Yet another alternator - HELP!
 

Hi guys,
Had the 240d all loaded up, fueled, and ready for my 700 mile thanksgiving trip, and the battery was dead. I had to take another car. Wouldn't it figure.

I was able to determine that the car isn't charging. Battery is brand new, alternator is new, wiring is fine and even though it's the third alternator in 4 months, the car had been running just fine for a month or so. Voltage regulator is also brand new.

The only thing I can think of is occasionally, maybe once a day, the glow light flashes on for about 1 second while the car is running. It's always done this. I figured something was flakey with the plugs or the relay, but it didn't seem to hurt anything. My theory is that when the plugs fire, they tax the alternator to the limit and that's what's blowing alternators. What do you guys think of this theory? Any help would be greatly appreciated.

Kip Amore
-thumbing a ride-

I assume that you have tested your charging system with a voltmeter, and even better an ammeter, to tell you if you are, or are not, charging.

I also assume that you tested all of your alternators and the wiring.

When the glow plugs 'fire' they do draw a lot of currrent, but that alone cannot damage your alternator, merely make it work hard. Unless it's a dead short, the alternator will produce full output and just stop producing more.

If the alternator does not have enough capacity to power the glow plugs or whatever electrical load is on, the rest of the power comes from the battery.

If the battery, alternator, regulator and wiring are all good, it is possible that you have a current draw while the car is switched off, such as a trunk light that doesn't shut off, or some other 'sneak' circuit that is supposed to be off, and that is draining your battery when the engine is off.

Best Regards,
Jim

Just to make sure, you might want to check to see if the grounding wire from the engine is bolted securely to the body. It is a braided cable that comes from the engine to the body. It is located near the firewall, between the engine, and the brake booster.

hope this helps

Check for voltage on the glow plugs when they should be off. If so, you will never get the battery to stay charged when you use the headlights, the total draw is more than the battery puts out (until, that is, the glow plugs burn out, anyway).

If it stays on, replace the relay.

Peter

Wild guess:
 

#A. Glow plug relay bad, causing a draw after shutdown.
#B. Possible shorted alternator.
#C. A current draw while the car is switched off will kill the battery every time.

The fact that you have purchased three alternators within the past four months needs some investigation. Where were these purchased from and was it possible for the seller to confirm that the alternator was not functional when it was returned? Are you quite sure that the alternator has failed each time? Is is possible that the voltage regulator has failed and the
alternator(s) are still functional?

The likelihood of three bad alternators within four months is remote. It is far more likely that you have a problem with the regulator(s) and/or a problem with the grounds. As was previously stated, follow the cable from the battery to the grounding point and be absolutely sure that there is no corrosion there.

Do you have charging data when each of these alternator/regulator combos was installed? Is this data within acceptable limits (13.5V above 2000 rpm)?

I don't have an ammeter, but I test the alternators by measuring voltage across the battery terminals. Not charging =10.5-11 volts. New alternator and new voltage regulator = 13.4-14.1 volts, and it goes up with RPM. I also test by starting the car and removing the positive battery cable. Lights and wipers off, not charging. Lights and wipers stay working, charging.

I didn't test the body/engine grounds because I assumed them to be working. Wouldn't the car have all kinds of wierd electrical symptoms with bad grounds, charging system aside? A $5 battery ground and an auxillary engine ground couldn't hurt it, so I'll do that next.

When the charging system dies, I put the Diehard on a charge overnight and it takes it and stores it as it should. I'm pretty ruled out on the battery itself.

Where is the glow plug relay and how I can test it armed with my voltmeter?

Thanks a million guys,
Kip Amore

I have always heard that removing the battery while the car is running - on an alternator system (not the really old generator cars) - will destroy the voltage regulator. Not sure if its a transient that gets it or an overvoltage condition or what - but I would discontinue that test where you pull off a battery cable while the car is running.

Glow plug relay is on the driver's side fender under the hood, a bit towards the front of the car from the middle of the wheel. It has a cover over the socket and plug connectors. If you are getting a battery drain from a glow plug relay problem, you could disconnect the glow plugs connector after startup and make your drive. This would get old after a while, but might serve as a temporary workaround.

Ken300D

Burn out the regulator every time
 

by pulling the battery cable with the vehicle running.

Yeah but...
 

The problem with the car is not that the battery drains, but that it isn't charging to begin with. What is blowing out the alternators (or voltage regulators or both)? What can cause this?

In other cars, I'd say "voltage spike" or "worn brushes" or simply replace it and forget about it. With this car I've replaced it but it's still blowing out? Suggestions?

Kip Amore
-stumped-

And also...
 

Is there any way to check the voltage regulator. It's looks like a single transistor, which means I should be able to read a .7volt drop across 2 legs, but not all three. Been a while since basic electronics. Any other methods would be appreciated.

Kip Amore

Hmmm
 

A bad field wire will kill an alternator. :(
Run a new field wire, easy and cheap. :)
Run an auxiliary ground from the block to the battery. :)
Check starter solenoid for loose connections, and replace the solenoid if needed, a loose and arcing cable will surge the alternator hard. :eek:

In the plug on the back of the alternator there are 3 wires. I assume the biggest one is the one that goes to the solenoid. Is that the field? Thanks,
Kip Amore

Question and answer.
 

What year is your 240D?

The field is a small wire.

If the pigtail to the alternator is in rough condition, that could be your problem.

Please do not check an alternator by removing the battery from the circuit, there is a reason there is a warning not to do this -- blown diodes almost always result because they cannot withstand 20+ volts when the regulator loses control of the charging circuit from no reference voltage. This can also cook the regulator instantly, so you will then get no current or voltage at all.=.

I sincerely doubt you have gotten three bad alternators in a row. Please take the alternator to a parts store or repair shop and verify output before proceeding.

Once you know you have a good one, make SURE it is actually connected when installed. Corroded connections in the three wire plug or a missing output wire (or misplaced) so you never connect to the positive battery post will result in a "dead" alternator because it has not field current.

And, I will repeat, you MUST verify that the glow plugs aren't staying on. They draw more than enough current to prevent battery charging (on the order of 40 A on a four cylinder engine while the alternator can only produce 35 or so) and the battery will slowly (or quickly at night) go dead. Easy to check with a voltmeter, you should have 0V on the glow plugs with the engine running. If you have 12V, get a new relay, the old one is bad. If the glow plug light flashes once in a while, it may be cycling on and off because there are fragments of contact floating around and shorting things out.

Bad glow plug relay will be very hard on the alternator, the aren't really meant to run a full output continuously!

Peter

Bad ground....
 

A buddy of mine runs an alternator/starter shop. Per him, about 99% of recurring alternator problems can be attributed to a bad ground, either engine or chassis. The bad ground may or may not cause any other electrical problems, but will blow the diodes in the alternator very quickly. Even running an extra ground circuit is very cheap insurance.

SteveM.

Once you know you have a good one, make SURE it is actually connected when installed. Corroded connections in the three wire plug or a missing output wire (or misplaced) so you never connect to the positive battery post will result in a "dead" alternator because it has not field current.

Hi ho, back up the train here Charlie. My alternator (77 240d) has a 3 wire plug thingy that goes into the alternator. And that's it! I always assumed the main alternator output wire was one of those three in the connector. Am I missing something important here. The alternator has the regulator that screws to the back, and it has a large capacitor across the back, but it has NO BIG OUTPUT WIRE like every other alternator in existance. Please clarify. Thanks, this could be the ticket.

Kip Amore

Mine is the same - two heavy wires and one
 

light one. The two heavy ones are tied together at the starter solenoid where the battery connects and, according to the diagram(Haynes manual) of the alternator, they are also tied together inside the alternator. The small wire continues thru the firewall and maybe goes to/comes from the charge light. Is this light gauge wire supplying the alternator field current? Someone clear this up for me please.

Jim :confused:

Bump - anyone?
 

Looking at the wiring diagram again, looks like one side of the charge light goes to the fuse block and the other side goes to the diodes in the alternator, not the ones that charge the battery. I believe the field is controlled by the regulator & I can't find a schematic of the regulator in the Haynes manual so I don't know where the sensing voltage for the regulator comes from....Help, please.

Jim again _ still :confused:

Sensing voltage is the battery line, power to energize the field coil comes from the ignition switch through the regulator.

Peter

74' 240D with similar problem
 

I have a 1974 240D with a similar problem, although I haven't replaced multiple alternators yet (luckily).
I charge the battery up completely overnight, drive to work, and by the end of the day the battery is completely dead. Replaced the alternator, and the problem still occurs. I thought it might be a trunk light on, so I checked that - nope, switch works fine. No other indication whatsoever of current draw. I haven't checked (or even found, for that matter) the glow plugs relay, so I'll check that tonight.
How do I measure the voltage across the glow plugs while the car is running (or off, since a bad relay would presumably cause this large current draw while the car is sitting when I'm at work)?
Could the problem be in the starter circuit in the dash? A grounded short in the switch mechanism, perhaps?
Or could it be in the ignition switch wire connections?
Again, no lights are on when the car is off, no apparent short (no obvious arcing, etc.), but the battery (brand new) is completely dead within 6 or 8 hours every time.
HELP!!!!!

Answer:
 

A simple test light will tell all.
With the car warmed up to roughly 60C - 90C.
Check for power at the glow plugs, with car running.
Power is bad in this case = bad relay.

For Ed
 

Ed,
Leaving a dome light or a trunk light on should take a LONG time to drain a battery, especially one as big as would go in the MB diesel. I couldn't say for sure, I would suspect a dome light would probably run for WEEKS. Lights and wipers are a different story, as is a dead short. There is one thing to NOT rule out - you got a bad battery brand new. It happened to me once, and it can happen to you. That would certainly explain it. As would a high draw item like glow plugs.

Kip Amore
-still walking-

Tested for power at the glow plugs when key is off
 

Well, last night I tested for power at the glow plugs with key and starter knob off - no power (good there). Tested again after car was started and warmed up - no power (good there). Still haven't found the glow plug relay, but I DID find a blown 16 amp fuse on the little fuse box that sits on top of the inner fender in the engine compartment (not the main fuse box, but just forward of it). There are two 16 amp fuses in it, but I don't know what they connect to. It sounds like I'm probably looking for a direct short somewhere.
Any advice as to the quickest and easiest way to track down the source?

Also, I tried two different brand new batteries, both of which became quickly discharged, and I'm currently using the other new battery in my 1986 Ford F250 diesel pickup, so I know its not a bad battery.
Thanks,
Ed

The only way to narrow down where the power is being drawn is to pull the fuses one by one and test. If you have a handheld multimeter, you can set it to check amperage draw, and put it across where the fuse used to be. Some circuits, like the clock, will always draw something. I couldn't say, but I'd suspect the clock circuit draws 50 milliamps. But since your battery is draining fast you'll see 1 amp draw at least. Definitly the best place to start are the two blown fuses.

Once you find the circuit that's drawing the power, you need to find out why. That's going to be a whole 'nuther problem. Others on this sig will have more specific info than I will when you find the problem area. It's going to end up being a broken wire or something simple though.

Kip Amore

Will someone please advise where to find
 

a schematic drawing of the Bosch regulator on my '77 alternator? I'm still having trouble making sense of how the charge light iluminates when the alternator is not charging but does not lite when it is charging.... One side of the bulb is grounded and the other goes to the D- connector on the alternator, which is also one input/output of the regulator.

I know - if it works, don't worry about it - I just wanna know HOW it works. :confused:

Actually, it doesn't work. I've had alternators fail in 3 different MB chassis, and none of them illuminated the charge light when they failed. You know the alternator isn't charging when the lights go dim. That's about it.

darn.. i didnt know about this... and i've usually done this to my car before.. when it wouldnt start... i would grab the battery from our van.. and use that.. and start it.. then while engine running.. remove the battery.. and replace it with the weak one.. thinking that the alternator would charge the weak battery!..

no wonder my alternator went south after that!...tsk..tsk..tsk...

i guess we learn new things everyday!..:D

tnx!

Battery Idiot Light
 

OK so when does the light come on???? Mine comes on when I start the engine but I have had a recent dead battery and it never came on.

grounds
 

Check grounds by going from centre of negative terminal on battery to clean area on block with your meter.I would prefer right to alternator case but too dangerous on this design with engine running. You should read virtually no voltage drop. Have friend crank engine and do same test again while cranking. Normally you may see a small voltage drop but I would start to check out cables and grounds if above about 1/2 a volt. The cables and grounds etc are quite old on most of our cars. Guess our babys are senior citizens of the car world now. Also would not hurt to check for any voltage drop between engine block and clean area of body shell. again with the engine running. Quick, fast, painless checks as you already own a meter.

grounds
 

Check grounds by going from centre of negative terminal on battery to clean area on block with your meter.I would prefer right to alternator case but too dangerous on this design with engine running. You should read virtually no voltage drop. Have friend crank engine and do same test again while cranking. Normally you may see a small voltage drop but I would start to check out cables and grounds if above about 1/2 a volt. The cables and grounds etc are quite old on most of our cars. Guess our babys are senior citizens of the car world now. Also would not hurt to check for any voltage drop between engine block and clean area of body shell. again with the engine running. Quick, fast, painless checks as you already own a meter. :sun_smile Yes i have seen just the alternators inability to charge being the only noticeable symptom of a marginal ground.

Did some more searching and learned how the charge light works
 

I crossed a line while tracing the wiring diagram in the above quote. As I mentioned in an earlier post(this thread), one side of the light is tied to battery + 12v thru the key switch via the fuse block(on my '77). the other side goes to the D- at the alternator/regulator and supplies external exciitation current while the llight is iluminated. When the alternator starts charging, 12v+ is applied to this same, point causing the same potential on both sides of the bulb so that no more current flows thru the filament and the light goes out.

Someone let me know if I finally got it. :D

Did some more searching and learned how the charge light works
 

That's pretty slick. Except that doesn't explain why the light doesn't work when the alternator isn't charging. My light works (the light comes on when starting) but failed everytime to tell me there was a problem with the alternator.

Now that I think of it though - if I had a bad ground, that would mean the alternator couldn't put it's current back out to the battery. That would mean that one side of the indicator light wouldn't have a ground reference (or the same ground referernce) as the other. Then the light wouldn't light. Maybe that's a "feature"! If your charging system dies but the light doesn't come on you know you may or may not need a new alternator/regulator, but you definitly have a bad ground between the engine and the battery.

Kip Amore

Which brings up something I forgot to include,
 

on my OM616, if the bulbs filament is open, the alternator will not charge. I suppose I could put a resistor in parallel with it so there would still be a path for the excitation current for the alternator?? :confused:

Copy/paste to address bar. Block diagram of charge circuitry
 

file:///C:/Documents%20and%20Settings/JIm/My%20Documents/alternator.jpg

:rolleyes:

Someone let me know if I did something wrong with my
 

last post as I do not understand the reply. Just trying to learn this computer/Mercedes stuff....

Aha!
 

Finally got some time and some daylight to examine the Benz this morning. Sure a ****, the is NO GROUND strap anywhere from the block to the body, much less the block to the battery. Aha! Eureka! Will the engine still be grounded to the batt? Maybe yes, but probably not, and almost certainly not in the way that the alternator needs to "see" it. Could be the car never had one, could be that it rotted away during my car's 12 year storage slumber. At any rate, good diagnostic advice on this SIG, and thanks to all who helped.

And since it's been on my mind these last couple of weeks, I think I'm going to install a traditional amp gauge in the car. Maybe in one of the heater vents, or maybe in the console somewhere where it can be easily removed if I want to keep the car stock. This problem (and expense) could have been greatly simplified with a regular old gauge. They don't call the idiot lights for nothin'.

Kip Amore

Without a ground strap the speedo cable housing carries all the current. I have personaly seen one smokin when the key was turned on and the glow plugs kicked in, never mind the current from the starter.

You pasted a file that you have on your computer. Nobody can access this file because it is not a web page. Unless you have the diagram posted on the web, nobody can see it.

You could "attach" the file to a post on Mercedes-Shop. However, it is normally difficult to read attached diagrams.

Ground strap will run from one of the starter bolts (61x engines) or one of the bell housing bolts (60x engines) to the frame on the passenger side. Without a ground strap they usually don't start (no starter action) unless both bushings are out of the shift linkage, in which case the starter grounds via the shift linkage. This can cause smoke from the shifter inside the car, along with erratic starting, welded shift linkage, etc.

Peter

Thanks, Brian. Another lesson learned. Appreciate everyones patience.
 

... :o