harming alternator wiht poor shutdown? - PeachParts Mercedes-Benz Forum

coachgeo · #1 03-17-2009, 07:50 PM

300SD

was reading in another forum and it mentioned with this engine if you power off the key but the car keeps going you will toast your alternator.

welll.. thats what happens...... I turn it off...... car keeps going..... I have to accelerate the engine to ..... I assume build vacuum so the vacuum shut off can do its job and power off the motor.

am I harming my alternator?

toomany MBZ · #2 03-17-2009, 07:56 PM

I don't know about the alt, but fix the shut off issue. I've had a similar issue, I doubt it harmed the alt. The light comes on because the key is turned off.

#3 03-17-2009, 08:11 PM

No harm at all. With the key off, the alternator field coil is not powered. It will sit there spinning but not charging.

This "another forum" sounds like they need to check their facts.

tangofox007 · #4 03-17-2009, 08:25 PM

Quote:

Originally Posted by coachgeo

300SD

was reading in another forum and it mentioned with this engine if you power off the key but the car keeps going you will toast your alternator.

Go back to that forum and ask them how it can be that aircraft have a switch that permits the alternator field to be turned off separately from the engine.

coachgeo · #5 03-17-2009, 08:26 PM

Quote:

Originally Posted by ForcedInduction

No harm at all. With the key off, the alternator field coil is not powered. It will sit there spinning but not charging.

This "another forum" sounds like they need to check their facts.

no Im sure they got their stuff right. I extrapolated wrong.

In Unimogs this does happen and has been discussed over and over. In a recent topic the discussion was about a transplanted 617 into a Mog and I went "hmmmmmm"..... "wait". "is this a Mog issue or an MB issue"...... "I better check" soooooo here I am checking.

Glad to hear this is not an issue with my SD.

It helps elemenate a potential problem and help diagnose another.

tangofox007 · #6 03-17-2009, 08:37 PM

Quote:

Originally Posted by coachgeo

In Unimogs this does happen...

How?

coachgeo · #7 03-17-2009, 08:40 PM

hopefully one will chime in and fill us in.

I belive its more of an issue with 406 and 416's which I do not have. Have just read threads on it before.

Simpler=Better · #8 03-17-2009, 09:24 PM

It can't hurt the alternator, that doesn't make sense.

Gene Horr · #9 03-17-2009, 10:30 PM

Quote:

Originally Posted by tangofox007

Go back to that forum and ask them how it can be that aircraft have a switch that permits the alternator field to be turned off separately from the engine.

Well, that design is for a specific purpose, though. That's so you can disable the hour meter in order to cheat on your rental time.

Ducking and running....

johnbonds · #10 03-17-2009, 10:40 PM

Quote:

Originally Posted by coachgeo

300SD

was reading in another forum and it mentioned with this engine if you power off the key but the car keeps going you will toast your alternator.

welll.. thats what happens...... I turn it off...... car keeps going..... I have to accelerate the engine to ..... I assume build vacuum so the vacuum shut off can do its job and power off the motor.

am I harming my alternator?

Short answer: No. But, not a good idea to then panic and turn it back on. I don't think it will hurt the Bosch alternator, but it will toast some regulators by burning out the diodes (that convert the AC of the alternator to DC current) if there is a heavy current possibility, as in a boat system with lotsa battery power. If you turn it off, leave it off until you get the engine stopped. That will keep you from any possibility of hurting the regulator (which is unlikely anyway with a charged battery).

John Bonds
Charleston, '82 240D daily driver (and a diesel driven sailboat)

Diesel911 · #11 03-17-2009, 10:58 PM

If you Vacuum Pump is putting out enough vacuum you might want to check for cracked hoses. Also there is a shutoff switch down near your steering lock behind the the Ignition Lock that can wear out and cause a vacuum leak; also hoses/tubing down there to.

May be the story got twisted as most Auto Repair Manuals tell you not to remove the Battery Cable from a [U]running[U] Engine and say it will damage the Alternator or Regulator. I have not tested this!

Simpler=Better · #12 03-17-2009, 11:51 PM

Quote:

Originally Posted by Diesel911

May be the story got twisted as most Auto Repair Manuals tell you not to remove the Battery Cable from a [u]running[u] Engine and say it will damage the Alternator or Regulator. I have not tested this!

I have done this with a '93 Ranger, '91 Hyundai Sonata, and '82 300D without noticeably shortening the alternator/regulator life.

#13 03-18-2009, 12:01 AM

When the key is off, power to the field coil is off. Without power to the coil the alternator is incapable of generating any current because there will be no magnetic field.