I had my old 2g Alternator catch fire some some months back, and did the 3g upgrade after some difficulty with the harness below. Some of it got burnt, some had a chew from a mouse maybe. I soldered and patched it up as good as I could, and it ran solidly for quite a while.. Now, the cold season hit and I hit the heater and defrost and the cab turned into a sauna. Bad heater core with the telltale coolant drip on the passenger floor. I just got a hose barb and looped the heater hoses together temporarily until I can get a new heater core. I'm hoping this is all coincidence, and something didn't melt down because of the heater or coolant getting into something. Anyways, when I went out to go to work, I find no power anywhere to to the truck. No lights, no ignition, Nothing. The battery is good, I tapped the starter solenoid and the engine cranked. Multimeter shows there is power across the megafuse, as well as downstream from fuse link J on the old alt wiring harness. Has anyone any ideas where else to check? I replaced the ignition switch on the column, it was old and kinda hard to turn anyways and read that sometimes that can be a culprit but no joy there. I did replace fuse link J to be sure, and did find when I skinned back a good bit of insulation that the wire underneath was corroded. I cut that back further and found clean wire and soldered in the new fusible link, but nothing has worked. The entire truck's power seems to be dependant on this old harness, and I'm guessing one of these yellow wires? Is it possible to just bypass the whole thing and make a loop at the socket on the wheel well? I'm looking to find a replacement so to have one not so raggedy in there, but they seem to be obscure. Does anyone have a proper name? A parts number?? The truck is a 1986 F150 4wd I6, I did an HEI distributor and added a plain jane carter YF carb. As always, you have my eternal gratitude! Thanks!

This dirty archaic bugger.. what is it called!?

 

Also thinking back to the last things I did if it helps matters with solving the power issue.. I had the heater on, the cab steamed up violently and I pulled over to air out the cab and wipe down the windows. I never turned off the truck, but I shut off the heater. Drove maybe 6 miles home and shut the truck off for the night.. Next morning all this cropped up.. There isn't some kind of fuse hidden out somewhere that maybe blew when the heater turned off is there? I never seen anything weird with the headlights, dash lights or anything to lead me to believe something was going on with the alternator. Voltmeter was stable up till the end if I recall around 11.5v or so maybe? I do run HID lights.

 

Your best hope to find the problem is for it to stop working again. When you have no power, you then need to get a meter or testlight, and start poking around while someone is trying to start it. You must have a load on the circuit for this testing to be valid.

You can start at the battery, probe the posts while you try to crank it. If you have 12v while trying to crank it, it's good. You can then move to the battery wire terminals with the meter, and try to start it again. No or low voltage means one of the terminals is bad. Good voltage means they are good. You can keep the + testlead on the battery, and move the negative lead to the metal on the engine block. Then try to start it again. No or low voltage means you have a bad negative battery cable or connection to the engine block. If that is good, you can move down to the starter relay terminals. Try to start it again. You can leave the negative on the engine block or move it back to the battery. Same as before, no voltage at the starter relay means a bad battery cable or a bad connection at the starter relay.

You can keep poking around till you find you have no voltage when trying to start. When you have no voltage, you are downstream of the problem. Your alternator should have nothing to do with cranking and running, it just charges the battery. If the battery is dead, then the alternator can be a problem.

 

Where is this megafuse? There should not be power across a fuse.

Only charge current.

All power from the truck comes from the battery terminal of the starter solenoid and the multiple fuse links off this terminal.

 

. Right. In that weird harness pictured above,
one of those yellow wires was cut as per the diagram. I may have gotten the wrong one as the color markings are quite faded. Anyways, there was no power much like now, until I reconnected one of them.

 

Thanks a million Franklin! I’ll get on it as soon as I get a helper.

 

This is a megafuse. Nothing like that in your picture and they were not used OEM in your application.

All primary power distribution is off the starter relay and there will be multiple fusible links. Check for corrosion and intermittent connections.For the most part, a yellow wire is always battery +. The harness you pictured only has power distribution for the alternator.

 

I had to add this from the wrecking yard to handle the 3g alternator.

 

IIRC on the 3G wiring that MEGA fuse is on the heavy wire from the ALT out put stud to the starter relay and dose not supply power to the truck.
If you are not getting power to that fuse look where that cable is bolted to on the relay.
There should be the cable from the battery bolted to the same lug check for power at that lug.

My guess is that you will not have power there so that would point to the connection of the battery cable at the battery.
I do have to ask the battery is fully charged?
Dave ----

 

Just a quick followup, power has been restored to the truck. the thinnest yellow wire in that bundle, the Y/LG one must have a corroded or dead spot in it somewhere. I took a test jumper and clipped it from a splice joint to the battery just to see if the underhood and cab light would power up, and it did. Truck started with no issues, all systems go as it always did.. Now, the question is.. Do I just cut out that bit of wire, and solder in a new bit and hope for the best? Or, if something in that plug truly is interfering with flow of power, where do I get the power from? Is it safe to just migrate that wire over and splice it into the yellow/white wire under S203 which is still intact and hot yet going nowhere?

 

I would replace that section of wire and try to keep the layout somewhat original. Then it won't be too far off the standard diagram. You already modified it a little bit for the newer alternator.

 

I tried replacing the wire. No joy was to be had.. I tried probing with a test light up and down the wire and it's dead. as is it's neighboring "Large Yellow" wire that goes to S202.. So what would you recommend? I've went to the few wrecking yards here to try to find a replacement, but only turned up trucks that had the external regulator box and a plug similar to this set, but not quite the same wiring in them. Either the R/OR wire is dead at the S203 splice to C610, or C610 needs a priest. S203 from Fuse Link J is hot in the Y/WH wire. I wish I could just find another harness here just to dissect the one I have at the rubber splices and do an autopsy.. The curiosity of what I might find has now gotten me engrossed. Anyways. my choices as it seem are now limited here. Can I, or rather, should I just take that Y/LG wire and tap into the Y/WH wire? Maybe put a small inline fuse in it? The differences in wire sizes here have me a little spooked honestly. I don't know what kind of voltages we're talking about, and well.. there isn't any such thing as a small meltdown of wires.

 

Here's the factory diagram. I am going to have to study this for a little bit and re-read your posts again.

 

It's hard to see the wires in the carpet picture in your post. But you need to take the large yellow wire that goes to the grey plug C610, and tie to the wire leaving fusible link J. Do you see that in the diagram? The wire with the word "shunt" is gone I am assuming.

 

. Shunt wire is indeed cut and capped off. But the large yellow wire is also dead. The small red wire wire coming from grey plug is hot, as is the yellow wire that goes to the old alt plug. The black wire that was labeled “shunt” also is getting juice, but as said it’s capped off and set aside.