Intermittent alternator and dashlight failing '87 W460. How do trace fault?

I did some digging and learnt a bit. I think you have a 1986 G300D ?

Option 1:

This is one of the top causes on W123 diesels.

• The alternator needs initial "excitation" current through the dash charge warning light to start generating power (a classic Lucas/Bosch design feature).
• When you turn the key to ON (engine off), the light should illuminate. If the bulb is burned out, missing, or the socket/circuit has poor contact, there's no excitation path → no charging even with a brand-new alternator.
• Fix: Check if the battery/charge light comes on with ignition ON (engine OFF). If not, replace the bulb (it's in the instrument cluster). Also clean the bulb holder/contacts. Test continuity across the bulb circuit.

Option 2:

Many aftermarket or remanufactured alternators come with cheap/faulty internal regulators, or the regulator fails soon after install.

• The regulator (with brushes) is a common weak point and can be replaced separately without buying a whole new alternator.
• Fix: Remove the alternator (or access the rear), unscrew the regulator cover, and inspect/replace the regulator (Bosch part or equivalent is inexpensive). Many owners report this fixes "new alternator not charging" issues.

Option 3:

The alternator has a multi-pin connector (often with two thick red B+ wires and smaller wires including the D+ excitation wire).

• Loose, corroded, broken, or miswired connections (especially the thin D+/excitation wire) prevent charging.
• Previous owners sometimes modify wiring, or the plug gets damaged.
• Fix: Unplug the connector, clean contacts, check for voltage at the D+ pin (should get ~12V with key ON from the dash light circuit). Inspect the red wires for good tight connections to the battery/alternator output.

Try the first option, since you clearly have NO dash Bulb installed... and if you then need to check the connection from the alternator to the bulb (option 3), see if this helps:

The multi-pin connector on the alternator of a 1986 Mercedes 300D (W123 with OM617 engine) is a 3-pin (or 3-prong) plastic plug from Bosch. It connects to the rear of the alternator and handles the main output plus excitation.

Typical Configuration

• Two large/fat terminals (often the bigger slots or pins): These carry the high-current charging output (B+).
  • Wires: Usually two thick red (or red/white) wires.
  • Function: These are paralleled for higher current capacity (one often routes to the starter positive post, the other to a nearby terminal block or directly toward the battery positive). They should show constant ~12V battery voltage at all times (key off or on), as they're direct battery connections via heavy cables.
• One smaller/skinny terminal (the narrowest slot): This is the excitation/indicator wire (D+ or sometimes labeled as the warning light circuit).
  • Wire: Thin blue (or white/blue in some descriptions).
  • Function: Provides initial excitation current from the dash charge warning light bulb to "wake up" the alternator. With ignition ON (engine OFF), you should see ~12V (or slightly less) here from the instrument cluster circuit. Key OFF: no voltage. This wire also feeds the tachometer signal on some diesels and lights the battery warning lamp if charging fails.

The plug is keyed/asymmetric so it only fits one way— the two large pins are typically side-by-side or in a row, with the small one offset (often the smallest slot is for the blue exciter wire).

Visual Layout (text representation, viewing the plug face/male side on the alternator or female harness side)

  Large pin (B+)     Large pin (B+)
       |                 |
       -------------------
              Small pin (D+)

• Exact orientation can vary slightly by year/production, but the small pin is consistently the exciter (D+), and the two larges are B+ outputs.

Quick Tests at the Connector (unplugged, multimeter)

• Key OFF: ~12V on both large pins (to ground); 0V on small pin.
• Key ON (engine OFF): ~12V on large pins; ~12V (or bulb voltage drop) on small pin → confirms dash light circuit is feeding excitation.
• Engine running: Voltage on large pins should rise to 13.5–14.5V if charging works.

If your new alternator has spade terminals instead of a plug (some aftermarket do), the main B+ is usually a large stud/bolt on the back, and D+ is a smaller spade (often marked "D+" or "61"). The harness plug may need adapting, but stock is the 3-pin as above.