83 300D Alternator Exciter Wire Troubles

[Dr.Livingstone]

Hi everybody, I recently bought another 123 for a song and drove it 4 hours home. Initially it charged just fine, starting and stopping in rapid succession with no apparent issues. About an hour away from home we stopped for food and came back out to a dead battery. A jumpstart got us back on the road and we arrived home shortly before dark, thankfully.

Having experienced charging issues on every other W123 I've ever owned I wasn't too terribly surprised, so I dove in as soon as I could. I don't have a multimeter handy, so a test light was fabricated from a 12v bulb and some crimp connectors. Unplugging the harness from the back of the alternator, I found constant battery power at the 2 red wires and nothing at the blue exciter wire with the key on. My charge indicator on the gauge cluster turns on when I turn the key, but still no power at the connector. I pulled apart the ignition switch and jumped the red/black wire (keyed power) with a constant power also in the switch and listened for relays to click and found it to be working.

I took the car to O'Reilly's on a whim to get the alternator checked and the voltage regulator tested bad, though I would assume this is the result of the exciter not doing its job. My next order of business is to trace and visually inspect the wire (as I am unable to actually test continuity), but short of finding a break in the wire I'm kind of at a loss. Is there anything I'm missing or misunderstanding about the charging system? Any help or suggestions would be much appreciated!

[Dmitry at Pelican Parts]

Hello and welcome to the forum! I've attached 2 DIY tech articles from our site that I hope you find helpful in troubleshooting and addressing your charge issue. If you have any further questions, feel free to post a Comment just below the article and someone will get back to you with more detailed info. Best of luck!

Mercedes-Benz W123 Voltage Regulator Replacement | 300TD 1977-1985 | Pelican Parts DIY Maintenance Article

Mercedes-Benz W123 Alternator Replacement | 300TD 1977-1985 | Pelican Parts DIY Maintenance Article


-Dmitry

[Frank Reiner]

Originally Posted by Dr.Livingstone

Hi everybody, I recently bought another 123 for a song and drove it 4 hours home. Initially it charged just fine, starting and stopping in rapid succession with no apparent issues. About an hour away from home we stopped for food and came back out to a dead battery. A jumpstart got us back on the road and we arrived home shortly before dark, thankfully.

Having experienced charging issues on every other W123 I've ever owned I wasn't too terribly surprised, so I dove in as soon as I could. I don't have a multimeter handy, so a test light was fabricated from a 12v bulb and some crimp connectors. Unplugging the harness from the back of the alternator, I found constant battery power at the 2 red wires and nothing at the blue exciter wire with the key on. My charge indicator on the gauge cluster turns on when I turn the key, but still no power at the connector. I pulled apart the ignition switch and jumped the red/black wire (keyed power) with a constant power also in the switch and listened for relays to click and found it to be working.

I took the car to O'Reilly's on a whim to get the alternator checked and the voltage regulator tested bad, though I would assume this is the result of the exciter not doing its job. My next order of business is to trace and visually inspect the wire (as I am unable to actually test continuity), but short of finding a break in the wire I'm kind of at a loss. Is there anything I'm missing or misunderstanding about the charging system? Any help or suggestions would be much appreciated!

With the engine running did the charge indicator go off? If so, then that wiring is likely OK.
Because of the resistance of the indicator bulb, the voltage at the alternator end of the blue wire (where you connected the test light) is probably too low to illuminate the test light.
If the voltage regulator is the cause of low charging voltage, then, although the battery is not being charged, the charge indicator can still be extinguished because it is 1) connected to a separate set of diodes, and 2) is not connected through the regulator.
Stanley

[Diesel911]

You could of course remove the Alternator and have it bench tested. If it tests OK that would be a good indication you have a wiring or other issue.

[SD Blue]

How did O'Really's determine that the voltage regulator was bad? Was it removed and bench tested?

If it was checked on the car, was the DC voltage reading high? If so, then it is likely due to a failing diode and not the regulator.

A decent multimeter, measuring DC and AC voltage across the battery will tell you what is wrong with the alternator.

[Diesel911]

I bult one of the below alternator test cords to bypass the wiring and charging light to see if the alternator works.

Someone over on benzworld is the only other person I know that also built one.

I cut off an alternator connector when I was at the Junk Yard. It comes apart so you can service the wires.
Post #13 has a picture of the actual cord.
http://www.peachparts.com/shopforum/diesel-discussion/379680-no-charge-idle.html

[optimusprime]

Doctor did you solve the problem?????? Thanks to Pelican Parts for there help .

[BillGrissom]

Easiest diagnostic is a cigarette-ligher voltmeter. HF has for $5, or splurge $15 for a digital one on Amazon (Equuis, etc). If >14 VDC with the engine running, you alternator is working. Should be ~12.6 VDC w/ engine off, if battery is charged. Even better, I use a clamp-on DC ammeter, but few shops have such. I have a Black & Decker charger w/ an "alternator test" feature. It simply looks for an AC signal riding on the BATT voltage.

In my experience, most "won't crank" issues are due to a bad battery. I had 2 in the last few months. I have a Battery Brain on most of my cars. When it trips, I suspect the battery, then test at Autozone. One was just 2 yrs old, so free replacement (for 2nd time) since bought ~4 yrs ago. The other was in my 300D (Frame 74) and 6 yrs old. Replaced w/ an H8.

Re alt charging. The "boot-up" wire that comes from the dash bulb can be accessed at a black round 1-pin connector that is clipped to the aft side of the coolant reservoir in my 300D's.