Forum,
Just found this site. It's fantastic. Thought I'd stump you all with an elctrical gremlin problem:-)
My 91 300TE 4M w/66K mi., showed classic signs of battery end of life. Long trips okay short trips, dead battery over night. Not just low, DEAD. Clock stopped, radio CODE when turned on, no dash warning lights at start position, nada. Put in a new Optima battery, same thing occurs. Changed the fuel filter the same day as I put in the Optima cranked it a bit getting the fuel up to the CIS, etc., moved the car around the driveway and parked it. Next morning, dead.
Testing procedure as follows: Turn all light switches to off, ACC to off, radio off, shut doors. With fully charged battery in the clean and well tightened cables, open fuse box and test each fuse from its top end to ground with multi-meter. Four circuits are using voltage. C, D, 9, 10. Switch multi-meter to DC amps highest scale is 200mA. Pull fuses from four circuits. Place one test lead in bottom of fuse holder on on top tang. 10 and D show 0.00 current. How can this be? Circuits 9 and C go off the scale - being digital it displays a 1 at far left of LCD. How can this be?
I thought I'd try this next procedure but don't know what I discovered:
With multi-meter in DC amp mode 200mA scale, I put the red test lead to the top of the fuseless fuse holder in #9 and the black test lead to ground. I got a reading of 124.45 mA and it started to descend very slowly, as if I were draining a battery. Did the same to "C" and got a reading of 0.42mA and it started the descending thing. I switched the meter to DCV and used the same procedure and got readings of 7.34DCV and 4.32VDC respectively. Now I'm totally confused.
I must be doing something wrong. How much current should the clock and the various memory circuits be using? What is the proper placement of the test leads?
I feel a bit like Alice must have after jumping down the rabbit hole!
Any help would be appreciated.

Not sure if this will help, but the trunk pin switch for the trunk light can break and keep the bulb on but I would think it would take longer than overnight to drain a battery with just that little bulb.

Alon

First, you should test the battery with a battery tester that is capable of putting load on the battery.

Second, your not going to read amperage in a ckt. if you are not drawing any current. Take a simple ckt. with one bulb, a switch and a fuse. With the fuse removed you will read voltage on the live side of the holder. On the load side of the holder your not going to have any reading at all because you broke the ckt. by removing the fuse. With the fuse in place and the light not turned on, your going to read voltage only. You will read voltage up to the open switch, but you will not read any current because the light is not on to draw any. With the light on, you will read full voltage up to the bulb and on the other side of the bulb, you will read zero. You will also read current on the ckt. because the light is now on drawing current. To trouble shoot a ckt. You have to know how the ckt. would normally operate. You would check the ckt. at different points to check for voltage drop and would need to know if a voltage drop should occur at that point from any resistance or current.

I would check the meter on a known good ckt. To make sure the meter is working properly and is being used properly.

Good Luck.