I know, you vets already know this.
But just maybe, someone will read this and not go what I just went through.
The car sat for 6 months, finally got her running again.
Had the battery disconnected, charged it b4 putting it in, charger said it took the charge.
Went to start it, started fine.
Turned her off (should have let it warm up) and next time I tried, she wore the battery down. I kept trying.
Then the starter made bad noises. It was fried.
New starter, old battery.
On the freeway, I find out the hard way that the fuel guage no longer works. How convienent. Ran out of juice on a dangerous spot. Tried to start it, ran the battery down, hitched to gas station, hiked back with diesel, primed her.....
and the new starter was toast.
Had it towed, put in another starter, got a new battery.
I believe those two starters would be alive today if the battery had not been dying.
And that starter swap is a miserable bear on busy roads.
I hope I save someone some serious grief!

__________________
Think Alternative Energy!
300CD '80 (now gone but not forgotten...)

how long were you cranking it before you decided to stop trying and hitch a ride?... maybe you fried your starter with all the cranking you did?

__________________
Mike

----------------------------------
1975 200 - Sold (no pix);
1978 200 - Sold - http://www.pbase.com/hboy/redbaron
1979 300TD - Sold
http://www.pbase.com/hboy/greenwagon
http://www.geocities.com/hboy726/300TD.html
1985 230E - now my daily driver...

true-you didn't crank it for longer than 30 seconds at a time, did you? That's enough to kill the starter, ESPECIALLY if it's not a good one. If you did, don't feel too bad, you are certainly not alone. I think a bosch rebuilt is around $140? and probably worth it. The problem with the starters is there is a great deal of heat created inside and with heat being continuously generated inside, it can't dissipatethe heat fast enough. The second starter being warm already didn't help. Their untimely deaths were possibly related to heat issues AND a lower voltage/higher current flow with an older battery. By reading your post I couldn't tell - were you changing the starter on the side of the road or did you take it to a safe place first. I sincerely hope not the former. I've done that in the past I'm ashamed to say, I'd rather work in a cold, windy rain than do that today! Not worth the risk, IMHO.

__________________
'82 300SD - 361K mi - "Blue"

"Good judgement comes from experience. Experience comes from bad judgement."

listen, look, .........and duck.

Jimmy Joe,

You haven't proved it to me that weak batteries damage starters. Weak batteries will cause less heating in the starter than good batteries. Starters are run in overload condition and thus will overheat if used too long.

Starters are damaged by over heating. It destroys the insulation and current starts to leak thru it. Then the starter pulls alot of current but does not give enough torque to crank the engine fast enough.

When the engine is hot, the starter is also hot and can not dissipate internal heat fast enough. So cranking a hot engine like when you run out of fuel overheats the starter much quicker than when the engine is cold.


I think you just overheated your starter from too much cranking and that is what did the damage, not a weak battery. It appears that your battery was good enough to overheat your starter, so maybe it wasn't as bad as you thought it was.

P E H

Maybe, maybe not. Let me put another twist on this. It will take a given amount of power for the starter to turn-over the engine. Weak batteries generally produce less voltage under a load such as running a starter. To produce the power required for the starter to turn-over the engine with a weakened battery producing less voltage there would have to be more amps running through the starter and the increased amperage may cook the starter worse than with a fully charged battery. Just a theory.

Similar situation when you jump a car with a dead battery and let the alternator on the car charge a dead battery. The alternator tries to put out its maximum amperage capacity for an extended period of time. It is an acid test for an alternator.

Be nice to your car and don't ask it to do these things, maybe it will be nice to you. That was the scientific part of the message.

Twitch,

Your theory holds true only for AC (induction) motors not DC motors such as starters.

The current draw on a DC motor will decrease as the voltage decreases. The output torque is proportional to the current and that is why the cranking speed decreases as the battery discharges.

AC motors draw more current as the voltage decreases because they try to put out the same amount of mechanical power. The motor speed decreases, the slip speed increases and the back EMF is reduced. So AC the motor draws more current, overheats and burns out if the low voltage is applied too long to an AC motor under load.

P E H

Correction- Ohm's Law is: Current = Voltage / Resistance. Therefore lower Voltage from a weak battery results in lower current being delivered, assuming the resistance of the starter remains constant.

Dave

This is only true if the AC motor has a Series Winding- ie- the Field Winding is connected in series with the Armature Winding. These are typically your household appliance motors- mixers, vacuum cleaners, electric drills- they all work harder as you increase the load or decrease the voltage. Also, they are all very noisey.

Other AC motors such as in your fridge have a Parallel Field Winding, so they run at a constant speed, and make a lot less noise. If your house AC power takes a dip, like during a storm, you'd probably notice your fridge motor slow down, but your vacuum cleaner would speed up.

Dave

Yup, thanks, I wasn't thinking about that pesky Ohm's law. I was thinking Volts X Amps = Watts and trying to keep the Watts constant. Volts decrease, amps decrease, watts decrease, starter slows, got it. As the two of you pointed out it was a misconception and the wrong law.

I have never really studied AC motors much. I understand matching the speed of the motor to the cycles of the AC current for torque. The stuff about how start capacitors shift the phases for torque at different speeds is a little past what my intuition can justify, need to read. As a kid I took apart some capacitors and it just didn't make sense.

The warning on the alternator still holds.

I am very sensitive to overcranking, and I just crank for 10 or 15 seconds at a time. The second time, yes the engine was hot, but the first time, it was a cold engine. I would crank, let rest, crank....That doesn't mean I wasn't heating up the starter, of course.
It was amazing how few cranks that I got out of the battery, then it would start cranking slowly, and when the battery was dying, it seems like this was the point where I killed the starter. When it was just barely moving. You know, that last crank that you shouldn't do because it is too slow to start, but you just want to GO! My dumb ass.
Yes, Pete, I did do it in an unsafe place. Not on the freeway, but on a busy road nearby. (Amazing how heavy that thing is, and how little extra room there is to get it up in there.) I was just out of options and low on cash. I hope I never have to do that again....

__________________
Think Alternative Energy!
300CD '80 (now gone but not forgotten...)

doesn't something on the other side have to go up(resistance ain't gonna change[well - maybe some as the current heats the windings])? Maybe I failed that question on the test....

__________________
Jim

'49 170?(donated to church in Darmstadt '58)
'58 220S(crusher, after '73 fire[San Antonio])
'72 280SE 4.5
'77 240D
'81 300SD
'83 240D parts car

As far as I know:

DC motors draw maximum current, and produce maximum shaft torque, at minimum speed. Current and torque is proportional to applied voltage. More voltage = more current.

Field winding(s) control strength of rotor's magnetic field, armature windings control strength of the static field. Torque is produced by the angle between the amature and rotor magnetic fields acting on the shaft. The commutator 're-aligns' the rotor field so that it can continue to produce torque, otherwise the rotor would turn through 180 degrees until the fields aligned, and then stop. Not too useful without commutation.

DC motors have series, shunt or compound field windings. Shunt or compound are most common. Series-wound have a nasty habit of trying to overspeed to destruction if the field current is too low.

Back EMF drops the armature current as rotational speed increases.

AC induction motors work differently. The armature windings produce a rotating magnetic field.

The rotating field induces current in the field windings, which produces a magnetic field. Again, the angle between fields produces torque, but no commutator is required because the field is rotating. The rotor fields 'chases' but never 'catches' the armature field.

AC inductons motors also draw maximum current at minimum speed (Locked Rotor) and back EMF reduces current when they are spinning.

Within their design range, lower voltage will raise current to maintain torque (power). When stalled, AC induction motors do not produce much torque, so they have a second set of rotor windings, or sometimes a second armature winding with a phase shift, produced by a capacitor, to help starting.

Many household AC induction motors are either "capacitor-start" or "capacitor-run" design.


For further reading and enjoyment,

http://www.engin.umich.edu/labs/csdl/ME350/motors/

Best Regards,
Jim

Thanks, this helps a lot, I can relate the commutation in the DC motor to the rising and falling fields around the windings of the AC motor. Commutation was driven home on mechanical rectification in DC generators.

Holy crap you guys get crazy!

Getting back to starters...Mean Green makes an awesome gear-reduction starter for around $300.00. True, it's pretty pricey but it spins my 2.4 over twice as fast as the Bosch even in the cold. Granted, there were some problems when I first got it. It was the first starter for a 2.4/3.0 they had sold and it was a bit tight to the flywheel, but they've got it sorted out and it works perfectly now. It's so efficient that I went from a huge John Deere tractor battery with 1200 CCA to a series 24 battery.

Jim

P.S. I've got an almost new, rebuilt Bosch for sale!

i had the same problem with my starter.. drained my batt with all the cranking.. then fried my starter... even with a strong batt.. the starter would turn quite slowly.. had it serviced.. and decided to put in a high speed reduction starter... now i am very happy with the results.. i should've done this a long time ago.. considering the reduction starter (bnew) is half the price of a bnew starter for the MB.

__________________
Mike

----------------------------------
1975 200 - Sold (no pix);
1978 200 - Sold - http://www.pbase.com/hboy/redbaron
1979 300TD - Sold
http://www.pbase.com/hboy/greenwagon
http://www.geocities.com/hboy726/300TD.html
1985 230E - now my daily driver...