Charging system basics + questions

[Kip Amore]

Howdy fellas,
You guys will know this stuff in your sleep, but I'm the diesel newbie and I just gotta know: how exactly does the battery idiot light work in the 123? My charging system took a crap last week (stranding me roadside twice!) so I swapped the voltage regulator. BUZZZZ wrong answer. I've got an alternator on order now.
As I understand it, the charging system is like any motor - alternator spins, internally rectified to DC, regulator regulates by reading battery voltage, wiring goes from alternator to battery + (stopping at solenoid along the way). Yes, I checked for continuity from alternator to battery. Yes, the battery is new and takes a trickle charge on the charger like it should.
My question is if the car wasn't charging, why didn't the light come on? The light itself works when I crank the car. How does this light "know" to come on at low voltages?
Last question: my method of testing the charging system is to turn the wipers on and disconnect the battery with the engine running. I know not to do this on a gasser (possible surge blows out ignition system). Is there any reason why I shouldn't be doing this on the Benz?

Thanks,
Kip Amore
77 240d

[TonyFromWestOz]

Originally Posted by Kip Amore

Howdy fellas,
You guys will know this stuff in your sleep, but I'm the diesel newbie and I just gotta know: how exactly does the battery idiot light work in the 123? My charging system took a crap last week (stranding me roadside twice!) so I swapped the voltage regulator. BUZZZZ wrong answer. I've got an alternator on order now.
As I understand it, the charging system is like any motor - alternator spins, internally rectified to DC, regulator regulates by reading battery voltage, wiring goes from alternator to battery + (stopping at solenoid along the way). Yes, I checked for continuity from alternator to battery. Yes, the battery is new and takes a trickle charge on the charger like it should.
My question is if the car wasn't charging, why didn't the light come on? The light itself works when I crank the car. How does this light "know" to come on at low voltages?
Last question: my method of testing the charging system is to turn the wipers on and disconnect the battery with the engine running. I know not to do this on a gasser (possible surge blows out ignition system). Is there any reason why I shouldn't be doing this on the Benz?

Thanks,
Kip Amore
77 240d

Yes

I know not to do this on a gasser (possible surge blows out ignition system)

Like you say, an alternator is an alternator whether it is on a Gasser or on a diesel. You can easily smoke your diodes, regulator & any other electronics (computer if fitted, CC amp, Stereo, etc)
EXPENSIVE to repair.
Just use a voltmeter.
Measure battery voltage - engine off = 12.0 - 12.6 V is normal
Measure battery voltage - engine idling = 12.6 - 14.0V is normal
Measure battery voltage - engine idling with headlights, heater on = 12.6 - 13.6V is normal
Measure battery voltage - engine at >2K RPM, regardless of load = 13.6 - 14V is normal
Note that the alternator will charge a battery to ~15V then drop the volts back to 13.6 once charged.

[autozen]

Yep Tony's right. By removing a battery cable, you remove the load which allows alt voltage to go out of control. You can smoke alot of things. Apparently the car won't restart after you shut it off. I'm assuming the car doesn't die, because once a diesel is running, you can throw the electrical system away. You may have a defective battery. You may have a dead cell. I don't care if it is new. I would not rule it out of the diagnostics.

Peter