I have a 2003 B2300 and in the past I have had a few times where I would drive the car, go in somewhere, then come out and it would start and immediately die. I verified it was a fuel problem by spraying carb cleaner in the throttle body and it would run for a second or two. I got an inertia switch and a relay and kept them in the car. I have never been able to listen to see if the fuel pump was running because it has always been in an area that isn't quiet. However one day I replaced the relay and later replaced the inertia switch and it stopped. However tonight it happened again. This time I had a multi-meter and a spare relay that has wire soldered to it for testing (it was for debugging a fuel pump problem on my dad's 4.0L Ranger).

Referring to the attached image showing the electrical diagram for the fuel pump system,

Pin 1: Voltage supplied in Start/Run position
Pin 2: Fuel pump relay, control. This is grounded (turning on the relay) by the ECU to enable the fuel pump
Pin 3: Battery voltage supplied at all times
Pin 5: Voltage output to the fuel pump

I was able to verify that pin 1 of the relay is hot when the key is in the run position and that pin 3 of the relay is hot at all times. However when I first turn the key on, or when I try to start it, pin 2 of the relay does not get pulled low by the ECU. It stays at the same voltage as pin 1 which indicates no current is flowing in the pickup coil.

A few things I am not clear about:

1. It appears that the ECU is not turning on the relay (pin 80 of the ECU should ground pin 2 of the relay to turn on the relay). Is there any sensor input that would tell the ECU to not turn on the fuel pump relay?
2. What is pin 40 (VPWR) of the ECU (see the attached wiring diagram)? It appears that output of the fuel pump relay (and input to the inertia switch) is connected to this pin. Is it an input to the ECU that tells it that the fuel pump is getting power?

If there is no input to the ECU that would tell it not to turn on the fuel pump, then I assume this means either a bad wire/connection, or a bad ECU.


Fuel Pump Relay (full)

Fuel Pump Relay (zoomed)

 

The below is ASSUMING there is no rapid flashing THEFT light on the dash with key on, that means PATS is activated so no fuel delivery

For safety reasons the ECU(PCM) only grounds the Fuel pump relay, pin 2, for 2 seconds if RPMs are 0, i.e. key on engine off
This is in case of an accident and a fuel line is broken, engine would stall, no fuel, and fuel pump would shut off at 0rpms automatically, inertia switch is there for the same reason but for impacts or roll overs

So if you are going to test the Ground on pin 2 then you need to be quick about it, put a test light between slots 1 and 2(no relay), it should light up for 2 seconds EACH TIME key is turned from OFF to ON, so repeat key off and then on as much as needed


ECU pin 40 VPWR is the monitor circuit for fuel pump power
When the ECU grounds fuel pump relay it expects to see pin 40 VPWR show 12volts, that confirms relay closed, if it doesn't see that then you get code P0231 Fuel Pump Secondary Circuit Low
Yes, fuel pump power is "secondary" lol

When you turn on the key fuel pump relay pin 1 gets 12v and as you said that passes thru the relay coil and to pin 80 on ECU, computer "sees" that 12v, if not then you get code P0230 Fuel Pump Primary Circuit Malfunction
Yes, the relay coil is the "primary" circuit :)

And just FYI if you test voltage at inertia switch or relay pin 5 after the first 2 seconds with key on, you should see 5-8volts, no AMPs just volts, that comes from the pin 40 VPWR monitor circuit
That voltage can be confusing so just a heads up, no amps so can't power fuel pump, its just voltage from the circuit inside the ECU


PATS(passive anti-theft system) will disable fuel pump relay ground(pin 80) AND fuel injector grounds, so no fuel system, but it should also disable starter relay, so no crank
THEFT light on the dash should come on with key on, and then go off, PATS disabled
Fast flashing THEFT light after key on means no fuel system, PATS active
If your THEFT light bulb is burned out you could be chasing your tail, there is no work around for PATS, its actually a very good system to disable a vehicles engine

 

Thanks for the detailed information!

I didn't have clips to hold my multi-meter probes when I was trying to measure the voltage at the inertia switch. So I had one test lead on the inertia switch connector and then I would reach over and turn the key on and quickly grab the other lead and stick it on a ground. By the time that happened, the battery voltage was off and it was back to the 5-8V you mentioned. Also, there appears to be a mistake in the service manual which may have been why I was thinking that the ECU was not grounding one side of the coil to turn on the relay. What I posted above shows pin 1 as the hot (when key on) and pin 2 as the control line that the ECU grounds to turn on the relay. But the relay socket pin-out shows pin 1 is the control and pin 2 is the hot (when key on). So I think the relay socket pin-out diagram is the correct one. I rolled the window down and was able to reach through and turn on the key while having a finger on the relay and I could feel it click. It seems to only stay on 1 second before it clicks back off. I always thought it was more like 2 seconds. But anyway, I was able to get some clip leads for the meter and found that there is battery voltage supplied to the inertia switch connector while cranking the engine. Not only that, but I measured the impedance on the inertia switch connector pin that goes to the fuel pump and I would expect a few ohms for the fuel pump and it was several k ohms. So I think my problem may be as simple as a bad fuel pump.

However there is one thing that is confusing me. I had a wire in the truck when it initially broke down. I tried to jump pins 3 and 5 on the relay socket in order to turn the fuel pump on and it got very hot very quickly. Yes the wire was undersized, but obviously something was drawing current on that lead. I then came home and made a more appropriately sized jumper wire and now when I install this wire, it does put 12V on the inertia switch connector, but there is no current flow like there was before. I know I was jumping the correct positions because I can see the dark marks on the relay socket from the wire getting so hot. I can't think of anything that might explain this.

 

There should be NO 12volt at inertia switch while cranking engine, Computer only does the 2 second thing with key on because of 0rpms, but also will not activate fuel pump relay until RPMs are above 400, cranking speed is 200rpms

Relay pins 1 and 2 are interchangeable electrically, its just a coil of wire, but for testing purposes its good to know which is which
Its not really a mistake in the Shop manual, because it doesn't matter which is which except for testing
All relays are this way for 1/2 or 85/86, there is no polarity, as long as one is 12v and the other 0v(ground) then relay will activate

This is the universal numbering for micro relay: http://www.gtsparkplugs.com/images/m...lay-wiring.jpg

Car makers often use this numbering: https://www.12voltplanet.co.uk/user/...pin-layout.jpg
And some use BOTH, lol, in different diagrams

If fuel pump was failing then it will draw more AMPs causing wires to heat up, and when it fails completely no amps, so no wire heat
When you jump the relay pins 3 and 5 can you hear the pump running?
Its not quiet

 

I guess that is true (that there should be no 12 volt at the inertia switch while cranking the engine because it is below 400 RPMs), but I could have sworn it did. But I was doing so many things I could be mistaken. I will check again.

And of course you are right about the coil polarity not mattering. It mattered to me when I needed to know which one was a fuse protected hot wire and which one was switched to ground to energize the coil and close the contact, but of course that is easily determined by just pulling the coil out and measuring the voltage.

I have seen that universal numbering before (30, 87, 87a, etc), but I can never remember which number is which without looking at the relay diagram, lol

That is good to know. So maybe the fuel pump was failing and that is why that small jumper wire got hot. It definitely is pulling no current and making no sound, even when 30 & 87 (3 & 5) are connected. And since I have verified that when they are connected the voltage is making it to the inertia switch and the inertia switch is 0.2 ohms, I think it is safe to assume the fuel pump is dead. When I am in my garage or driveway I can really hear the fuel pump. But I was in a parking lot near a busy highway so it was difficult to tell if it was making no sound or if I just couldn't hear it. But before I had it pulled home, I went back down there at night and confirmed it was making no sound.

Thanks for your excellent (as always) comments.