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Alternator/low voltage

9K views 47 replies 11 participants last post by  nobby 
#1 ·
1976 450slc
Normal voltage output when no accessories are on. Low voltage when any accessory is on.
Alternator is good. Ground good. Belt is tight.

This forum has been helpful in the past, so I'm hoping to get advise on my problem.
I have low voltage whenever I have an accessory turned on – headlights or auxiliary fan or blower fan.
Here is what happened:
This summer, I had a dead battery. After a couple weeks of driving around town, dead battery again. I could tell starting motor going slower towards the end. Test at autoparts store says battery fine, alternator only putting out 12.5 volts.
So, I had it rebuilt at the local electric rebuilder. Car seemed to work great. 3 weeks later, dead battery again.
Took to my indy mechanic and he checks things and says bad alternator. I take alternator to rebuilder and bench tests fine under load. 13.8 amps. He says it’s the car.
So back to indy who puts it back on and still low amps when an accessory is on (ok when no accessories). I take car to rebuilder who does some tests and thinks it’s the aux fan.
Back to indy and unhook fan, same issue with headlights on. So we decide to try another alternator from autoparts store. Same results as original alternator.
So to summarize:
13.8 volts at idle with no accessories. Checked at alternator and at battery.
Turn headlights on, 12.8 volts. Aux fan, 12.6 volts.
Belt is tight. Indy says idle is fine and all systems check out, including ground. Everything is stock. No energy leak when car is off. He’s stumped.
I have a voltmeter on my charger. I confirm the volt readings at idle with no accessories, and with either accessory on.

Any ideas where to start to diagnose? My first step is to buy a good voltmeter.
 
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#37 ·
Bad news fellas.
The ground strap connection to the transmission was too tight for me to remove. So I removed the frame side, cleaned and reattached. I also bought a new strap that I attached to another location on the tranny and to the same frame mount.
I also connected a jumper cable from the alt. case to a point on the frame. I had a good connection because I was able to start the car when the ground straps were disconnected.
I now have 2 engine ground straps and Jumper cable. Same issue.
800 rpm, 13.8 volts. I turn on accessory and it drops at least .5 volts (headlights or blower). Aux fan or rear defrost will drop around .9 volts. Raising RPMS to about 1000 will return to 13.8 volts on the .5 loss. But the .9 loss I have to go to at least 1500 rpm. More than one accessory, forget about it.

So...
2 alternators.
2 batteries
2 battery ground cables
2 engine ground straps (both on)
 
#39 ·
Bad news fellas.

800 rpm, 13.8 volts. I turn on accessory and it drops at least .5 volts (headlights or blower). Aux fan or rear defrost will drop around .9 volts. Raising RPMS to about 1000 will return to 13.8 volts on the .5 loss. But the .9 loss I have to go to at least 1500 rpm. More than one accessory, forget about it.
QUOTE]

These numbers are still a little higher than I would suspect, but not that bad... unless lights and fan equals 1.4 total volts, then it is too much in my opinion. What problem is this causing? At 1000rpm, you are back to 13.8, which is nominal for charging system. Sounds like you have improved the situation and just need to drive the damn thing! All this is out the window if the lights are too dim to see by at night when idling...

1968 may have hit on something I neglected to consider... Also, has anyone changed anything on the engine or alternator that would have changed the drive ratio of the accessory belts?

I bet if any of us measured the voltage from the battery positive to a ground while idling and accessories on, we would see a .2-.5 drop in volts...
 
#40 ·
Appreciate everyone's help.
Ears you may be right about just driving it, which I'm doing.
My problem is I'm driving around town 35/45 mph with the air on. Within a week or 2, i have a dead battery. Currently I'm charging the battery every couple days. It shouldn't drop down to less than 12.5 volts with a couple acccessories (yep, lights and fan drop it way down).
vehicle is pretty much stock, nothing different added for 20 years. Only thing recent is the motor/frame mounts.
I think my next step is to the auto electric rebuilder to get his thoughts. He originally thought it was the aux fan that pulled too much amps, but even with that off, lights (w/brights) and others pull it down, too.
I'll keep everyone posted.
 
#45 ·
I saw that coming the moment I hit reply!
Would be fun, tho.

I talked to the rebuild guy's helper who is stumped, too. Going to check with the boss on monday.
Mechanic gave me this Bosch testing document. Might take a stab at the output and regulator tests. Waay out of my comfort zone, so hopefully I don't burn down the garage.
 

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#47 ·
I have identical probs with my low miles 1998 SLK 230. It nearly died under full electrical load so I replaced the battery. New batt tests fine: 12.5 V at rest, 12.0 under light load. Alternator bench tests at 14.5 V under load and 13.5 unloaded. When installed on the car, voltage drops to 12.2 with lights on and 11.5 with all accessories except rear defroster on. Revving doesn't change this much, if any. I replaced the voltage regulator before testing the alt, just in case, but as I sqid the alt is fine.

Did anyone figure out this problem?

Ernie
 
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