Alternator or something else? (1985 300TDT) - PeachParts Mercedes-Benz Forum

azitizz · #1 01-03-2011, 10:14 AM

Quote:

Originally Posted by Diesel911

Harbor Freight sells digital Volt/Ohm meters (millimeters) for $5 or less when they are on sale.
You will need to charge up your Battery so you can get the Engine started and running and set the meter to the proper DC voltage range with the red probe on the + Battery terminal and the black probe on the - battery terminal and read the voltage.

You need to be getting around 13.5 volts for the Battery to charge.

Sounds like you have already looked up some threads and know that slipping belts, loose Alternator Pulleys, worn Brushes, poor Alternator wire connections, poor Battery terminal connections and poor Engine Ground strap connections can all cause the Alternator not to charge.

As far as rebuilding the Alternator goes there is no how-to-do it with pictures that I have come a crossed.
I have had mine apart to the extent that I changed both of the Bearings and it is no more difficult than the Delcos I have had apart with the exception that I needed a Bearing Plate and a Gear puller to pull off the front Shaft Bearing on mine.

I do not know if you if that was the only way to get the Bearing off because since I already had those Tools I used them.
A few members have replaced bad Diodes in there Alternator but I have never done that so cannot comment on that.
At least one member has changed the Slip Ring.

Most have limited their Alternator repair to replacing the Brushes, Voltage Regulator or bearings.

Another way to go would be to Charge the Battery; get the Engine started and drive to the nearest Auto Parts Seller that will test your Charging System for free. They will Tell you if it is charging or not.

Hi there, thanks for the tips. So just wondering, When I do a voltage test with the meter, I would be placing the probes on the battery terminals?

Wouldnt that simply give me a reading of what the battery is putting out and perhaps a mix of what may be coming out of the alternator?

My thought would have been that I would have to disconnect the battery and test the alternator separately. But I guess the car would stop running though if the alternator wasnt working..or would it?

compress ignite · #2 01-03-2011, 11:06 AM

It'll "Blow Out" the Diodes, instantaneously.

Testing with the engine running at the Battery Terminals (Alternator connected)
Gives you the Alternator Output.

Rebuild Bosch 120 Amp Alternator (Pictorial)(Almost ALL Bosch alts are similar):

http://www.oichan.org/mods/alter/alter2.htm

Rebuild Bosch 143/150 Amp Alternator(Pictorial):
(Getting the Alt OUT of the V-12 Chassis is Tres Difficile,You'll not be as Unlucky)

http://w140repair.com/Alternator_Bearing_Replacement.htm

MultiMeter (Fluke DMM as an Example) usage tutorial:

http://mechatronics.mech.northwestern.edu/design_ref/tools/multimeter.html

Diesel911 · #3 01-03-2011, 01:25 PM

Quote:

Originally Posted by azitizz

Hi there, thanks for the tips. So just wondering, When I do a voltage test with the meter, I would be placing the probes on the battery terminals?

Wouldnt that simply give me a reading of what the battery is putting out and perhaps a mix of what may be coming out of the alternator?

My thought would have been that I would have to disconnect the battery and test the alternator separately. But I guess the car would stop running though if the alternator wasnt working..or would it?

Putting the Probes on the Battery without the Engine running and the Key off will tell you the Battery Voltage that on a charged Battery should be 12.50-12.75 Volts.
Ounce the Engine is going and if the Alternator is charging the Voltage will go up to what ever the Voltage Regulator is set up to charge the Battery at. That be at least 13.5-14.50 but most seem to be closer to 13.5 volts than 14 volts.
If it is charging OK the next thing to check is to see if it will continue to charge properly after the Head Lights are on and some of the accessories.

On my Car the charging light never comes on no matter how many accessories I use (my AC does not work so that is not one of the accessores I use); my charging voltage stays at 13.7 and as long as the Engine is running my Charging Light never goes on.

Some members when they are running their accesories and Head Lights when they idle the Charging light goes on but quickly goes off as soon as the Engine rpms go up.
I do not think that is normal but it seems to be OK as long as it does not deaden the Battery.

azitizz · #4 01-03-2011, 02:20 PM

Ill be making that Alternator voltage check as soon as I have a decent charge again on my battery.

From what Ive read here and there, it seems like the voltage regulator brushes often cause insufficient charging. Does it sound like that could be the case?

Diesel911 · #5 01-04-2011, 01:00 AM

Quote:

Originally Posted by azitizz

Ill be making that Alternator voltage check as soon as I have a decent charge again on my battery.

From what Ive read here and there, it seems like the voltage regulator brushes often cause insufficient charging. Does it sound like that could be the case?

Yes. That is the reason some have suggested taking off the Voltage Regulator to check the Brushes. If either or both of the Brushes has 2mm or less sticking out the Brushes or the Voltage Regulator should be replaced.

azitizz · #6 01-04-2011, 08:44 PM

So I charged the battery and started the car. Before starting and after I unhooked the battery charger, I was getting a reading of 14.4 V. Then I started the car and let it idle with my foot pressing lightly on the gas to keep it from stalling in the cold weather.

Once it was good to stay idling on its own I tested the voltage and it was 12.4 V Or something close to that. I tested again after I turned the car off and it was reading 13.7 V What do you think that means?

My guess is that with the few accessories being used, clock, dash and dome light as well as the stereo display that it is taking up some of the juice and so the reading is lower and from the alternator. But thats a guess

Its a bit low for the alternator by what one of the previous posts suggest where it should be regulated at. But What I wonder is why the voltage wouldnt simply show whichever one is higher. I would think testing two separate charges that are connected to one terminal would simply give the highest reading. But again, I'm no electrician

By the looks of it, the voltage regulator can come off without having to remove the alternator. Is this right? This to me would be the logical start. But Im open to suggestions.

funola · #7 01-04-2011, 08:55 PM

Quote:

Originally Posted by azitizz

So I charged the battery and started the car. Before starting and after I unhooked the battery charger, I was getting a reading of 14.4 V. Then I started the car and let it idle with my foot pressing lightly on the gas to keep it from stalling in the cold weather.

Once it was good to stay idling on its own I tested the voltage and it was 12.4 V Or something close to that. I tested again after I turned the car off and it was reading 13.7 V What do you think that means?

My guess is that with the few accessories being used, clock, dash and dome light as well as the stereo display that it is taking up some of the juice and so the reading is lower and from the alternator. But thats a guess

Its a bit low for the alternator by what one of the previous posts suggest where it should be regulated at. But What I wonder is why the voltage wouldnt simply show whichever one is higher. I would think testing two separate charges that are connected to one terminal would simply give the highest reading. But again, I'm no electrician

By the looks of it, the voltage regulator can come off without having to remove the alternator. Is this right? This to me would be the logical start. But Im open to suggestions.

Did you rev the engine above 1500 rpm when you measured the 12.4 V? If you really did get 13.7 volts after shutting the car off and the battery charger was not connected, something is wrong. Fully charged battery voltage chould be about 12.7 volts.

azitizz · #8 01-04-2011, 09:00 PM

Quote:

Originally Posted by funola

Did you rev the engine above 1500 rpm when you measured the 12.4 V? If you really did get 13.7 volts after shutting the car off and the battery charger was not connected, something is wrong. Fully charged battery voltage chould be about 12.7 volts.

I reved it above 1500 only to keep it from stalling on me before it was warm enough. But I'm doing this solo so I had no way of taking a reading and reving it at the same time. Oh wait, I guess i could have done this using the throttle in the engine right? Duh.....

...So I just made the measurement and revved it up to probably at least 2500 or even 3000 rpm and the voltage never budged. It seemed to slightly climb weather or not the engine was revving or idling. It was showing about 12.5 V.

Then when I shut it off it almost stayed the same but climbed slowly to 12.7V over a period of about 3 or 4 minutes. I Imagine it continued climbing but I havent been out to check again.