![]() ![]() The ecm powers up the fp relay providing power to the fuel pump to start the engine. 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. 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. Now notice the wire attached to that pin is a gray wire of circuit 120 (the very same circuit that powers the fuel pump). That is where the ECM monitors for adequate oil pressure. Notice it is labeled Oil Pressure Input and not FP Input. 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 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). When the sensor detects oil pressure it changes the resistance which is monitored by the guage or light on one side of the switch. Notice the location of pin A that leads to the Inst Cluster. Notice that it is similair to a relay/crossed with a variable resistor. Now lets look at the Oil Pressure switch. That is the circuit the ECM uses to energize the relay to close the switch to power the fuel pump. Now notice circuit 465 going from the request terminal of the FP Relay to ECM pin# F6 Labeled FP Relay Control. Just as you had stated they are wired in parrallel with eachother utilizing the same 12v power source as well does the ECM. ![]() First look at the relationship of the fuel Pump Relay and the Oil Pressure switch. 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. Now we can use their own wiring diagram to prove that to be total nonsense. 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. Which serves my purpose well so you can see it too. To my amazement the diagrams are exactly the same. We then went to my shop and got on my AllData Online computer to compare the data. ![]() We found a wiring diagram of the Fuel Injection and Fuel Pump Control System that illustrated our point. 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. He asked me for proof and I think I did a pretty fair job of providing it. He also claimed that the switch is not monitored by the ECM in anyway. Thank you and here is my previous posting.I posted this in reply to an AF Advisor who insists that the OP switch is only along for the ride and only serves as a backup fuel pump power source in the event of a fp relay failure. Please review the information yourself and please let me know rather or not you find it as cut and dried obvious evidence supporting the fact that Oil Pressure switch does serve as a fuel cutoff. I personally know that the OP switch does infact serve as a safety fuel shut off and to prove my point I made the following posting in another thread. The vehicle in question here is a 94 C-1500 with a 5.7L TBI Vin code 'K' engine. ![]()
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