Mercedes 300D Alternator Issues Again

I've got charging issues on the 300D again. Let's see if we can fix it.
Link to Regulator:
amzn.to/2xvGeXJ
Link to Titan Ratchet:
amzn.to/2W2agfJ

[@danpicocfp6276]

Thanks for this video. Runs through logical steps in diagnosing a charging system. Keep good going!

[@jeremybanks719]

Dealing with the same thing right now with my 1985 300sd. I've replaced the VR, sprayed electric cleaner in the alt and god it was crusty. Tested the wires tighten the terminals in the alt plug. Cleaned contacts at the junction box, then I jumped the alt from bat to the field to excite the reg. Just a tap of the wire is all it took and the alt started changing. Every so often I got to jump it and it works as it should.

[@joelucchese9165]

Exact same problem with my 300sd. Changed regulator, not charging at idle, but starts charging with increased rpm. Thanks for taking time to make video. Very helpful!

[@ColinQuinnuniverse]

Thanks for walking through your process. Many times I have just swapped the regulator on the w123 and charging was restored.
Dealing with a no charge issue on a recently acquired w123. Have a back up regulator on the way. Going to test wiring as you have. Hope I don't need a new unit.

[@williamgrissom9022]

The Motorola-style alternator needs boot-up current to enable its internal power source. That current comes from the dash bulbs shown, plus the low-fuel lamp in 1984+ 300D. I think just from the "Batt" lamp in 1983-. Same deal in 1970's Chevy's. If you replace with LED bulbs, they don't provide enough current for boot-up, but once you rev the engine >1000 rpm, residual magnetism in the alternator rotor is usually enough to boot-up without the lamp current, especially in an older alternator. The Owner's Manual even warns to drive directly to the dealer for an emergency bulb replacement if the "Batt" lamp doesn't light on key-on. I don't think your voltage test at the 1-term round connector (bootup wire) on aft side of coolant reservoir tells much. That just insures that at least one of the dash lamps which supplies boost current is working and that the Vreg in the alternator is trying to draw current, i.e. drawing black lead down to gnd voltage. Switch the multimeter to amp mode and measure bootup current. Filament bulbs probably draw >200 mA (2.4 W). Once booted up, the Vreg drives that wire to 12 V so the dash lamps turn off (dV = 0 V, unless something else is grounding them). LED bulbs might draw only 10 mA. Strange that once you forced bootup w/ the jumper wire, the bulbs didn't stay off. Another possibility is the alternator wasn't getting grounded to the engine block thru its case. I had that once in my Dodge Dart. Sanded the aluminum and cleaned the bracket and viola it worked again.
An advantage of these is that you can swap in a $10 brush-regulator module in-car, which is great since M-B designed the alternator-bracket-from-hell, then surrounded it with parts that prevent easy access to the bolts (AC HP tube runs up R side of engine, EGR valve, heat shield in 1985 CA cars). Alternator output is on 2 parallel wires to that screw block in front of the battery. Perhaps they needed two 56 terminals (spade) at the alternator to handle the current. The left two screws are BATT+. The smaller right screw is "start" wire to starter's solenoid, I think direct from key switch (no relay) via NSS. If you jumper the "start" to "BATT+" at those screws, the starter will engage, even if not in "P" or "N". Don't stand in front, set parking brake (always).

[@v.dembsey3355]

For other troubleshooting, the purple wire in that power junction box goes to the glow plug relay. A bad glow plug relay can stick "on" and just run the glow plugs constantly which the alternator can't keep up with. P.S. I had that exact alternator and it came with a bad voltage regulator right out of the box. Maddening.

[@mjmcomputers]

I’ve heard of them sticking and so far haven’t had that issue.

[@v.dembsey3355]

@@mjmcomputers There is a mod for the glow plug relay in which you solder a little .10 cent resistor to the board which makes the glow plug light on the dash glow whenever the glow plugs are being powered. People often think the glow plugs go out when the light goes out but they actually keep glowing until the relay clicks open again. The resistor basically tells you if the relay has failed closed.

[@Fngsouth]

Good video . thanks , interesting paintwork 😁

[@cal48koho]

I agree with Wingding. Alternators internally rarely fail and the brushes are usually the culprit. The MB regulator has brushes integral which is wonderful. I always just replace brushes to fix alternator problems. Some alternators are impossible to replace brushes without a special tool however so you have to go with a reman, like on most Toyotas.

[@mjmcomputers]

New regulator didn’t fix the issue, ended up buying a reman.

[@nicolasr.3843]

Bonjour , merci . Vidéo intéressante .
Au revoir et bonne route .

[@wingding6758]

Alternator Brushes..... I always carry a spare set in the glove box. Those moving parts are the first to go.

[@wingding6758]

You can get them for about $15 .. In the end you did say the brushes were good.. So... that changes everything if that is true.. But that is the first thing that usually goes.