Do alternators begin to charge at a certain RPM?
 

I noticed a while back that the headlights on my new (to me) '95E300D would suddenly get brighter shortly into my morning commute.

Last night I finally got my DVOM out and I clipped it to the lighter socket this morning to watch the voltage. Sure enough, the voltage jumped from around 11V to typical charging voltage of around 14V+/-, after I had been driving for perhaps 20 - 30 seconds.

I have not observed this phenomenon closely enough to note if the voltage increase occurred when I reached a certain RPM for the first time or not. But my '88 5 series BMW will not start charging until the throttle is blipped up past 1200 - 1500 rpm or so, which is common for that older 5 series Bimmer.

Maybe it's characteristic of Bosch alternators? I know that when the diesel starts, there is no surge in RPM at all, it just goes up to its relatively slow idle speed and stays there, and because I baby it until it's warmed up it doesn't see much RPM. I'll be observing more carefully but I just wanted to ask.

Rgds,
Chris W.
'95 E300D

No, this is not a characteristic. The alternator should charge a little, even at idle. I would want to know that the battery is in good shape first. If it is weak the base voltage would be lower.

You should do a voltage drop test to see that you have good connections, and you can easily remove the two screws from the regulator/brushholder on the back of the alternator to remove and inspect it. If it is an older alternator, the brushes may simply be getting worn. You can click on Fastlane above and buy a new regulator/brush unit for about $14. As long as the bearings aren't noisy, if the brushes are worn, replacing this inexpensive item will normally put you back in business for a long while.

Good luck,

Your glowplugs stay on for a while after you start the engine -- the lights will brighten noticably when they kick off, usually within a minute or less.

You should have 12V more or less with the GP off and engine running at idle, and yes, it's normal to have very little charging at slow idle. However, the alternator should put out 13V just above idle, not "wait" until 1200 rpm!

Peter

psfred NAILED Your condition, the AFTERGLOW on your car stays on for upto 120 seconds after starting. The glow system take 90% of the max alternator output.

hey guys,
I had to take my truck to sears yesterday for a bad battery. I asked him to check my alternator first to make sure it's not bad before I got my battery replaced. But he told me that it can't be done unless there's a new battery in place first because the old one wasn't holding charge and was pretty much dead.
My question : was his statement correct ?
more over what's an easy way to check the alternator ? just putting the DVOM across the battery terminals to read the voltage ? with the car off the DVOM should read around 12V, the battery output, and with the car running it should ready between 13.1 to 14.1V, the alternatory output ?
and if the alternator is not charging correctly how does one figure that out with the DVOM ? I'm thinking if the alternator is faulty but a brand new battery is put in, wouldn't the voltmeter read atleast 12V with the car running, which is the output for the brand new battery ?
thanks for the reply.

Benz300:
His statement was not correct. You're still going to see a difference in voltage regardless of the starting point unless something catastrophic has happened to the battery. The alternator is checked exactly as you describe, but a good battery will read 12.4v -13v. If it only reads 12 it is discharged somewhat. Check the battery voltage with the car off, across the terminals with your DVM. Let's say you get 12.56v. Then with the car running, check it again. You should get 13.5v - 14.5v If you're getting 12.56v or less your alternator is not charging. A new battery will give you a 12v+ reading initially, regardless of alternator condition, but you can still check it as previously described as it will steadily discharge the longer the vehicle runs. Keep in mind too that if you determine there is a problem, the first thing to check is the condition of the connections both to the alternator and the battery itself.

Alternators do only charge after a certain RPM if the exciter current is not there, but should be 'excited' after the first blip of the throttle.

A healthy battery reads 12.60 or better at rest and surface charge has bled off. That's accomplished simply by letting the car sit for 6-8 hrs. before checking with the VOM.

You can get a fairly good idea how well off the brushes might be and the alternator when you start the car up cold in the morning; testing with the VOM. You can also do as Larry Bible suggests and simply pull the brushes for examination. It takes more time to setup for this than it does to pull, examine and reinstall

I was seeing about 13.90 in an orig. 13 yr. old Bosch alt. After driving 30 miles or so, it would read around 13.25-13.35 - in the summer. Alts. lose a bit of punch when things have warmed up.

I decided it was time to change out the alt., due more to it's age & not wanting the better half stuck out on the road. I replaced the 80 amp Bosch AL66X with a 100amp Bosch AL166X.

That was 6 mos. ago. After the upgrade, I was seeing 14.35 at idle when cold. Am still seeing this same reading taken on a cold engine at idle.

It's not the afterglow...
 

I'm familiar with the afterglow period, and in fact my sudden increase in voltage occurs well after I hear the glow relay click and see another (smaller) brightening of my dash lights, etc.

On these winter days I don't crank the starter until the afterglow period has just ended - right after I hear the relay, etc.

But I understand what the pros say - the alternator should charge even at idle - mine doesn't - but it charges just fine after it "wakes up". Now I've just got to see if I can pin the wakeup to a certain rpm, or, change the brushes, etc.

Rgds,
Chris W.

DC measurements 210 E300
 

my 98 E300 has been sitting for two weeks. Using a fairly reliable multimeter, checked the DC at the post under the hood in 3 stages:

before starting (no key in ignition)
12.4v

immediately after starting (no climate control or accesssories running)
13.5v

after one minute (Engine idle changes - GPs turn off?)
14.1v

DC - warm engine
 

15 minutes of idle (temp gauge just below 80c), DC still reads

14.1v

Chris:

Most likely brushes making poor contact until the alternalter is running pretty fast.

Pull the voltage regulator and check them.

Peter

The voltage regulator is designed to maintain 13.5-14.5 volts. The range is dependent on regulator tolerance and the temperature of the regulator - the higher the temp. the higher the voltage.

Field excitation current is provided by the battery. Since most automotive alternators will not self excite, if you disconnect the battery and try to bump start the car (manual transmission only) it will not start because the ignition system will never be energized.

Each alternator has a design amp output (at spec voltage) versus rotor rotation speed. Delco alternators are typically rated at 5000 rotor RPM (don't have data for Bosch, but it's probably similar), and with typical drive ratios of 2.5:1 the alternator should be capable of max output at about 2000 engine revs.

If the current load on the electrical system is greater than alternator capacity at a specified rotor RPM, voltage will drop, and as load is increased eventually the battery has to pick up part of the load when alternator voltage drops to battery voltage.

Because the regulator voltage is higher than fully charged battery voltage, if not saddled down with excess load, the alternator places a back voltage on the battery, which charges it.

Duke

Alternators are self exciting, and self starting, sometimes. Sometimes when they get old they don't start right away. I had an old one that left the alternator light on until it started, monitoring the battery voltage verifyed that. When it doesn't start at all you'll have to fix it, probably by replacing the voltage regulator, or maybe the rectifyer.