More battery/alternator troubles - PeachParts Mercedes-Benz Forum

Knight Rider521 · #1 02-07-2005, 06:14 PM

Recently I have been having alot of problems with my charging system. After replacing the battery and having the alternator rebuilt, it seems I am still having problems. Here is the problem. I checked voltage of battery with car off, read 12.6 volts, with car on only read 12.4 volts. It seemed as though the alternator was not supplying enought current to the battery. But when I disconnect the negative battery terminal, voltmeter reads 14.4 volts at idle. Went to mechanic and explained the problem. He told me that voltage regulator does not supply between 13.6 to 14.4 volts to the battery constantly because that would overcharge the battery. The battery hopefully should have been fully charged, because the entire weekend I drove only during the day, with no accessories on whatsoever. He informed me that it is completly normal for the voltmeter to show low 12s as a voltage reading, if the battery is fully charged, to avoid overcharging the battery. But the problem is that the more load I put on the electrical system, compressor at max speed, headlights, radio, with 2 500watt amplifiers, and rear defroster, the closer that 12.x gets to 12.0. With all these systems on, the alternator read 12.1 volts. This seems like the opposite way the alternator should be functioning, correct?? The higher the load, the more voltage needed to maintain battery supply. Alternator shop said that something is wrong with the battery and that I should have it swapped out, but the battery is only a week old?? What is going on here??

jbaj007 · #2 02-07-2005, 07:53 PM

What RPM are these reading?

Knight Rider521 · #3 02-07-2005, 11:37 PM

Readings were taken at idle, about 600rpm.

ozzy · #4 02-08-2005, 06:22 AM

Quote:

He told me that voltage regulator does not supply between 13.6 to 14.4 volts to the battery constantly because that would overcharge the battery.

What a load of BS, depending on rpm, load and charge in battery you should have a reading on your volt gauge of at least 13 v, anything less is not acceptable. If you have cleaned and checked all your terminals and grounds, next step would be to check voltage output at the alternator (the big fat terminal) anything less than 13 v would suggest alternator has failed.

pmckechnie · #5 02-08-2005, 07:36 AM

First thing is NEVER unhook a battery while the engine is running. 2 things could happen and both are something no one would want. First the battery could make an arc and ignite the gases around the battery and cause it to blow up. Second, if there is a problem with the regulator and it is OVER charging you could actually TOTAL the car by distroying all the electronics. SO NEVER unhook the battery while the engine is running.

Now on to your problem.
The voltage at the battery with the engine running and everything else off and fully charged battery should be 13.0 to 13.8 Volts. Domestic cars will be 14 to 14.8 volts.
With the engine at idle, check for AC voltage. It should be less than 1V. If it is more, than the rectifiers are bad in the alternator. Now back to DC volts at the battery, turn on the lights and the AC fan to high. The voltage will drop but it should stay above 12.5V. Rev the engine to 2000 RPM and the voltage should go back up to 13.0 to 13.8 V. If not, check the alt belt first. It should be tight enought that you can't turn it by hand.(Be careful, it will be hot). If the belt will slip when turning it by hand, it is toooo loose. Tighten it and recheck.
Let us know what you find.

Paul's Auto Electric

Knight Rider521 · #6 02-08-2005, 08:00 PM

With the engine at idle the AC voltage read .1 volts. After switching the headlights on and with the fan on defrost, the DC voltage on the battery only read 11.8 volts. At 2000rpm, voltmeter only read 12.1 volts. I was not able to test/tighten the alternator belt tonight, but will have that done by tommorrow. The thing is that I bought the battery only 10 days ago, had the alternator rebuilt 3 days ago, and checked the wiring/made my own wiring harness but got the same results. Alternator shop suggested that something was wrong with the battery, but now I just don't know what is going on.

Knight Rider521 · #7 02-09-2005, 03:31 PM

I went by the mechanic again today and he tested the battery and told me that it was working properly, he told me that something in the system was draining the battery, so I decided to try and determine the cause of the drainage. I read on another post how to go about determining the fuse location, so I proceeded to pull fuses one at a time measuring the amp draw each time. As soon as I disconnected the positive side of the battery I noticed that the voltmeter started at -.400 amps and went up a pretty much a constant rate of about 1/100th of an amp per second. This continued with all the fuses I pulled until I got to fuse #13. The fuse card says that the diagnostic, hazard warning system, clock, central locking system, control unit memoroy of seat adjustment system, and electronic radio are all run of this fuse location. Oh yeah, after having the battery disconnected for maybe only 10 minutes, the voltmeter went all the way to 5.0 amps when I got to #13. I am almost positive that the vacuum pump is the cause of this battery drainage because I started having problems with the driver's side lock around the same time (after a girl rear ended me). I will search for the location of the vacuum pump and provide an update.

Duke2.6 · #8 02-09-2005, 04:41 PM

"Went to mechanic and explained the problem. He told me that voltage regulator does not supply between 13.6 to 14.4 volts to the battery constantly because that would overcharge the battery."

You need to find another mechanic. This guy is clueless!

Duke

mpolli · #9 02-09-2005, 04:46 PM

I don't see how a drained battery would have any connection to low alternator output. Did you check the belt as previously mentioned? According to Paul's test below in post 5 you have a bad alternator, or belt is slipping. I would get the alternator tested by someone other than the guy who fixed it, to get a second opinion.

MP

Knight Rider521 · #10 02-09-2005, 10:13 PM

Yes, I checked the alternator belt today, it had plenty of tension. I will have the alternator output checked by another company.

G-Benz · #11 02-10-2005, 10:52 AM

Just adding to what everyone else has said:

With ignition off, your battery output should be a nominal 12V or just a bit over. That is the voltage output of the battery, period. Anything less indicates a weak battery or one that has been drained and not sufficiently recharged.

With the engine running at idle and all accessories off, you should be seeing 13.8-14.V volts. Only the alternator will be providing that type of voltage, not the battery. If you are seeing anything less, then the alternator is not providing enough juice to drive the accessories and simultaneously recharge your battery.

If you have confirm belt tightness, then most commonly, either the voltage regulator has failed or the brushes in the alternator have worn out.

You have mentioned that you are running high watt amps...these require a pretty healthy appetite of current, and the stock 80-amp unit may not be up to the task. Alternators are the first to go when someone brings in a car with a mega-watt sound system that has charging problems.

You might consider upgrading to the high-performance alternators, which are designed for heavy-load accessories. They start at 100-amp units up to 250 amps, and I have seen setups in SPL sound-offs that run several in tandem!

mpolli · #12 02-10-2005, 12:30 PM

Good point about the high amps. I missed that in the orig post. Those 500W amps are likely the problem. I would yank those puppies out. Those would be more appropriate in a pink Hyundai Sonata with a big spoiler on the back and 12 inch wheels. Or put in 2 more, take out the engine and the car can just bounce down the road.

MP

G-Benz · #13 02-10-2005, 03:44 PM

Quote:

Originally Posted by mpolli

Good point about the high amps. I missed that in the orig post. Those 500W amps are likely the problem. I would yank those puppies out. Those would be more appropriate in a pink Hyundai Sonata with a big spoiler on the back and 12 inch wheels. Or put in 2 more, take out the engine and the car can just bounce down the road.

MP

Nah...I ain't hatin'...I used to compete in sound-offs with my VW...it required a custom high-output alternator and I ran two batteries!

Just saying if you are running 1kW amps, you may be overtaxing your alternator. BTW, their current draw is continuous regardless of your volume setting, and during peaks, power demand most likely forces the system to starve all other components in the car to feed the amps!

Pro competitors use huge capacitors to store excess energy and deliver it when your amps require it under those conditions.

Also, I hope your amps are at minimum being fed by an isolated 5-gauge lead with a 35-amp fuse directly coupled to the battery. Not much other than ABS and other major components require that type of current feed. You don't want to risk blowing a fuse while driving and putting yourself at risk with a major component being disabled.

MB design requires engineering the total package when integrating a new feature. Putting in high-wattage amps requires the same thought process...

Chevota · #14 02-10-2005, 05:27 PM

I agree with G-Benz 100%. If any car I drive isn't charging at 14.5V I start to wonder what's wrong. Keep in mind that even some all oem cars cannot always keep up at idle, with all the accessories and lights on. It sounds to me like you're more knowledgeable and mechanically inclined than you mechanic, I bet you solve it on your own. It sounds to me that your alt is just simply too weak for your car. Maybe a different more powerful alt, or having the rebuild shop boost yours?
I like doing things different and on the cheap, I’d put a high amp unit from a junkyard chevy in there. The boneyard by me charges $18 for an alt.

mpolli · #15 02-10-2005, 08:28 PM

Quote:

Originally Posted by Chevota

If any car I drive isn't charging at 14.5V I start to wonder what's wrong.

I think 14.2V or even 13.9 should be enough.