[CalifOkie]

You know what MT-2500.... I just noticed that you are a AF Advisor. Meaning that you give advice and assistance to alot of people which is very commendable. It is also because of that fact I must take it upon myself to educate you... I called a buddy of mine and we went down to the local dealership (yes, on Saturday night) and we researched the matter in the GM database. We found a wiring diagram of the Fuel Injection and Fuel Pump Control System that illustrated our point. We then went to my shop and got on my AllData Online computer to compare the data. To my amazement the diagrams are exactly the same. Which serves my purpose well so you can see it too. But first let me say that I did check in Alldata under description and operation of the Oil Pressure Switch and it does say that it serves as a backup incase of fuel relay failure... Now we can use their own wiring diagram to prove that to be total nonsense... Please open your allData program and look at this diagram located under Powertrain Management/ Electrical Diagrams/ Fuel Injection and Fuel Pump Control for a 1994 c-1500 w/ 5.7L... First look at the relationship of the fuel Pump Relay and the Oil Pressure switch. Just as you had stated they are wired in parrallel with eachother utilizing the same 12v power source as well does the ECM. Now notice circuit 465 going from the request terminal of the FP Relay to ECM pin# F6 Labeled FP Relay Control. That is the circuit the ECM uses to energize the relay to close the switch to power the fuel pump. Now lets look at the Oil Pressure switch. Notice that it is similair to a relay/crossed with a variable resistor. Notice the location of pin A that leads to the Inst Cluster. When the sensor detects oil pressure it changes the resistance which is monitored by the guage or light on one side of the switch. That half energizes the switching of the other half ( So that switch can fail and store a PO520 and the guage or light still work properly on later models utilizing the OBD2 diagnostics systems). Now at this point it seems that the ECM powers up the fp relay and then oil pressure opens the OP switch and both power sources run at the same time powering up circuit 120 through the fp fuse to the fuel pump. That is where the mistake is made... Now look at the ECM pin# B12. Notice it is labeled Oil Pressure Input and not FP Input. That is where the ECM monitors for adequate oil pressure. Now notice the wire attached to that pin is a gray wire of circuit 120 (the very same circuit that powers the fuel pump). Now if you trace that wire it leads to splice#127 where it monitors the voltage supplied to the fuel pump.Now use some common sense... How can oil pressure be monitored on a wire that is powered up by the fp relay and the OP switch at the sametime? It can't... This is how it works... The ecm powers up the fp relay providing power to the fuel pump to start the engine. Then when oil pressure comes up and the OP switch closes the additional 12V power source causes a spike in the current flowing to the fuel pump that is monitored at pin# B12 of the ECM. The ECM then cutsoff fp relay request at pin# F6 and disables the fp relay. At that point all power flows through the OP switch to fuel pump allowing pin# B12 to monitor oil pressure and not fp relay voltage. So you should be able to see now that if the OP switch failed the engine would die from the fuel pump being shut off. Also if the engine was to suddenly stop ( as in a crash) the fuel pump is immediately killed very similairly to Ford's fuel inertia switch system.. IT IS A SAFETY ISSUE TO PREVENT FIRE AFTER A COLLISION.
So right there is the proof on one of your own wiring diagrams. It further proves that Mitchell and AllData has the information but they only understand about half of it. You can now consider yourself educated with some GM knowledge on this matter anyway.
I am sorry if I appear to be trying to make you look bad, I honestly am not. I simply find it imperative to help you. You do some fine work and help alot of people and I just want to insure that the info you graciously provide is accurate.