When my daughter turns off her '93/940 and the headlights are turned on the lights go off. There is still a drain on the system because the next morning the battery is dead. Do I look to the switch for the drain or is there somewhere else I should be looking?

Remembering to turn the switch off as well works but once you turn off the car she forgets to pay attention to the switch.

 

I leave the lightswitch on all the time on our 92 740 and 87 240. no drain.

if you have a digital multimeter, put it in its "AMPS" mode, usually you need to move the red + lead to a different socket on the meter, and switch it to 10A or whatever scale. now, lift the positive terminal off the battery, connect the + lead of the meter to the battery + terminal and the - lead of the meter to the positive cable to the car. DO NOT HOOK THE METER UP ACROSS THE BATTERY IN THIS AMPS MODE, ITS A DEAD SHORT.

now, hooked up as I said, in series with the positive lead, you likely see some current leakage if there is any. might be an amp or two, might be a few 10ths. to flatten a good fully charged battery overnight takes a several amp drain.

Next, pull all the fuses one at a time and check the amp meter each time, if the current drain stops, bingo, you found which circuit its on... of course, there's stuff thats not fused.

 

Not sure I did it the same way but we set the meter to volts and put it between the positive post and the positive cable and it was drawing 1.0## volts. Pulling the PCM fuse dropped it to .8## and pulling the radio dropped it to .7##. The ## noted were constantly fluctuating so something kept changing its drain on the system.

There was no drain change by just adjusting the light switch. Also the PO changed added a switch that went to the compressor for the AC system. Not sure why they disconnected the wire from the heater/ac controls unit on the dash!

 

those voltage readings are nearly meaningless. use DC Amps to do that measurement

 

She took it to AutoZone before the door got torn up and they said it was the alternator since the battery was 30% charged. Took off the alternator and brought it up and they put it in their test machine and it passed all the tests 3 times. Then my son took the battery up and they said it was fine. Sure was hoping it wasn't a drain!