hello,
my boy just picked up a 91 f150 5.0L and he was sitting in town truck died and would not restart. towed it out to our place, he said distributor was bad, i said no way, guess the kid he got it from insisted bad dizzy partly why he sold it, started checking, key on fuel pump runs 1 or 2 sec and quits, fuel on rail - push shroeder valve, fuel sprays out - haven't tested pressure yet, grabbed haynes manuel had him start running though test procedures, found coil primary to resisance to be not steady and constantly varing with coil on bench from 0 to way up over sev 1,000 ohms just sitting there, so i grabbed a new coil, he put that on, nothing, no start, no spark at coil hi tensile lead or plug 1 lead, so put light on coil primary for crank/flash test, nothing, dead, so pulled primary connector try for voltage on plus side, came in at 6 volts battery at 12.3 vdc, so i figure short in coil primary wires, too darn cold to cut the harness open right now (North Dakota and -10 and wind this weekend) so i grab jumper wire and clip into coil primary + and go to batt plus, he jumps in starts right up and runs fine! yeah. can't very well run to school with redneck jumper wire, so start looking around eec and fuel pump relays laying on fender well, not in box like they should be, test connector laying on fender well not where it should be, question is, do you think wires could be problem or eec relay shot, one, don't know if it was fuel pump or eec had case off of it even and looked bad, should i just replace both relays or cut into the harness to see if there is a short in there? thanks guys.
maybe i can just bypass that little bit of the harness if i could figure out right pin to tap into for the coil plus to relay? is it? your thoughts? you guys are top notch, sorry for long post.
micheal

 

The EEC Power Relay & the Fuel Pump relay have nothing to do with spark.
Sounds more like a bad Ignition Switch.





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thanks much, any way i can test for a bad switch before i rip into steering column to replace it? what else should i look for, or is it not too bad to put a switch in and give it a whirl, don't think i have had a switch out of one of these, at least not for quite some time.

thanks much.
Micheal

 

You do not take the column apart to change the switch. You may have to drop it a little.
The switch is mounted on top of the column down near the floor board.
Note that the plug may be bad also that plugs into the switch. A bad contact pin in the plug can have a voltage drop also.

 

wonderful. thanks very much, we will give it a look as soon as it warms up a bit. very cold up here right now. thanks much.
micheal

 

little update, got a couple things done but still no spark. replaced both fuel pump and EEC relays under the hood, they were both well abused by someone in the past, laying on fenderwell and open, cap actually lost and coil open and exposed, so that was just cheap insurance, took woping 2 sec to plug in new relays, PO didn't know much about cars though. my boy put in the new ignition switch last nite, old one tested bad on the bench after we had it out, but.... testing coil primary still was not good. Key in Run, only had 6.8 V to coil + cranking, only had about 2 V to coil +, so obviously, no spark coming out of new coil yet. what do you think, fusible link for the EEC relay, or worse, PCM computer? going to check fusible links tonight, guess they are all over by the battery? can i just tug a bit on them to find a bad one? truck runs when we jumper from coil + to batt + and runs good, just can't quite get enough volts in the primary of the coil to fire the plugs. thanks for your help. what should we investigate next?
micheal

 

As noted above the EEC Power relay and the Computer have nothing to do with power to the coil. So the fuse links to the EEC power relay has nothing to do with the power to the coil problem.

Sounds like you may be using a meter of some sort. Try getting a test light with a bulb in it for the power test and a meter will lie to you in some circuits.
I think you will find you do not have power to the coil in the run or start positions.

Does the headlights work OK?
How about the radio?

If you have a bad fuse link it would be fuse link "P" or "M" (the wiring diagram can not figure it out).

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i didn't even ask him if the radio / lights etc worked, i know the cab light and dash guages move when he turns key on, i will check more carefully tonight. so, in finding these fusible links, he can see the yellow wires with the ring terms hooking up to the solinoid, but from this last diagram, it appears each yellow supplies 3 fusible links then, is that accurate? looks like have to split some loom open to actually get to them then? does that seem true? replace with fusible link again or can one put a fuse in? i am guessing not, otherwise they would probably be in there in the first place. thank you so much for the help and diagrams etc.
i appreciate it. yes i am using a DMM, but will probe it with a test light tonight also.
thanks again.
micheal

 

Yes it looks like you have two yellow wires feeding three fuse links each.


Here is 12-3 that was referred to in the above diagram.


Here is the location of the fuse links.

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alright another update. Thank you Subford for all your help thus far. my boy got the harness opened so we could test the fusible links. found one (it was a single yellow from solenoid to yellow in the harness that didn't show any continuity, so he cut that one out and we will replace it tonight, but that wasn't for the EEC or coil power, from EEC relay yellow wires to rings that go on the solenoid studs shows continuity, so i would guess that means fusible links are still ok, darn it, now what to next? we thought probably unplug the PCM plug in the engine compartment and see if it starts with that unplugged, does that sound like a good next step? He opened the coil wire loom back to where it goes into the bigger bunch and so far the wires look good, no cracks or broken insulation. could this be pointing to a bad computer, or could it still be the dizzy with a bad PIP pickup or something?
thank you very much, all. this is good for him - he is 17 and usually doesn't have much patience, so this drawn out troubleshooting is hopefully teaching him something as long as we can make it run finally in the end.

 

It will not start with the computer unplugged as the injectors will not fire (they are grounded by the computer to fire). Unless of course if you stray starting fluid into the air intake and then it will try to run with the computer unplugged.

You must get power to the + side of the Ignition coil before you will have spark. The EEC Power relay and Computer have nothing to do with this power getting to the coil.
The power leaves the starter relay on the passenger fender, goes through a fuse link to the ignition switch on top of the column down near the floorboard and then to the ignition coil + POS side.

Note it is not shown on the 1990+ wiring diagrams but the 1989 truck did have a fuse link on the drivers side between the ignition switch and the ignition coil. I think it was dropped before your year but maybe not from what I am hearing from you.

To test a fuse link you just try to pull it apart and if the fuse link stretches it is bad.

 

thanks for the input again. how about problem in that round connector on the firewall? what about broken / compromised wire on that connector? wouldn't happen to have pin out for that would you? maybe got a bad pin in there or something???? the ignition system seems so simple except when some little issue like this surfaces. thank you again. we will keep after it until found.
any idea where the '89 fusible link on the left fender would be? never know, but i wouldn't be surprised to find one over there.

thansk much.
micheal

 

Yes there could be a problem with that big round plug.

No body as far as I have seen has a pin-out of the that big round plug.
Here is a photo of it if it will do you any good.
But I do not know what wire goes with what pin number.


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when my boy replaced the ignition switch i had him put it back in the same spot as the old one, could that be mis adjusted slightly causing no power to the coil? what is the procedure to see if it is in the correct spot? thanks again! man that is sure a lot of wires in that plug isnt it! dang.

thansk very much.

 

By the way the wire going through that big round plug for the Ignition Coil is a red wire with a green stripe. But I guess you already knew that.