NOW I'VE DONE IT...

I'm at my parents old empty house with my kids - they're swimming and I decided to work on my truck - fix an electrical problem where the headlights short out on low beams.

FIXED that problem -

BUT - in all my hacking around - MY TRUCK WON'T START.

It turns over just fine - but no spark at all. had been running like a top, I clearly upset something in the wiring.

It's an 88 f150 - 302 4wd.

I know this is wishful thinking but can anybody help me narrow down which wires I need to be following????

there's a big round zillion pin connector that goes through the firewall - I had that removed to find my headlight wire...

I'm ASSUMING in mucking with that, I lost contact in what ever wire signals "run" to the engine - or that turns the engine off when you turn the key off... maybe not in the connector (I've since tried re-seating it) - maybe in some of the million other connectors in the region...

IS there such a wire that is a "run" signal????

I'm SO far out of my element here it's not even funny - but I've got to get my kids home!!!! Wife's travelling on business - I'm stuck!!!

I looked at the coil hoping to find such a run wire - but there are just too many for me to understand what's going on.

I'll take ANY and ALL suggestions -

I don't even have cash or credit card for a cab!!! There's no food in the house - TOTALLY stuck!

 

Do you have a meter or anything with you? If so my first suggestion is pull the connector off your ignition coil, it should only have two wires, one should be 12v, the other should read a good ground. If you don't at least have that, you'll have no spark.

Also how did you determine no spark? can you hear the fuel pump run when it starts?

 

I am not an expert or anyhing close. With this disclosure, have one of the kids turn over the motor and start pushingmoving some of the wiring. You may get lucky and and have it start/locate the problem. Move some of the wiring you were working around to see if it is rubbing on metal and therefore grounding. Double check the connections you took apart to ensure they are seated correctly. Check the coil by pulling the wire and see if you have a spark. Of course check your fuse box. Good luck

 

Ok - I took another look at the coil - there's a large flat plug with three wires in it. Two of the wires are bonded together.

I pulled that plug - and yes, thank GOD I have a voltmeter with me - No voltage on either the single or double wire.

Turn the key to "run" and I DO get 12 volts on the single wire...

I'm not sure that helps or complicates matters...


I check for spark by pulling a spark plug - grounding it - and having my son crank the engine. No spark that I can tell ( willing to accept I may not have grounded it perfectly - but - I'm pretty sure there's no spark.


With that wire to the distributor powering on in the run position... I'm worried... Shouldn't I be getting a spark? I can't imagine why if the coil is powered, why it wouldn't spark.

But - am I even looking at the right wire???


Again - truck ran FINE until I mucked with the wiring...



UPDATE 2:

To confirm there's no spark - I took one for the team - stuck a screw driver in the plug wire and held the metal part. I confirmed there is in fact no spark at all. Felt nothing.

 

Well if you get 12v to the coil with the key on you're good at that point. Next step is check your TFI ignition module for 12v and a start signal when the truck is cranked. This module will either be on the side of your distributor or remote mounted in a heat sink on the driver side fender. I'll try to scan my ignition system diagrams into my computer real quick to give you the best info possible

 

It is possible that it isn't anything you did. Pull the distributor cap and confirm that the rotor spins when you crank the engine.

 

Ok, so these are for a '93, but it's the best I can do on short notice. Your system should be very similar, so use these as a guide. Hopefully they help,
like I said check your 12v at the TFI module and also the pin labeled "start" in the diagrams when the truck is being cranked.

EDIT: Forgot to mention, idk if you're familiar with these diagrams, but the numbers next to the wires in the first diagram correspond with the numbers pointing to the pins in the connector views in the second diagram, so you know which pin is what wire in the distributor connector and the ign. module connector. But the connectors will be different if your module is on the side of the distributor.


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OK - still nothing - sun's going down fast and I don't have a flashlight...

this )#(#)'s

I check the "start" pin on the tfi - it gets about 9 volts when cranking...

I'm assuming that's good enough.... ???

And with a slap to the forehead - I realized I have my Chilton manual stuffed behind the front seat -

BUT - it does me no good. I followed some sketchy instructions for testing the TFI module (Very unclear) - and now I'm at a dead end.

It told me to stick a needle in the wire with a test light - into the wire that runs from the tfi to the negative side of the coil. Other lead of testlight to ground.

Then crank engine and watch test light.

Well - I did that -

The light is supposed to flash...

Mine just lights very dimly... how fast its' supposed to flash I don't know - is my dim light just fast flashing??? - I don't know.

But according to chiltons - if the light lights and doesn't flash - proceed to the wiring harness test...

well THAT sounds very promising... I do suspect something in my wiring harness, so I'll take my very dim light as "lit" and proceed to test my wiring harness...

but... as is too often my luck - there IS NO WIRING HARNESS TEST IN THE )#W(*$)#(*$ MANUAL....

SOOOO - if a dim light while cranking on the wire from the TFI to the neg coil connector MEANS anything to anybody ---- I'd love to hear about it.

getting dark - I'm so screwed.

 

Pull the coil wire out of the disributor, hold it 1/4" from ground and crank it. You could be losing spark at the rotor or cap.
I've had mixed results with the test light on coil - test. Some test lights have a bulb that the filament glows for a split second when it shuts off and looks like it just dims. It will also dim when the battery voltage drops while cranking, kinda hard to tell the difference.

 

If you look at the diagram I posted, as you can see that wire you attached to your test light is a ground wire that returns to the PCM, it has a 22,000 ohm resistor in series which would limit any voltage on the ground thus limiting return current to the PCM. Instead of checking your voltage with a test light, take your meter disconnect the tfi connector, set the meter to ohms put your neg. lead on the neg. terminal of the battery and probe that coil wire at the tfi, have someone crank, if the meter goes from O.L to 22k ohms, the grounding function of the PCM is working, if not then your computer is not grounding the coil. (AKA not firing the coil)

 

Don't want to sound rude or stupid , but have you checked all your fuses ? And Does this truck have the plug in ECM ground by the battery ? I have had a few Ford cars of this vintage towed in with the main ground plug loose ( T birds were bad for that ) . Just a thought .

 

Ok - did that, and got nothing. Got some numbers jumping around while the key turned... - but when I cranked, it stayed at 0.L

Where IS the pcm just out of curiosity - is it in the cab or the engine compartment?

 

well I finally managed to arrange for a ride.

I don't have a flashlight here - and the mosquitos are INSANE. (swamp nearby).

I'm going home defeated. Not a good feeling!

I'll have to get a ride back and start fresh in the morning.

Thanks for all the help so far guys.

 

Is there an easy way to tell if I've got the3 plug in ECM ground by the battery? What would it look like?

I will check the engine ground as well.

 

Here's where I'm at:

Still no spark.

12v to positive terminal on coil when key in run position.

Test lead on neg side of coil shows steady dim light when cranking engine, no blinking.

Tested line 11 (per the diagram posted) from the neg coil side through the 22k resistor to the pcm - have continuity all the way through between coil, tfi and the PCM - shows 22k resistance. I'm comfortable that circuit is solid.

There is a solid ground connection from the PCM to the chassis.

tested voltage to TFI connector - pins 1-2-3 per Chilton manual - pin one is only 4.5 volts, others are good. Chilton manual basically leaves me hanging there.

Saw an article about testing TFI modules - using an LED between pin 6 and ground - crank engine - see if it blinks. I do not have an LED to test with -so used my digital multimeter. Minimal voltage until cranked, then I DO get lots of steady fluctuation that I would assume equates to an led blinking.

According to that article - that proves my TFI module is bad...

I'm trying to find info on testing it further, but sounds like I've got to yank the distributor to take the tfi off...

Which - keeping in mind this all stated with me fixing a headlight short - really sounds like the wrong road... yet... I'm at a loss.

Could I REALLY have just lost my TFI right NOW after fixing a short???? I'm not a real strong believer in coincidences at times like this - but... I'm also at a dead end.

The truck HAS been a little hard to start when cold starting lately - crank and no start- stop - crank again starts instantly... but other than that it's been running perfectly.

Thoughts?

Any idea what to look for to see why I'd only get 4.5volts on pin one of the TFI??? that's the part that makes me doubt I've got a bad tfi...

REALLY desperate here - Monday's coming fast, and I need my truck!!!!!!