larsulrik

(92 RS 305 tbi) So, my dangling former smog pump wires shorted on the block right at the negative battery cable-to-block area. This in turn burned through some of the thick red alternator wire next to it and shorted it. This phenomenon blew my gauges fuse, my radiator fan motor (maybe a coincidence?), and the fusible link for the thick red alternator wire at the little junction block next to the battery.
I fixed the exposed wire shorts, replaced the gauges fuse and fan motor and everything seemed to be fine. After about 15 minutes the fusible link burns through again and the volts drop to 11 and very very very slowly creep downwards over 2 or 3 quick trips until it no longer starts on its own. I replaced the fusible link instead of repairing it and it got red hot after a few seconds. I turned it off before it possibly burned through again. No other wires in the battery junction block got hot at all and no other wires are shorting anywhere that I can see. The battery and alternator have both tested GOOD multiple times.
What am I missing? Maybe a starter fusible link? It's too late to check right now...

Pontiacguy18

this happened to my brothers 96 mercury courgar. I'm not 100% but if the starter wont even turn over I think your PCM is screwed up and I would bring it to a dealership, they can fix it for about 200. Good luck

pizza_guy

Do you by chance have another alternator to swap in. Probably not otherwise you probably would have tried that.

Did you take the alternator to the "parts store" to test it. Personally, I'm not real impressed with their ability to "test" things off the car. Try measuring the voltage at the alternator output with the car running. Shouldn't go over 15V. A 'scope would be even better to check for spikes, but a little difficult to get ahold of for the casual mechanic.

I'm confused how the shorted smog-pump wires could be related to the alternator-to-battery wire. Did they physically melt together? That might have cause all sorts of electrical hell. I would also investigate the fan wiring, all the way to the relay. Electrical failures are rarley a coincedence. One bad part can take out another part pretty easily.

Wait, was it a pink/black wire that shorted? That's the 12V power to most of the underhood acc., and would make sense to draw a lot of power from the alternator. Any length of time directly shorting the alternator like that is a good recipie for death.

When all else fails, I try disconnecting things one at a time until I find something odd. Start with your fan, since it's a possible issue. Just unplug the relay at the driver-side firewall and try that.

Sorry pontiacguy18, but I think your on the wrong track. The ecm is the ground for 95% of the cars electronics, not the 12v source. That way, if you short the 12v line like poor larsulrik has, you don't fry your ecm. Oh, and our thirdgen cars have ECMs, not PCMs. And taking your car to a dealer is like lighting your wallet on fire, which is why he is here to try and fix it himself. And I doubt a ford product has anything in common electrically with a GM product.

larsulrik

One thing I noticed today is that it did not blow again. However, I didn't use the a/c and the fusible link was cool enough to hold onto. I just tried the a/c and the wire became too hot to touch in a few seconds. Is that maybe normal since so much current is going through the small fusible link?
Pizza guy, the smog pump wires got rubbed over the years and shorted to the ground off the engine which melted the jacket on the alternator wire and ate through a couple of strands of the copper. As far as I can see no other wires were affected. I wondered if a starter link also frying could cause the alt to put too much power through the thick red cable and in turn the small gauge of the fusible link.

sofakingdom

iTrader: (1)

Sounds to me like the alternator is fried.

Specifically, sounds like it has a shorted diode; and it's putting out a whole bunch of AC as well as the DC it's supposed to be generating.

Take it off and go get it tested.

larsulrik

Update : No blown starter links. No visible problems. Can anyone go to their car and confirm that the alternator fusible link gets really hot with the a/c and fan on? That'll at least take some of the guess work out.
Seems inefficient to drop the wire gauge right before the battery. My blazer alternator wire has no link and goes right to the battery. I'm thinking of doing that since I can't find any problems. Should make the charging system more stable anyway right?

sofakingdom

iTrader: (1)

No the fusible link should NOT get hot.

"Inefficient" isn't the issue. It's a FUSE. Its job is to melt when something goes wrong, to protect other more expensive devices, and above all, to protect the rest of the wiring from getting burned up (and possibly even setting the car on fire).

Hacking it off will NOT make the charging system more "stable".

The fusible link getting hot is a SYMPTOM of a problem, not the PROBLEM itself. Leave it alone, and find the PROBLEM, which is SOMETHING ELSE; and when you fix the real problem, the fusible link will stop getting hot.

Get your alternator checked.

Crasher0083

So what was the cause of this problem? Did it ever get resolved?

larsulrik

Lol I remember this happening but I don't remember writing all those silly presumptions like replacing fusible links with just wire. If I remember correctly the wires that had rubbed raw and were shorting on the block had apparently gotten so hot that some of the wire coating higher up on the wire inside the plastic looms had also melted and was shorting against another wire and half of the wire strands in some of that wire had already snapped so a lot of current was going through a few thin strands of the copper and causing the heat and the slow fusible link blowing.