'96 5.0 F150 Won't Start


#1

My daughter was out behind the wheel with the wife driving for the first time after she picked up her permit just a few days before. She bumped a curb and the truck died. It is not the inertia switch and it is getting fuel. There is however no spark. I checked the rotor, cap, wires, TFI and coil all OK. The scan code reader attempts to read codes and comes back to say “failed”. There is a blown fuse in location 22 the owners manual says it is for distributer pickup, ignition coil, Powertrain control system relay coil and TFI module. I replaced the fuse and it blew again. I am going to replace the relay and try another fuse any other ideas? My daughter is getting desperate to drive again and the nagging is going to get so bad I may have to resort to actually paying someone else to fix the truck. Help!


#2

First off, you should not continue to replace the fuse and guess at a relay. I’m assuming here the fuse is blowing instantly when the key is turned on? If so, this means a direct short.

Here’s a link to a wiring schematic. Figure 24 if I remember correctly. Not much of a schematic but better than a poke in the eye with a sharp stick.
http://www.autozone.com/az/cds/en_us/0900823d/80/0a/3a/41/0900823d800a3a41/repairInfoPages.htm

What I would recommend in this case is disconnecting the ignition module plug, the PCM relay, the ignition coil, and the noise capacitor. Note the red/green wire goes to all of those items.
Replace the fuse and then start connecting those items again one at a time, checking the fuse after each item is connected. This may at least determine which circuit the problem is in.

If the fuse blows instantly with all of these items disconnected this means there is a direct short in the wire harness between Fuse 22 and any one of those items on the red/green wire circuit.

Doing this over the net can be a brain drain so you might try some of that and see what happens. Continue to post back with more info and we’ll try to wade through it. :slight_smile:


#3

ok4450 has good suggestions. Going out on a limb… the fact that the code reader is failing to communicate with the PCM and the fact that this occurred after a big bump makes me think bad PCM.


#4

I think ok4450 & mr Josh have this nailed, I seem to recall something similar I heard about 18 months ago, same scenario truck got bounced and quit. I heard later that the shock bounced the PCM relay contacts causing internal damage. If my memory serves me correctly the contact bounce also took out the PCM Diode.

For wiring diagrams - http://arrc.epnet.com/autoasp/framerq.asp

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#5

While you’re looking around, check things in the distributor too. If you have a vacuum advance unit, which you may not have, the wires connected to the ignition pickup will get frayed from moving back and forth. Bumping should not effect frayed wires enough to matter, but who knows what is wrong for certain. The other posts seem to show experience in this stuff.


#6

I did try disconnecting everything except the PCM made no difference. I was leaning toward the PCM but would like to get some other ideas before I replace a $150 part not being sure if it is bad for certain.


#7

Is there a test for the diode? Do I just use an ohm meter? Should it have a different reading depending upon the polarity

Thanks Guys


#8

If you’re testing a diode, you should have continuity in one direction, and infinite resistance (open) in the other direction.


#9

Think I have it nailed down. Though this was not a text book method to diagnose the fault. I disconnected all of the items fed off of the fuse. Last new 20 amp fuse was inserted, key on, there goes the fuse. I then disconnected and removed the computer ready to go buy a new one. Before I went I wanted to try one more time so I removed the 30 amp fuse for the trailer lights and turned the key. It did not blow the fuse but when I moved the wire bundle that the distributor feeds off of there was an arc against the front corner of the valve cover not enough to blow the fuse but enough to identify where the short is. So while I did needlessly remove the computer I fortunately did not needlessly buy a new one. I’ll get it all back together tomorrow and see if it starts again.

My daughter was relieved that she did not in fact break the truck but I am not letting her off that easy. It was working when she left the house died while she was driving and it had to be dragged home. Not a good omen for the first time behind the wheel. She’ll hear about this for a while, maybe forever.


#10

Oh comeon, man, she’s uncertain enough about her driving skills as it is, you’re going to throw that in her face, too? :slight_smile:

Good to hear that you were willing to roll up your sleeves and find the problem.


#11

That did it!!! I’m glad that is all it was but I can’t beleive it took me three weeks to figure it out.