battery not charging, battery and alternator test good... - PeachParts Mercedes-Benz Forum

wilburtual · #1 04-05-2011, 01:51 AM

1977 240D
when car is running i am getting 11.94 V at the battery, battery when car is not running is 12.4 V
I had both the battery and alternator tested at 2 auto parts stores and they both said that they tested good... the voltage regulator in the alternator is brand new, thats what i changed first thinking it was that...
battery ground cable and battery to starter cable "seem" to be in good shape, i removed and cleaned the ends/contacts.
when i removed the alternator, the pully does have a little drag on it when turned by hand, not a clean spin, sorta like it catches up a bit in some places when being spun,, so i first thought that might be a sign that it is bad,,, but 2 stores tested it and is "passed" on their machine...

Skippy · #2 04-05-2011, 02:00 AM

If you haven't been running it in a non-charging state for long, it's very likely that your battery is still good. As for the alternator that passes a bench test but clearly fails on the car, this would tend to point to a wiring problem. It also sounds like your bearings might be starting to go, but unless its causing the belt to slip badly while in the car, this isn't the direct source of your lack of charging.

nickofoxford · #3 04-05-2011, 02:13 AM

Check the plug-in connector that goes into the back of the alternator. Both of my 123's have had a sloppy connection at that point making a no-charge situation like you have.

Does the alternator light come on when the car is running? Im pretty sure the alternator light and brake lights will come on with a dead alternator.

If so, the alternator is not putting out current.

If not, you probably have bad connections at the battery. Possibly a bad ground connection to the body. Even less likely a bad "ground strap" connection the engine to the body. Im pretty sure the engine ground is by the drivers side motor mount.

nickofoxford · #4 04-05-2011, 02:16 AM

Forgot to mention, the alternator must be fed power to make power. It has to be "energized" by the battery before it can actually make current to charge the battery. That power would come from the connection on the back of the alternator and would be something I would check for with a meter. Im sure someone on here knows what pins show voltage.

I agree with Skippy, the fact that 2 stores tested it as good would tell me you have an electrical problem somewhere on the car.

Skippy · #5 04-05-2011, 02:17 AM

The regulator has one mode of failure that will cause a "no-charge" without illuminating the alternator light. If this were the case, I would think it would cause the alternator to fail the bench test.

Diesel911 · #6 04-05-2011, 02:36 AM

Check the Alternator Connector as previously stated for corrossion and fit.

With the Key off the 2 large Slots (Red Arrows in the pic) should always have Positive Battery Voltage as they are eventually connected directly to the Positive Battery Terminal.

With the Key off the single small Slot (Blue Arrow in pic) should have no voltage at all.

When the key is turned on the single small Slot will get about 1/2 of al volt less than Battery Voltage.
This is the circuit that goes to the Battery Charging Light. If there is a burned out Light Bulb the Alternator will not charge. The Alternator needs that little bit of current flow created by the Bulb in order to stat charging.

One source has that the Battery Charging Lignt Bulb needs to be 3 watts.

SD Blue · #7 04-05-2011, 04:11 PM

Also, check for any corrosion between the voltage regulator and the alternator housing. This is something that caused my alternator to not charge. The corrosion I found was on the regulator, where it grounds to the alternator chassis. A little cleaning with an old typewriter eraser (mild abrasive) and a dab of silicon grease to block any future problems.

Diesel911 · #8 04-05-2011, 05:31 PM

Quote:

Originally Posted by SD Blue

Also, check for any corrosion between the voltage regulator and the alternator housing. This is something that caused my alternator to not charge. The corrosion I found was on the regulator, where it grounds to the alternator chassis. A little cleaning with an old typewriter eraser (mild abrasive) and a dab of silicon grease to block any future problems.

This is true but the OP did say that the Alternator worked during 2 different Auto Part store tests so at that time the Regulator was OK.

wilburtual · #9 04-05-2011, 05:42 PM

thanks for all the info guys! i'm gonna go poke around with a voltmeter later today and check those alternator wiring harness and the battery connections, i guess i'm not sure if the battery dash light works or not, haven't really noticed... i'll check that too
i'll report back with my findings.

bluetypewriter · #10 04-06-2011, 10:13 AM

I had a slow battery drain for months. I never got a chance to use a voltmeter to figure it out. Then winter came and I let the car sit for a few months.

I tried starting it up and it wouldn't turn over. I checked my glow plug relay and it was melted around the fuse contacts. I guess the relay had gone bad without blowing the fuse, causing a slow drain on the battery.

I grabbed a replacement from a PnP and ever since then my slow battery drain has disappeared.

Diesel911 · #11 04-06-2011, 12:21 PM

Quote:

Originally Posted by wilburtual

thanks for all the info guys! i'm gonna go poke around with a voltmeter later today and check those alternator wiring harness and the battery connections, i guess i'm not sure if the battery dash light works or not, haven't really noticed... i'll check that too
i'll report back with my findings.

When you first turn your key on all of your warning Lights including the Battery Charging Light should go on.
All Cars I have seen with warning Lights have this feature.

lutzTD · #12 04-06-2011, 01:04 PM

If you still dont find it, try getting the test done with the alternator in the car and not on the bench, the chain part stores have a real nice handheld machine to test the system in the car.

wilburtual · #13 10-14-2011, 03:07 PM

so my ongoing charging quest with this car continues... been on the back burner the last couple months,,, to rehash, the alternator was removed and tested at 2 different stores and tested OK, i had the battery charged and tested yesterday since its been sitting, and the store said it tested OK. I finally poked around with a volt meter at the alternator harnessd and got the following results:

alternator harness with key off: large wires = battery voltage (11.99), small wire= no voltage
with key on- large wires= battery voltage (11.99), small wire= 1.75 v
with engine running after GP shut off= large wires= 12.65, small wire= 14.5-15v, at this point the reading across battery terminals is 11.57v

when poking around with meter probe on the back of the alternator harness, the readings seemed to be varied depending on where i made contact (at the wire vs deeper in touching closer to the alternator pin itself... sometime 11.75 ish to 12.75 ish.. this leads me to believe that the harness wires/contacts maybe be suspect.. they are pretty corroded, the wire connect contacts are corroded and the harness contacts that plug into the alternator seem pretty worn, and opened up where they may not be making a tight solid contact.

my questions: where do i get a new alternator harness? i've looked around online but cant find a place that sells it.
is there anything else that could cause a higher voltage at the alternator but lower reading at the battery?
do those voltage readings when the car is running seem right?

one other thing i think i should mention is there is an electric fan on the radiator that seems to run all the time if the key it on. not sure if that is normal or some thing the PO put on..

vstech · #14 10-14-2011, 06:58 PM

likely the fan is on due to a jumper on the a/c dryer being in place likely he was trying to keep the a/c working better, but it could be to mask an overheat problem with the car for sale.
that will cause a pretty big draw on a weak alternator.
corroded connections can be replaced at the auto parts store if they have alt pigtails for sale, I could pull one from one of my cars as I'm parting them out, and ship it to you also...