OK Gurus--Diagnose this battery problem
 

In the 91 350sdl, it has a new bosch alternator and a fresh battery. Both correctly installed. These are the givens.

As long I drive the car everyday, it is fine. However, when i let it sit for 4 days, the battery gets too weak to start the car.

My hypothesis is that there is drain on the battery somehow. Kinda like a leaking artery that is sucking the life out of the battery.

Can you suggest EASY ways to find the problem? All suggestions are welcome.

I think your hypothesis is right on the money. Do you own a Digital Multimeter?

It sounds expensive. What is it?

Multi Meter
 

It used to check volts,amps, ohms, etc.... can be expensive, but you do not need a Fluke meter, just a cheap one.... under $20.00. I had the same problem once, pulled the radio fuse and no more drain on battery, replaced radio, this was on my Fiat. Good luck.

Some things...
 

FWIW, you can get a decent DMM (digital multi meter) from Sears for $50-75. The model I own (I'll get you the number when I get home) can measure DC current up to 10A for brief periods of time.

You can use the amp meter to determine if there is a constant current draw (10A or less) on the battery.

You might also just disconnect the power antenna...it may be frozen up and not shutting off it's little motor.

edit-I've never seen <$20 meter that would measure any current other than milliamps.

Likely culprits are power antenna, electric vacuum pump for the door locks, and light on in the trunk. Yeah, get a multimeter and search here for "current draw" to find out how to track that sucker down.

Buy the meter and set it on high current setting (buy one that can measure up to about 10 amps. it usually has a seperate plug for that function) Disconnect a battery terminal (battery should have some charge for this) and see if current is flowing. If it shows more than about 200mA leave it there (connected), if not, switch to lower setting. You should see something, because the clock draws a little bit, maybe 30 mA? anyway, if you have maybe 100mA draw or more you have a problem. Revove 1 fuse or relay at a time. This may very well tell you which circuit is leaking, as the current flow should drop as soon as the offending circuit is disconnected.

Probably around $25. Any auto parts store should have one as well as wally world etc. You can get an analog one for less than $10, it won't be nearly as useful but probably fine for this problem.

Follow exactly what Pete was saying above, with the following modifications/additions:

1) For safety, disconnect the negative battery terminal and put one probe of the ammeter on the battery terminal and one on the cable end. Make sure everything in the vehicle is shut off and the doors are closed.

2) If pulling one fuse at a time does not find your culprit, disconnect the alternator wires from the back of the alternator and see if the meter drops below 50 mA. If so, then you have found your problem.

When I go looking for these problems, I don't use the big expensive meter that can measure 10A. The little Radio Shack meter that is the size of a pack of cigarettes is much more convenient. It will measure up to 1A. Your current leak is certainly less than this amount (probably 100 to 500 mA).

If accuracy is not too much of an issue
 

Harbor Freight has a digital meter that will actually read up to 10 amps for 3.99 on sale. The 10 amp side is not fused so you may have to buy another one if you switch on the glow plugs...;) It is not too accurate but close enough for this kind of tracing. What you are looking for is a drop in current draw as you remove each fuse. Minor drops i.e. milliamps are probably not your problem. .5 amps or higher is probably more likely for a four day drain. I'm certain that someone here could do the calcs of exactly what the drain would have to be to reduce power over X period on a certain size battery.... (see head swimming) oh never mind...

I'd add stereo and alarm system (if installed) to the list of likely culprits.

funny because the power antenna is indeed broken. i'll be happy to disconnect it if you tell me how. also, assuming i have a good enough DMM, what is the procedure for checking for constant current draw.

you guys are the best. do you realize that if i took this car to the dealer, i might be tempted to never come back for it after they tell me how much it's gonna cost. <Group Hug!!!>

actually, check this out if this is a clue.

sometimes, the radio does not work. when this happens, the "ABS" light and another light on the dash comes on. Now this goes on for about a minute or two, then the two lights go off and the radio comes on.

Clearly, it's a gremlin of sorts. Could this be a clue? Inquiring minds want to know.

The ABS and dash light issue seem to be a loose ground wire kind of thing. Have you reviewed the service records to see if and where someone may have been poking around under the dash.

You may have several issues though. FIRST (and I'm suprised no one mentioned this) you may have an internally shorted battery. Although normally internally shorted batteries would discharge faster than yours is indicating. I'd rule out the battery because you can get this done for free at your nearest Sears (Kmart now? :D ).

Once you have determined the battery is good, THEN I would follow the above good suggestions. Remove each fuse one at a time and check to see if your indicated current flow from the meter drops significantly.

Sometimes we car buffs ignore the obvious or easiest solution and start beating our heads to find the answers.

meters
 

Do not buy an old fashioned cheap analog meter that has a sweep hand. Buy a new cheaper digital meter with numbers. The primary reason is when reading glow plug values you require a useful resolution down to under 1 ohm value. Plus the accuracy when reading voltages etc is so good. That is almost impossible on the analog type without spending a fortune. In canada our canadian tire stores put one on sale often for around 10.00 that is more than adaquate for the occasional user. Also is one of the better examples of a products decline in price with time. Used to buy flukes (a famous brand) when this type of meter first introduced and other varients. seem to remember 200-300 dollars apiece as well. About 10 years ago gave a batch of new fairly cheap meters to my son-in-laws, uncles, and friends for christmas. Almost only gift that is totaly remembered where it came from as all these years later still average several calls per year of how do I use this for a certain application? Or worst asking for interpretation of what they have found. When giving these out I was not certain it was a really good ideal but time has proved I was wrong. . All of them became electrically literate to some extent. Of course for the first few years the phone rang a little more often than usual. Reminds me that better buy a batch for the grandchildren and hope their fathers do not borrow them permanently. :sun_smile

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If you do it this way you won't even have to purchase a voltmeter. May take a couple weeks though.