Does a GM oil pressure switch serve as a fuel cutoff ?

[MT-2500]

Originally Posted by ImpalaSS1965

On some 350s, there are actually 2 Oil pressure sitches. One bell Shaped , one lead, that sends info to the dash light. I don't believe this one impacts the ec system. or the fuel pump. Then there is another more traditional shaped switch (2 or 3 prong) that does serve as a fuel pump shut off.

On the 305, the single 3 prong switch serves both these functions.

My wiring diagram that I purchased does not match reality.

Also, in the Haynes manual, I can't even find a referece to an oil pressure switch.

I have never run into that setup.
What year and truck ans size and engine code?

[ImpalaSS1965]

The 2 oil pressure switches (1 1 lead, 1 2 pin) are on a 350 G20 Van.
The 3 pin is from a 1989 Caprive 305 Vin E I believe.

[MT-2500]

Originally Posted by ImpalaSS1965

The 2 oil pressure switches (1 1 lead, 1 2 pin) are on a 350 G20 Van.
The 3 pin is from a 1989 Caprive 305 Vin E I believe.

Both of them us a oil pressure switch tied to fuel pump.
But it is not a cut off or a fuel pump saftey kill switch.

It is wired in with the fuel pump relay to aid and supply power to the fuel pump in case the fuel pump relay quits.

If fuel pump relay is working right engine will run with or without the oil pressure switch working.

[MT-2500]

The only GM engines that used the oil pressure swith to power the fuel pump engine running was on early 80 model V6 engines.

It was a wide blade 3 prong oil pressure switch that supplied fuel pump power to fuel pump after engine started and oil pressure came up.

[danielsatur]

I don't know about a GM, I had to put two gallons of gas in a 2000 Ford Taurus for the electric fuel pump to turn on, a one gallon gas container wasn't enough.

see F.O.R.D. Taurus
http://www.automotiveforums.com/vbul...d.php?t=970835

[prose0304]

will a intermittant hard start be a symptom of a po520 code? i pulled this code from the history on my scanner ,but i have on check engine light on. thanks

[MT-2500]

Originally Posted by prose0304

will a intermittant hard start be a symptom of a po520 code? i pulled this code from the history on my scanner ,but i have on check engine light on. thanks

Welcome to AF on your first post.

To get you started on the right foot.
With a problem on yours best to start a New/thread/post.
Go to proper forum and upper right of it look for and hit the New thread button.
Hit us with all info.
Like what they ask when you go to the parts place for parts.
Year make and model and engine/engine code and transmission and mileage.
Also what work has or has not been done on it.
If check engine light still on after code clear you still have a problem.
Recheck for present code and post back code no.
Hard to start always confirm good fuel pressure up to specs.

[SolusUltra]

Summarizing all the information above:

There are 2 switches in parallel that control the fuel pump.
This results in exactly 4 combinations of switch states that lead
to 1 of 2 possible fuel pump states.

State FP Relay OP Switch Pump
1 off off off
2 off ON ON
3 ON off ON
4 ON ON ON

That’s it.
For a normally operating car, the states are:

State 1 – key off, or key has been on for more than 2 seconds but is not cranking
State 2 – this state only occurs if something is wrong
State 3 – key on for less than 2 seconds, or cranking but oil pressure < 5psi.
State 4 – engine running, or cranking and oil pressure > 5psi

Note: if either switch is on, the pump will be on. The other switch cannot turn it off. In
The event of an accident, the ECU will see that the engine is not running and shut off the relay. The oil pressure switch will also be off. Since both are off, the pump will shut off.

Now, what if something goes wrong?

A. Normally switches don’t fail by being shorted. If either switch is shorted, the fuel pump will keep running, even after an accident. But this is unlikely. If a switch is shorted,
the pump will keep running the battery down when the car is shut off and you would not be reading this thread. You would be reading “why does my car hum and run the battery down when it’s off?”

B. If the fuel pump relay won’t close, the oil pressure switch allows you to limp home. As stated in one of the comments above, this mode will cause long cranking times since the fuel pump won’t start until after the oil pressure gets high enough during cranking. In this situation, the oil pressure switch can shut off the fuel pump in the event of an accident. The ECU could recognize that the fuel pump relay didn’t turn on when the key was turned on, and set a code.

C. If the oil pressure switch won’t close, you may never know it until the fuel pump relay also fails.

[brcidd]

My 77 Vega would stall as I rounded curves (on exit ramps) with low oil (Vega's were always low on oil) so to keep it running, just straighten up your curve..or add oil ....the FP relay was only activated during engine crank...same car would run with no FP if you jacked rear end up higher than carb....easy to do with a floor jack---lower car, engine stopped...

My '95 Chevy Dually would cut -out -then not start- found out it had a bad OP switch that was partially shorting to ground (really messes up FP circuit) internally from being melted from those extremely hot exhaust manifolds (454's do that) -finally saw erratic OP guage as my hint.. same truck had the plastic rotor inside distributor cap melted later on)

I've seen 3 astro vans seize their engines due to low or no oil pressure- proof that FP will run and lock up with no oil pressure. I wired my own to be like the Vega cicuit above...I would rather have the engine shut-off and coast to a stop, then to seize while driving......same end result (while driving) IMO.

With loud fuel pumps, you can often hear them run 2-3 seconds after engine shut down, while oil pressure diminishes down to point below 4psi and OP switch opens.

[Modeworld]

"You need to plug-in the oil pressure switch to make the Fuel Pump Relay to work properly under critical condition" - this is the conclusion that I made after a few weeks of headache for sorting out why my 1995 VW Golf had intermittant engine cut off problem.

About a month ago, my VW Golf engine would shut down when I slowed it down upon turning. You could hear "tick-tick" sound from the fuel pump relay immediately before cut-off. The engine couldn't start immediately but 5 to 10 minutes later, it could start again. My first thinking was a faulty fuel pump relay but it didn't fix the problem with a new relay.

I sorted out later that my oil pressure switch had not been plugged in properly; there is a rubber barrel covering the electric terminal of the pressure switch, it was hard to tell there was a loose contact upon visual inspection. I was lucky to find when I checked for all electric contacts. I figure my mechanic did a lousy job when he replaced some parts earlier; very likely he just plugged in the rubber barrel without making sure that the plug did sit properly too.

The intermittant problem didn't appear again once I re-plug the oil pressure switch. I think the pressure switch does serves as a backup in case of fuel relay failure.

I once thought the ECU cut off the engine when it senses low or no oil pressure, it was until I read this post that the engine was cut off due to lack of support from the oil pressure switch connection. If the engine was cut-off by the ECU in case of engine overheating etc, it has to be immediately and I would not hear the "tick-tick" sound generated by the fuel pump relay.

[MT-2500]

Originally Posted by Modeworld

"You need to plug-in the oil pressure switch to make the Fuel Pump Relay to work properly under critical condition" - this is the conclusion that I made after a few weeks of headache for sorting out why my 1995 VW Golf had intermittant engine cut off problem.

About a month ago, my VW Golf engine would shut down when I slowed it down upon turning. You could hear "tick-tick" sound from the fuel pump relay immediately before cut-off. The engine couldn't start immediately but 5 to 10 minutes later, it could start again. My first thinking was a faulty fuel pump relay but it didn't fix the problem with a new relay.

I sorted out later that my oil pressure switch had not been plugged in properly; there is a rubber barrel covering the electric terminal of the pressure switch, it was hard to tell there was a loose contact upon visual inspection. I was lucky to find when I checked for all electric contacts. I figure my mechanic did a lousy job when he replaced some parts earlier; very likely he just plugged in the rubber barrel without making sure that the plug did sit properly too.

The intermittant problem didn't appear again once I re-plug the oil pressure switch. I think the pressure switch does serves as a backup in case of fuel relay failure.

I once thought the ECU cut off the engine when it senses low or no oil pressure, it was until I read this post that the engine was cut off due to lack of support from the oil pressure switch connection. If the engine was cut-off by the ECU in case of engine overheating etc, it has to be immediately and I would not hear the "tick-tick" sound generated by the fuel pump relay.

Welcome to AF on your first post.
But.
VW Golf is a long way from being a GM.

[jfishtaker]

jfishtaker
Hey folks, I have a 1995 RV with 7.4L engine, engine starts right up and then the power to the fp shuts off. Turn key on power to fuel pump for 2 to 3 seconds. Changed OP switch no change. Check volts to fp 9, after two seconds 0. Any suggestions. New fuel pump, relay,ECM, new sensor on motor except EGR. Rebuilt TB pressure. The only question I have on op switch purchase at Advance Auto BWD brand(Oil Pressure Switch), ACDelco actually call theirs fuel pressure/oil pressure switch, should I have paid more and got the ACDelco brand. I talk to Advance Tech, said they were same except brand name??

[j cAT]

autozone pressure switch should work, 9 volts to the fuel pump is too low . why? make sure grounds are also good

[MT-2500]

Originally Posted by jfishtaker

jfishtaker
Hey folks, I have a 1995 RV with 7.4L engine, engine starts right up and then the power to the fp shuts off. Turn key on power to fuel pump for 2 to 3 seconds. Changed OP switch no change. Check volts to fp 9, after two seconds 0. Any suggestions. New fuel pump, relay,ECM, new sensor on motor except EGR. Rebuilt TB pressure. The only question I have on op switch purchase at Advance Auto BWD brand(Oil Pressure Switch), ACDelco actually call theirs fuel pressure/oil pressure switch, should I have paid more and got the ACDelco brand. I talk to Advance Tech, said they were same except brand name??

As said 9 volts is to low.
You should have battery voltage at fuel pump.

Oil pressure switch only feeds voltage to fuel pump after oil pressure come up and only if fuel pump relay fails.



2 second prime up key on and then shut off is normal.
Then when engine is in start or run rpm signal from VCM should turn on relay and should feed 12 volts to fuel pump.

Have you checked for voltage to fuel pump engine cranking or running?

[jfishtaker]

jfishtaker

>Check volts while cranking, drop from 9 to 6 volts,????
>Can not find a bad ground so far. Check for weak connection in wire harness, wish this was a Outboard motor I could fix it!!
>Will try a hot wire(with fuse) to the purge side at relay and check voltage at fuel pump connection, if volts are will try cranking and see what happens, hope I do not smoke something.
>There is a red wire in wire harness at the fuel relay labeled purge, is location I will connect hot wire??