fuel pump and oil pressure circuits

[eran tomer]

I'm trying to figure out how this circuit work.
if both wires from C of the oil pressure switch and A3 of the relay are live and spliced- doesn't it make a short or sparks?
and if there's no oil pressure, the pump is still getting power from the relay, so what makes the engine to stall?

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[Schurkey]

I'm trying to figure out how this circuit work.
if both wires from C of the oil pressure switch and A3 of the relay are live and spliced- doesn't it make a short or sparks?

In normal operation, both wires have essentially the same voltage on them--therefore it's not a "short" circuit, and there'd be no sparks unless the insulation on the wire is broken and the wire grounds on something.

When one wire has voltage but the other doesn't, there's most-likely a switch open--either the oil pressure switch contacts, or the contacts within the fuel pump relay. So again, no "short" circuit, no sparks. If one wire doesn't have voltage because it's broken...sure, it could short to ground at the break and cause a spark, which would likely pop the ECM-B fuse.

and if there's no oil pressure, the pump is still getting power from the relay, so what makes the engine to stall?

There's nothing in the fuel pump wiring and parts that can turn the pump off with no oil pressure, IF the fuel pump relay system is working properly.

If the engine stalls with no oil pressure, the fuel pump relay system is probably faulty. DOES THE FUEL PUMP PRIME when the key is turned from "Off" to "Run"? Could be the ECM, could be the relay, could be the wire harness connecting them, including crappy contact with the relay where the relay plugs into the socket.

But maybe the engine isn't stalling from lack of fuel. Maybe it's stalling due to some other fault. Engine stalls, oil pressure goes to zero--instead of oil pressure goes to zero and then the engine stalls.

 

[eran tomer]

so the oil pressure switch is not intended to protect the engine from loss of oil pressure as some people believe?

 

[Schurkey]

so the oil pressure switch is not intended to protect the engine from loss of oil pressure as some people believe?

Yes, it does not protect the engine from loss of oil pressure.

 

[GoToGuy]

Think about this, a mechanical fluid pump that's direct connected to engine rpms. A" low oil pressure " switch that illuminates at 4 psi or less. A oil pressure transducer that indicates oil pressure in psi in instrument cluster gauge.
There is no control overlap. A loss of oil pressure will stop the fuel pump resulting in shutdown.
Ask me how I know at 65 mph.

 

[WICruiser-97]

Your diagram does not tell the whole story as the A1, B1, and B2 are part of a control module (ECM). The description cut off at the bottom of the page explains how it works.

If A1 does not get power from the ECM the fuel pump relay is not pulled in to complete the fuel pump circuit that bypasses the oil pressure switch. I beleive the A1 terminal is provided power when the key is turned on but it is timed so it only provides power for a few minutes and after that it "turns off" such that if the fuel pump is not getting power through the oil pressure switch the pump shuts down.

As a result if you loose oil pressure the engine will stop getting fuel. It may be too late to save the engine but that's another story.

 

[Schurkey]

Think about this, a mechanical fluid pump that's direct connected to engine rpms. A" low oil pressure " switch that illuminates at 4 psi or less. A oil pressure transducer that indicates oil pressure in psi in instrument cluster gauge.
There is no control overlap.

I don't understand this. Seems unrelated to GM fuel pump wiring.

A loss of oil pressure will stop the fuel pump resulting in shutdown.
Ask me how I know at 65 mph.

A loss of oil pressure does not stop the fuel pump if the pump is still getting power via the relay.

Your diagram does not tell the whole story as the A1, B1, and B2 are part of a control module (ECM). The description cut off at the bottom of the page explains how it works.

If A1 does not get power from the ECM the fuel pump relay is not pulled in to complete the fuel pump circuit that bypasses the oil pressure switch.

WHICH "A1"? The A1 at the ECM, or the A1 at the relay?

A1 at the ECM provides power to B3 at the relay, which then grounds through A1 at the relay and it's harness wire, which pulls the relay contacts to run the pump.

I beleive the A1 terminal is provided power when the key is turned on but it is timed so it only provides power for a few minutes

About two seconds for the "prime" cycle. Some TBI engines had a "Hot Fuel Module" that ran the pump for ~20 seconds.

and after that it "turns off" such that if the fuel pump is not getting power through the oil pressure switch the pump shuts down.

Yes--IF there's no RPM signal from the engine sensor--Pickup coil/ignition module on TBI, crank sensor on Vortec. If there's an RPM signal delivered to the ECM, the relay stays engaged to power the pump.

As a result if you loose oil pressure the engine will stop getting fuel.

No. The relay is still engaged if the ECM has an RPM signal, so the pump still runs.

The pump can run from the ECM/Relay side of the circuit, or it can run from the oil pressure switch side of the circuit. In normal operation, the current is divided between the relay and the oil pressure switch so that neither one carries the full amperage load. But if one fails, the other takes over. If it's the relay side that fails, you can expect longer cranking time because the engine has to build oil pressure before the fuel pump will run. If it's the oil-pressure side of the circuit that fails, you'd never know the difference...until the relay side fails and then the engine won't run.

 

[WICruiser-97]

The A1 I was referring to was the ECM not the fuel pump relay.

I am not familiar with the RPM signal providing power through the ECM but my knowledge may be dated from the days before the ECM received an RPM signal from the distributor. I know that if you delete the ECM but do not "fix" the pump relay power source such that it relies on the oil pressure build to turn the pump on it can cause lack of fuel thus preventing starting. This is typically the case when the engine is swapped to a carburetor and older type distributor without an ECM.

 

[Schurkey]

The A1 I was referring to was the ECM not the fuel pump relay.

OK.

I am not familiar with the RPM signal providing power through the ECM but my knowledge may be dated from the days before the ECM received an RPM signal from the distributor.

The ECM has ALWAYS gotten an RPM signal, either from the pickup coil via the ignition module, (all the way back to 1980 1/2) or from the crank sensor.

The RPM signal does not power the pump directly. The RPM signal tells the ECM that the engine is either cranking or running, and therefore needs the fuel pump relay turned on. The ECM drives the fuel pump relay via "quad drivers", not by routing the RPM signal to the relay.

 

[elplatano]

I don't understand this. Seems unrelated to GM fuel pump wiring.


A loss of oil pressure does not stop the fuel pump if the pump is still getting power via the relay.


WHICH "A1"? The A1 at the ECM, or the A1 at the relay?

A1 at the ECM provides power to B3 at the relay, which then grounds through A1 at the relay and it's harness wire, which pulls the relay contacts to run the pump.


About two seconds for the "prime" cycle. Some TBI engines had a "Hot Fuel Module" that ran the pump for ~20 seconds.


Yes--IF there's no RPM signal from the engine sensor--Pickup coil/ignition module on TBI, crank sensor on Vortec. If there's an RPM signal delivered to the ECM, the relay stays engaged to power the pump.


No. The relay is still engaged if the ECM has an RPM signal, so the pump still runs.

The pump can run from the ECM/Relay side of the circuit, or it can run from the oil pressure switch side of the circuit. In normal operation, the current is divided between the relay and the oil pressure switch so that neither one carries the full amperage load. But if one fails, the other takes over. If it's the relay side that fails, you can expect longer cranking time because the engine has to build oil pressure before the fuel pump will run. If it's the oil-pressure side of the circuit that fails, you'd never know the difference...until the relay side fails and then the engine won't run.

Sorry to revive an old thread, this is the closest to the answer I am looking for and I want to ask you because you seem knowledgeable in this area. I am attempting to use a remote control relay kill switch system, however it only comes in the 4 pin version(all pins except 87a). As I understand this relay 87a is the power coming from the oil pressure sensor and is only used in case of relay failure.

So in theory, if I use a 4pin relay the system will function normally. Only downside is, as you stated in this thread, in case of relay failure the engine will die instead of having that redundancy circuit.

Any input you may have is super appreciated.