I constantly have to jump my '92 240. It has a new battery,which is the second since this problem started, and a new Alternator, which I had checked just in case a few months ago and it was all good. Furthermore, it only drains the battery when the engine is running. So, every couple days -- if I drive it regularly -- I have to charge it or it won't start up, but if I drive maybe once and then go back to college for a month, when I come home it will crank right up. I have no idea what this could be.

 

Your alternator is obviously not charging... Is the BAT light on the dash ON, then OFF as the car starts? The bulb has to work for the alternator to kick in. Have you used a multimeter across the battery posts as the car is idling to see the voltage put out by the alternator? It's the easiest way to check--should be around 14V...

 

Yes, the BAT light is on when I first crank it, but it is very dim. I checked the multimeter and it read 12.1 volts at the battery.

 

Disconnect your battery and clean all your connections esp. the ground. Do the same for the alternator, reconnect battery after doing both.

I've been chasing all the grounds and connections in my '89 and '92 as they are all problematic (dim lights on one side, wipers came on when lights came on, car died when switching from low/high high/low beams, etc.

This is to be expected with 20+ year old cars and should probably be standard maintenance with these). While you're at it, do the lights, windshield washer, etc. I use a wire brush on all parts until they're nice and shiny. Next up, conductive grease (not tune up).

Even if this doesn't solve your current problem you'll have to do it eventually.

 

I already cleaned the connections with battery cleaner/corrosion remover and with a wire brush. Thanks, but I need to fix this current problem, because recharging every night just in case is annoying.

 

the red 'bat' or alternator light has to come on when you first turn the key on, and it has to go out when the engine starts, or the charging system is not performing properly. the light is actually part of the circuit, its connected between the ignition switch and the alternator D+ input.

fully charge the battery, disconnect the charger, wait an hour.

measure the voltage at the battery, it should be like 12.6V if its warm, and less if its cold (as low as 12.0V in freezing weather). now start the car, and measure the voltage of the battery, it should be about 1V to 1.5V higher, if it is, the charger is working.

if its not, try some things. measure the voltage between the alternator metal body and the battery + terminal, if THAT reads 13.5V or whatever, then your alternator isn't properly grounded.

measure the voltage between the alternator D+ (the smaller red wire) and the engine ground, that should be battery voltage. B+ (the big red wire) to engine or alternator ground should 13.x volts, if its not and the D+ is powered, then your alternator or its brush/regulator assembly is probably bad.