Trying to understand the function of the oil pressure switch in a L98...

Will the oil pressure switch only work as a back-up switch in the case the fuel pump relay dies?

or

will the ECM power the fuel pump relay only at a start of the motor for a few seconds and then the closed oil pressure switch will provide 12v to the fuel pump relay?

Thanks for help

 

The first scenario is correct. The ECM will operate the fuel pump relay:

A) Whenever the ignition has been off for 20 seconds or more, and is then turned on (prime cycle);

B) Whenever there are distributor reference pulses (moving pickup coil reluctor) and for two seconds afterward.

The auxilliary oil pressure switch is simply a redundant path for power in the event of failure of the relay or ECM output to the relay.

 

hi Vader

Your answer is incorrect.

The second scenario is the correct one. The relay is only used to prime the fual pump. It is easy to test, unplug the fual pump relay after the car is started, the car should continue to run.

Second test, stop the motor, plug the relay back in. Wait twenty seconds. Unplug the connector on the Oil Pressure switch. Now start the car, I bet it will not continue to run.

 

Actually Bruce, your answer is incorrect. Disconnecting the oil pressure switch will not shut the fuel pump off, unless the fuel pump relay is defective.

 

Have you tried it.

The fual pump on my TPI350 works as I have documented. The operation was verified my the Chevy dealer who installed the motor in the car. If a TPI injected V8 loses oil pressure then the engine should stop running. The TPI system on my 82 is wired per 1989 specifications. I know it works that way, because I was pleasent when the dealer went though a complete system check out back in 98.

 

Actually this same question was asked not so long ago and it made me curious, so I tried it on an LB9 Firebird and a handful of other FI GM vehicles. Hardly an exhaustive experiment, but every one ran without the oil pressure switch.

 

Disconnecting the oil pressure sending unit will not stop the oil pressure, it will simply stop registering the oil pressure at the oil guage & the ECM since the oil pump is internally driven. Also the fuel pump relay is used to power the fuel pump through the fuel pump signal going to pin "A" of the relay from the ECM. BTW this signal also goes to the oil pressure switch by a wire splice from the tan/white wire going to pin "A" of the relay. The relay also gets its signal to prime the pump through the dark green/white wire to pin "C" of the relay from the fuel pump relay driver in the ECM.

 

I believe the debate is whether or not disconnecting the oil pressure switch will stop the fuel pressure.

 

Whether it be a debate or argument, I know it didn't stop the fuel pressure on the one's I tested in my area including my own. However there is a lot of misinformation being spread in this debate or argument as the case may be.

 

I'm not sure if I'm reading that right.

The ECM turns the fuel pump relay on (during priming or normal running) with an output from pin A1, which goes to the coil on pin C of the relay. The NO contact of the relay is on pin A, which is connected to the +12V source on pin E when the relay is energized. Pin A then powers the fuel pump. Pin A is also connected to pin B2 on the ECM, which is an input for the fuel pump voltage sense signal, not an output. If this signal is lost, the ECM sets code 54.

The fuel pump is also redundantly powered when the oil pressure switch closes, which also presents the fuel pump voltage signal to the ECM, of course.

I could dig up Vader's favorite schematic, but I think he'd have more fun posting it himself.

 

have a look

 

"There should be about 12 volts on CKT 120 for 2 seconds after the ignition is turned "ON", or any time references pulses are being received by the ECM."

That statement has nothing to do with the oil pressure switch. It has everything to do with the ECM sending a 12v signal through the GRN/WHT wire, CKT 465. The service manual is telling us that the ECM turns on the fuel pump relay while receiving references pulses (yes, and for 2 seconds after the ignition is turned ON).

I've been building new TPI harnesses for a few years now and sometimes have people request that I do not include the oil pressure switch. I test every harness on a 350 TPI engine running on a test stand in my garage. The oil pressure switch is not necessary to run the fuel pump. I don't see any room for argument.

Although not necessary, I would not personally exclude it.

 

In other words, only ONE is required to run the pump, doesn't matter which really. Bad relay, you get long starts until oil pressure comes up and the switch closes. Bad switch, who cares, the diagram clearly shows 12V can get to the pump through either the relay OR the switch, since the power splice for the switch is AFTER the relay "ouptut" of 12V to the pump.

Edited for clarity.

 

Correct. But your first sentence can be confusing.

You need either the fuel pump relay or the oil pressure switch. Best to have both, but not required. At least one of the two are required.

 

It's showing exactly what I'm saying. Pin B2 on the ECM is an input, not an output.

 

From my understanding, there are two switches for oil. One is an oil pressure sending unit for the gauge, the second is a safety switch for if oil pressure drops to 0 it shuts the fuel pump off to keep from

1) either damaging an engine in the event of an oil pump failure

or

2) in the event of an accident and the motor is off, it will kill the power to the fuel pump.

 

That's the misconception we're trying to dispense with.

 

Well, I can say that the original engine that was in my Firebird had two switches, one that went to the gauge, and another that went to the fuel pump power lead, if I unplugged the one that went to the gauge, the car stayed running, if I pulled the second one, it died.

 

In other words, your relay was dead.

Just look at the diagram posted until you get what it's showing. Once you understand it, you will understand why the oil pressure switch will NOT have any bearing on the engine running, *given that all wiring/components in the fuel pump circuit are working correctly and in good shape.*

Hint: wire coming in from the upper left corner of that diagram is constant 12V from the battery.

The oil pressure switch is a failsafe in case the relay fails.

There is only one oil pressure switch on these cars, and thats for the fuel pump. The gauge uses a sending unit, big difference.

 

It's a back-up switch! See PIN A1 at the ECM (fuel pump relay drive)
The voltage with the engine running is described as B+, so the fuel pump relay is driven by the ECM all the time the engine is running.

Thank you all for your help!

 

anyone wired it up to be a required part of the fuel pump operation?
I thought about that last year after I wired up my remote start/alarm, my old *** engine could use a little extra security..

 

And I thought we put this one to bed about three years ago. Unless you got one of the extremely rare AC/Delco Time Lapse Wiring Harnesses that has already begun to alter itself, all ECM-equipped and most PCM equipped vehicles have the same fuel pump circuit. Those wire harnesses were reportedly only installed on the elusive 1986 model year L98 cars with the T-5 and T-Top bodies - You know, the ones with a carbureted TPI...

I'm going to have to contact IROCZTWENTYGR8 and ream him a new one. Obviously, the feature of the board is malfunctioning again.

 

I don't think it is a case of the search function malfunctioning again, more like a case of not being utilized properly.