Alternator question - PeachParts Mercedes-Benz Forum

MarkM · #1 12-09-2008, 12:21 AM

I just did extensive research on this site about replacing/upgrading my stock alternator ('84 123 wagon). I just put in a new duralast battery last week, and today, first cold day (about 16 deg F) and the car would not start after work. Slow cranking.

I have volt meter permanently installed in the car. When the car is running, I get between 13.2 and 13.8 volts depending on speed. I dont have a big powerful stereo, but it does have a couple of amps,...I dont have any other accessories, (no heated seats, no oil/veg heater, etc.)

Am I to understand that my alternator should be putting out over 14.0 volts during highways speeds...preferably up to about 14.4? As I said, I get 13.8 v max. ....is that likely the cause of low battery/failure to start.

Mark

JimSmith · #2 12-09-2008, 01:17 AM

A stronger alternator may help a battery that gets assaulted often by cold starts and then short, city driving with the lights on, followed by being allowed to cool down again. But the voltages you report are about normal for the car. If anything, I would consider removing the brushes and checking them for wear. When they get short they don't make as good a contact with the rotor, and you don't get the performance you might expect.

The other issues affecting cold starting are discussed routinely. I think we may need a "sticky" thread for cold weather starting of 615/616 and 617 engines since this is an annual topic, and the problems are always the same:

1. Glow Plugs
2. Fuel (injectors, fuel quality/additives for cold weather flow, filters/water)
3. Starter and battery - speed is critical - no Diesel starts with a groaning slow cranking speed in cold weather
4. Engine oil - thicker oils add drag to the chore of turning the engine over by the starter. Synthetics are highly recommended based only on my personal experience.
5. Compression related issues - valve adjustments, frozen rings, or just plain wear - low compression makes it more difficult to sustain ignition since the air is too cold.

There are many aspects to the conditions noted, but basically if the engine has air, fuel, glow plugs and compresssion it will start.

Good luck, Jim

Paulc66t · #3 12-09-2008, 08:32 AM

I was having charging issues with my 300sd recently as well. The battery is relatively new-2yrs and the alternator is newly rebuilt. Last week the battery light would come on every so often. I had installed a volt meter to monitor the situation and noticed it was typically in the 13V range most of the time. The other day it was getting worse with the voltage getting lower on average and the light coming on more often.
I was able to easily fix the issue when I removed the connector to the alternator to inspect it. There was a bit of corrosion. I had cleaned it when I replaced the alterantor but apparently not good enough .I was not home and didin't have anything to clean it. Just pulling it off and pushing it back on was enough to help push the voltage to the 14V range. Later I cleaned it and all seems good so far.

yellowbenz · #4 12-09-2008, 10:43 AM

Quote:

Originally Posted by MarkM

I just did extensive research on this site about replacing/upgrading my stock alternator ('84 123 wagon). I just put in a new duralast battery last week, and today, first cold day (about 16 deg F) and the car would not start after work. Slow cranking.

I have volt meter permanently installed in the car. When the car is running, I get between 13.2 and 13.8 volts depending on speed. I dont have a big powerful stereo, but it does have a couple of amps,...I dont have any other accessories, (no heated seats, no oil/veg heater, etc.)

Am I to understand that my alternator should be putting out over 14.0 volts during highways speeds...preferably up to about 14.4? As I said, I get 13.8 v max. ....is that likely the cause of low battery/failure to start.

Mark

I don't think your alternator is working if your output is less than 14 volts. I think you are just measuring the voltage of the battery. Make sure your belt tension is adequate. If that's not the problem think about having your alternator checked at one of the national auto parts places. Why did you replace the battery?

tobybul · #5 12-09-2008, 01:24 PM

The alternator is not that difficult to remove. I would probably take it out and have it tested. AZ and Advance test them for free.

If you do decide to get an alternator, try upgrading to a higher amp. The stock is 65a. I found a decent 90a in eBay for $85. Can't imagine you need anything bigger.

Edit: Actually, they can test the alt in place if you want to bring the car to them.

LUVMBDiesels · #6 12-09-2008, 02:24 PM

Quote:

Originally Posted by tobybul

The alternator is not that difficult to remove. I would probably take it out and have it tested. AZ and Advance test them for free.

If you do decide to get an alternator, try upgrading to a higher amp. The stock is 65a. I found a decent 90a in eBay for $85. Can't imagine you need anything bigger.

I agree with upgrading the alternator if it is bad. However, get the Bosch AL129X it is 115 AMPS. It puts out a higher voltage at idle 14.4 and does not have the voltage drop associated with the undersized alts that we were stuck with. It was able to power everything in my 300SD from the fog lights to the rear defroster without going below 14.4volts.