Fuel pump not powered after oil pressure sending unit closes

Hi Joe, I found a thread online about the confusing matter of what powers the fuel pump in these cavaliers. I've copied the obd2 related info and I'll paste it here. The reason being is I've got the car to stay running by putting 12v to that third pin in the oil pressure sending unit (darn Grey wire again. Circuit 120) I'm going to pick up a new op sending unit tomorrow and see if this is the final fix. I'll paste that description now.

On later models utilizing the OBD2 diagnostics systems). Now at this point it seems that the ECM powers up the fuel pump relay and then oil pressure opens the OP switch and both power sources run at the same time powering up circuit 120 through the fuel pump 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 fuel pump Input. That is where the ECM monitors 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 fuel pump relay and the OP switch at the same time? It can't. This is how it works. The ECM powers up the fuel pump 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 cuts off fuel pump relay request at pin# F6 and disables the fuel pump relay. At that point all power flows through the OP switch to fuel pump allowing pin# B12 to monitor oil pressure and not fuel pump relay voltage. So, you should be able to see now that if the OP switch failed the engine would shut off 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 similarly to Ford's fuel inertia switch system. It is a safety issue to prevent fire after a collision.

Well Joe, long story short, found a chafed wire at the oil pressure sending unit wire harness just after the wire loom ended. It looks like it shorted out on the block. Repaired wire and immediately the theft light illuminated, and communications returned through the dlc to my scan tool. (No codes present, no MIL light on except during the bulb check) engine fired right up. Smooth as silk. It'll take me about 2 hours to re-wrap wiring harnesses and put it back together but that will give my niece time to come pick it up. Once again thank you sir, your knowledge and patience was a true blessing. If you can send me some info on how I can provide support to this web site, I'll gladly do my best to help ya'll.

Just a little history on the GN. My father bought it new in Mar of 1986 from a Chevy dealer in Springfield. MO. For my mother. Aug of '88 I was in Florida working Civil Service for the Navy when talking on the phone to him one evening he mentioned that they were planning on trading it for a new van. I couldn't let that happen. A week later I was on a flight back to S.W. Missouri to pick up my GN. It had 38,000 miles on it. As of this evening it has 61,230. Other than normal stuff. Pads, belts, hoses etc, the only work I've done to it is a new flow master exhaust system and pulled the turbo 2 years ago and sent it off to Arizona to have it rebuilt. Seals dried out. Still have the original window sticker and sales agreement from when my folks bought it new. I've always liked the '87 GNX but I wouldn't trade this for two of them!

Joe, just wanted to let you know that the Cavalier is running good still. I just talked to my niece this evening and she said it hasn't missed a beat. She asked me to pass on her gratitude and thanks to you. That goes for me too. Dave