Bronco Forum - Full Size Ford Bronco Forum banner
1 - 14 of 14 Posts

·
Registered
Joined
·
4 Posts
Discussion Starter · #1 ·
I was driving on the interstate and boom, my truck dies and I have to get towed home. I notice the fuel pump isn't pumping, so I replace it. Still nothing. Change the fuel pump relay, check the lines, put in a new filter, nada.

I start chasing down the electrical. Im getting 7.5V and 3.52 at the pump. I start working forward and the wires look great. No issues that I can find. I pull the wires and test the power at the inertia switch. 3.52 going in.
My questions include, Shouldn't it be 12v at the inertia switch? What are the chances it is the ECM? What else could it be?

Thanks everyone. I have been looking for answers everywhere and can't find them.
 

·
Registered
Joined
·
5,003 Posts
@WildBillBronco - You could help us a little by telling us what year your Bronco is, as I don't know, I'll take a guess and guess it's a '92-'96. So, here's the schematic of the fuel pump from the inertia switch to the fuel pump. Yes, you should have 12v to the switch all the way to the pump.

And the 12v source schematic:


1996 Ford Bronco '94 EVTM picture | SuperMotors.net

Don't forget to check ground G100

Look for more schematics in my SuperMotors if you need more information, if you don't have a '92-'96 - TELL US WHAT YEAR IT IS!!!
 

·
Registered
Joined
·
5,003 Posts
It's a 94 EB with the 351 and auto.
Maybe I should get a job as a psychic reader! That's exactly the schematic I provided. Let us know what you find. Good Luck.
 
  • Like
Reactions: WildBillBronco

·
Registered
Joined
·
2,492 Posts
I start chasing down the electrical. Im getting 7.5V and 3.52 at the pump. I start working forward and the wires look great. No issues that I can find. I pull the wires and test the power at the inertia switch. 3.52 going in.
My questions include, Shouldn't it be 12v at the inertia switch? What are the chances it is the ECM? What else could it be?
Yes it should be 12 V across your battery terminals, at the inertial switch, and at the pump.
As Mikey350 mentioned, the ground is the other half of the electrical circuit.
To rule out a bad ground, run a ground from the neg battery terminal to your multimeter, and then test for power at the inertia switch. Do you know that you won't have power at the inertia switch when the engine is not running, unless you jumpered the fuel pump relay, or grounded out the fuel pump test pin of the DTC Diagnostic Trouble connector?

167638
 

·
Registered
Joined
·
2,492 Posts
Correction, you should also have power to the inertia switch, while the pump is priming, for a second or two when you turn the ignition switch to ON/Run.
 

·
Super Moderator
Joined
·
29,143 Posts
Yo Bill,
167640

G100 @ driver side front of engine compartment on upper radiator support.
 

·
Registered
Joined
·
2,492 Posts
The ECU doesn't send power to the inertia switch. Therefore, if you have less than 12 volts to the inertia switch, it is not because of the ECU.

With the ignition key OFF, if you back probe the fuel pump relay, ONE of the four pins should have 12 volts, all the time. If not you have a bad/dirty connection/broken wire/ blown fuse. What you get there is what you should get to the inertia switch, (when the fuel relay is energized). IF there is a difference, you have a bad fuel pump relay, or crappy connection between them.
 

·
Registered
Joined
·
2,492 Posts
I suppose that if you get voltage to the inertia switch, when the fuel pump relay is NOT energized, that could happen if there is an internal short in the ECU.
 

·
Registered
Joined
·
5,003 Posts
Pin 8 on the PCM is used to tell the PCM that the fuel pump relay energized when told to energize, and that the relay did indeed send 12v. to (or toward the inertial relay and then to the pump) the pump. The PCM sees 12v. on pin 8, and that tells the PCM that the fuel pump relay energized. If the fuel pump relay does not provide 12v. to the pump, you will get a code 543. Code 543 is the fuel pump monitor circuit.
 
  • Like
Reactions: BroncoJoe19
1 - 14 of 14 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top