Drawing current? - PeachParts Mercedes-Benz Forum

dieseldaddy · #1 05-13-2002, 02:47 PM

OK, I have been searching this topic for a while and always come to some stumbling block.

I have a 1981 300tdt and I purchased an Optima "red top" battery with 950ca and 850cca to replace the 550ca battery from Texas life. ("Hot" in Michigan is "Cold" in Texas!)

Problem is, I have had to FULLY recharge it 6 times since the beginning of February (when I purchased it).

The voltmeter reads 12.0-12.5 when sitting fully charged, and 13.8+/- when idling.
That tells me that the alternator is doing it's job...right?

Here is the history: My wife's car, driven most days, short 4-7 mile trips. At least once a week we drive it for extended periods on longer trips, but still nothing major. I have recently been taking it to work (26 miles each way) once a week, and we DO drive it all weekend.

THE ONLY THING I can think of is that when I installed a new CD player, I have to rig up a wire for the channel memory directly to the battery... could this be the culprit?? hard to believe...

Just learned that the PO had some trouble like this, but it was more hard cranking issues...with a 550ca battery i can imagine! THAT problem seemed to remedy itself with the purchase of my new battery.

Questions#1: Could this be the Voltage Regulator even if the alternator is giving us these good readings? How do I test it.

Question #2: How can I test to see if there is something (like a stuck glow plug?) drawing current while the car is off?

**Please dumb-down your responces a notch or two as I am electronically challenged. Thanks~

ANY suggestions gladly accepted!

The Warden · #2 05-13-2002, 04:39 PM

Actually, the short drives could be doing it. You know that a diesel takes a lot of juice to crank. However, the alternator's not going to charge the battery all the way up in 4 to 7 miles of driving.

From what you've said, I'd guess that the alternator's working okay; however, you could remove it and take it to an auto parts store to have it tested if you so chose. Most shops do it for no charge, and it's nice to know that it's performing to specs. They may be able to test the voltage regulator too...but if you're getting any current at all to the battery, the voltage regulator's probably okay (it usually either stops giving out juice altogether or full-fields the alternator and cooks your battery).

Unless the ground's bad or it's staying on all the time, the CD player shouldn't be playing a factor. It will draw some current (just enough for the clock and memory presets), but it should only draw a very small amount...definitely not enough to drain the battery in any less time than 3 or 4 months of no driving whatsoever...

I'd try putting another new battery (a battery that's drained too many times won't charge back up to snuff) in and try driving it around more, and see if that'll help. Just a suggestion, though...if the battery's new enough, you might be able to get it replaced under warranty.

Good luck!

Robert W. Roe · #3 05-13-2002, 05:24 PM

A stuck glow plug would likely kill your battery within a few hours. My trunk light wasn't shutting off, and my battery would be dead every 4 to 7 days or so, and this is with a brand new red top Optima. The "old" battery was dying in 3 days or so. It wouldn't hurt to check for lights that aren't shutting off, but in a wagon, I would think you'd notice it. Could be the "memory" wire actually is powering more than just a few watts of draw. Good luck, Bob

#4 05-13-2002, 05:53 PM

dieseldaddy:
I had similar problems. Just purchased a new battery, was not driving the car yet, and after about two weeks the battery was run down. If you can get your hands on an ampmeter,(rated for 5 amps or more and if possible digital) and connect it between the battery post, and the positive cable. Very important, the key has to be off, and all lights accessories etc off. Draw on battery should not be more than .075 amps, or 75 milliamps. This is typically the clock requirement and because it is electronic it is intermittent. The CD changer memory draw should not require no more than an additional 10 to 20 milliamps. If it isn't Start pulling fuses until the current draw goes away. I found a car phone circuit (old type) that had not been fully disconnected. The other item I noticed is that the servo motor for the heat will run after the car is shutdown, but I think it times out and shuts off, or is suppose to.

PeterG

79 300SD 75K miles

Billybob · #5 05-13-2002, 06:11 PM

Peter's diagnostic procedure is the way to go, check first for total current flow from battery, and then each circuit. His method of removing fuses will work as long as you check every fused circuit as there may be more than one drawing current. You can also check each individual circuit by removing the fuse and attatching the amp meter across the fuse contacts to read the actual current flowing in a given circuit. A cheap 5 or 10 amp meter and a couple of small alligator clips from Radio Shack will do the trick if you don't have a multi-meter yet.

#6 05-13-2002, 06:35 PM

13.8vdc is a good voltage level from the alternator. Below is the voltage vs. charge state. As for checking for current draw from the battery, you can disconnect the positive lead from the battery and use the Amp function of the meter to check for any current draw. Make sure that you have the meter leads properly connected to the meter for the Amps function and connect the Positive meter lead to the battery post and the Negative meter lead to the Pos battery cable. Make sure that the CD memory backup wire is attached to the battery cable (at least touching the Neg lead of the meter). The reading you should get should be in milliamps (.0XX Amps) and extremely low, maybe in the neighborhood of or less than 70milliamps (.070A). I am not sure what the current draw in my 300CD is so that value is a WAG (Wild @$$ Guess). You can then have an idea of what juice is being sapped from the battery. Disconnect the memory back up wire and start eliminating the other sources if the reading is still high with the memory back up removed. Pull fuses if needed. Maybe someone with the same year and model will be good enough to take a reading on their car as a comparison for you. I’ll try to do it on my 85 300CD tonight.

BATTERY CHARGE STATE at 80 deg. F
V.O.M Value/Approx. State of Charge
12.65 100 %
12.45 75%
12.24 50 %
12.06 25 %
11.89 Discharged

dieseldaddy · #7 05-14-2002, 03:16 PM

Alright, thank you EVERYBODY for your responses!
Again I am amazed at the knowledge and willingness to help a brother in need.

Well, I am 99.92% sure I found my problem. Back a while, I had a silmilar problem, and I did not remember it until last night when I got my FREE replacement battery (new 7-year warranty to boot!) and installed it only to have it not even 'click' when engaged.

The REST of the story goes like this:
One of the PO's had apparently had either an alrm system or a CB or something under the dash and the wiring was a total disaster!!
I noticed it when I was fixing the interior dome light and had to remove the below-the-dash panel to get at the ground.
Well, there is ONE white wire down there that apparently attaches to the ignition switch that when attempting to wire for alarm/cb/whatever, they just cut the wire covering and wrapped another wire (no connection, tape, or anything) around the original wire. This had vibrated loose, but not off, which would be giving the appearance of spotty behavior.
Tried to start the car and nothing, tightened the wired connection and VROOOM! (actually," va...va...va....rrr.rrr.rrrrrrrrrrr.oooom") we were fixed.

Somehow, I had pushed that to the basement of my brain because I did not remember this until I had replaced the battery and it sould not start. I got back under the dash and sure 'nuff, the wire was now totally off. Tried to start the car, nothing, wrapped the wire back on and she started right up!

Tonight I am going to make a *real* connection and soon I will attempt to return the whoel wiring mess back to as original as I can. Whew, sorry for the long description, but it sure is good to know what was causing what I thought was a "drain" was actually just a loose/disconnected wire!!

Thanks again everyone!
~Brian

#8 05-14-2002, 03:30 PM

Always be thankful for the easy fix!!

dieseldaddy · #9 05-14-2002, 03:39 PM

You can say that again!

I am the type of person who immediately jumps to the conclusion that I have the one vehicle problem that absolutely cannot be solved...

Then I realize that I blew a fuse, or forgot to put it in park...

I am really beginning to enjoy and appreciate the mechanical quality of these cars, and am STARTING to realize that there is almost always an "easy fix" for whatever is ailing her.

(Not bad from someone who prior to Feb. 2002 never even changed the oil on my lawn mower...

)