1991 F-150 Not Starting, Starter Turning Over

So I have it narrowed down to two possible problems, both of which are unlikely in my opinion. So I turn the truck off to fill up gas. Fill the gas up, get back in. Turn On, try starting. Nothing. Go under hood see my starter relay is busted. Walk to the awesomely close advance auto parts, buy new one. Replace. Starter turns over, but truck doesn’t start. Now, on the starter relay before, one of the connections was missing. There was 4 connections, one for battery, one for starter, one for starter switch, and one that says ‘I’ which i honestly don’t know what its for, I assumed it wasnt needed since the starter relay before didn’t have it connected. Now, unless my truck is just being a bxxxh and wanting more new parts, I think it might be that connection screwing me. It started fine a couple of minutes before i turned it off for gas and only drove maybe a 1/4… it almost sounds like either the alternator is out (haven’t checked to see if spark plugs sparking yet), or that the fuel pump isn’t working which I hope to god isn’t the case because I filled my tank up and I heard the pump is inside the tank. So now I am taking the bus to work until I can get my truck fixed. Any ideas what is it? Could it be that missing terminal on the starter relay?

Edit: not sure if both tanks have seperate pump, but my fuel tank switch is broken so I couldn’t switch it over to see if other tank works anyway.

Ford usually ties the main power bus wire, that supplies power to the truck on the battery side of the solenoid you replaced. Since you are able to get the starter going it seems you reconnected the connections to the solenoid correctly. I assume that other electrical things are working correctly, like the lights and blower. One thing to be aware of is the smaller wires tied to the battery side of the solenoid are most likely fusible links that are designed to blow out in the middle when too much current is passed through them. They can even look okay on the outside of them when they blow out. You don’t seem to have that issue here.

I would ordinarily guess that the “I” wire would stand for ignition but since it wasn’t connected before then that doesn’t seem the case here. You do need to see if the ignition system is working. It would be good to see if power is getting to it. You could also take a spare spark plug with you and plug it into one of the wires and then place it on a good engine ground point and then see if spark is getting to the plugs when you try to start the engine. If you think you have a fuel problem spray a small amount of starter fluid into the intake and see if that helps get the engine going. When you turn on the ignition and listen, you might be able to hear the fuel pump turn on and verify it is getting power at least.

There should only be one fuel pump the selector switch determines which tank is used to supply the pump. As far as the alternator, even if it was bad it wouldn’t prevent the engine from starting. The battery just wouldn’t get charged while the engine was running.

Yes, the small wires connected to the stater solenoid with the battery (+) cable are the fused links to everything electrical in the truck and apparently one of them is left disconnected, broken or burned and they can burn inside the insulation. Flexing the fuse link wire will usually give an indication that the wire inside is burned.

a coworker told me to check the fuel filter as well… which isn’t a bad idea considering i was filling it with gas, and when it was low some gunk might have gunked it up. I will double check the connections and see if they are burned out when I get home, and I think I connected them to the starter side not the battery side, which is how it was before, but the guy that owned the truck before me did some ‘custom’ electrical work which I finally finished tracing all the changes he did and doing it right. He may have changed it around. I was in the process of adding custom airplane-type switches for the headlights and all the wipers/etc to make it look nice inside the cab, and I don’t think that is the problem since I utilized existing wires for most of the stuff.

Both the headlight switch and the wiper switch include a ground wire. Your efforts to install airplane style switches might have resulted in a short to ground which took out fuses and/or fusinble links.

i checked fuses, they are all fine, and the switches I used included a grounding terminal not the cheapy 2 terminal switches.

Go under hood see my starter relay is busted.

Could you be more specific? What did you see?
A fourth connection to the starter relay (solenoid, right?) should be for the neutral safety switch, but you already said it had started without it connected.

When a switch has a grounding terminal it is lit, which doesn’t indicate that it is of higher quality than an unlit model but it is more expensive.

Ok, so update. Truck still isn’t fixed mainly because when I don’t have the tools I need i have to go buy them with a friend, because no vehicle, then by the time I do that I have work the next day. So today I checked fuel pump. It sprays fuel out when I disconnect line after filter so its not that, at least that far up the line. I also hear it turning on. Need sparkplug socket to see if they are sparking, so that is yet another tool I am going to get haha. Took video on my phone of it trying to start…

Ok, so I see that it is turning everything over. That means the starter is working right? I also need to get some sort of testing device for the stupid fusable links, but it SHOULD start even if they are down I thought.

My plan now is to try and push start it with my friend. If that starts it I know the problem is the starter. If it doesn’t start it then I know it is something with the sparkplugs. Any help is appreciated, I need to get it running again soon so I don’t have to rely on other people for rides haha.

Don’t waste your time pushing the truck. Remove the coil wire from the distributor cap and shove a screw drive into the end and hold the bare shaft ot the screw driver near (1/4 inch) the engine while someone cranks the engine over. If the ignition is working you will see a steady stream of sparks. If no sparks occur you must use a wiring diagram to test for power and ground to the ignition and computer.

Looking at the video it shows the starter is indeed working. You can hear it and see the fan belts moving. Pretty simple. To see if you have a fuel problem spray a small amount of starter fluid into the intake to see if that helps the engine fire up. If that doesn’t do anything then you need to check the ignition system. Put a spare spark plug into one of the lines and place it on a good engine ground point. Crank the engine and see if you have spark getting to it. If you have spark but no engine response then you need to check the compression of the cylinders.

If you do not have a spark at the plugs, plug wires, or coil wire you need to consider a faulty ignition module.

Your truck should be one of the infamous TFI-IV models with modules that were prone to heat failure. The symptoms vary from intermittent bucking or stalling to outright failure which prevents the engine from even starting.

You also need to check for battery voltage at the ignition coil with the key in the RUN position.
No power could mean a faulty ignition switch. (the electrical part)

No reason to panic yet. The ignition and engine management on this era of Ford is simple so take slow, measured steps in diagnosing it and the answer will appear.
Skip the pushing the truck bit and avoid any shotgun approach by replacing parts on a guess.

On my 70’s Ford truck the “i” terminal bypasses the ballast resister in the circuit which supplies power to the ignition coil when the key is in “start”. This is to insure a good hot spark during starting.

GeorgeSanJose, I might try hardwiring it see if that works, but the ignition coil has power uploading another video

So what I need to do now I guess is take spark plugs out and see if they all failed at once? Maybe something really gunky gunked them up? Or maybe the thing that spins just isn’t supplying voltage to the spark plug coils.

Also I apologize in advance for the video it was raining and I was trying to keep dry by leaning under my hood, and also I can’t think of the right part names sometimes so I just say something wrong.

Has this engine ran since you replaced those plug wires.

If you did replace the wires for this problem…and those wires have never actually started the engine…they may be the problem.

In the first video that you posted it seemed to me that the engine was struggling too much to turn over. You may have gotten a couple of wires mixed up.

I’d recheck the firing order and trace those wires to be sure they are in the correct spot on the distributor cap.


No, those are the wires that ran it since I bought the truck. I have only taken one off at a time in order not to confuse the firing order.

So took spark plug out. Never done this before, but when I connect the sparkplug to a live 12v source then ground out the edges of it where it screws into the motor it should spark right? Because it wasn’t sparking. Hoping its my sparkplugs now.

Edit: added pic to demonstrate what i mean

Edit again:
may be distributor, just in case going to replace distributor and all the spark plugs and coils.

No your normal 12 volts is not enough to create a spark at the plug.

You must have the plug body grounded to the engine or frame and the plug wire from the distributor attached to the plug.
Then have someone crank the engine over as you watch for spark.


Before replacing anything I would suggest checking for battery voltage at the ignition coil. I think (memory is a bit fuzzy) that the hot wire at the coil should be red with a green tracer.

There should be power at that terminal with the key in both the run and start positions.

The main ignition coil lead should tie to the center of the distributor cap. The coil creates pulses of electricity at many thousands of volts to fire the spark plugs. The center of the disty cap connector to the rotor that rotates on a shaft. The rotor aligns with the spark plug connections on the cap and the ignition fires a pulse to the plug.

You didn’t provide any feedback on the coil test by checking the minus side of the coil. You should have voltage pulses there while cranking the engine. If you just have a steady light then something is wrong inside the disty.