HOW TO CHECK IF ALTERNATOR IS CHARGING - PeachParts Mercedes-Benz Forum

Ryan Z · #1 12-16-2009, 05:57 PM

Used the car this morning, no problem. Went out this afternoon, low battery. If I get it started, or get a new battery, can I somehow use a multimeter to see if it's charging.
Went through at least 3 alternators, this one is on over a year.
Hope it's just the battery. Any ideas?

kerry · #2 12-16-2009, 06:00 PM

Multimeter on the battery with engine running at high rpm's will show you the output of the charging system. Should be 14.x volts.

toddyvol · #3 12-16-2009, 06:19 PM

Autozone, Advance Auto Parts, etc.... will check the battery and the alternator for free if you can drive it to them.

Diesel911 · #4 12-16-2009, 07:35 PM

Quote:

Originally Posted by Ryan Z

Used the car this morning, no problem. Went out this afternoon, low battery. If I get it started, or get a new battery, can I somehow use a multimeter to see if it's charging.
Went through at least 3 alternators, this one is on over a year.
Hope it's just the battery. Any ideas?

On most Multimeters turn the meter to the 20 VDC (20 Volts DC) and put the Red Probe on the (+) Battery Terminal and the Black Probe on the (-) Terminal of the Battery that will allow you to read Voltage.

I think 14+ volts is a little optimistic; on older models like my 84 I would consider a minimum Charging Voltage as 13.5 Volts.
If you happen to have a Voltage Regulator on your Alternator that puts out 14+ Volts rejoice.
Also, not if the Voltage goes over 15-15.5 Volts and continues that way you will overcharge your Battery; bad Voltage Regulator or it indicates your Battery is used up inside.

Diesel911 · #5 12-16-2009, 07:40 PM

You did not say if your Battery/Charging Light on the Instrument Cluster was glowing Red (indicating not charging) or not.
If that Lignt Bulb is burned out your Alternator will not charge as it need the little bite of current flow caused by the Light Bulb to charge.
To test it turn you Ignition Key to the first position and you should see it glow Red. After your Engine is running the Red Light should turn off when it starts to charge.

Ryan Z · #6 12-16-2009, 08:37 PM

Quote:

Originally Posted by Diesel911

You did not say if your Battery/Charging Light on the Instrument Cluster was glowing Red (indicating not charging) or not.
If that Lignt Bulb is burned out your Alternator will not charge as it need the little bite of current flow caused by the Light Bulb to charge.
To test it turn you Ignition Key to the first position and you should see it glow Red. After your Engine is running the Red Light should turn off when it starts to charge.

Red light never came on until Monday and Tuesday, not today. After all these alternators I get upset. Charging battery, going to Autozone in morning.

Junkman · #7 12-16-2009, 08:44 PM

Be sure to have the new alternator bench tested before leaving the store If you have one of AZone's lifetime warranty products. Pre-testing may save you an install.

#8 12-16-2009, 08:54 PM

I have a volt meter that plugs into the cig lighter that is very handy to check on the alternator while driving. I think I got it at a walmart for about $15 when my alternator was acting up.

kerry · #9 12-16-2009, 08:58 PM

Quote:

Originally Posted by Craig

I have a volt meter that plugs into the cig lighter that is very handy to check on the alternator while driving. I think I got it at a walmart for about $15 when my alternator was acting up.

I bought one of those for each of my cars. Very useful

Junkman · #10 12-16-2009, 09:11 PM

Quote:

Originally Posted by Craig

I have a volt meter that plugs into the cig lighter that is very handy to check on the alternator while driving. I think I got it at a walmart for about $15 when my alternator was acting up.

Useful unless your cig lighter is useless.

(Off topic): A friend of mine claimed that car companies were going to do away with cig lighters because people no longer use them.

My response: In that case, they should also eliminate turn signals.

#11 12-16-2009, 10:16 PM

Quote:

Originally Posted by Junkman

Useful unless your cig lighter is useless.

(Off topic): A friend of mine claimed that car companies were going to do away with cig lighters because people no longer use them.

I think they've just renamed then instead. I have two female "cig lighter" connectors hard wired under my dash to support a semi-permanent installation of my GPS and a charger cord for my phone. I only have a couple of inches of wire showing. The W123 lighter location interferes with the radio controls if you plug anything in.

vstech · #12 12-16-2009, 11:38 PM

Quote:

Originally Posted by Craig

I think they've just renamed then instead. I have two female "cig lighter" connectors hard wired under my dash to support a semi-permanent installation of my GPS and a charger cord for my phone. I only have a couple of inches of wire showing. The W123 lighter location interferes with the radio controls if you plug anything in.

A-freaking-MEN!
that and the outlet turning off when you shut down the car is SOOO annoying!

benhogan · #13 12-17-2009, 09:46 AM

Quote:

Originally Posted by kerry

I bought one of those for each of my cars. Very useful

great stocking stuffer idea from Santa to me! Any chance you can take a pic of it so I'll know what I am looking for when I go to Wally's?

Graplr · #14 12-17-2009, 09:58 AM

Quote:

Originally Posted by Ryan Z

Went through at least 3 alternators, this one is on over a year.

Where are you getting your alternators from? I've had this problem before. Next time mine goes out I'm getting a quality Bosch rebuild. Not an ebay, autoparts store or used special.

How is your glow plug relay? I think my GP relay going bad is what took out my alternators. I'm not positive but mine was bad and I replaced it and haven't had any charging issues since after going through 3 alternators (the one on the car when I bought it, a used one from a car I parted and one ebay "rebuilt" high voltage unit).