Relay, fuel pump, oil pressure switch

Hi everyone. Got a weird technical issue with a friend. He has an aftermarket fuel pump and harness along with an oil pressure switch. Relay is SPDT and wired accordingly. 30 is 12v 87 is power to pump 85 is trigger to oil pressure safety switch 86 is ground P terminal on OPS connects to pole 85 S terminal connects to starter relay I terminal is key power Got it hooked up and apparently when he connects the wire to the starter relay the fuel pump runs constantly. This should not happen with an oil pressure safety switch. My thought is that more than likely he is picking a source from the starter relay than is 12v constant, instead of 12v while cranking. Given that the P and S circuit at OPS are connected with no oil pressure, this would mean it as in the relay/pump is given full time power. In the event that I am wrong my other two ideas are that the relay is bad, or oil pressure switch is bad. Any way to rule those out? Thanks

Oh dear, another oil pumper. That practice should have died years ago. Yes I know some vehicle manufacturers implemented it as OEM (tho usually as a backup) but they later ceased, and some fools refer to the fly logic "why do so many people do it then?" (I don't know - ask them!). Assuming you are using a switch for safety in case of an accident, oil pressure is not that way to do it. Pumps can run on for way too long - I know of engines whose switches remained engaged 40 seconds after stopping. If you are using it to protect the engine in case of oil failure, fuel cutting is not the answer. Not only can it be dangerous, but it can also cause far greater engine damage - especially on carbied or surge-tanked engines - ie lean burn & holed pistons; and some argue injector damage). If oil pressure engine protection is the intent you use an ignition kill but that is never AFAIK implemented in vehicles for safety reasons (tho time delay circuits with a manual overrides have been used). In short, oil pressure controlled fuel pumps are, in general, undesirable as both safety or engine protection methods. The best simple OEM and add-on systems use the alternator charge light, viz: ... as per fuel pump which is probably worth a read. Alternatives - especially where the alternator charge-light circuit may not be able to supply the +12V current required for the relay coil (assuming people don't want to add a MOSFET or transistor buffer) includes air flaps (if fitted to EFI etc systems). Otherwise oil pressure is better than an IGN only powered fuel pump, but as above, that is not generally appropriate for engine protection nor safety and it is considered dangerous. (Why does low oil pressure usually trigger an idiot light and NOT an engine kill?) Of course EFI EMS's should have fuel pump control tho some do not (eg, old Bosch and VAGs). (Even some of those still use the oil pressure switch as a backup - IOW even if the EMS kills the pump, the pump can keep running...) And FYI - tho using an SPDT relay as per the above diagram requires only the relay (and charge-light circuit) to provide fuel during cranking and engine running, I prefer SPST wiring using diodes (even tho I use SPDT relays for all SPST relay applications). That enables other controls to be added - eg, a momentary switch for per-crank priming or toggle switch to milk the tank etc. And note that should the alternator fail, the relay can be activated by disconnecting the alternator's voltage regulator - ie, disconnect its D+ or L output by disconnecting the alternator's small connector if it's a newer all in one internally regulated type, or disconnect the voltage regulator if an external type. The dash charge light and other connected "IGN-on test" bulbs should be enough to energise the relay for limp-home mode. For SPST with diode examples see here tho below is a better quality and corrected version (the original battery symbol was incorrect). FYI - similar and additional info exists under UIBI aka Ultimate Intelligence Battery Isolator threads. In my case the UIBI was inspired from the above simple SPDT relay diagram - a battery isolator being exactly the same in principle as electric fuel pump & choke etc controls but without the crank priming (except when dual batteries are desired for cranking).

BTW - if your oil pressure switch is a typical single wire grounded NC type, the pump will only be on when you DON'T have oil pressure. If it's the same but NO and closed with oil pressure (uncommon tho some vehicles use them; ie - the fail safe type of implementation) then your 86 should be to IGN +12V, not GND.    Only if you have a +12V oil pressure switch that closes under pressure is your wiring correct. Those typically 3-terminal switches exist and are commonly sold by those that like making money - especially if they reap the rewards from holed pistons and flamed cars.

oldspark wrote: BTW - if your oil pressure switch is a typical single wire grounded NC type, the pump will only be on when you DON'T have oil pressure. If it's the same but NO and closed with oil pressure (uncommon tho some vehicles use them; ie - the fail safe type of implementation) then your 86 should be to IGN +12V, not GND.    Only if you have a +12V oil pressure switch that closes under pressure is your wiring correct. Those typically 3-terminal switches exist and are commonly sold by those that like making money - especially if they reap the rewards from holed pistons and flamed cars.

Thank you for the reply. It is a 3 wire sensor for this purpose. I wonder if it would be possible to use something like a time delay relay, or a delay timer module, in addition to the oil pressure switch, in my wiring somewhere? The goal is for 3-5 second cycle before starting, but then also have power while cranking and running. .

Duplicate the first pic I posted with 87a to the starter (ST or solenoid) for the pump during cranking. (I take it that the vehicle is carby, not EFI?) Only if the carby/s is/are dry might that not be enough then a manual enable could be used - eg, shorting 87 (or 30) to 87a with IGN on. Otherwise a VAG type tachometric fuel pump relay triggered by points/ignitor/igCoil- tho they usually only have a prime time of ~1 second (they keep it short if using a 555 because the prime on time equals the turn off delay after the last spark). I still strongly suggest connecting 86 to the charge light circuit instead of the oil switch. It the alternator is typical as used until the 1980s and probably 1990s then there should be no issue.

The circiut should function as desired, configured the way you describe it in your OP. If the pump runs with the engine running, with the S terminal at the pressure switch disconnected, and pump does not run with oil pressure at zero, but key on, then the issue is with the S terminal. I would check terminal 86 at the relay for voltaqe to a known good ground like the battery terminal. If you have a "dirty" ground that has some (not necessarily 12 volts)voltage on it, the relay coil will energize through the ground in the starter solenoid windings. And of course check that the S terminal has NO voltage on it when the starter is not engaged. You have a very simple, reliable, safe, used for decades, electric pump control circuit. It should work just fine. My guess is that he has the S wire on the wrong terminal on the starter relay, or starter.

Noting that oil pressure to control fuel pumps is not generally considered safe. Furthermore it's caused a few nasty accidents as well as engine destruction albethat mainly in competition vehicles and generally when carburetted. I simple terms, engine protection (ie oil pressure switches/lights) and fuel pump control (mainly a safety issue) are two totally separate things. Sure - oil pressure control is safer than straight IGN +12V control, but since cars have had suitable prime & engine-running signals for decades, why not add a mere relay instead of a special oil pressure switch (and optional relay) plus a priming mechanism? But let the OP decide which method is preferred since the pressure switch already exists.