Friday, February 20, 2015

1978 Mercedes 300D Alternator Refreash Time!!

(posted on and

Lets see, we are going to cover very basic electrical here! Now I know we have a zillion threads on here on why is my battery not charging? Why do I have electric gremlins? What is there no warning light? And the zillion others.

Well here is why you have those issues! This is my Bosch 55amp Part number 0120489526 14v Alternator from my 1978 Mercedes Benz 300D. This is the ORIGINAL alternator!! My issue was that I woke up with a dead battery! I checked and had 11.2 volts at the battery, I had assumed my cheap o’reilly battery crooked again. A huge argument with the “Manager” resulted in my not getting a warranty replace so I threw the battery in my 82 which I know has a good charging system. Thankfully the battery held it’s charged now it time to figure out why! *A word of advice in Arizona, be prepared to be discriminated against. Every Navajo here treats you as though you just scalped their daughter!!

I had no signs of a bad alternator and the VOM showed a 12.5 charge to the battery at idle. Low it is but it should still keep the battery charged. There were also no flashing of the battery light or it not illuminated. I was noticing an issue when the climate control was on, it wasn’t operating properly.

First thing I decided to do was pull the alternator, as I was sure it was original and it was quite corroded. Even though I didn’t see any signs of it being bad, I opted to start there and work towards the fuse box.

Here we get to the simple basics of electricity in your car. You see the alternator is grounded to the bracket, which is grounded to the block thus grounded to the body of the car. Thus back to the battery.

IF there is any corrosion, oil, etc. on any of these mating surfaces it will cause the parts to not ground. On a battery system that is in your car, in order for the electrical to operate correctly this circle of life needs to be fully operational and this must start with good clean grounds! Without this, you will have tons of electrical bugs going on with the car. 

This is normal maintenance, as your car is opened to all sorts of elements! It is more an age item, then a wear item due to mileage. This car is approaching 40 years old! And has been sitting for more than half of those years. So it needs to be done!

First we will start with the bracket and were it mounts to the block, the first thing I noticed was the nuts were not tight at all! I was expecting a PITA situation trying to get them off. Well after that was removed we see an issue! From the bracket being lose and 40+ years of grime we see fresh oil on the shiny mating surface. This will cause a ground issue.

Now with the bracket off, let’s look at all the points where the alternator mates with the bracket….errr…corrosion, grease, uneven areas. Again will not make a good grounding surface. We also see all the hardware is corroded and greasy. How is this supposed to ground the alternator to the block?

Now let’s look at the alternator itself. Lots of corrosion! Still wears its tag! Explain that!


So how does the alternator charge and feeds the electrical system in the car? The umbilical cord! The mating surfaces of the male and female plugs look like this. Err full of grease, corrosion, oil and who knows what. No wonder I am having strange gremlins and no charging. Also with this being the Positive side of the battery, all this gunk can also cause arcing which can lead to worse nightmares then just a no charging battery from the negative side of the battery.

Now let’s pull the voltage regulator! Now let’s unscrew this! Hmm not too bad, worn yes but not bad…but what’s this? The ground point of the regulator is corroded. Well that explains the low charge voltage.

Then we look at the alternator itself. Bearings aren’t making any noise, dry yes but not bad. The commuter where the brushes ride isn’t horrible does show its 40 years.

Finally analysis and plans for phase two. The alternator is fine, the only issues are age. Corrosion, grease, dirt, oil etc. The area that are meant to wear such as the brushes and commuter have worn. What will be done, is strictly labor. Plans are to clean all the pieces, sand/file/steel wool all the pieces that are responsible for grounding. Clean all the electrical connections with deoxit and shine them. Go to the hardware store and see if I can find replacement brushes. Then the alternator should give me another 40 years of service plus my expense will be more then like under 20$ if I find brushes or under 45$ if I have to buy a regulator.