Electrical Diagnosis. Fun :) - PeachParts Mercedes-Benz Forum

pj67coll · #1 12-27-2006, 09:13 PM

I've been reading thru numerous posts about problems with the battery light comming on. It seems I might be looking at either 1) A new voltage regulator, 2) A new belt or 3) some kind of warning module?

I'm electrically ignorant so can someone please tell me if any of the above three things would cause the symptoms that started today.
My battery light pulses when the car's at idle. This morning it would go out as soon as I'd step on the acellerator but by evening it only goes out when the revs get high enough on the freeway. At the same time the brake light comes on as well. Also pulsing in time with the battery light though it's much fainter.

I initially thought slipping belt from the way it was pulsing but now I'm thinking possibly one of the other two issues. Is there something I'm overlooking?
Since 2001 the alternator has been replaced three times, the latest being 20K miles ago in September 2003.

- Peter.

Dubyagee · #2 12-27-2006, 09:29 PM

Where are you purchasing your alternators from? Check the voltage at idle with a multimeter. You should have more then 13.5 volts at the battery when idling. Also post the voltage you have at the battery. The battery should have 11.5 volts or more when not running. Buy a load tester to verify your batteries condition.

Matt L · #3 12-27-2006, 09:54 PM

If a battery shows 11.5 volts when not running, I buy a new one. Mine shows about 12.8.

Dubyagee · #4 12-27-2006, 10:01 PM

Most engines ( including diesels) should start at 11.5 volts at the battery cold. If the battery passes a load test cold, it should be suffecient for starting.

pj67coll · #5 12-28-2006, 12:49 AM

Thanks for the input folks. But I wasn't really asking if the battery was bad. I'll check that at Autozone tomorrow. I'm more concerned about the other issues that would cause the light to glow like that. Starting is not an issue. At least not yet...

- Peter.

pj67coll · #6 01-13-2007, 07:37 PM

Ok. It finally died on me at work on Wednesday so I jumped it and today got to looking closer at the problem.

My V-Belt seemed real loose and possibly glazed. I adjusted the tension on the alternator and got it tightened up. I test drove it and the battery light still pulses a little at idle but goes out as soon as I start moving. But now I can hear the belt making a noise which it wasn't doing before. Have I overtightened the belt? I thought it only made noises when it was too loose.

Also. I have a replacement belt from fastlane that I bought some time ago just in case. I see that it's about 2mm narrower than the one currently on the car. Will it work?

- Peter.

sailor15015 · #7 01-13-2007, 07:46 PM

Check your main chassis to engine ground. When my tranny was replaced about ten years ago, the mechanics got the bell housing bolts mixed up (different lengths) and the one that was the main chassis ground was too short, barely catching a thread and finally worked its way loose. This made things in my instrument cluster go nuts. My fuel gauge and temp gauge would vibrate with the same rhythm as the pulsing of my brake and battery light.

At the time I didn't know I had issues with my bell housing bolts so I just added an extra ground strap to the engine and that solved the problem. Later, when I found that the bolts were linked to my vibration issues, I fixed the main strap.

Odds are that's not your problem, but it still wouldn't hurt to check and clean the battery to chassis and chassis to engine grounds.

Jkid · #8 01-13-2007, 07:58 PM

I assume you've at least pulled out your voltage regulator to see that the brushes are wearing evenly? I sounds like the regulator. I don't know if you've check the voltage at the battery, but it sounds like the alternator is working at higher RPMS>
I think that the alternator belts should have about half an inch of deflection when pressing down between the two pullies.
If it's never been done, I would 1) clean the wire connections at the back of the alternator (use electrical spray, sandpaper or a wire brush until you get to nice clean metal and put some electrical grease on the connections). 2) Do the same thing at the junction box. 3) Replace the neg. battery cable and cut the pos. cable back an inch or so and put a new clamp on. These things are very quick and easy to do and cost less than 10 bucks. The best thing would probably be to replace all of these wires, but that's a little more involved. After 20 years or so the wires tend to get pretty nasty and it's hard for current to flow through them.

pj67coll · #9 01-13-2007, 09:46 PM

Thanks for the info folks. Just got back from Autozone and the battery tested good. So. I'm going to look more closely at the grounding issues tomorrow. After I return from a junkyard. Hopefully with a new battery tray. Mine is in dreadful condition and I figure while the battery is out may as well try and get a replacement. I ordered a voltage regulator from Phil Friday so hopefully I'll have that early next week. I'll pull the old one then and if necessary replace. But I'm still not happy about having a belt that's narrower than the ones currently on the car. Any ideas on that?

- Peter.

winmutt · #10 01-14-2007, 02:07 AM

Quote:

Originally Posted by pj67coll

Thanks for the info folks. Just got back from Autozone and the battery tested good. So. I'm going to look more closely at the grounding issues tomorrow. After I return from a junkyard. Hopefully with a new battery tray. Mine is in dreadful condition and I figure while the battery is out may as well try and get a replacement. I ordered a voltage regulator from Phil Friday so hopefully I'll have that early next week. I'll pull the old one then and if necessary replace. But I'm still not happy about having a belt that's narrower than the ones currently on the car. Any ideas on that?

- Peter.

3 alts since 2001? You have other issues, get a multimeter and start looking for a short. I Used to go through alot of alts when I was pushing alot of amps into my amps. The regulators just cant handle load like that. I am betting you have short somewhere.

pj67coll · #11 01-14-2007, 06:53 PM

A short? Quite possible. The car has had quite a hard life and the way the radio (which I removed) was wired doesn't fill me with confidence about the electrical systems in general. It was direct feed from the battery, thru one of the holes in the firewall with the grommett pulled out and just sliding along the oil pressure line I think. Anyway. Right now I'm looking to try and clean up the mess under the battery tray before I put a ''new'' junkyard replacement tray in tomorrow. I have no actual electrical skills so troubleshooting such problems may be more than I can handle.

- Peter.

leathermang · #12 01-14-2007, 07:15 PM

Find the places which your car uses as grounds...
battery to engine,
engine to body,
battery to body,
etc

take those wires apart and clean the places they meet... like with a powered brass brush in a cordless drill.... clean all contact surfaces and the bolt holding them.

my123ca · #13 01-14-2007, 08:45 PM

Quote:

Originally Posted by pj67coll

.

Also. I have a replacement belt from fastlane that I bought some time ago just in case. I see that it's about 2mm narrower than the one currently on the car. Will it work?

- Peter.

What is the size of the belts? I have 10 x 1035 on mine. Your alternator could be misaligned resulting in an uneven wear on the regulator and bearings. The alternator will not last if it is that way. Get an original bosch alternator, the aftermarket ones that I tried lasted just over a year.

pj67coll · #14 01-15-2007, 07:58 PM

Leathermang. The only ground I can find on the car is the engine to body. At least from the bell housing to the body under the booster. I guess the negative terminal cable that connects to the body is the battery to body connection right? I cant find any other connection.

However. It seems that the problem was the voltage regulator after all. It arrived from Fastlane via UPS today and after installing it I went for a drive and the charge light didnt come on once. Not even when Idling.

123ca. The replacement belt I have is only 10X970 which I guess is the wrong size. I'll have to take another look at Fastlane and if I cant figure it out I guess a trip to the dealer is in order.

- Peter.

jnc19610 · #15 01-15-2007, 09:01 PM

Ok... a quick electrical primer....

A 12 volt battery has 12.6 volts, fully charged. 12.4 = 75%, 12.2 = 50%, 12.0 = 25%, anything less is discharged.

At a high idle, a fully charged battery should show about 13.2 to 14.5 volts with the car running. Less is bad (battery won't charge), more is bad (battery will burn up)

On a battery charger, like a 6 amp, you can charge up to 15.5 volts to 'top off' the battery. If it gets hot stop, if it takes more than 12 hours, stop. If it never reaches 15.5 volts, stop. (charging actually wears out batteries, but it is good for them to be 'topped off' occasionally)

Normal alternators have 6 diodes. 3 negative, and 3 positive. If one diode goes bad, then the alternator light will glow. (leaking AC in to the DC circuit)

The alternator light normally goes out when the alternator voltage exceeds the battery voltage. (if the battery voltage is higher than the alternator voltage, then the light glows full.)

Put a wrench on the alternator pulley or power steering pulley. If you can 'slip the belt', then it's probably not tight enough. (personally, I like slightly loose belts, I'd rather replace the belts, than burn up bearings from too tight belts)

So.... if it glows just off idle, it's probably a glazed over slipping belt. Replace it. If it glows all the time, then it's probably a bad diode. You can drive them that way, but..... (it's better to fix it)

Personally, I'd never buy a 'rebuilt' alternator again. Find a compent rebuilder in your local area, and get what you have tested, and if needed repaired. It's mostly bearings, diodes, brush packs, and some compenent soldering.