Thought i had a bad PCM, nope.....bad GEM, nope....replaced all fuses/relays, still crank/no start. Have spark, fuel pump runs if jumpered at the relay terminals.

checking the flow of voltage with key “on”, from fuse 111....thru the PCM diode, etc. everything has power and when I get to fuse 18, both terminals have 11.8v, with the fuse out. This tells me that there is a short, without digging deep into the wiring does anyone know where the issue might be? Where these two wires are routed?

1999 F150 4wd auto short bed 4.6l Triton

 

What is the history of this truck and what led you to this point?

 

Your owners manual tells you Fuse 18 feeds the fuel pump relay, the MAF sensor, the fuel injectors and the PCM. The IAC valve is also fed from a splice for the fuel injectors.

 

Indeed it does. However, in order to see battery voltage on BOTH socket terminals of a removed fuse means *something* is very wrong. Repair and known history, if any, is probably going to be useful to figure out what is going on.

 

The information on this forum has been amazingly educational so far, I was about to give up but I read a bunch of posts on here, grabbed my meter and started digging.

The gauges on the truck were going nuts, odometer would go blank then come back on after a few minutes. My daughter was driving it and all of the gauges went nuts again, all dash lights , MIL, everything came on then went dark. She made it to her friends house, turned it off, then it had a crank/no start condition. THEFT light flashes, but I get no check engine light when I turn the key to “run”, and connecting my scan tool to the OBD port I get a “cannot link” error. I also have no voltage at the 2 pins on the OBD that are supposed to have power. (I have it written down out in the truck).

I replaced every fuse and relay, I have spark at the plug wire ends, fuel pump runs when jumpered at the terminals.

I started measuring voltage at all of the fuses that are supposed to get voltage with key to “run”....got to fuse 18 in the BJB, and have 11.8v at both terminals with the fuse REMOVED....I read that fuse is for PCM, fuse pump, and mass air flow sensor I believe. Trying to track the wires, to see which one goes where, right now it appears that they both go into the big harnesses that go into the cab behind the fuse box.

I keep doing more reading on this every day....does anyone know if a bad connection behind the gauge cluster can cause a no-start condition? Resulting in the odometer being inop

 

Wondering if the PCM diode is bad, but it wouldn’t be clicking when key turns to run, if it was bad?

 

If the diode were bad (open), the PCM relay would not energize and BJB fuses F18, F23, and F24 would not be HOT IN RUN AND START. The diode's inclusion in the circuit is for insurance purposes only. It prevents the PCM power relay from energizing in the event the battery terminals are reversed thus protecting the PCM from catastrophic failure in the event of that mistake.
Short answer, yes, that is among the possibilities. Both the PCM and the cluster must power up together and establish two-way communication with each other in order for PATS to prove out successfully and allow the engine to be operated. If the cluster does not work, neither does the truck.

An unsuccessful PATS attempt on your era of truck will result in 1) a fast-flashing THEFT lamp and 2) an engine that will not operate because the fuel injectors have been disabled by software.

Define "inop". Does that mean completely blank or does it display all dashes? The latter is the indication that the cluster and PCM did not establish 2-way communication during the defined timing window.

Observations and recommendations:

Note: Ensure battery is fully charged before making voltage measurements. Low system power will cause unusual operation or failures.

1. The initial premise regarding BJB F18 is probably a red herring. There are plausible reasons for those readings that I now recognized given additional context.
2. Most likely, the issue is due to instrument cluster malfunction. If it doesn't work, the truck is a lawn ornament.
3. Pull the kick panels on both sides of the cabin and inspect your grounds for loose, corroded connections or body rot.
4. Verify that the following fuses are HOT when indicated.
A. CJB fuse F02 ALWAYS HOT.
B. CJB fuse F30 HOT IN START & RUN.
C. CJB fuse F06 HOT in RUN.
5. Post your specific results if further analysis is needed.