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Fuel pump problem

276 Views 4 Replies 3 Participants Last post by  jimmykicker
So recently, my truck developed this issue where it was stalling out, but would just do it for a second. Sometimes 4 or 5 times in a row. Sometimes not for several minutes. It came to a head the other day when it was really doing it bad. I tapped on the ECM, and it died. I'm thinking great. ECM is cooked. Swapped to another. Same. No start. My fuel pump is external, so I ran one of the wires of a jumper cable set to the pump from the battery. Truck lights right up. So definitely not the pump, and since it runs, I highly doubt the ECM.

Now, I'm no stranger to the wiring of these trucks whatsoever. My truck is an 84, and I installed the EFI system on a carbureted model from a 1991. Below I have the schematic of the ECM and associated components. I've been told that the truck should run with "either" the relay or the fuel pressure switch, as that is redundant so the truck doesn't quit on you. I was under the impression that the oil pressure switch prevents the truck from pumping fuel in case of an accident? But looking at the diagram, it could be redundant. Question here is what is the function of pin A1 and B? They are "fuel pump relay", and "fuel pump feed" respectively? That is the difference here? Also the orange wires that come from ECM B fuse are going to two "12v bat" labels on there. While obviously this is a constant 12v input, what is the function of that 12v in those pins(B1 and C16)?

Feeling pretty sure the problem resides in the ECM B fuse. I'll point out that on my truck that is on a standalone fuse box on the passenger side. My 84 didn't have an ECM B fuse, so that standalone box and the ECM harness is effectively is connected to what passes for the ECM B fuse. Those wires go through a factory harness to their under hood or under dash components. Well, we all know what is right there where that harness exits into the engine compartment. Exhaust. My truck has headers and it gets gates of hell hot there. Even though it was wrapped (poorly), I still think I have a burnt wire. I'm going to pull the whole harness and make sure all is kosher.

But I really need to know what those wires are for if anyone knows. I really want to understand this circuit better because it doesn't make sense. The A1 wire for instance. That is obviously a trigger wire to make the relay kick to on. What makes that wire hot? B2, fuel pump feed, goes directly to the pump and the output wire of the relay? HUH? Is "fuel pump feed" actually sending a 12v signal to the ECM (an input) that tells the ECM that the truck's fuel pump is on? If it is an output, it appears to me the pump would get a signal from that and make the pump relay and oil pressure sensor irrelevant? Not that the wire could handle that current.

I'm glad I have confused you thoroughly. Here's the diagrams.

Thanks for your input!!

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I'm not sure on the real early trucks, but most have the fuel pump relay wired to either ignition or battery and the relay control is grounded by the ECM.
Looking at the diagrams it appears that in 84 they actually provide 12V from the ECM to the relay since the other side of the relays switch circuit is grounded. Guess it doesn't matter which way they do it since that side of the relay is such low amperage.
I image that the board in the ECM completes a path from B1 or C16 to A1 when it is needed. Most likely when the ECM receives a pulse from the pickup in the distributor or input from the oil pressure sender at B2.
Do you have power to and from the ECM B fuse?
GM uses a positive trigger for the fuel pump relay. This is a bit of a safety thing, in the case of a collision and if the fuel pump trigger wire were to be pinched and shorted to ground, this would cause the fuel pump relay to be shut off, instead of being held on. Yes, it's an unlikely scenario, but it's a safety thing. GM has done this for many years, I'm not sure it's ever changed, except for maybe late model returnless systems that use PWM to control the pump, but that's a different control scheme entirely.
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