alternator test (voltage across battery): instant response? - PeachParts Mercedes-Benz Forum

ktlimq · #1 02-28-2008, 10:34 AM

Below is what I did.

start engine
wait 1 min at idle speed
2500 RPM for about 5 seconds
idle

I observed voltage across + and - battery terminals.

The reading was 12.40 V. It did not change at all when the RPM increased to 2500.

I followed the above procedure (except [start engine], since the engine was already on) with headlight and fog-light on. The reading was 12.21V. It did not respond to 2500 RPM either.

Did I do something wrong? Should I keep 2500 RPM for several minutes to see voltage increase?

The car is 1992 300D.

pawoSD · #2 02-28-2008, 10:44 AM

No, your alternator may not be charging. It should go up to about 14 volts. However, if you have an afterglow-glowplug relay, it will stay around 12 volts for 2-4 mins. To make sure thats not the case, start the car and let it run like 5-6 mins and then check it. If its back up to 14 volts or so (or close) then that means you have afterglow that uses a lot of power the first few mins after startup.

I think even with afterglow on revving it to 2500 should have shown a difference on your meter. I'd do the above check first though.

pimpernell · #3 02-28-2008, 10:52 AM

Quote:

Originally Posted by ktlimq

Below is what I did.

start engine
wait 1 min at idle speed
2500 RPM for about 5 seconds
idle

I observed voltage across + and - battery terminals.

The reading was 12.40 V. It did not change at all when the RPM increased to 2500.

I followed the above procedure (except [start engine], since the engine was already on) with headlight and fog-light on. The reading was 12.21V. It did not respond to 2500 RPM either.

Did I do something wrong? Should I keep 2500 RPM for several minutes to see voltage increase?

The car is 1992 300D.

Another poster brought out the issue of the afterglow process, which is a good possibility. I have a 96 e300d, and when starting the engine from dead cold, around 35 degrees, the afterglow process will usually last 1-2 minutes. During that time, with the lights and rear defroster on, my voltage will read 11.8 volts. As soon as the afterglow process is completed, it bumps up to 13.5-13.8.

MB-Owner-in-ind · #4 02-28-2008, 11:44 AM

I would strongly suspect a bad alt regulator. Not sure of the capacity of the alternator but I feel it should be enough to overcome the load you presented to it including glowplugs.

ktlimq · #5 02-28-2008, 01:19 PM

Quote:

Originally Posted by MB-Owner-in-ind

I would strongly suspect a bad alt regulator. Not sure of the capacity of the alternator but I feel it should be enough to overcome the load you presented to it including glowplugs.

It is 70 A.

#6 02-28-2008, 03:50 PM

I would like to know why you are checking the voltage in the first place. Has the alt light come on? Do you have some reason to question if your charging system is functioning ok? I would have to say that if you had no output from your alt that the batt voltage would be below 12v. I personally have never seen a batt that can hold a steady 12v+ after starting and with headlights on unless it is brand new or recently charged so I would agree with the regulator theory.

ktlimq · #7 02-28-2008, 04:37 PM

I just did it after running the engine for 5 min.

12.34 V at idle. 12.35V at high RPM.

I removed the battery from the car and charged over the night. The first test was the first engine start, and the test I just did was the second engine start after recharge.

Below is the story.

morning, 2 days ago
some explosive sound, beeping sound, radio went off, the lights on the instrument panel blinked, unable to re-start the engine after a turn off

afternoon, 2 days ago
jump started, ran engine for 15 min, battery seemed to become weaker instead of becoming stronger, all electrical things went off while the engine was running,
traveled home in the dark in the rain/snow without lights, blinkers, speedometer, wiper, etc.

evening, 2 days ago
received free replacement of the 1 year old battery at Pepboys

morning, 1 day ago
no problem

evening, 1 day ago
I could starting the engine. After about 40 min driving, ABS and SRS lights came on. I tried stop-and-restart of the engine to eliminate those lights, but I could not restart the engine. Jump started. All the electrical things soon went off while the car was running.
I purchased a battery charger, and recharged the battery.

ktlimq · #8 02-28-2008, 04:44 PM

Interestingly, when ABS and SRS lights came on, the light that indicates that the battery is not being charged did not come on.

pawoSD · #9 02-28-2008, 04:49 PM

It would appear you need either a new voltage regulator....or a whole new alternator. Depending on the age/miles on the alternator I'd just replace the whole thing.

KarTek · #10 02-28-2008, 04:50 PM

A battery is 13.2 volts or about 2.2 volts per cell. Like the other said, when the engine is running and the battery is fully charged with little load on the alternator, the voltage across the battery terminals should be at least 14 volts.

I can't tell you what's exactly wrong but what you're describing sounds like classic alternator failure - everything runs for a while after charging the battery or a jump start then it all goes south.

If you heard a pop, possibly one of the rectifier diodes let go in the alternator.

Every vehicle I've owned that had a bad alternator, the light never came on. I think it's just connected to a circuit in the alternator that's energized when the car is running and it's not really indicating if the thing is putting out any power. It's more like a "broken fan belt" indicator.

pawoSD · #11 02-28-2008, 05:08 PM

The battery light is dumb on these cars....when my alternator failed the only way I knew it was that my voltgauge that I have added to my car stated 12.3 volts while driving instead of 14.1 The battery light never lit even when I was losing all remaining juice in the battery.

oldiesel · #12 02-28-2008, 07:53 PM

when you replace the alternator,the 143amp alternator from some of the newer cars will bolt right on,add a couple of heavier wires to carry the extra current and your electrical system will be much happier. I dont know why they only put a 70amp alternator on these cars,it really is borderline even when everything is working right.Heck even my old Jetta came with a 120amp and it has very little in the way of electrical load. My $.02 Don

argus445 · #13 02-28-2008, 08:20 PM

I just replaced the voltage regultor on my 82 300d and it charges correctly at like 14 something volts w/o any load with everything on it pulls down to like 12.9 or so (lights,ac,radio etc full blast) so i'd suspect the factory alternator was marginal at best

ktlimq · #14 03-02-2008, 12:32 PM

The alternator was replaced.

13.69 V at idle with lights etc. off

By the way, is my current battery reliable? It was almost totally discharged once.

#15 03-02-2008, 12:40 PM

Quote:

By the way, is my current battery reliable?

I have accidentally discharged mine overnight twice without lasting ill effects. Once should be fine if it is fairly new, if it is more than two years old you should have it load tested to be sure.