Almost all the idiots were lit dimly when my girlfriend brought the car home. Checked around. Bad ground and alternator was the gossip. Ok. Put in new alternator, lights went out, yet no 13.8-14. Rev engine a little for a little and boom hello voltage. Turn off same thing consistently. So question;
Should the voltage be 13.8-14.1 immediately? I would think.? If so I got a bad alternator from autozone. NEXT at first I only had 13.2 when it finally came up, remember initially dim idiot lights, that went away as soon as we changed alternator but after cleaning all connections real good I got 14.1, now when I turn key no more idiot light initially then go out like they used too. No bad connections no forgotten wires. Checked coolant sensor by shorting all good, window washer lever sensor lights light just not the intialazation of the left lamps, brake etc. ABS, SRS and check come on when key first turned then out after start. They all used to light. I don't know the norm at this point?

summary: 1. should alternator voltage be 13.8-14.1 upon start up immediately?
2. Why would pre start up light check crap out after cleaning all connections and getting great ground and voltage. Just can't win...

Yes, the voltage should be in spec as soon as the alternator gets up to speed.

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Prost!

Voltage may or may not come up at first. Depends on the car. For instance, a diesel with after glow will not show normal voltage until the glow plugs go off. I would recommend that you tell us what you are working on.

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84 500 SEL (307,xxx miles)

Sorry, 1990 300E 94k, Gas 3.0. Got new alternator coming tomorrow, got Core back of original and will pull VR (maybe rebuild for spare) test all before installing. So I'll have 3. Orig, current and new replacement. So far we all agree voltage up as soon as rev. Or start. We also need to know if the headlight High Beam bulb is burned out ( which it seems be now, does it effect the left line of the idiot lights not come on during key up? I can't understand why they were working, cleaned all grounds and connections got voltage up beautiful and lost bulb test key up on left side....test coolant and brake lights and they light, just no more intialization test,on the left. Only clue, high beam light not working. Thank you in advance. Kenny

On a 126 car the "initialization test" is done by the alternator via the battery light. I assume the 124 is the same. If the battery light does not come on, it is probably caused by the alternator in this case.
I don't believe the high beam is part of that test. It's not on the 126.

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84 500 SEL (307,xxx miles)

Hmmm. The early 90's. All sorts of new gizmos. Lots of small alternators. In general.

13.2 seems fine. Thats more than your battery can hold.

Its car specific. Some don't do as well at idle. Especially that era. Once again. In general.

You could pull as many fuses as possible. See if it comes up. Def look at how many amps it is.

Test it on a machine would be more accurate.

Somewhere out there is a chart showing charging capacity at different rpm.

And lastly. You could convert to one that charges better at idle, if you like to pump the bass in stop and go traffic.

But charging is a slow balanced process. As long as that battery stays charged. I wouldnt worry.

The key to the "Batt" light coming on, when the key is first turned, is via a ground [negative] provided by the alternator - when the alternator starts turning along with a running engine, that ground signal turns to a positive - the Batt light goes out. With that said, check the main ground cable to the engine - be sure the engine has a good ground.

Also note the Batt light circuit supplies a positive to the alternator which is an "exciter" signal - that "gets things started" with the alternator first beginning to generate electricity. If it does not get "excited", it can't start generating electricity! So be sure the wiring going from the alternator to the dash is good as well - good clean connections along the way. A burned out Batt light bulb or broken wiring connection to that bulb would keep the alternator from working.

To read more about this, search google.com for the words...
Bosch Alternator Exciter

Yes indeed. Bad alternator rebuild. Installed another after testing before I left autozone. Voltage comes up ASAP to 13.5, all intialization lights come on all fixed. I also the believe the rough start has gone away, so 3 birds.... High beam indicator NG, separate issue, probably bulb. Long story short, the alternator failing causes weird stuff even if you just turn key.. Thanks to all.