How to check if alternator working
Thread Starter
Junior Member
Joined: Jan 2016
Posts: 68
Likes: 4
2008 R500 5.5 l
How to check if alternator working
My battery gets discharged occasionally, and it's not necessarily after being parked for too long. I am under the impression that my alternator is not doing an excellent job.
1. Is it possible that an alternator gets degraded with time? I mean it charges but not as good as it should?
2. How can I diagnose it?
Thank you.
Pierre
1. Is it possible that an alternator gets degraded with time? I mean it charges but not as good as it should?
2. How can I diagnose it?
Thank you.
Pierre
Senior Member
Joined: Mar 2017
Posts: 258
Likes: 50
R320, Astro, JSW, ST4
Watch this:
Key on (before starting)
Dash display shows trip and overall odometer reading
Press R(eset) button in middle of dash 3 times, display should change to show voltage.
(Hopefully showing 12+ V)
Start the car while watching voltage. It should not dip below 11V (off the top of my head).
If you were to try cranking starting with say 11V and starter drew the line voltage below 10V I think it stops the starting attempt to prevent battery damage.
Anyway assuming cars starts, the voltage should come up to 13.8-14.1 after the alternator adjusts for accessory load.
If so, alternator is working more or less okay.
Alternators fail from wear in the commutator ring or burned diodes in the voltage rectifier.
Or, brushes which follow the commutator can wear out, or ...
Key on (before starting)
Dash display shows trip and overall odometer reading
Press R(eset) button in middle of dash 3 times, display should change to show voltage.
(Hopefully showing 12+ V)
Start the car while watching voltage. It should not dip below 11V (off the top of my head).
If you were to try cranking starting with say 11V and starter drew the line voltage below 10V I think it stops the starting attempt to prevent battery damage.
Anyway assuming cars starts, the voltage should come up to 13.8-14.1 after the alternator adjusts for accessory load.
If so, alternator is working more or less okay.
Alternators fail from wear in the commutator ring or burned diodes in the voltage rectifier.
Or, brushes which follow the commutator can wear out, or ...
Senior Member
Joined: Mar 2017
Posts: 258
Likes: 50
R320, Astro, JSW, ST4
Okay, glad I could help.
I have had my alternator die, it's a little awkward to replace.
I don't remember which warning comes on the dash, but it will give you warning for sure.
And if you do end up crossing that bridge, be particularly careful about the replacement part.
There are numerous variants.
I have had my alternator die, it's a little awkward to replace.
I don't remember which warning comes on the dash, but it will give you warning for sure.
And if you do end up crossing that bridge, be particularly careful about the replacement part.
There are numerous variants.
Thread Starter
Junior Member
Joined: Jan 2016
Posts: 68
Likes: 4
2008 R500 5.5 l
Ok.
I'm now left wondering why the battery sounds weak sometimes in situations like I've been driving for one hour, I stop the engine for a brief moment, and restart. It starts but the starter runs slowly.
It does not while starting in the morning.
I'm now left wondering why the battery sounds weak sometimes in situations like I've been driving for one hour, I stop the engine for a brief moment, and restart. It starts but the starter runs slowly.
It does not while starting in the morning.
Super Member
Joined: Nov 2017
Posts: 558
Likes: 114
From: San Francisco Bay Area
2006 R500
Your starter is on its way out. My starter was doing that, and replacing the starter cured that issue.
Alternatively, it could be a prelude to engine failure. Last month my car started doing that again (hard to start when hot) and then finally it wouldn't start anymore. Took a breaker bar to the crank and it wouldn't turn at all. No other symptoms other than an add 1 quart of oil at next refill warning that would occasionally pop up. I did have three injectors fail on me (stuck open, flooding the cylinders) about 3 months ago. Planning to replace it with the 5.4 AMG engine.
Alternatively, it could be a prelude to engine failure. Last month my car started doing that again (hard to start when hot) and then finally it wouldn't start anymore. Took a breaker bar to the crank and it wouldn't turn at all. No other symptoms other than an add 1 quart of oil at next refill warning that would occasionally pop up. I did have three injectors fail on me (stuck open, flooding the cylinders) about 3 months ago. Planning to replace it with the 5.4 AMG engine.
Last edited by Tsumi; May 12, 2022 at 04:12 PM.
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Thread Starter
Junior Member
Joined: Jan 2016
Posts: 68
Likes: 4
2008 R500 5.5 l
Senior Member
Joined: Mar 2017
Posts: 258
Likes: 50
R320, Astro, JSW, ST4
^^++ It does take more amps, both when car is dead cold in the winter (thick oil), and when it's at operating temperature (piston/ring/cylinder clearances closing to spec).
And if you're taking a lot of short trips, starting repeatedly will tap the battery down making it worse.
The more the OP wrote, the more it sounded like tired starter to me.
And if you're taking a lot of short trips, starting repeatedly will tap the battery down making it worse.
The more the OP wrote, the more it sounded like tired starter to me.
Thread Starter
Junior Member
Joined: Jan 2016
Posts: 68
Likes: 4
2008 R500 5.5 l
^^++ It does take more amps, both when car is dead cold in the winter (thick oil), and when it's at operating temperature (piston/ring/cylinder clearances closing to spec).
And if you're taking a lot of short trips, starting repeatedly will tap the battery down making it worse.
The more the OP wrote, the more it sounded like tired starter to me.
And if you're taking a lot of short trips, starting repeatedly will tap the battery down making it worse.
The more the OP wrote, the more it sounded like tired starter to me.
