Battery or alternator problem? - PeachParts Mercedes-Benz Forum

Flop · #1 12-12-2006, 12:39 AM

Hi guys,

Just got a 1980 240D yesterday. It was running fine, but when I turned the headlights on during the drive, they seemed dim. Once I got home, I turned the car off and got my volt meter. The battery read 11.2 V. After sitting for 24 hours, the battery read 12.2 V. I started the car and checked the battery while it was running and the voltage kept dropping. When it hit 9.4 V or so, it slowly started climbing. I turned the car off, went inside, and scoured the threads here. Still not sure what the problem is, but I'm leaning toward the alternator. I checked the battery about an hour later and it read 12 V.

After reading many posts, I think maybe the alternator belt is a bit loose, but can't figure out how to tighten it. I scoured the posts again, but couldn't find a description for this car. Can anyone tell me how to do it?

Thanks much.

PanzerSD · #2 12-12-2006, 01:19 AM

Well, here are three things that you can do that are relatvely easy and cheap.

1. Change/tighten belts, or while you're at it, change the belts and hoses too!
2. Replace the voltage regulator. ( cheap like borsch)
3. check ALL your ground points. and of course clean the battery connections.

The fact that the car even starts and glows tells me the starter and bettery are alright.

I bet if you do those things you'll narrow it down to i'm sure, a fatally flawed alternator or dead battery.

justinperkins · #3 12-12-2006, 01:23 AM

Warning: I've never troubleshooted a battery/alternator problem on an MB, so this is more just common troubleshooting tactics I'm telling you here.

Could be a bad battery, bad alternator, or bad connection somewhere. I'd start by checking all your connections for a good connection (remove, clean and re-attach) and check the belt tension (you'd probably hear a loose belt). Check your ground strap and check for continuity in all the common places to assure yourself the the battery voltage (engine not running) is the same at the battery as it is everywhere else you have a hot wire. If it's not, you've got a connection problem.

Once you do that, with the engine running check the voltage at the battery (you've got a good connection now right?). If it's not 12.5, then you probably have an alternator problem (or voltage regulator, not sure if that's integrated with the alternator or not). If it is 12.5, then you either fixed the problem with a bad connection or you still have the problem in which case it's a bad battery.

All that being said, I'm leaning towards your alternator or a bad connection since you initially noticed this problem with the engine running.

300SDog · #4 12-12-2006, 01:24 AM

Another culprit to your electrical probs can be the external regulator on the alternator. It's the bottle cap looking thing on back of the alt, 2 phillips scews secure it. Look for worn teeth or weak springs when you check it out.

Meanwhile here's how you tighten the Alternator belt..... There are 3 x 17mm bolts attaching the alt to the bracket - loosen them all. The 13mm fancy collared nut on the end of the tricky "L" bolt should be removed and discarded, you have to bend the ears on the bracket to do this - usually they break off. It's a joke of German engineering that MB ever designed it this way in the first place, probly the worst designed component on your 240D.

Then just pry the alternator away from the block to apply tension to the belt. You can do this with your hand or use a wrench for leverage. Start with the 17mm nut that holds the dopey "L" bolt and tighten all 3. Dont worry about the 13mm collared nut, it wont be missed. Many folks snap off the "L" making it into an ordinary nut'n bolt combo for simplicity.

Shorebilly · #5 12-12-2006, 08:21 AM

G'mornin',

First, look around here, and find the thread in checking your alternator with a voltmeter....and do the test....

Second, fully charge your battery....if you don't have a Battery Charger...get one, you will need it sooner or later....

Third, clean your battery terminals.....purchase a tool if necessary

Fourth, most Auto Parts stores will test your battery free of charge.....have the battery tested.....unless you own, or can borrow a resistence type of battery tester.....

Fifth, check your ground straps...connections clean and tight...straps in good condition.....

SB

Flop · #6 12-12-2006, 09:05 AM

Thanks for all the great info, guys. I'll start checking all of these things. I really appreciate it.

vstech · #7 12-12-2006, 10:17 AM

Quote:

Originally Posted by 240Dog

The 13mm fancy collared nut on the end of the tricky "L" bolt should be removed and discarded, It's a joke of German engineering that MB ever designed it this way in the first place, probly the worst designed component on your 240D.

Sorry, I gotta disagree.
while it is certainly a poorly designed part, it does make it simple to tighten the belt.
FIRST!!! Get some PB blaster, and gently spray some into the end of the nut, and the threads of the thin bolt.... let it sit for about 1/2 an hour. be sure the nut is well lubricated! it is such a light duty bolt, it will just snap if there is ANY rust on the thing. I would loosen it up first and without any tension on it, spin it well by hand to be sure it will turn. then put a new belt on... of course to do that you will have to remove all the other belts, so change them too. That is the most poorly designed area of this car! once the belt is on, just tighten up the thin bolt/nut, until the belt is tight, and then tighten up the three 17MM bolts on the back of the alternator.
John

carnut · #8 12-12-2006, 12:40 PM

I agree with VSTECH, just make sure the adjuster bolt and threads are lubricated. without that adjuster, getting proper tension by prying is at best a guess. Can you rotate the alt. pulley by hand against normal rotation? If you can, its too loose.

vstech · #9 12-12-2006, 12:50 PM

Quote:

Originally Posted by carnut

I agree with VSTECH, just make sure the adjuster bolt and threads are lubricated. without that adjuster, getting proper tension by prying is at best a guess. Can you rotate the alt. pulley by hand against normal rotation? If you can, its too loose.

in addition to this test, if you can rotate the pully by hand, more than likely the belt has been slipping for a while, and the belt will be so polished, that no amount of tightening will suffice. ya gotta replace the belt at that point, so Just go ahead and replace them all.
John

Flop · #10 12-12-2006, 01:29 PM

Forgive my ignorance here....

Is the funky nut in question the thing sticking out from the alternator at about the 10 o'clock position when looking at the alternator from the front of the car? I was trying to determine how this thing worked last night, but couldn't see it well enough with my poor flashlight. If it's what I think it is, it looks like a really long nut. Is the idea that turning this nut counter-clockwise pulls the alternator further out and thus tightens the belt? I gather from various posts that you need to loosen all the other bolts first. This is starting to make sense..... Loosen the bolts that hold the alternator in place (3 of them, I believe) and then use the funky nut to move the alternator out until proper tension is achieved. Then tighten the 3 bolts back up. Is that it?

vstech · #11 12-12-2006, 01:34 PM

just about.
CCW will loosen the bolt... and the alt.
yes 10:00 pos is the bolt, fitted into the alt frame bracket... USE PB BLASTER!!!
otherwise you will be stuck with a broken adjuster, or paying upwards of 150.00 for replacement parts.
John

Flop · #12 12-12-2006, 01:55 PM

What is this PB Blaster of which you speak? I assume it's like Tri-Flow or WD-40. Where does one purchase this magical product?

vstech · #13 12-12-2006, 01:58 PM

Quote:

Originally Posted by Flop

What is this PB Blaster of which you speak? I assume it's like Tri-Flow or WD-40. Where does one purchase this magical product?

it is nothing like WD-40, it works! wallyworld, any parts supplier, everywhere. look for the white and yellow can with what looks like old time newspaper ads all over it. big letters PB Blaster on the can with testimonials all over it. you will have no problem finding it.
John

Flop · #14 12-12-2006, 02:20 PM

Thanks. I'll get some tonight. The bolts to loosen the alternator - where exactly are they? I believe I saw in other posts that there are 2 or 3 and they are 17mm or 19mm.

vstech · #15 12-12-2006, 02:37 PM

Quote:

Originally Posted by Flop

Thanks. I'll get some tonight. The bolts to loosen the alternator - where exactly are they? I believe I saw in other posts that there are 2 or 3 and they are 17mm or 19mm.

they are 17 MM and they are all on the back of the alternator. one is right up against the block of the motor, one is outboard of the alt. on the bottom. and the third is on the stamped bracket that sits on the alt. it will slide in a groove the other two just let the alt. pivot. If you look at the front of the alt, you will see the nuts that they thread into.
John