Hi All,

My 1986 F-150 (4X4 C-6) is giving me some electrical problems. A few weeks ago I was towing a friends trailer and blew a fuse link. Trailer wiring was shorted to ground. The event cooked my stereo and the OEM digital clock. After replacing the link the lights flicker (headlights, dash lights, interior lights) the tach jumps around erraticly as it approaches 3000 rpm, and the turn signals flash very slowly when switched on. I had the alternator checked and its putting out a steady 14 volts under varying electrical loads and charging properly. (lights and air switched on and off etc.) The ammeter is not working either. Is there another under dash voltage regulator that could have failed. Any other ideas as to what may be wrong? I have a set of factory wiring diarams on the way / I gave up on the quagmire in the back of my Haynes manual.

 

Let's start with this problem. Turn on the signals, and walk around the truck and make sure all the lamps are on and blinking, including the dash indicators.

If they all are working, but still flashing slowly, I would check the voltage coming in on the flasher. It should be close to 12 volts.

 

All lamps are on and blinking when tested. I think the problem is more towards the current getting from the source to the individual components.

 

Do you have 12 volts going into the flasher? If not, I thought we could work our way backwards from there, possibly stumbling onto the problem. You have so much going on at once, it's hard to decide where to start.

 

Voltage to the flasher varies, depending on what other accessories are switched on. The alternator is putting out a steady 14 volts though. Will a partially burnt fuse link cause resistance to increase as current draw through it increases?

 

Theoretically yes. Any unwanted resistance is bad for the truck's electrical system, because the voltage it runs on is so low(12 volts). But can a fusible link partially burn through? I don't know. You would think it would blow completely out, but I guess anything is possible.

If you have one you suspect is giving a problem, put a voltmeter across it while you have some loads turned on. If it reads more than a volt across it, I would be suspicious of it, especially if it gets hot to the touch too.

 

Ok all of the fuse links checked out ok. Replaced the battery ground wire, swapped batteries, checked alternator wiring, and I still have flickering interior and exterior lights and slow flashing turn signals. I also disconnected all of the wires from the battery side of the starter solenoid except the power wire from the alternator, problem still exists. Any ideas on where to look under the dash? Removed each fuse one at a time while running and no difference. Also swapped out the coil with a spare and no change. I have a Haynes manual but the wiring diagrams are terrible (covers too many years to be accurate). A good wiring diagram for a 1986 f-150 5.8l 4bbl would help. Need the part that covers where the alternator connects to the battery and the rest of the truck's circuits pick up their primary power. I can't seem to isolate the problem. Thanks everybody.

 

You commented on this before, and said you were looking for better diagrams, but it's going to be hard to find some any better. All diagrams can be confusing, till you get used to how they are laid out. Believe it or not, the Haynes are not too bad. But I have to admit I was confused till I started using them.

The diagram of the alternator is on page 1 of 7. I have a simplier style diagram that shows how it's hooked up. Here it is:



If you need any help with the diagrams, just post your questions and I or someone else can walk you through it.

The above statement has me wondering if one of the smaller ground straps to the sheetmetal got melted in two. The engine block is grounded through the large cable you replaced. But the engine is mounted in rubber. So here and there, they have small grounding straps that connected the firewall and various sheetmetal parts to the block. Do you suppose you have lost a ground, possibly even to the fender where the regulator mounts?

 

Well after a lot of wire chasing this weekend without any results, I took a step back form the project and thought it through again. All of the symptoms were pointing to a low/fluctuating voltage situation. Flickering lights, slow turn signals, stuff like that. As I mentioned earlier, I had stopped by a local repair shop and had them test the battery & alternator. The mechanic watched the meter while I switched the lights, air, blower motor, etc. on and off to see if there were any voltage changes. He said everything looked good.

After spending Saturday and part of Sunday trying to figure out what was wrong it occured to me to make SURE the alternator was good. I removed the unit and took it to the parts store where they bench tested it and ..... sure enough it was only putting out about 11.8v. Bought a new one, bolted 'er in, and Bingo! problem solved. I feel like a dummy for not think of it sooner . I'm just happy it wasn't some single wire shorted somewhere in a wire bundle hidden under the dash.

Thanks to all who replied with ideas, and remember - check the obvious stuff first and then double check it.