Is my W126 battery failing? - PeachParts Mercedes-Benz Forum

quaid · #1 01-02-2009, 01:10 PM

Car is a 1985 W126 500SEL that I installed the 5.6 ECE Euro V8 into a few years back.

Before I go crazy trying to figure this out, maybe someone can give me a second opinion.

I recently did the 'Saab' aternator upgrade, because I'm running alot more electrical equipment, electric fans, electric waterpump, modern sound/media installation etc. I used the alternator from a late model 9-5, and its got a 130A rating.

My battery is a Bosch Silver, and is the correct size for the car (it fills the battery tray) but is about 3 1/2 years old.

In the cold weather we have been having recently here, the outside temps havent been much above freezing. The car is sluggish to turn over after its been sat for anything over several hours, and i'm frightened that its going to leave me stranded somewhere.

Battery measures 12.5v with car off.
At cranking, battery measures around 10.2v
Once started, the battery measures 14.3v or so.
I checked for equipment draining the battery when the engine is off, and its around 20-30ma, so thats ok.

I've googled about how long these Bosch batteries should last, and most people reckon longer than the 3.5 years that i've had mine for.

Any suggestions welcome

Ben.

pawoSD · #2 01-02-2009, 03:08 PM

Battery health seems ok based on the voltage.....usually 12.5-12.6 is good.....might want to get it "load tested" though.....10.2 during cranking seems normal too. Maybe its a failing starter motor causing the slow cranking....

tinypanzer · #3 01-02-2009, 07:38 PM

Have you checked the water level in the cells?

I don't know if that battery is considered "maintenance free" or not, but I have been noticing that more and more batteries are going back to the old style where you can actually check and add water.....

Use DISTILLED water, and don't smoke...

-tp

tinypanzer · #4 01-02-2009, 07:39 PM

One other thing, are you sure you don't have too viscous an engine oil? That will give you exactly the problem you are describing, though it doesn't sound like you are getting quite that cold. But, if you're running 20w-50, then it certainly could be the issue.

-tp

mpolli · #5 01-02-2009, 08:24 PM

This guy has a decent troubleshooting page:

http://www.aa1car.com/library/voltage_drop_testing.htm

It is a little wordy, but basically the idea is:

Measure the voltage on the battery posts (not connectors) while cranking.

Then measure the voltage at the starter (or as close as you can get) during cranking.

If they are the same then fine, but if they are different then look for where the voltage "drops". This would be at the battery clamps, cables, and/or ground connections.

quaid · #6 01-03-2009, 08:01 AM

Thanks to all who've replied.

The battery is a 'maintinance free' type, so no chance to check the cells.

I went to my local parts store this morning to check the warranty on the battery. The guy put his load tester on it and declared it as good as new. He said that still had almost 720CCA capacity, and that maybe my starter is showing its age.

It was -1'C here before I left, and although it didn't struggle to start today, I did notice that the oil pressure was slow to reach maximum on the gauge.
I picked up a new MANN filter and some synthetic 5w40 oil to see if that helps things along. My last oil change was around 8 months ago, and I haven't done too much mileage since. I think it was part synthetic 10w40 that I put in last time.

Cheers,
Ben.

pawoSD · #7 01-03-2009, 11:02 AM

Its probably a combination of thick oil and a weakening starter....

quaid · #8 01-04-2009, 09:11 AM

On the look out for a new starter, it looks like two types are available.
My engine was manufactured in '87, so has the standard 'direct drive' type starter installed. If I get the starter for an '88 onwards car, its a gear reduction type. Is this worth ugrading to?
It is a little more money, but if it means I get a faster cranking speed with more torque, I guess that can't be a bad thing. Especially since I have the high compression motor.

mpolli · #9 01-04-2009, 01:54 PM

On the one hand that starter is pretty old. On the other hand, in my experience if the starter is bad then it doesn't do anything. It sounds to me like a voltage drop somewhere which are very common, especially at the battery clamps, also where the clamps connect to the cable, also ground connections. I am just saying consider that when you put in the new starter it might not fix the problem. I have had that experience before.

quaid · #10 01-04-2009, 07:23 PM

Yeah, thats normally my experience also, they either work or they don't.
The manager of the parts store said that he does see weak starters though too. He knows that I don't buy parts like this from him, I use a mail order German specialist, so I don't think he was saying this to get a sale.
I made sure that the connections at the battery end were good and clean, but I didn't check the starter end. I'll have to do a voltage drop test too before I go spending money on a new starter.

tinypanzer · #11 01-05-2009, 07:05 PM

The ground strap from the starter to the chassis is a good place to check. At least on my car, it gets BAKED by the exhaust and also winds up with various engine fluids on it.

-tp