Alternator output question
 

Today I measured the battery voltage to see if the alternator was doing ok. Here is how my test came out.

With the car not running the battery voltage is about 12.5 volts.

Start the car and at idle the voltage goes to about 12.75 for the first three minutes (my glow plug relay has the three minute afterglow feature)

After three minutes the battery voltage jumps up to about 13.71 volts at idle

My thoughts are the alternator and voltage regulator are ok....

What does the forum think?

Many thanks,

Glenn

As long as the battery voltage stays between 13.5 and 14.5 at higher RPMs, I'd say that you're just fine.

How about with AC and headlights on?

Sixto
87 300D

I don't what the current draw of the AC but alternator output tests need to be performed under load, at least high beams if not lights and blower motor.

~13.5 V is good.

Obviously measured at operating RPM of alternator. I don't what the design spec is for these but I'd guess 2,000 RPM would be good.

Voltage measurements without meaningful current draw (load) reveals very little.

Afterglow is a pretty good load. Above resting voltage after engaging the starter motor and during afterglow, means that it's putting out some serious current.

Is that the term for the glow plugs continuing once started?

Yeah I'll bet you that's pretty close to a dead short. What is it 50 amps? I mean on the old 115 with those external wires connecting the series glow plugs you'd get a nice cherry red glow. Possibly 1400 F or so.

Forgot to say I did have a load on it when testing it. I had the high beams on.

Worry about the maximum voltage too. Turn everything off, wait until afterglow finishes, and rev up the engine to 2500 or so. Ensure that you don't exceed 14.5V. That shortens the life of your battery. Not much more and you end up with a habit of carrying a lot of spare bulbs.

So, you have an 1983 Mercedes 300D that you are someone installed a Glow Plug Relay with the Afterglow feature?

My favorite parts place shows a 65 Amp Alternator for your year and model.

The tech data book has an 88 amp hour Battery for Diesels. 90 amp hour for Canada, Norway and Finland.

What does the voltage drop to when the Glow Plugs are applied and the Engine is not cranking or running?
Next, what is the Voltage when Cranking (the Glow Plugs are also on during cranking if the wiring is stock)?
In general (on a regular Car Battery) the further down the Voltage drops when under a load the shorter the Battery life is.

Not for Sweden??? :)

Sixto
87 300D

Yep, I had to replace the glow plug relay about a year ago and the one I got had the afterglow feature. Actually although a lot of people like it, I'm on the fence about it

Afterglow is for pussy cars with weak compression. If you don't *need* it to get started and running off to your destination on the coldest days you drive, delete it. Just an opinion.

The afterglow relays I'm familiar with work with a coolant temp sensor. If the engine's warm, there's no afterglow. In fact, to Scott's point, afterglow relays will flash the glow light for much less time than a traditional glow relay or not at all when the engine's warm. I don't know if that translates to less actual glow time before the relay cycles off before the starter cranks. Without a temp sensor I believe you get maximum glow cycle every time.

Sixto
87 300D

M-B disagrees with you. The 606 has afterglow from the factory.

...or for cars with more emissions controls and concerns, like 606.