65 Stang. All in the sudden ALT light lids on when driving in a city. What was odd, the light seems to be working totally opposite way: it stays on even when the key is removed, goes off when turned on (OIL light turns on OK at this moment and goes off when started…), and ALT also goes on while the engine is started. So, lots of ALT light just when it is not needed! ALT light turns brighter as the engine is accelerated. Belt tension is fine and all wires seem to be connected to the alternator. Just got p...ed off and disconnected battery cables... Could it be ignition lock or alternator/regulator related issue? Or some wires loose or grounded some how some where? Any great ideas/educated guesses? Thanks

 

I was waiting for someone to answer this because I have no freaking idea of how this is being caused...

Thinking out loud...

Would a blown diode in the alt rectifier cause this?... by allowing back-flow?

 

fuse maybe? or problem with voltage regulator?

 

Did this just start happening all by itself? I had a similar situation when the wiring was all jacked up at the voltage regulator. I never discovered what caused it b/c I cut out all the junk and started over. When I finished, everything worked right. I know this does not help, but get a good set of wiring plans and verify wires if you have done any electrical work. If it just started happening, you may check out the voltage regulator.

FYI, you may get more help/hits if you post this in the technical section instead of the general discussion section.

 

I have tons of diagnostic instructions if you have the equipment to carry them out?

The Alternator output is the first thing to consider.

 

Hi again, and thanks for your concern. Problem was solved. A guy from a local car electric shop handed me right away a new rectifier a.k.a diode pack (some 5$) as I explained the symptoms. Something I should be honest with: alternator in this strangely behaving Mustang is Lucas made with integrated control box, probably from some British origin car dating back to70´s or 80´s. It has been a simple and obvious change when hard to get or expensive original control box and/or alternator has gone bad (note: here in Finland…). It takes only two wires to be connected to the back of the alternator and that’s about it. Anyway, I believe original Ford approved parts have the same principal way of functioning. IDK the reason why something was burned, yet I found a melted drop of soldering material inside the devise. Sounds like this whole story has nothing to do with Mustangs due to crappy Lucas? Maybe, but lesson learned: keep your car original and do not settle for less. Hope this lesson also makes this a "general discussion" applicable...