Delay in alternator kicking in? Battery dies. - Page 2 - PeachParts Mercedes-Benz Forum

#16 11-30-2002, 09:43 AM

Never knew the alternator starting it's self thing. I guess I take a lot for granted, thought it started spinning and put out juice, aboviously much more complicated than one would think.

Sounds like the consesus is, bite the bullet and buy an OEM or equivilent unit.

Good luck,
Joel

Richard Wooldridge · #17 11-30-2002, 02:17 PM

Hi JoshMb,
I have seen this problem many times and usually it is a slipping belt. The load from the alternator is very heavy at first, and the alternator will load the belt until it slips, causing the voltage to be low. After the initial starting depletion is taken care of, the load is less, and the alternator speeds up and the voltage rises to normal. Try replacing the belt and/or retensioning it! The alternator you installed is probably identical to the one you took out, except rebuilt using an OEM frame and rotor, so it's probably fine.

Richard Wooldridge
'82 300D/4.3L V6
etc..

JRBrown · #18 11-30-2002, 09:49 PM

JoshMB,

If you haven't done it already, you'll want to give the battery a good charge from a battery charger. Cables disconnected of course.

Richard,

I too thought that the belt might be slipping, but I checked mine with a Krikit at install and post install.

What is very interesting is that if I don't go above 2000ish rpm, the alternator won't light off. Time does not appear to be a factor. We confirmed this one winter morning (-5 degrees in MO) when I thought I had throttled up for light off but hadn't. When I returned to the car after about 10-15 mintues it was trying to die and I noted the voltage was still holding at 12. I revved up and it fired off. Started purring like it had a fresh sauscer of milk.

By the way, in my case, the alternator case is nothing like the Bosch/OEM case. Voltage regulator on the back of the alternator was not Bosch either.

It works, It was cheap, but I'd probably not do it again. OEM to manufacturer's spec almost always wins out. But I do have to re-learn that one every once in a while.

#19 12-01-2002, 02:00 AM

As a former Automotive Battery and Electrical Technician perhaps I can shed some light here.

Generators have residual magnetism, alternators do not. Score one for generators. However, generators do not put out enough juice at idle to say so, whereas alternators do. Score one for alternators. Greater output at almost all RPM's is why OEM's went with alternators to begin with (that, plus alternators are lighter). Chrysler (the inventor) started out with alternators in their top-of-the-line and bottom-of-the-line cars in '61, the Imperial and the Valiant. The test went so well they went with alternators across the board in '62. Every other OEM followed very soon thereafter.

Since alternators have no residual magnetism, it is possible to reverse your battery's polarity if you let it go dead. Incidentally, it's impossible for a charged battery to freeze, as the Specific Gravity is around 1.280, but a discharged battery (Specific Gravity near 1.000) can freeze overnight.

Jim Anderson is right, an alternator is self starting, provided you have at least a modicum of juice available to excite it initially. A good-but-dead or a bad battery doesn't cut it. He's also right on the alternator light as being essential; it's all part of a series circuit. The bulb must be good and working properly.

dpetryk is also right; your alternator should start charging the second it starts spinning.

I must respectfully disagree with Mike Murrell, however. Although my car is a Benz, my truck is a GMC 6.2 L diesel and I have had an Auto Zone Lifetime Warranty alternator on it for several years with excellent results. I forked over some extra bucks to get a 94 Amp unit instead of the OE unit which was 78 Amps. My training and experience both tell me that you can never have too big an alternator, especially in this part of the country! The way Auto Zone has treated me is A+. Considering how long I keep my vehicles, that Lifetime Warranty is pretty sweet!

I have found that a perfectly good alternator is often blamed for problems caused by a bad battery. To accurately test a battery it must be fully charged. Poke each cell with a Hydrometer and look for 1.260 to 1.280. This will tell you if all six cells are taking a charge evenly. If it's still in the game at this point, then you need to have a shop put a load on it and test its output voltage while under load. Then, and only then can you tell what kind of battery you're actually dealing with. Remember, to work properly, a battery must do three things: accept a charge, hold a charge when not being excercised daily, and give up its charge when needed. When a battery can only do one or two of these tasks, it's time for a new Die Hard.

A generator produces direct current, while an alternator produces alternating current which is changed to direct current by means of an internal diode rectifier. When your alternator idiot light comes on it usually means that one or more diodes are shot. Often this is done through careless or improper battery jumping, while either giving or receiving a jump start. A split second is all it takes to start frying diodes.

Hope I haven't confused you too much, Josh! Best wishes.

JoshMB · #20 12-01-2002, 08:13 AM

Thanks Medical Dave,
Those are the kind of posts that I love. It explains it all to me and I get the reason behind why things work. I'll test the battery, but Autozone put their machine on it and said it was good. I'll swing by another shop just to make sure.

Richard, I will check the belt, I did have a slipping problem on the old alternator. I don't think I do any more, but I know that all the belts need to be changed. I have been avoiding it because this belt is the deepes of three to change and I have to remove/replace all the others to get to it.

JRBrown, I'll give the battery another charge, but why do the cables need to be disconnected? I thought that if you placed the negative on another grounded part of the car, it was good? Am I mistaken?

Thanks again for all the help. I'll be sure to let you all know how it goes.

Josh

JRBrown · #21 12-01-2002, 11:24 AM

JoshMB,

My reasoning for removing both cables is that I once witnessed the aftermath of did happen when the negative cable isn't "secured" in such a manner that it doesn't accidently make contact with its battery terminal during the charge.

What the gent came back to a few hours later was a cracked battery case, battery acid in the engine compartment, on the fender and bumper. Needless to say, a mess. The negative cable was found in contact with the terminal.

The ground cable found its way back to the negative terminal. I suspect the cable insulation's memory on that cold day caused it. The spark must have fired off the hydrogen sulfide gas in the battery. But of course, it could have been charging a very bad battery.

So unless I can secure the ground cable well. I just spend a few more minutes pulling the postive and go ahead and give everything a good cleaning and a coat of vaseline.

Medical Dave,

Thanks for posting. My curiosity in this area far and away exceeds my expertise, so I spend a few dollars here and there for books and manuals and try to get smarter.

With the "GEN" lamp in the preexcitation circuit, is it possible that the preexcitation occurs (which will turn the "GEN" light out), but that the excitation circuit hangs up. When I read Bosch's Technical Instruction book on alternators, there is some discussion regarding post preexcitation where residual magnetism aided by preexcitation, induces a slight voltage into the stator winding. This causes a small flow of current in the rotor winding which finds its way back to the stator, and this process repeats itself until the alternator is fully excited and desired voltage level is increased. Any thoughts on what would account for the odd behavior mine exhibits (will not generate proper voltage without first throttle engine rpm past about 2000)? I'm fairly certain that is is not a battery issue as this is the vehicle's second battery (4 years old) and this behavior was identical with the previous battery.

Richard Wooldridge · #22 12-01-2002, 04:51 PM

Hi there,
Medical Dave, I must take violent exception to a comment you made - you stated that you can reverse charge your battery with an alternator - IMPOSSIBLE! The diodes in an alternator ONLY allow it to put out a positive voltage! I suspect you have forgotten a statement taught in electrical "machine" classes - at least it was in my college class - that a DC generator can be field flashed to put out an opposite polarity voltage, due to the fact that it only has the residual magnetism to get it started and if the magnetic field is reversed by applying the opposite voltage to the field winding for a few seconds, the generator (and regulator) will happily put out the opposite voltage. I have personally done this, so I know it works. If you attempt to produce the opposite voltage with an alternator, which is an AC machine, the diodes will rectify the sine wave and always produce a positive output, unless the rectifiers are physically resoldered into the circuit in the opposite direction, which is usually impossible due to their construction. Automobile alternators are all 3 phase AC machines, with 6 diodes connected so that the output is a positive DC voltage with very low ripple, due to the 3 phase design. Their field winding must have a low voltage (around 3 volts for most alternators) applied, usually through the idiot light circuit, to function, although typically the integrated regulator designs don't even need the lamp circuit connected to operate.
I'm still confident that Josh MB's actual problem is that the alternator belt is glazed and allowing the alternator pulley to slip on startup, a condition that only a new belt will fix. I have seen this problem many times, and it really isn't obvious sometimes. Usually, if neglected, it causes the alternator to overheat and burn the diode stacks after awhile, because the alternator is putting out maximum amps at a low speed, even though it's unable to put out the proper voltage, which means the fan cannot cool it properly. Please replace the belt, JoshMB!

#23 12-02-2002, 01:29 AM

JRBrown,

You're absolutely right; H2SO4 (aka Sulfuric Acid) is wicked stuff. Don't get it anywhere near your clothes or you might soon be running around in your birthday suit!

For charging purposes cable removal is ok, but certainly not necessary. The way to avoid sudden battery expansion (as in 'explosion') is to allow the gas generated as the battery charges to vent freely and gradually, as opposed to building up and then releasing suddenly. The best way to do that is to remove the battery caps and then set them down lightly on the cell holes for the duration of the charging process. This will allow the gas generated to vent away as soon as it is produced. The only caveat here is to totally prevent sparks from occuring in this area on top of the battery while it's charging/venting. If you leave the caps on tight while charging you're in for a nasty surprise, as you may have acid in your eyes the next time you remove them, or they may self-remove, spraying that nasty H2SO4 all over everything.

I agree w/ Richard Wooldridge concerning your charging glitch. You more than likely have a loose and/or defective belt. What you're observing as the cut-in point of your alternator is actually the RPM point at which it actually starts turning at the correct speed for that engine RPM. In other words it's slipping up to that point, and then it finally catches up to where it should be speed-wise.

Many years ago I was a Professional Wrench at Sears Roebuck in Ann Arbor MI where I ran the Battery and Electrical Department. One day a guy came in who was about ready to pull his hair out. He had the same symptoms you have. I put his car on the VAT 28 (state-of-the-art equipment at the time) and observed something I had never seen before: when we revved his engine to a steady 2,000 RPM his alternator output started out way low but then gradually came up to the specified value. I told him I had never seen this before but that I thought what was happening was that the belt was slipping, not enough to squeal (which it wasn't) but just enough to behave in this erratic manner. We tightened his belt just a tad (too tight will fry your alternator bearings) and retested it on the spot. It was perfect! We had located his problem and I was a little smarter than when I went to work that morning.

Richard Wooldridge,

Thanks for your post. I'm glad you caught my mistake. I went back and read my post and realized I hadn't said what I intended to say. It is true that "alternators have no residual magnetism". I should have ended the sentence right there. "It is possible to reverse your battery polarity if you let it go dead". This is also a true statement, but it's not related to alternators, but it is to generators. You're absolutely right; an alternator is not going to charge your battery backwards, as a generator can.

Whenever you install a generator (obviously this is going to be on an older vehicle) you must momentarily touch the Battery and Armature terminals on the voltage regulator with both ends of a wire so as to properly set the system polarity.

I agree wholeheartedly with you on your call re: JR's charging problem (see first part of this post).

Two good calls, Richard. Sorry for the confusion. I'm kinda new at this forum stuff. Please be patient w/ me.

Richard Wooldridge · #24 12-02-2002, 02:22 AM

Hi there Medical Dave,
I hope I didn't come on too strong on my previous post, I suspected you just had a momentary lapse on the generator/alternator business, but wanted to make sure it was clarified for any other readers. I do think you are correct on the history of automotive alternators, however. One interesting thing is that a generator actually can be made capable of producing large charging currents at idle, but the engineers couldn't do that on automotive engines because if they made the pulley ratios such that the idle currents were high, the generators would "throw" the armature windings at higher engine speeds. For instance, my brother has a 1960's Ford Diesel tractor that will put out over 60 amps at idle, but it won't rev much higher than 2600 rpm, about half what the usual car engine will. Alternators have rotating fields, which are wound with much lighter wire and positive retaining designs, so they can spin a lot faster. This allows a smaller alternator pulley, contributing to the slipping belt problem. Those people that are goosing their engines to make the alternator start charging are just tightening the belts by centrifugal force to get the alternator up to speed...

Regards, Richard Wooldridge