1999 F150 4.6L 4x4 extended cab 4r70w trans

I have the familiar cranks but won't start syndrome ...
let me start by saying this truck has many electrical issues ..
theft circuit has already been cobbled and bypassed with remote starter ...
there are wires hanging everywhere ... steering column half torn apart ...
I have inherited this project because the idiot who did this has skipped town
and left my sister with this tin can full of splice and dice.

so not knowing really where to begin I figure it all starts with the keyswitch fuses and relays .
there are no blown fuses and relays all function by bench testing. ..

now the 5 amp PCM fuse at location 20 on the inside fuse panel is not blown ..
but there is no power going to that fuse in any of the 'off' , 'on' or 'run' positions .

other symptoms are as follows :

cranks but will not start
fuel pump will not run unless I jump pins 30 and 87 @ pump relay
but even with  those terminals jumped and the pump running
it still does not start
pin 86 is hot with key in run position .
PCM does not seem to be supplying the ground connection to pin 85 .
my test light at 85 flashes once for about a 10th of a second
when I move the key switch from on to run
but the relay immediately reverts back to its normally open position . ..
I have tried four or five relays that all bench test just fine and they all do the same thing ..
A lightning quick flash and that's it

sorry for the long-winded diatribe ..
but there's so much wrong I don't know where to begin.
I have searched and read a bunch of threads on the subject ..
but what and when the PCM does what it does ..
is interdependent on so many different variables that I can't quite wrap my head around it .
then add the fact that there is some kind of aftermarket remote starter ..
that would hypothetically have to initially override the theft system ..
not to mention I've never heard of this system with the computer chip in the key thing ..
and I don't know if it's stopping any thieves .. but it sure seems to play hell with the owners. .. HA !!

but anyway there's no power to that 5 amp fuse so i guess let's start there ..

any help , irrelevant banter ..
or even verbal abuse will be more than welcome .


1999 F150 4.6L 4x4 extended cab 4r70w trans

 

CJB F20 should be HOT ONLY IN START. Probably not your issue as that circuit is not critical to PCM operation or engine starting.

Also, the 99 was equipped with the PATS system which will prevent injector operation if the PATS system is triggered (fast-flashing THEFT light).

System start-up, how it works, condensed edition:

When the key switch is turned from OFF to RUN, power from fuse F111 <edit - corrected> in the BJB goes through the switch, CJB Fuse F30, the PCM POWER DIODE, and energizes the PCM POWER RELAY.

When the relay energizes, power from BJB Fuse F02 is routed through the relay to three fuses in the BJB, F18, F23, &F24. F18 is the one that supplies actual operating power to the PCM so it will boot up (old computer term).

During the PCM's power up cycle, it energizes the fuel pump for 2-3 seconds by grounding pin 85 of the FP relay which energizes the relay, routing power from BJB fuse F10 to the fuel pump circuit (inertia switch, pump).

A good indication of whether or not the PCM has powered up is to watch the MIL. It should come on when the switch is turned from OFF to RUN (not START). If it doesn't come on, then either the PCM isn't getting power, the +5Vref power is shorted (usually to ground), or the PCM is defective.

That will give you a good idea of what needs to be checked and in what order.

 

@ projectSHO89

absolutely awesome post ..
of all the posts and threads i have read on the subject over the past two or three days
this is by far the most accurate and concise explanation of the starting/fuel system I've come across ..
the Mods need to make that a sticky ..
thank you very, very much .

just a few quick questions before I go apply the knowledge you have given me to do some troubleshooting ..

When the key switch is turned from OFF to RUN, power from fuse 110 in the BJB is goes through the switch
my BJB 110 as for power windows ..
so I'm assuming you meant 111 which is the 50 amp ignition switch fuse

then either the PCM isn't getting power, the +5Vref power is shorted (usually to ground)
what do you mean by the +5Vref .. where / how .. do I check that.

and with reference to the PATS ... he left the dashboard cluster uninstalled ..
allegedly waiting for a shifter cable that he never ordered or installed
I will install the dash cluster and check this out ...
but I intensely hope that it's not the PATS combined with an aftermarket remote starter causing the issue .. what a troubleshooting mess that will entail ... God only knows where that wiring even is or how it will come into play ..

and once again , thanks a lot
cheers n beers

 

It's a tough situation to inherit.
You need to understand the operating systems, be able to read schematics, test with volt/ohm meter functions and logic things out.
With the PATS system, the dash has to be hooked up to work or the PCM inhibits the fuel for starting.
With your situation there is no telling how many issues there are so it take point by point testing to get all circuits back into operation.
One thing for sure, the theft system is working on this truck.
Good luck on it.

 

The +5VRef is a regulated +5V power supply generated inside the PCM. It's used by several of the engine sensors, most notable the TPS which is where you can usually measure it. When it gets shorted to ground, it can prevent the PCM from operating correctly.

Yes, you are correct about the large fuse, it should have been F111. Misread the diagram. I'll change it in the earlier post.

Yeah, it's always a mess taking over someone elses half-finished project. It's even worse if they didn't know what they were doing.

 

HA !! so true
the only way anyone is going to steal this truck is with a wrecker.

 

LMAO ... it would appear that he had absolutely no idea what he was doing.

do you have (or know where I can get) some kind of diagram / schematic , that represents all this ...
I have bought the Haynes manual and the truck still had the owners manual in the glove box .. but they don't seem to contain any schematics that show or explain the specific sequences that you were talking about .

 

well , believe it or not.. it's running .
and the main culprit was the uninstalled – dashboard unit .
the other part of it was a weak battery in the remote starter switch.

for some reason when I reinstalled the dashboard unit all the idiot lights were on without even inserting the key .
apparently the last time someone tried to start the truck with the remote it failed to start .. but they never shut it back off .
I guess this left the dash unit and the computer in the on position .

so i went and bought a new battery for the remote starter ..
and used the remote start button to shut electronics off .

now it starts with the key .
I guess I was rollers mode starter buttons switch into the glove box and tell her not to mess with it ... and to just use the damn key ... at least for right now until I can get someone who knows anything about remote starter systems to have a look at it ...
The weather is breaking up here in snow country and she doesn't need to preheat the truck

and thanks again for all the help guys

 

!!!!important side note... if cluster is not installed antI theft is activated and truck will not start! Also ,ecm and anti theft and cluster all need to be paired (initialized)using ford specific software which can only be rented or leased from ford. But that's only necessary usually after one of those components is replaced

 

If a few things are wrong at same time, you should check wires and grounds. Just visually check first. https://www.ford-trucks.com/forums/1...problem-2.html

 

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