neothe1

So, after my recent fiasco with the battery draining for no good reason, I took the A4 to the mechanic who claimed he had rebuilt the alternator by changing some sort of diodes in it. The car doesn't die anymore, but now the guy tells me that the positive terminal is heating up like crazy.

I know that there are a lot of solutions around (change the alternator, clean the contacts, change the cables going to the terminal, change the battery, etc.), but I'd like to know whether it's possible to diagnose what precisely I need to change, since it'll probably be stupid to fix something that's not actually broken. Any thoughts?

a4audi4fun

Electrical stuff only gets hot because of excessive resistance so you need to determine where the bad connection is. Start with the connector clamp to the battery terminal; make sure the whole inner surface of the clamp and the whole terminal are clean; then ensure that there is a good amount of surface connection between those two. If that doesn't resolve the issue, then check the cable is solid at both ends as heat could be being transferred to the terminal (though that is unlikely). Lastly the cell connected to the terminal post may be shot and generating heat now that you are putting full charge current through it - that one will need a battery replacement.

neothe1

This is great! I now definitely have the confidence to walk through the problem. Thanks.

tytower

Well I would add that resistance is futile .
I think it far more likely you have a near short somewhere which has been draining current for quite some time causing the alternator to work so hard it went bad.
Use a multimeter on Amps setting. If you don't know how then learn or have someone who does know ,and is not looking for the work, do it.

Take the positive terminal off and with some aligator clips tentatively put the multimeter leads between positive terminal and positive cable end . Tentatively with meter set on maximum (usually 10 amps) touch it on and watch the meter . Get it off if it goes to Overload.
If it does not then get someone to turn the ignition on and check the reading .
If it goes to overload at any time it means the current flowing exceeds 10 Amps. Don't use the starter at this point.
So if an overload appears then note the position of and remove all fuses.
Put them back in one at a time and watch the meter ,You are looking for a circuit that is drawing a lot of current thats where the short will proably be .
Assuming you are not blowing fuses atm then it must be drawing current close to the fuse limit and this can happen by fitting extra spotlights ,having a caravan or trailer with extra lights etc etc.
With the starter it draws something like 80 amps so if the car is not starting quickly requiring a long period of starter turnover then that will heat up everything.