The battery on my 79 300TD wagon isn't charging at full capacity. I have already changed the battery and the alternator and have also cleaned the chassis ground connectors and crimped down the cables a bit to get better contact and the battery still only charges at around 10.4-11.7 and every once in awhile it will hit 12.2 (for only a few seconds) but it should be between 13-14. What am I missing?

Voltmeter definitely accurate? Does the battery go dead? Polarity at batt correct?

Unplug glow plug relay and see if the voltage goes up.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I'm pretty clueless about this stuff voltmeter stuff, unfortunately. I think it was off, I think it should read at around 700, right? And it was around 500. The battery does eventually go dead and needs to be jumped.

I will unplug the glow plug relay and see what happens. Thanks.

This might be a stupid question, but... regarding the relay. I'm reading this tutorial about replacing the glow plug relay https://www.pelicanparts.com/techarticles/Mercedes-W123/32-ENGINE-Glow_Plug_Relay_Replacement/32-ENGINE-Glow_Plug_Relay_Replacement.htm

Would I disconnect the battery and then unplug the relay like in steps 5 & 6 and then reconnect the battery and check to see of that battery voltage goes up?

A fully charged battery after it sits a bit will be approximately 12.5-12.7 volts. Depending on which spec you read a minimum charging rate is 13.3 volts; however, more average is around 13.5-13.7 volts but some go up to 14.5 or so volts.

When you first turn your Key the warning lights should go on including the low charge light. If that light is not going on when you first turn on the Key it could mean the bulb is out. The significance there is that the Bosch system needs the bulb working to charge properly.

Low or no charging can be caused by too loos belts.

You can remove the voltage regulator and see if the brushes are worn down too short. If the brushes are 2mm it is time to replace the brushes or the voltage regulator.

Make sure your Ground Strap is not broken and making good contacts. It is under the car about where your feet sit on the US drivers' side.

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84 300D, 82 Volvo 244Gl Diesel

When I turn the key on, the battery indictor light does come on. Does that mean that's okay?

I just put in a brand new alternator which has a new voltage regulator on it so that should be fine. And yes, the ground strap has been cleaned up and is making good contact.

I'll double check the belts again but I think those are fine too.

That means it has a working light bulb.

Is everything fine now or is it still not charging?

If the Battery is no good the Alternator also will not charge correctly.

If you get an alternator from some local place ask them to run test it before you leave the store with it.

Assuming you have a bosch alternernator when you disconnect the electrical connector you should get battery voltage at the 2 large slots all of the time and no voltage at the small slot with the Key off. With the key on you should get just a tad less then battery voltage from the small slot.

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84 300D, 82 Volvo 244Gl Diesel

Start the car first, check voltage, then unplug the glow relay and check again. The glow plugs us more amps than the alternator can produce at idle.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

So I started the car and the voltage was at 11.7 and then I unplugged the glow plug relay and the voltage stayed the same. No change. So I suppose that means it isn't the glow plug relay, right?

You can measure voltage at the glow plugs. There should be none when they are off. Pulling the plug on the glow relay will disconnect the glow plugs regardless. Perhaps measure voltage at the glow plugs then disconnect the relay.

AutoZoo tried to condemn my alternator when the glow plugs were glowing and pulling measured voltage down. Also, I've had bad voltage regulators new brand name out of the box.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

You are correct. The alternator or regulator could be bad out of the box. It happens sometimes. Did you check for the specified voltages at the alternator electrical connector yet? I'm wondering if you have a broken wire in your charging circuit.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

I have not, I will though. One thing that's kind of strange though... as I initially stated when I first posted,

"the battery still only charges at around 10.4-11.7 and every once in awhile it will hit 12.2 (for only a few seconds) but it should be between 13-14."

Now, it does the same thing, usually hovers between 10.4-11.7 and occasionally gets to 12.2 but now it will also hit up to 15-16 for very brief moments and then drop back down to 11.7.

That sounds like a bad regulator to me. If the other test I mentioned checks out, then I'd suggest you pull the alternator and take it back to where you got it from and have them bench test it. Then in the likely event that it fails, you can get another one under warranty.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.

Is it fixed yet ? .

I only use BOSCH or HELLA branded regulators for exactly this reason .

When you start the engine, as soon as the red charge lamp goes out the voltage should rapidly increase to at least 13VDC off idle .

Very often the entire fault is the regulator, even if a freshly 'rebuilt' alternator as none but BOSCH puts in the $ BOSCH / HELLA regulators .

It's also very easy to mis install the regulator so one brush isn't riding correctly on the slip ring, this causes the brush to wear *very* rapidly .

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-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

I actually took it back to my mechanic who installed the alternator a month ago and he replaced it with another new alternator and voltage regulator. It has only been a couple of days but so far so good. I'm getting the correct voltage numbers now so let's hope that continues.

Oh, and both alternators and regulators were/are Bosch. I guess some of the comments above were right, sometimes you just get a bad one right out of the box.