What is function of Oil pressure sending switch?

[azu]

Is this just for the dash gauge or does it do more than that? This is on a 95 SE.

[J Wikoff]

On some years, I forget which, if the sensor doesn't see enough/any pressure, it cuts power to the fuel pump.

[ChilinMichael]

Well, when mine went bad and read 0, the car was still running fine...think it's safe to say they didn't incorporate that into the 95'.

[Mechanical Mike]

95 still used the oil pressure switch to send power to the fuel pump. Most of us believe it was a secondary source of power so if the fuel pump relay failed the engine would still run. Unfortunately we occasionally see the sender short out which blows the fuse & kills both sources of power causing the engine to stall.

[ChilinMichael]

...and a little side note. My vin 95 was incorporated in the TSB of vin's that have cavity a5 on the wiring harness in the engine bay....transient voltage from fuel pump to lighting system. So that makes 3 sources of power LOL! I had to cut mine as it caused the lights to come on/stay on at random and TSB said to do so.

[azu]

Unfortunately we occasionally see the sender short out which blows the fuse & kills both sources of power causing the engine to stall.

I know that!!! Happened to me 2 weeks ago!

[Mechanical Mike]

That was for other readers, yours was the first one in quite a while.

[myfirstbonnie]

Ok, to clear this up:

The oil pressure switch provides a second route for the current to get to the pump. NOT a sencond supply. They are tied to the same fuse, but follow different wires.

So, from the 1995 FSM volume 2, page 6E3-A-10 titled 3800 (VIN 1) (VIN K) & (VIN L) (SFI) Driveability and emissions.

Red line: the path of current through the relay

Blue line: the path through the oil pressure switch



If you don't believe it works, do what I did.

1. Disconnect the oil pressure switch and start the car

2. Pull the relay at the same time the switch is disconnected and the car will not start

3. Plug the oil pressure sender in with the relay pulled and the car will start, but it must build 4psi of oil pressure before the switch closes

For troubleshooting this issue, refer to page 6E3-A-41 in the same FSM. Lower right corner, there is a Notice: If original complaint was "cranks but will not run" make the following checks:


1. Engine idling at operating temp, oil pressure normal, disconnect fuel pump relay, Engine should continue to run, Does it?

Yes, no problem
no, faulty oil pressure switch

[dpcookson]

For troubleshooting this issue, refer to page 6E3-A-41 in the same FSM. Lower right corner, there is a Notice: If original complaint was "cranks but will not run" make the following checks:


1. Engine idling at operating temp, oil pressure normal, disconnect fuel pump relay, Engine should continue to run, Does it?

Yes, no problem
no, faulty oil pressure switch

I don't have the FSM, so maybe I'm out of line here, but I don't understand how they went from "cranks but will not run" to Engine idling.
Just noting this, no relevance to the post.

[myfirstbonnie]

It is referring to the customer complaining of moments when the car will crank but not always start. One of those issues where the car won't start and then when it gets towed to the dealer, it starts for them. One of those intermitant issues.

[Mechanical Mike]

Thanks for correcting me. We're saying the same thing but 'source' was a poor choice of words.

[J Wikoff]

So was I wrong about it being able to cut power to the pump? Does the dual path just exist as a troubleshooting aid?

[myfirstbonnie]

That and a way to run the car if the relay dies while you are driving or stop somewhere. Imagine if you were driving down the road and the relay dies. Just the opposite of the Toyota syndrome.

If people notice that it takes a few extra seconds for their car to start, the fuel pump relay could be bad. You actually have to crank the car an extra couple seconds for the oil pressure to reach 4psi before the switch will close and allow the pump to come on, ONLY IF THE FUEL PUMP RELAY IS BAD.

[Coryjames]

Hey RJolly87, the only gauge I notice move at all was the oil pressure. The others stayed about the same but I do not know about the tachometer. I probably would have noticed if it was jumping though since I've had experience a crankshaft sensor dying on me once.

Also, I didn't see a security light doing anything; I'm not even sure if it has one. The only light different light I saw was the "check gauges" light.
Another piece of information I can provide is when I was having it towed behind my friend truck; when ever I have the key on and when the car would roll it would periodically beep a bunch to tell me to check my gauges. I actually turned my car completely off just so it would stop beeping at me while being pulled.

And yeah, I would only want a gold plated computer from Antarctica. But before I start switching part arounds, I was wondering how I could test which computer it could be before I start switching things out and getting a PCM from the junkyard. If it even is a a computer anyway. is there anything else beside the PCM and ICM that would do this?