1991 Ford F-150, 4.9 L. Dead battery.
Jumped the truck and noted only about 8 volts showing on the gage.
Installed the new alternator but there wasn't enough power to start the truck. When I tried to connect the jumper cables, I got a little sparking. I started the truck, but heard some strange sounds coming from the driver's side of the engine compartment--a small whoop whoop sound. The speedometer needle moved up and down (no idea why this would happen). I turned the truck off and went to remove the cables--that is when I noticed the small wire on the starter relay was smoking. Now, I have no horn, headlights, fuel pumps,interior lights,etc. When I jump the starter relay, the starter engages, but truck does not start. I checked the main relay (beside the fuel pump relay under the hood) and there is no power going to it. I also checked the wires between the starter relay and the two plugs going into the alternator. Continuity is good--even the one that was smoking.
I have also changed the starter relay.
Any ideas?

Wires from starter relay to alternator (1991 F-150, 4.9L).

 

I believe the "wire" with the green sheath is a fuse link. Someone let the smoke out....It needs to be replaced.

 

Not trying to be a jerk, but did you make sure you had the jumper cables/battery cables hooked up the right way? When wires fry after hooking up power its usually because the wires were installed backwards. Or you have a serious short somewhere.

 

There are one or two additional ground wires coming off your negative battery lug - the smaller wires. One goes to the PCM, and the other to various relay blocks. Also the black plastic connectors at the battery for those wires are a source of failure and/or intermittants.

 

You are not being a jerk. I have seen that done and the results are usually funny.
In this case, no I did not put the red on the black and the black on the red. :-)
I suspect a serious short somewhere too. The "whoop whoop" noise and the spedo needle jumping were new to me though. I thought that the spedo was mechanical--so no idea why it would have been affected. The noise was weird...not very loud, but there nonetheless.

 

Yes, it has a fused link. However, I still have continuity in all of the wires associated with this connection.

 

With a meter you may have continuity, but under load it's most likely another story. A stranded 14 gauge wire will measure continuity with a meter if you have one strand connected end to end. Try to pull a 15 amp load through it then it turns into a toaster. It's obvious that fuse link has been compromised. Replace it. As a test: rig up an inline fuse and some clips, then see if your power returns.

I use the term continuity to indicate the ohm value is very low. If you are using a circuit tester that uses a light to indicate the circuit is good, they will lie to you as to the true integrity of a power circuit.

 

That link likely prevented a fire which is what it is designed for. Replacing it is a small price to pay. Glad it worked. Sandy

 

Excellent point! I will see if I can find a new one. Meanwhile, I pulled out my ECU and I think it may be fried. I am not an electrician (pretty obvious), but one of the square "thingys" appears to have been damaged. I'll see if I can post a photo.
So, given the melted wire and 14-ga fused link issue, add on the possibly damaged ECU, and absolutely no power anywhere beyond the starter relay... it seems that I still may need to find the cause (a grounded wire????)?
This is the same truck that had a rat/squirrel nest on top of the engine that caught fire and burned my injectors and injector harness. The injectors and harness were replaced (thanks to a member on here from Canada that provided the harness that is no longer sold), but I was getting the check engine light pretty often after the truck ran a few minutes, stinky exhaust--sometimes white in color, poor idling (usually after the check engine light came on). I still have the original catalytic converter and have been told that some of these issues could be cause by a bad cat. The electrical issue is the newest problem though.

 

That actually looks like the stuff they spray on the board post production to kind of cement the components to the board.
When these processors blow the usually knock out a chunk from the case.

Randy

 

Sorry to see the troubles continue Jim.

I will agree on replacing the fusible link. Also think something looks hot right with the coating on that I in your ECU. Perhaps power things up without the ECU connected to see if the replacement link survives. Lots more parts up here in Saskatchewan if you need them!

 

Thanks to my northern neighbor!
I went ahead and replaced the 14 ga wire and soldered the other wires and a new connector. No joy. Thinking that I may not have done that correctly, I went to a local junkyard and collected two other identical wire assemblies. Again, no joy.
I have removed the ECM and tried to see if there was any power to anything, using the home-repaired wires and again with the two I got from the junkyard. There is no power to anything. No headlights, etc. is puzzling because these will come on, normally, even when the ignition switch is in the off position. In fact, the headlights will normally come on when there isn't even a key in the switch.
I have checked the obvious (obvious to me, anyhow), including making sure there is a full charge on the battery, checking battery connections, checking ground wires.
I can jump the starter relay to make the starter turn, but aside from that, there is no evidence of electricity elsewhere.
Plan to pick up a new computer control relay today, but I am not real optimistic about that since there is not power to the relay that is there now.
Also considering replacing the ignition starter switch, but not holding out a lot of hope there either.

 

Great...now I can't get that song out of my head--"Who Let The Dogs Out?" Agree that the fuse link is supposed to prevent further problems; however, when I took it apart though, the wires were very covered with green corrosion, so I am not sure that there was a good connection. That whole wire assembly has now been replaced with a repaired fuse link and with two junkyard acquisitions: no change after all three attempts.
The struggle to revive my faithful Ole Blue F-150 continues. Yepee ah yo!

 

are you still using the same battery that you mentioned in the original post ?
if so, have you had it load tested ?
i have scrapped many batteries that showed 12+ volts after charging but couldnt pass enough current to power a 12v test light.

 

Is that pic in the first post your alternator wiring harness? If so, I would suspect shorted diodes in your alternator. That alone won't kill all the power, but it would burn up the associated fusible link.

Since you still don't have power, the only general suggestions I can add are check your battery ground connections and every yellow wire under the hood should have battery positive at all times. All that power comes from various fusible links located at your starter relay. Check them all!