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So theoretically if the car loses oil pressure, the fuel pump gets cut? How does the car start in the first place? Is there some sort of delay that will allow the fuel pump to get power while the engine is off? (or in the process of starting?).
Anyone know?

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Originally posted by Orlando_87GN:
So theoretically if the car loses oil pressure, the fuel pump gets cut? How does the car start in the first place? Is there some sort of delay that will allow the fuel pump to get power while the engine is off? (or in the process of starting?).
Anyone know?
There is a two second "bypass" that is triggered by the ignition switch. When you first start the car, the parallel circuit powers the fuel pump, but it goes off after two seconds. After two seconds, the oil pressure switch is the only active path, and if you lose oil pressure, the fuel pump shuts off (in theory).
George W
Raleigh, NC
 

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Whoa...there is no safety cut off switch if the engine loses oil pressure. It will run til it blows up.

As above, there is a back up to start the car if the fuel pump relay goes out. After oil pressure comes up from cranking the engine, the circuit is closed and the pump will get voltage.

The ecm supplies power for two seconds and then cuts it off if the car is not started. If the car is started voltage is once again supplied to trigger the fuel pump relay. If the relay fails, there is a back up source of power thru the oil pump pressure switch.

My cars have no oil pressure switch because I removed it when I put the guage in. It runs fine.

If you look at the wiring diagram, it's a simple circuit.


[This message has been edited by Steve Wood (edited January 18, 2002).]
 

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Now I am confused. Nothing new though.
 

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I just edited my post to add a bit...just pull the connector off your pressure switch and you will see I am right...

then replug it, and unplug the fuel pump relay and it will run that way, too..
 

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Okay, I will try it again.


there is a pink/black wire from the fp/inj fuse that splits into two wires, same color. One wire goes to the oil pressure switch and exits the switch as a gray wire which goes to the fuel pump test lead and thru a connector, directly to the fuel pump.

The second wire goes straight to the fp relay and exits as a gray wire also connected to the fp test lead and also straight to the fuel pump.

The first wire that went to the pressure switch will supply power to the pump when the pressure switch is closed by oil pressure.

The second wire that goes to the relay will pass voltage thru the relay and on to the pump when the relay is triggered. The trigger wire to the relay comes out of the ecm (A1-C2) and is a dark green wire. This wire carries voltage for about two seconds when the key is turned to the run position. It also regains power when the engine starts-probably when the tach indicates 450 rpm or so...I am guessing.

Therefore, we have two independent circuits that will supply power to the pump as long as the fp/inj fuse is good. One by passes the relay and is simply a straight shot thru the oil pressure switch and the other uses the relay. The oil pressure switch provides redundancy in case the relay goes.

Often, when a car is slow to start in the morning, it means the relay is bad and the car has to wait until a couple of pounds of oil pressure is built up and the switch closes.

The car will run without the oil pressure switch if the relay is good, and vice versa...

Hope that makes sense.
 

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Originally posted by Orlando_87GN:
So theoretically if the car loses oil pressure, the fuel pump gets cut? How does the car start in the first place? Is there some sort of delay that will allow the fuel pump to get power while the engine is off? (or in the process of starting?).
Anyone know?
The fuel pump will shut off, in the event of an accident, because when the engine stops, it will lose both oil pressure, and the rpm signal which would otherwise trigger the relay. Guess I mis-stated the "safety" aspect of the circuit. Forgot about the rpm signal. Sorry. Thanks, Steve, for the clarification.
George W.
 

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yeah, but it's not much of a safety factor as the pump only stops if the engine is killed. Also, the fuel rail is pressurized so anything that opens the system will cause fuel to spray. One would think that the engineers would build in some sorta failsafe if oil pressure drops but they did not.

If the ecm trigger did not resume function after the initial two seconds and the circuit was then made by the oil pressure switch, it would tend to do this.

I have lost bearings and cams, tho, at much higher oil pressures than what the typical o.p. switch closes or opens.
 

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Strange" My car ran fine when I had the OP switch just hanging from the wiring harness' I had installed a guage unit & did'nt have a adaptor to use both at that time.
Is this why my car wont do 11's!
 

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That is what I said, the oil pressure switch is not needed...it is only for redundancy in case the fp relay fails.

I would get a new nut to hold the steering wheel and see if your car will do 11's.
 

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OWWWWWWWWWWWWWWWWWWWWWWWWWWWWW! OK. Now go to my charge light post & tell me how to fix it!
 

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Done...
 

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If you want the OP switch to act as a safety, unplug the FP relay. You'll need to crank the car forever till oil pressure comes up to start it, but it will die if the switch opens
 

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So, if you believe in magic, put a switch in the trigger to the relay and flip if off after it starts...actually probably would not take much to program a chip to time out and do it for you.

Might be handy to have when the oil cooler lines get tired and decide to slip out of the crimped on ends....that ain't that uncommon.
 

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Originally posted by Steve Wood:
So, if you believe in magic, put a switch in the trigger to the relay and flip if off after it starts...actually probably would not take much to program a chip to time out and do it for you.

Might be handy to have when the oil cooler lines get tired and decide to slip out of the crimped on ends....that ain't that uncommon.
Well I took a quick look and didnt see *ANY* fuel pump logic in there, but DID find a call looking for cranking pulses to be saved to a "B" accumulator which I ASSume if was incrementing could be used for a fuel pump enable. Otherwise its simply a buffered gate in the ECM. Logic: As long as this is high, lets keep this high too. Crank signal is high, lets leave fuel pump high too.

Other than that, I dont know that it can be done. I DO however have some other ideas for something else that I must check into.




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Jim Testa
1986 Regal T-Type
Stock block w/girdle, stock rods, TRW's, JE pins, 214/210 roller, TurboMotion port/polished heads w/1.77-1.50 valves, T&D roller rockers, Champ intake w/o EGR, PTE plenum, Accufab 70mm TB, MSD50's, TE61, PTE front mount, SMC alcohol inj, JT trans, 3000 vigilante, stock rear w/Hotchkiss arms, and kick ass tunes..

 

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I was thinking of a separate, simple chip to time out...
 

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Maybe a one shot triggered by the ECM command. Or a timer relay you can buy with a knob on the top, high current and about $50 with programmable delay.
 

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5o bucks..you believe in those rich texican stories? I was thinking 7 bucks, tops!
 

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Ummm Steve, that's already built with the timer inside and soldered by a Professional (machine) and tested. Not a 555 timer stuck onto a Radio Shack relay with Copper RTV, and
an old TV volume knob hacked into the dash for control.


Sylvania and other companies make them prepackaged and work on 12 volts. Very reliable and adjustable, the signals wire right to the socket with screw terminals and they make all kinds. Delay ON, delay OFF, one shot etc., multiple contacts too.

Or you can build your own, just like a hotwire kit, real easy...


[This message has been edited by salvageV6 (edited January 27, 2002).]
 

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Well, I have a EE genius/car nut down the street and I figured he probably has something laying on the bench to do it.


Hot wire kits for the inept are fine. For those of us that grew up with a soldering gun in one hand and a pump Daisy in the other.....it's a waste of $40.
 
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