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Alternator not charging battery at high rpms

Started by ajafri, 19 October 2016, 05:02 PM

ajafri

I finally got the alternator out after wrestling some rusted bolts, and I took the voltage regulator out. Do the brushes look worn enough to be causing the issue? I attached an image of it in this post.
1979 350se

daantjie

They look fine to me but replace it anyway they are nice and cheap.
Daniel
1977 450 SEL 6.9 - Astralsilber

rumb

And since the whole thing is out now, take it down and get tested.
'68 250S
'77 6.9 Euro
'91 300SE,
'98 SL500
'14 CLS550,
'16 AMG GTS
'21 E450 Cabrio

jmperkins

Ali,
I thought or assumed (oh dear) that you were going to drive it the auto parts store to have it tested while it was in the car...oh well and yes, the brushes look long enough but I agree with the others that it is cheap insurance to replace them as they are part of the voltage regulator assembly. Congrats on the removal.

ajafri

I decided against driving the car, didn't want it to stall in the middle of the road. I'll take the alternator to get it tested tomorrow and let you guys know what happens after that. Thanks.
1979 350se

Malpaso44

I recently had an issue with my alternator. It would only charge slightly when driving at high rpm. It would barely reach 12.8 volts and then back down to around 11.3 while idling. I already replaced the voltage regulator but still no luck. I removed the alternator and had Autozone and O'Reilly's test it for me. According to their machines it tested ok. I replaced the ground battery cable and checked the positive cable to make sure there were no corrosion or breaks in the wires. I know there are diodes in the alternator that can go bad intermittently and drain your battery. So I said to myself, enough is enough! I don't drive my girl every week but when I do I need her to treat me right. So I purchased an alternator with a lifetime warranty. I installed the new alternator and we are go. Your brushes look fine to me. Do yourself a favor and purchase another alternator if you can swing it. I'm just saying   ;)
Some are born great, some achieve greatness and some have greatness thrust upon them.

ajafri

Well, I finally got around to replacing the alternator yesterday, and nothing changed. The damn battery still won't charge. So now I'm assuming that either the battery has dead cells or there are issues with the wiring.  :(
1979 350se

jmperkins

Sorry to hear about this as it is indeed frustrating.... 12.8 volts output from the alternator sounds on the low side...you should have a voltage tester on the battery now when the engine is running with your new alternator and I would expect in low 13 voltage. If the alternator is doing its job, it doesn't even need the battery when the alternator is spinning while the engine is running....just my thoughts...

I'm assuming your fan belt to the alternator is tight and you have good connections at the alternator...

ajafri

Yes, all connections are fine. When the car is off, the battery is at 12.6 volts, and when the car is idling the voltage drops to around 11.8. However, when you rev the engine, the voltage drops to around 7 volts and the car stalls out.
1979 350se

jmperkins

11.8 when idling is way to lower...when running, it needs as a minimum a larger voltage than 12.6 or the battery is doing all the work so there is something definitely wrong with the charging system! I suppose its always possible that your new alternator is no good....

Alec300SD

Your symptoms sound like there is a loose connection.
The three prong connector, (black plug on the back of the alternator), may be losing electrical contact - especially when the engine revs.

This happened to my 1983 W126 300SD.
I crimped the female pins to establish a more solid connection and made sure there was a little more slack in the harness right at the alternator.

To test:  hook up your voltmeter to the battery , with the car idling, wiggle the connector and see if the voltage changes.
Be careful around the moving belts, etc.



78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

ajafri

Wiggling the connector in didn't change the voltage, however, when I disconnected the positive lead for the battery, the car shut off. I'm assuming I need to replace the alternator...again.  :-\
1979 350se