Fuel Pump Relay and Oil Pressure switch, how does it work?

Fuel Pump Relay and Oil Pressure switch, how does it work?

Postby N6KB [OP] » June 4th 2020, 8:42pm

Looking at the wiring diagram for my 2005 Astro, in a Chiltons manual, I see there are two possible current paths to power the fuel pump. One is through the Fuel Pump Relay and one is through the Oil Pressure Switch. I am guessing the Fuel Pump Relay is only actuated for a few seconds when the engine is getting started, and then once there is oil pressure the current to the fuel pump is supplied through the Oil Pressure Switch. Can someone who knows tell me if I've got this right? If I pulled out the fuel pump relay when the engine is up and running would it it keep running because the fuel pump is being powered through the Oil Pressure switch? How long does the Fuel Pump Relay stay actuated at startup?
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Re: Fuel Pump Relay and Oil Pressure switch, how does it wor

Postby Leeann_93 » June 5th 2020, 1:02am

It really doesn't go through the oil pressure switch. Chiltons ain't all that great.

Fuel pump prime cycle is 3 seconds when you turn the key to run. If you pull the fuel pump relay, the fuel pump won't run. That is how they tell you to relieve fuel pressure to replace the filter, etc.
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Re: Fuel Pump Relay and Oil Pressure switch, how does it wor

Postby N6KB [OP] » June 5th 2020, 8:46pm

Sure would like a wiring diagram that is correct. I know Chiltons is not perfect. When I look at the "Typical four-cylinder engine controls" in my Chilton book it shows a six cylinder distributor.
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Re: Fuel Pump Relay and Oil Pressure switch, how does it wor

Postby Rod's Trucks » June 5th 2020, 9:29pm

IIRC,
Leeann is correct.
The oil pressure switch does shut off the fuel pump if you loose oil pressure, but it does that by interrupting the required control signal to the fuel pump relay. That fuel pump relay control signal may go through other systems also, but ultimately the fuel pump relay is the what turns the fuel pump ON and OFF.
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Re: Fuel Pump Relay and Oil Pressure switch, how does it wor

Postby Mmusicman » June 6th 2020, 1:24pm

Rod's Trucks wrote:.. ultimately the fuel pump relay is the what turns the fuel pump ON and OFF.

I recently got in a conversation with a local "seasoned neighborhood mechanic" (near my job) where he was replacing a fuel pump on an Astro that had a full tank of gas. He told me he cleverly disconnected the fuel line and let the van pump out the gas.. turning the key off and on multiple times to have the fuel pump pump out the gas. (I guess the pump wasn't totally bad after all..lol) With engine not running and no oil pressure, the pump would only pump fuel for a few seconds. I asked him why he didn't just jump power to the fuel pump relay location! I'm not even sure my answer registered with him. True story.. lol

PS: This is one good way to test the pump and bypass the relay as well as all the controlling circuits.
Of course this doesn't work for poor pump connections or bad grounds.
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