My newly purchased '89 300CE isn't charging. 12.5V at the battery with engine off, 12.0 with it on so the alternator isn't supplying juice. I replaced the voltage regulator because the brushes were worn to nubs, but that didn't help. The battery indicator light in the dash isn't coming on with the ignition - in fact none of the lights in the instrument cluster are coming on except SRS. I pulled the cluster and checked the battery bulb and it's fine (6 ohms across contacts and it lit up when I touched it with a 9V battery).

I'm about ready to buy an alternator, but I wonder if something else is going on since the idiot lights in the cluster aren't coming on. Any suggestions?

since none of the dash lights are lit, I'd check the fusebox, and the ignition switch as culprits, but it could be a ground issue as well.

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check the wire that goes from the alternator (B+, or D) to the positive battery terminal. It may not be connected all the way. Also check the tension on the belt. If you replace alternator, consider upgrading to a larger 120+ amp alternator.

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1990 190E 3.0L

Google "alternator voltage drop test" First link is a youtube video. Good stuff.

• Good continuity from B+ to positive battery terminal and from D to where it connects at X4/10 terminal block.
• Belt tension seems OK - the alternator is turning with the engine on.
• 38k ohms from D at alternator to positive battery terminal when ignition is on. I'm not very good at reading schematics, but shouldn't it be pretty low resistance?

If I'm reading the schematic right, with the ignition in the run position I should get continuity from D on the alternator to fuse 7, but I'm getting an open. Between D on the alternator and the fuse is connector X4 and the ignition switch, so a bad connection from D to X4 would do it or a bad ignition switch. Although the seat belt buzzer and SRS light come on so would this rule out the ignition switch? Can anyone tell me where connector X4 is?

Thanks.

EDIT: Here is the X4 connector location (page 6, upper left) but it's not clear to me where this is at. Is it in the interior?

Here is the charging system schematic (page 59)

Here is a drawing showing where the X4 connector is (page 10), but it's not obvious to me where that is. Is it in the interior under the dash in front of the passenger seat?

Here is the schematic for the charging system (page 59).

I don't know if yours is the same... But on my 420, the Batt light turns on when a ground (-) is supplied by the alternator and that would be through a diode in the alternator.

If that diode was broken open, then no ground would be provided to the light.

If the alternator is working ok, then when first turning the ignition key and the alternator is not generating electricity, the ground is provided to the Batt (and other) lights. Then after starting, a positive voltage overrides the ground and the Batt light goes off.

Also in my owner's manual for the car, it says to check the alternator if the Batt light comes on OR if the Batt light does not come on when first turning the key.

Anyway if the diodes in the alternator were not replaced along with the voltage regulator, there may be a bad diode in the alternator. I don't know if you can replace just that - I prefer to replace the entire alternator as other parts get worn as well.

Update: I found X4. It's on the drivers side interior under the steering wheel and to the left.

• I have good continuity from the positive battery terminal to X4
• Open circuit from X4 to fuse 7 regardless of ignition switch position
• I'm getting 10k ohms between D on the alternator to fuse 7. (the battery bulb alone is 6 ohms)

This is pointing to the ignition switch or the wiring to/from the switch. I guess next step is to pull the switch.

I swapped out the alternator and now life is good I couldn't get my head around how a bad alternator could keep the battery light from coming on, but now I understand.

Did you get a new alternator or just use one that you've already had? IF the latter, then the next time you need to replace your alternator, you should look at doing the upgrade to the larger amp alternator. Its pretty amazing.

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1990 190E 3.0L

Actually to see if the alternator has failed in this manner [say you are driving a long distance], you would need to stop the car periodically, shut it off, then turn the key switch on to see if the Batt light does not come on when first starting the engine!

One way to solve this lack of notification of a problem is to install a voltmeter. They also make voltmeters which plug into a cigarette lighter.

Proper test is:

Verify battery is good and voltage makes sense
Voltage drop test
Check fuse
Use ammeter to check alternator

Following those steps will tell you exactly what is wrong every time, or that you can at least rule them out as root cause.