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Old 03-17-2020, 10:26 PM   #1
19Fordy
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Default Fuel Pump Oil Pressure Shut Off Switch Question

My 8BA uses fuel pump oil pressure shut off switch so that the fuel pump will stop pumping gas when the oil pressure drops below a certain level, like in an accident. I can turn the fuel pump on and off with a separate toggle switch under the dash.

For the last 6 months my engine will start and idle fine and will rev up fine without hesitation when in neutral. However, when you drive about 1/4 mile the engine will sound and feel like it's starving for gas and shut off. If I wait about 20 min engine will restart using starting fluid but then stall out again after moving a short distance.

I can see that plenty of gas is getting to the carb from the electric pump. Carb is a Holley 390 that has run fine for several years.

I have a gut feeling that the problem may be that the fuel pump oil pressure shut off switch may be malfunctioning internally causing the fuel pump to shut down since they are both wired together.
IS THIS A POSSIBILITY?

Please explain to me how this fuel pump oil pressure shut off switch works internally to help me figure this out.I have rebuilt the carb and you can see gas squirt out of the jets when you give it gas at idle. Engine revs up fine but dies once underway.

The photo below shows both the oil pressure sending unit and the fuel pump oil pressure shut off switch. Both units are installed in a horizontal position.

Fordbarner help is appreciated as this is driving me nuts.I even replaced the ignition system thinking that might be the problem. No fun when you can't drive your car down the road. Thanks. JIM
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File Type: jpg IMG_5722b.jpg (83.4 KB, 30 views)

Last edited by 19Fordy; 03-17-2020 at 10:36 PM.
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Old 03-17-2020, 10:46 PM   #2
JSeery
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

Try a jumper around the switch and see if the issue goes away.
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Old 03-17-2020, 11:30 PM   #3
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

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Originally Posted by JSeery View Post
Try a jumper around the switch and see if the issue goes away.

Damn.....Took the words right out'a my mouth! DD
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Old 03-18-2020, 06:28 AM   #4
51 MERC-CT
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

Something to consider when deciding to install an oil pressure shutoff switch as opposed to an impact shutoff switch.

Crash happens, and gas pedal or linkage is damaged so as to cause the engine to run/race wide open.

Which one is more likely to shut off the electric fuel pump?


I have witnessed two accidents like this, but neither of the vehicles involved had electric fuel pumps.
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Old 03-18-2020, 06:41 AM   #5
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

Not to uncommon for oilpressure switches to have an internal leak around the membrane.
At startup it works then slowly as it leaks stops working....if you wait for a while it will kinda reset and work again for a while.
Replaced many as customers comes in terrified their car looses oilpressure after a short drive.
If the car runs fine on the bypass switch...replace oilpressure switch.
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Old 03-18-2020, 01:16 PM   #6
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

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19Fordy, I find your post a little odd. You are a pretty sharp mechanic, but from your post you say you have “a gut feeling it’s the pressure switch”. Seems like that could be verified easily enough, by jumping the switch as stated by others, or just put a test light to it.

I have a new Holley brand switch that I did not use. You can have it for cost of postage. Switch opens at 5 psi oil pressure.

Last edited by JayChicago; 03-18-2020 at 01:29 PM.
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Old 03-18-2020, 02:03 PM   #7
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

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Originally Posted by 51 MERC-CT View Post
Something to consider when deciding to install an oil pressure shutoff switch as opposed to an impact shutoff switch.

Crash happens, and gas pedal or linkage is damaged so as to cause the engine to run/race wide open.

Which one is more likely to shut off the electric fuel pump?

I have witnessed two accidents like this, but neither of the vehicles involved had electric fuel pumps.
On most of the modern cars I have been around both are used (inertia switch and oil pressure switch). I use both, but if you were only going to use one, I would go with the inertia switch.
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Old 03-18-2020, 03:09 PM   #8
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

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On most of the modern cars I have been around both are used (inertia switch and oil pressure switch). I use both, but if you were only going to use one, I would go with the inertia switch.
Interesting, are you saying that two different type switches are used on one car and if so, which one?

Last edited by 34PKUP; 03-18-2020 at 03:35 PM.
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Old 03-18-2020, 03:25 PM   #9
19Fordy
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

Thank you Fordbarners all for the replies.

Jay, as silly as it seems, I never thought of jumping the pressure switch figuring they wouldn't wear out. The gut feeling just came to me as I was posting this question yesterday. Thanks for the kind offer but I ordered a new pressure switch about 20 min. ago before I saw your post. I have always been gun shy when it comes to auto electric wiring.
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Old 03-18-2020, 05:36 PM   #10
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

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Interesting, are you saying that two different type switches are used on one car and if so, which one?
I think he is saying that most modern cars use both. If either one detects trouble, power to the fuel pump is cut.

We could do the same. Just run the wire providing power to the fuel pump thru the oil pressure switch, then thru the inertia switch, then to the pump.

Here is a picture of the inertia switch just installed on my '37. I was planning to use an oil pressure switch, but then on advise from others here, decided an inertia switch is a better safety device. This switch was pulled from a junkyard Mercury Sable. Easy removal. Is now mounted in the trunk of the '37, on the spare tire platform support. BTW, those big red knobs were on the switch, I guess to draw attention to it's existence; I put them back on to serve the same purpose.
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Old 03-18-2020, 05:38 PM   #11
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

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Originally Posted by 19Fordy View Post
Thank you Fordbarners all for the replies.

I have always been gun shy when it comes to auto electric wiring.

Seems I remember you successfully did some headlight relay and re-wiring modifications a while back on your '40 without any problems. As long as you always try to think of automotive electrical circuits like plumbing, whereas voltage = pressure, resistance (wire size) = the diameter of pipes, amperes (current) = the flow of liquid in the pipe, and switches = valves, it becomes a little easier to visualize the fundamentals of how basic DC electrics work in a car. DD
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Old 03-18-2020, 05:54 PM   #12
mike in tucson
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

It is important to know the current capacity of the oil pressure switch. For instance, the stock GM 5.7 switch is packaged with the oil pressure sender and has very little current capacity...it is intended as a signal to the ECM. Some builders route the electric fuel pump circuit thru this switch and, similar to an overcurrent situation in a relay, burn the contacts. Such suggests for troubleshooting (like everyone is telling you) that you use a jumper to bypass the oil pressure switch.
By the way, the stock GM oil pressure switch is used to prevent the engine from starting until the minimum oil pressure is reached (3psi I think). It is a signal to the ECM that it is OK to start. An inertia switch is used in case of a crash to shut the fuel pump down to prevent fire.

Last edited by mike in tucson; 03-18-2020 at 06:32 PM.
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Old 03-18-2020, 07:29 PM   #13
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

Mike in Tucson raises a good point. The aftermarket oil pressure cut-off switches are designed to carry the full current to a fuel pump. But a switch off a modern car, if designed to just send a mini-volt signal to the ECM, would not work on our cars.

I guess the same would be true with inertia switches. The inertia switch I pulled off a 1990's car appears to be designed for full amperage to the pump. It has no. 14 wire, was located in the trunk near the fuel pump. But maybe other modern cars use an inertia switch designed to just open the control circuit to the fuel pump relay, and then would need to be wired that way on our cars.
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Old 03-18-2020, 07:52 PM   #14
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Default Re: Fuel Pump Oil Pressure Shut Off Switch Question

The oil pressure switch should be the trigger for a relay and not fully current to an electric fuel pump.
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