turn on the ignition and let the fuel pump run for a few seconds to get the fuel running to the carbs. But it would also take a good 10 seconds or so for the fuel pressure to come up far enough for the fuel pump to come on in your design. So how to you manage to start it in the first place with this?
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| Posted: 12-06-2024 11:04 pm |
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7th Post |
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JH12947 Member
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Tom - The 3 terminal switches (such as an Echlin OP6610) can be wired such that the normally closed (S) terminal takes power from the ignition "start" wire on the solenoid to provide power to the pump until the pressure is high enough at which time it closes the contacts with the switched normally open terminal for running. I've pasted below an excerpt from a thread on the OpelGT.com forum that fully explains the operation. I have this setup on my Opel, and it works well, but in that car the OP drops right off when the engine is shut down. Safety Switch contacts are "switched" by engine oil pressure ONLY: 1. CO/Common/"P"/e-Pump (+) Power 2. NC/Normally Closed/"S"/Ignition switch "Start" 3. NO/Normally Open/"I"/Ignition switch "On/Run" 1. Key in 'ON/RUN' position, engine off, no oil pressure, safety switch is static/unswitched (NC_CO NO): 12V available at safety switch NO/"I" contact, but not at NC/"S" and CO/"P" contacts . . . e-pump stopped. 2. Key in 'START' position, starter turns over engine, oil pressure rising, safety switch is static/unswitched (NC_CO NO): 12V available at NC/"S" and CO/"P" contacts to power e-pump . . . e-pump running. 3. Key in 'START' position, starter turns over engine, oil pressure rises to ~7PSI, safety switch is switched (NC CO_NO): 12V source for CO/"P" contact (e-pump power) is switched from NC/"S" to NO/"I" . . . e-pump running, engine starts. 4. Key released to 'ON/RUN' position, engine running, oil pressure above ~7PSI, safety switch stays switched (NC CO_NO): 12V source to CO/"P" contact (e-pump power) remains connected to NO/"I" by oil pressure . . . e-pump running, engine running. 5. Key is in 'ON/RUN' position, engine stops, oil pressure drops below ~7PSI, safety switch is switched back to static (NC_CO NO): CO/"P" contact (e-pump power) is switched back to NC/"S" by lack of oil pressure . . . e-pump stops due to no 12V at safety switch NC/"S" contact . . . ignition key not in 'Start' position!
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| Posted: 12-07-2024 03:20 am |
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8th Post |
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Tom Bradley Member
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Very nice. Thanks for the information.
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