Crankshaft position sensor on gen V 454 TBI 32PSI version

[big_al273]

the oil pressure switch and the relay are wired in parallel as far as supplying 12v to the fuel pump however the oil pressure switch also sends oil pressure signal to the ECM, the ECM supplies power to the fuel pump relay under the following conditions: for five seconds upon key on, while seeing a crank signal, and while the ecm sees the engine running. Now i'm sure the ECM will shut down an engine under a variety of conditions but since the ECM sees a signal from the oil pressure switch it would make sense that the ECM would shut off the fuel pump relay upon the loss of oil pressure signal and assuming that the engine really has lost oil pressure the oil pressure switch would also shut off leaving the fuel pumps unpowered, hence safety feature.

[VileZambonie]

Not sure how shutting off the engine while driving would be a safety feature....

BASIC SYSTEM OPERATION:

The fuel supply system begins with gasoline in the fuel tank. An electric fuel pump, located in the fuel tank with the fuel level gauge sending unit, pumps fuel to the fuel rail through an in-line filter. The pump is designed to supply fuel at a pressure above the pressure needed by the injectors. A pressure regulator located in the throttle body assembly keeps the fuel available to the injectors at a constant pressure. The unused fuel is returned to the fuel tank by a separate line.

MODES OF OPERATION:

The ECM uses voltage inputs from several sensors to determine how much fuel to give the engine. The fuel is delivered under several conditions, called "Modes." All modes are controlled by the ECM, and are described below.

STARTING MODE:
When the ignition is first turned ON, the Electronic Control Module (ECM) turns ON the fuel pump relay for two seconds, allowing the fuel pump to build up system pressure. On Heavy Duty models, a fuel module will override the ECM two second timer and the fuel pump will run for twenty seconds and then shut OFF if the vehicle is not started. This circuit corrects a hot restart (vapor lock) during high ambient temperature conditions. If the engine is immediately started, the fuel pump will continue to operate. If the key is left in the ON position, but the engine is not started, the ECM will turn the pump OFF. The fuel pump will remain OFF until the ECM detects the crank signal, or in the case of a faulty fuel pump relay, the oil pressure switch detects approximately 4 psi of oil pressure which will then bypass the fuel pump relay.

Fuel delivery from the injectors (which deliver fuel in all operating modes) is controlled by changing the amount of time the injectors are turned on or "pulsed" by the ECM. The ECM checks the coolant temperature sensor, throttle position sensor, and the crank signal, and determines the proper air/fuel ratio for starting. This ranges from 1.5:1 at -36°C (-33°F) to 14.7:1 at 94°C (201°F).

CLEAR FLOOD MODE:
If the engine floods, it may be cleared by pushing the accelerator pedal down all the way. The ECM then pulses the injector at a 20:1 air fuel ratio. The ECM holds this injector rate as long as the throttle stays wide open, and the engine speed is below approximately 600 rpm. If the throttle position becomes less than approximately 80%, the ECM returns to the starting mode.

RUN MODE:
The run mode has two conditions, called Open Loop and Closed Loop.

OPEN LOOP:
When the engine is first started, and engine speed is above 400 rpm, the system goes into Open Loop operation. In Open Loop, the ECM ignores the signal from the Oxygen sensor, and calculates the air/fuel ratio based on inputs from the coolant temperature and Manifold Absolute Pressure (MAP) sensors.

The system will stay in Open Loop until the following conditions are met:
The Oxygen sensor has a varying voltage output, showing that it is hot enough to operate properly. (This depends on temperature)
The Coolant temperature is above a specified temperature.
A specified amount of time has elapsed after starting the engine.
CLOSED LOOP:
The specified operating conditions vary with different engines, and are stored in the programmable read only memory (PROM). When these conditions are met, the system goes into Closed Loop operation. In Closed Loop, the ECM calculates the air/fuel ratio (injector on-time) based on the signal from the Oxygen sensor. This allows the air/fuel ratio to stay very close to 14.7:1.

ACCELERATION MODE:
The ECM responds to rapid changes in throttle position and manifold pressure or air flow, and provides extra fuel.

DECELERATI0N MODE:
The ECM responds to changes in throttle position and manifold pressure and reduces the amount of fuel. When deceleration is very fast, the ECM can cut off fuel completely for short periods.

BATTERY CORRECTION MODE:
When battery voltage is low, the ECM can compensate for the weak spark by:

Increasing the amount of fuel delivered.
Increasing the idle speed.
Increasing the ignition dwell time.
FUEL CUT-OFF MODE:
No fuel is delivered by the injector when the ignition is OFF. This prevents dieseling. Also, fuel is not delivered if no reference pulses are seen from the distributor, which means the engine is not running. This prevents flooding. Fuel cut-off also occurs at high engine rpm, to protect internal engine components from damage.

[Irish_Alley]

like vile said if you unplug the ops the truck will start and continue to run as long as the fuel pump relay is working. you can tell when a relay has given up if you ever ear a TBI motor crank for 5 or so seconds then firing up. that means the relay has quit and it need oil pressure to power the pump.

[big_al273]

if you engine has no oil pressure and you keep driving you will seize the engine ending in an unpowered coasting scenario, if the ECM detects low/no oil pressure and shuts off the engine your still coasting unpowered but now you don't necessarily have a blown motor to go along with it. As far as the oil pressure switch goes it was wired in parallel with the relay so that in the unexpected event of fuel pump relay failure while driving so long as you still have engine oil pressure you won't shut down in the middle of the road. Now insofar as what the ECM will do if it loses the signal from the oil pressure switch will take some more research however everything I've found so far has indicated that the ECM should shut down the fuel pump relay in a no oil pressure situation to prevent engine destruction. My original point however about not wanting the fuel pump to continue running after an accident still stands, although the fuel pump will keep running if the engine does

[Irish_Alley]

if the ecm kills the engine without giving you sometime to react (to pull over) you will lose power steering and brakes. for most we can handle this, but for some this would be a liability issue with gm cause they might not know how to tell the engine isnt working and they no longer have power brakes or if youre in the mountains and are now trying to stop a trailer while going downhill. as far as low oil pressure, if you wreck your motor cause you ignored a "check engine" light or the oil gauge being below 3 psi then thats solely on the operator of the vehicle. 

as far as the event of the relay failing while driving is kind of hard for me to understand. most times when they fail its due to arching and burning up the contacts. this is done when power is applied and when its turned off.

you have an 88 so it would take less time to reply to a comment than start the truck up and unplug the OPS or even start it with the OPS unplugged

[big_al273]

power assist brakes weather vacuum or hydroboost have an internal boost/vacuum reservoir that gives you 1-3 good assisted pedal pushes, this feature was implemented so that in the event of unexpected engine shut down you still have brake assist to bring the vehicle to a safe stop. Power steering is only needed in parking lots or downtown traffic, it's nice to have but it's a luxury not a necessity, cars are very easy to steer so long as they're rolling, even the flat bed tow truck (10,000 lbs deck capacity) at work can be steered strong arm style as long as it's rolling. As for unplugging the oil pressure switch i will be trying that after i get a new fuel pump relay (hopefully just need a relay) and stop feeling like death, I've been home sick the last two days

[MuddiGGEr25]

FWIW. I haven't gotten the OP sensor repaired or replaced yet.. mainly working on a billion other things. But yesterday I got the engine wired and started. It idled and moved around in the driveway under its own power with no oil pressure switch.