Cadillac Repair: 1981 eldorado biarritz, oil pressure switch, fuel pump relay

Question
Hello, thanks for your time, normally starts runs very well, hesitated one day like a low charge, I put it to the temp, -20 degs,2 hrs later I gassed up and battery was dead.Since then I replaced or checked:new batt and alt,[100amp]fuel pump,Injector andpump relays,Idlecontrol sw,Tps,new battery leads checked all grnd straps all fuses good,continuity to allstartup and fuel circuits good,traced allcodes diagnostics,eng.malfunction,switch test procedures,I know you are going to tell me to change the ECM,but my ??? is the oil pressure sw,could I jump it with 4or5 volts? She starts great with a spoon of gas in the throttle body but no current to pump or inj.tofeed the fuel.I put an ECM in about a yr. ago. Also I don't know if the Injector relay has a diode attached, I lost the original. Fuel pres. Okay.Thanks again for your time I appreceate it.     Les

Answer
Fuel pump test:

1. Get yourself a helper.

2. remove the gas cap.

3. have one person listen in the gas tank at the gas cap opening.

4. have the other person turn the ignition switch to the ON position ONLY so as the dash lights come on. Do NOT attempt to start the engine.

5. As soon as the ignition key is turned to the ON position, The person at the gas tank should hear the fuel pump run for TWO SECONDS and then it should STOP.

6. Does it ?.


If you do NOT hear it run, in most cases the fault is the fuel pump relay or the fuel pump..




As soon as you turn ON the ignition switch to start the engine a 2 second ONLY battery voltage signal is sent from the computer to the fuel pump relay and from the relay back to the fuel pump in the fuel tank. This runs the fuel pump for ONLY 2 seconds of which pressurizes the fuel system in preparation to start the engine.

Once the engine starts, The computer BYPASSES the fuel pump relay and  sends constant contionous battery voltage from the computer to the oil pressure switch of which is now CLOSED because the oil pressure is UP because the engine is running. Therefore the battery voltage flows thru the oil pressure switch back to the fuel pump to run the fuel pump constantly until you turn OFF the engine.