So I have a 1990 F150 with a 5.8 351, automatic, power everything, yadda yadda. It's my first truck (I'm 16) and I'm having a hell of a time getting things to work right.

I picked the truck up early last month. Ran awesome, couple little issues here and there. So far I've only really had to deal with electrical issues (lights were wonky, swapped the signal harness), but now I seem to be facing a few more problems.

To start things off I have a relay somewhere on the drivers side fender that is magically setting itself off, without the key in or anything. So when the relay goes off my side markers along with my interior lights all turn on, this usually lasts only for a second. Unfortunately this is enough to kill my poor battery. So my question here is, any idea which relay it could be? I've tried feeling for the click but i can't tell by just that, and with it happening when it wants its hard to check them all.

And..

So, because of my relay issue my battery is DEAD. Now, I dont have a battery charger, but when I hook the truck battery up to my car's battery and try to boost it the engine will turn over, but it won't start. First thing I did was pulled out one of the plugs to check for spark, which led to nothing. The gas gauge read 1/4 tank so I added a jerrycan thinking that the gauge might be wrong and I'm out of gas, the gauge now reads 1/2 tank so it cant really be that. What else should I check here? I dont have an unlimited bank account unfortunately so I can't just go out and buy new everything haha. Im confused.


Any help is greatly appreciated guys. Like I said this is my first truck, and I'm trying to soak up as much mechanical knowledge as I can, so I apologize if the questions appear to be stupid! Thanks in advance!

 

Nobody wants to help!

So today I went to try and troubleshoot myself and a buddy of mine ended up putting the booster cables on backwards. So now when I go to boost the truck as soon as I hook the cables up the starter relay goes nuts! As well as the overdrive button inside the cab. I really think I'm over my head here, but unfortunately I do not have the money to take my wiring issues to a mechanic lol. Does anyone please have any ideas at where I should be starting to fix?!

Thanks.

 

did you ground the spark plug when you pulled it out? if not then you wont see any spark. you need an actual spark tester. and your buddy probably screwed the computer by hooking the jumper cables up backwards. which starter relay? the one on the fender or the one on the starter? and i'm gonna try and see what relay your talking about for the lights. your saying the relay is causing the interior lights and marker lights to come on? are your hazard lights on?

 

By the starter relay going nuts I will guess that with the booster cables on and you turn the key to the start position the solenoid on the fender chatters. This is because of low voltage, you need bigger jumper cables or some way of getting more power to it.

Did you try to start it with the cables on backwards?
Or were they just put on and pulled back off?

The relay and the lights coming on sounds like some kind of after market alarm system.

 

Yep, grounded the spark plug, and I tried a few other plugs. Theres no spark in any of them. Could it just need new plugs maybe?

My computer eh? That sounds friggin expensive. What will I be looking at to fix that? Is it a big job?

The relay that I'm talking about it on the passenger side fender. And now, after the cables were hooked up backwards, as soon as I go to hook the cables up normally that relay starts chattering like crazy. So its as soon as the battery gets any power now. And, the key wasn't touched or anything, as soon as I saw he did it I ripped them off, so they were maybe on backwards for like 30 seconds.

Also, subford, the truck does have an aftermarket alarm system now that you mention it. I never thought to check that! I'll try to find that relay as soon as I get my other, more serious, electrical issue dealt with haha. Good call though!

 

i'm thinking ignition coil or ignition module. your computer may be alright, but dont take it away from the list of possibilities. do you have an ohm meter? hook it to the coil and check the primary resistance and secondary resistance. mainly to see if there is an open in the coil circuit internally. some parts stores can test the ignition module. or someone on here may be able to tell you how to do that. and depending on what type of alarm system you have can also have affect of whether you can start the engine or not. some companies make systems that you hook to the fuel system or ignition system to shut down so noone can steal the vehicle. just a possibility.

 

Go ahead and unplug the computer and leave it unplugged until you get the Ignition system working.
That way you will not run the fuel pumps and you will know it is not the computer stopping the spark while you work on the Ignition system.
The spark may even return when you unplug the computer and if so you will know it is the computer stopping the spark.

 

Also if you have no spark with the computer unplugged check the Ignition coil with a test light and see if you have power on the two wires.
This plug on the Ignition coil gives a lot of problems, you should see power going into the coil and coming out the other wire. You must have power on both wires with the key on and the engine not cranking.
If you have power on both wires and the tach side of the coil does not flash the test light while cranking you have a problem with the TFI module or the PIP sensor in the distributor.
If it is not flashing at the coil have the tfi module tested at an auto parts store. If it tested good then you may have a bad PIP sensor and you will have to replace the stator in the distributor or replace the distributor.

 

Alright. Thanks for the replies guys! Couple pretty silly questions here for ya though.

Where is the computer and what does it look like? (lol)
Ignition coil, where is it? (again, lol)

Sorry for the silly questions! But hey, Im trying to learn haha.

Also, how should I be checking power if my battery is right dead? Probably have to buy a new one eh?

 

Drivers kick panel.




Follow the big wire out of the center of the distributor cap and the other end plugs into the Ignition Coil. The two small wires at the coil is the ones to test.



Or get the battery charged.

 

Alright awesome! Thanks for all the patience and help haha.

 

Sure, you will have to unplug the Computer from under the hood.
The bolt is 10mm.
It comes out from inside the cab.

 

Oh alright. It's what you have circled in green there eh?

 

Yes it is what is circled in green but just unplug it for now.

 

So, I got the battery charging right now, and I'm trying to get this computer unplugged. The 10mm bolt turns but I'm not seeing any progress haha. It just doesn't seem to be loosening.