My 1999 B2500 just quit running and after a lot testing I found that the signal wire to the fuel pump relay is not getting a signal. I checked for broken wires and all are OK. I also checked wire number 40 from the relay to the ECM and it shows that it is OK. I also checked the fuel inertia cutoff switch and there is no power getting to it. Please note that this truck uses a regular key so there isn't a chip in the key. I have a complete wiring diagram book and have been going through it trying to figure out the issue but so far no good. The only symptom of any fuel system issues occurred about a week before it died. When I filled the tank up the fuel gauge read slow for quite a long time and then finally read Full. Other than that this issue it runs like a top (motor has been completely rebuilt and has about 10K miles on it). I am at a loss on this one. Any help will be greatly appreciated.. And if anyone can explain the electrical flow from the key to the ECM and relays that will be greatly appreciated.. Mazda's book leaves alot to be desired in this arena. Thanks in adavnce!

 

Welcome to the forum

The PCM(ECM) Grounds the fuel pump relay to activate it, Pin 80 on the PCM is the wire that does this, not 40

Pin 40 is the monitor wire the PCM uses to see if fuel pump relay is working, if PCM is powered up this wire should show 5-8volts, FROM PCM, not the relay

Pull out the fuel pump relay to test its slots
KEY OFF
Test each slot for 12volts
One slot will show 12v from the fuel pump fuse, fuse #9 in engine fuse box

Turn key on
There should now be another slot with 12v, this is from the PCM Relay, it powers the FP relay's coil

Diagram of fuse slots here: http://www.gtsparkplugs.com/images/m...lay-wiring.jpg

In 1999 I think Ford used the Micro-relays
Slots 30 and 87 are use for fuel pump power<<<<<send power to fuel pump
Slots 85 and 86 are used for relay coil power<<<<<closes relay when it has 12v AND Ground

So either 30 or 87 slot should have 12volts KEY OFF, the other slot will run to inertia switch
And either 85 or 86 should have 12volts with key on, the other runs to pin 80 on the PCM


The PCM only Grounds pin 80 for 2 seconds, with key on, its a safety thing, so you need to have someone turn key off and on a few times to see if slot 85 or 86 is a ground for those 2 seconds
Once engine RPMs are above 400, PCM Grounds relay(pin 80) full time

 

Thank you very much ! I have been on this forum countless times and read through many of the posts. Its great to see so many knowledgeable folks such as yourself who step in to help out when another user has issues. Such as myself :)

Your detailed breakdown helped me isolate the issue rather quickly. I was on the right track but didn't fully understand how the circuitry worked. Your explanation is exactly what I needed to know. Thank you very much !

Upon checking the wires further and using the information you provided I found that there were a few dead shorts in the wires going to the PCM. Upon inspection of the wiring harness I found that the plastic clip that supports the wiring harness (at the firewall directly behind the head) had broken and let the wiring harness drop down onto the EGR tube which melted the wiring harness cover and some of the wire jackets which shorted them out. I fixed these wires, temporarily strapped the wiring harness up, and retested the wires and then found that the PCM fuse had blown (due to the shorting of the wires). I replaced the fuse and everything worked again and the truck started up immediately (just as it always did). Thank you very much !!!

 

Good work, not an easy fix

Thanks for posting what worked and where the problem was found