Alternator charging - fiero fault - anyone advise?

[drfiero308]

Hi
Newbie here.
Just hoping someone can advise.
I bought a nice fiero 308GTB replica last week. It had a dead battery but jump started fine. The battery clearly was dead as a new one was required and simply connected the new battery in parallel to allow me to drive the car home (which it did fine for about 250 miles). Cabin volt gauge indicated between 12 and 14 volts - needle periodically flickered between these two displayed values.
I changed the battery over the weekend - which is a bit of a fiddle given the poor access. Car fired up fine but volt gauge showed 10v or so and, when driving around clearly was falling gradually. A multimeter across the battery terminals indicated no charge being received from the alternator.
In a fiero, the positive battery point has a feed to a nearby connector block, from which the main red cable links to the alternator. There is a fusible link in this cabling.
I poked around this connector block area and noted that a PO had cut the fusible wiring in the alternator cabling and then crudely wound the wires together and covered it in tape for some reason. I thought my poking around might have disturbed this connection and that this would explain why i had no charge to the battery. I reconnected this with a short 27amp length of cable and then reattached the battery.
Immediately, there were lots of sparks and the newly installed spliced cable simply melted.
Clearly I had a short that was earthing the battery directly through this cabling.

Reading around the topic as I can tell, then I could have failed diodes in the alternator allowing a direct connection to earth?
But just a bit confused as to why this happened now when before the battery install, even with the dodgy connection i found, the car was charging and running reasonably OK? Just a coincidence of problems?

Anyone have any experience of this problem or similar please?
Thanks
ian

Car is a 1987 Fiero V6 with aftermarket 308GTB body fitted.
The dash alternator light had never come on at any time

[GWD]

Hi!
It looks like you have found the problem. To me it is a diode bridge failure.
That wire you have replaced is marked red on my diagram.
I don't know why it was charging when you drove it home. Here is a wiring diagram about charging system of the
'88 Firebird. I don't think so there is any significant difference.
I hope it's helping to see the entire charging.
That 20 Amp fuse (blue mark) has played up (did not blow) on mine.

[GWD]

Uhm..I just read your post again and probably I misunderstood. So you have repaired and got melt the wire next above
the red marked one.Which is marked "S" at the multiplug. Then it's the solid state regulator.
Try to find a remanufactured alternator or second hand one.

[bhm1712]

If there is a direct short to earth you should be able to measure this with a multimeter set ot ohms. With the battery disconnected, measure between the positive cable where it terminates at the battery and the chassis or an earthing point. With a normal multimeter and not a mega, anything other than open circuit (usually displayed as Ol or oL) will mean its likely it is down to earth.

A diode can have different failure modes depending on the fault condition, whether it be over current or over voltage. It all points to a failed component (probably regulator) in the alternator. However, what i dont understand is the fact that previous to you discovering the dodgy taped connection and splicing in a new fuse wire, it wasnt melting cables. Double check your wiring before you fit the new alternator and connect up the battery.

[bhm1712]

Having said the above, on these old cars where the wiring loom and electrical components are not new, it may also be possible to have a low resistance reading due to older components and the associated insualtion being less effective than new.