'85 300TD charging system

Hello, I suspect I have a charging deficiency in my '85 300TD(230K miles). To-date I have replaced the brushes/regulator, belts, replaced dash light and cleaned bulb holder. Voltmeter testing at the battery shows 12.5 vdc at idle, 13.5 vdc at 3-4K rpm. The dash light will not light at all, but shows continuity from the multi-pin connector across the bulb circuit with holder in place. As far as symptoms go: I occasionally (once every six months or year) find that the battery is low and will not turn the starter. The climate control system does not react to any setting less than full on or full off and the heater motor takes a few minutes of driving to start blowing. Is it necessary to have the dash bulb light up in order to excite the alternator circuit? Is it correct to expect more like 14 vdc at the battery during idle? Any further diagnostics indicated? Any thoughts appreciated....

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Body and Interior Specific Aid

Chassis Model Number

W123

Similar problem with '85 300SD charging

My problems are very similar to the above post. I also have an 85 300SD, mine with 320,000 miles.
Labored start about ten days ago, suggested a battery problem, but in fact, the original alternator drained the battery, rather than charged it. An Autoparts store salesman found that the battery as good, but not fully charged. Later, the alternator was declared dead, even with new voltage regulator purchased from Mercedes Source.
Eventually the original alternator was replaced with a re-manufactured alternator. It checks out 100% on the bench, but fails to charge the battery when the engine is running at idle speed. At highway speed, the alternator puts out only about 13 v. Terminals and cable terminations have been cleaned and are bright. No voltage drops end-to-end with Digital Volt-Ohm meter.
I saw another post regarding electrical failures. That user found the Hazard Switch was the focus of many electrical circuits. I wonder if the charging circuit also goes through the hazard switch contacts.
The Service Advisor at the local MB dealer suggested the problem may be in the "over voltage protection relay", on the engine side of the firewall.
I do not have an electrical circuit diagram. Hopefully that would give a clue as to why the charging circuit is so flaky.
Any suggestions will be welcome. Car is out of service until the charging circuit is fixed.
Thanks for your assistance.

Suspect faulty alternator / glow plug relay

I know even though it is rebuilt. I have had a lot of problems with aftermarket rebuilt alternators. You might have to have it tested again (ie second opinion). If you are sure the alternator is good then suspect a possible battery drain while you are driving. I have seen this happen with glow plug relays staying on while the engine is running. Unplug the relay while the engine is running and check for voltage variation. This can be done with a simple volt meter. But to really test the alternator, you need a high amp clamp style amp gauge to really see what is going on with the charging circuit while the engine is running. Try to find a good automotive electrician (does not need to be a Mercedes mechanic) to help you.

GLOW PLUG RELAY IS SUSPICIOUS!

KENT : I think you put your finger on the culprit: the glow plug, gp, relay. I measured voltage at the power connection of the relay, under several situations:
1 Engine idling.. gp connected...11.41v.
I found that the wires were hot to the touch with the engine running and gp connected, confirming that there was power going to the glow plugs when they should have been de-energized.
2 Engine idling.. gp disconnected... 13.25v
3 Engine stopped..gp connected .. 12.70v
4 Engine stopped..gp disconnected.. 12.77v
Finally with battery isolated, engine stopped, battery voltage is 12.65v
Now the question is: why is the glow plug relay still energizing the glow plugs after the engine is running.
Does the ignition switch break the glow plug circuit when the key is free to return to the running position, or is this a function of the timer in the gp relay?

Until the problem is resolved, I will pull the plug to the glow plugs when the engine is running, to prevent the drain of the battery and frying of the glow plug wiring.
If worse came to worse, I could put a simple toggle switch in the power supply to the gp relay and switch off the circuit drain once the engine is running.
Again, your opinion is valued. Thanks

Good detective work

Glad you found the problem. Failed pre-glow relays are common and there are hard to fix or try to wire in a manual activation circuit. If you like the idea of having a reliable manual control glow activation relay check out my video where I explain how it works. CLICK HERE....

84 300 turbo diesel-charging system problem

I am having issues somewhat similiar to the post "'85 300TD charging system".

Issue: start car and drive to destination. Car will not start (turnover or click). Battery is 6 months old and restested at auto parts store. Alternator was removed and tested.

At idle with no load alternator charges at 14.5 volts. When blower is turned on it lowers to 13 and if lights are turned on it charges even lower. When I rev it up a little its more than 13 v.

A mechanic said dimmer switch is not working and dash board lights are always on. I removed it.

I watched one of your videos where you put a test light between the negative terminal of battery and the negative lead and then checked fuses. But the light didnot come on so no draw.

Seems issue is with charging not an overnight draw.

Voltage regulator? Glow plug relay?

Thanks for the website! Videos and this forum are great

Karl-Chicago

Try this test first

Next time you drive some where - start the car - then unplug the BIG wiring plug at the glow relay. Drive to your destination and see if engine will turn over.

1984 300 turbo, Glow plug relay

You were correct. I followed your instructions and it was the glow plug relay. I have a spare 84 300 turbo so I took the relay from that one.

I also bought a manual relay but have no installed yet.

Thank You for your help