Heater Kills My Alternator!!! - PeachParts Mercedes-Benz Forum

slimak9 · #1 02-23-2007, 05:35 PM

1990 300D 2.5 Turbo
I had my alternator checked/rebuilt recently, and now I also replaced my voltage regulator in hopes of correcting my problem, but with no luck.

The battery read about 12.6V at off.
At cold idle it goes to 13.8 - 13.9V.
I turn on the exterior and all interior lights, plus the radio and the read at the battery continues to be level at 13.7 - 13.8V.

As soon as I turn the blower motor/heater on (whether on its own or with the above items), the reading begins to drop to 13.21...13.20...13.19...13.18...13.17...13.16...and goes down at a steady rate.

The alternator light never comes on, and I'm left stranded with a dead battery after driving around, needing a boost, and then driving without heat to let the battery charge.

Any ideas??? Thanks.

#2 02-23-2007, 05:56 PM

I would check the current draw of the blower motor, if the blower is dragging it could be drawing lots of power. Do you have the correct fuse for the climate control system, does it ever blow? Just a guess.

Brian Carlton · #3 02-23-2007, 05:57 PM

The alternator is not keeping up with the demands from the vehicle.

The blower motor certainly demands quite a bit of power, however, not so much that the alternator cannot maintain charging voltage.

Did you run this test at idle, by chance? I might expect to see the voltage fall at idle with heavy vehicle demand.

Try the same test with the engine running at 1500 rpm or more. See if the voltage is maintained at 13.5V or so.

slimak9 · #4 02-23-2007, 07:14 PM

It's the correct fuses, and it never blew on me.

As for the test, I did run it a idle, with full load (lights, blower motor on high, radio), and it droped as low as 11.75V, then I reved up throttle and the Voltage came up to 12.5 - 13.0V range. I could see the headlights getting brighter as I did this.

The only thing I could think is that the belt is not tight enough, or it slips on the pully???

I ran the same idle test on my 1987 300D Turbo, and had no problems, constant 13.8V. And the best tension feels the same.

junqueyardjim · #5 02-23-2007, 07:25 PM

So many of these electrical problems are laid on everything but the guilty one, a dead or dying battery. There are lots of reasons that cause batteies to die, old age is a sure one. Looks to me like your system is working well enough, but it can't or won't take or hold a charge. With a full load on the system to say you are charging 12.5 volts. That is enough to keep the system up.

Brian Carlton · #6 02-23-2007, 07:41 PM

I agree with Jim.

The system appears to function normally. It will typically discharge at idle with a decent load because the alternator cannot keep up. The fact that the battery voltage drops like a stone when this occurs clearly indicates that the battery is not doing the job.

Replace the battery and the problem will be resolved.

Whiskeydan · #7 02-23-2007, 07:53 PM

It sounds to me like the battery's internal resistance is high making it a bit 'soft'.
It would be interesting to measure the blower motor current as well as the batt voltage without the eng running. Maybe the blower is due for some lub.

Or, just change the batt.

vstech · #8 03-01-2007, 02:36 AM

welcome back. 12.6 is low for a battery, but not too bad. the alt is clearly not working properly. voltage jump should be instant. are the belts new? I had similar issues, and I swapped alt bracket from parts SD and it worked fine. you may have a flexing upper bracket. a slipping belt, or a problem with the wiring circuit to the alternator volt sense circuit.
sorry I don't have the expertise to trace the problem if it's in the wiring.
if you have an impact wrench, you may want to swap the alternators between the cars and see if the problem follows the alternator.
John

slimak9 · #9 03-01-2007, 11:58 AM

I switched idler arm springs with my '87 with hopes of it being shot, but no luck. The belt is only 1 year old, and looks perfect.

I think my only option is to swap alternators with my '87. Although with a single garage this means having to do one outside in -10o Celcius weather.

I'll keep you posted. Thanks.

vstech · #10 03-01-2007, 12:18 PM

naw, pull the good running one into the garage, pull off the alternator, then start it and drive it out of the garage, put the problem one in the garage, and install the known good alternator in.
the car will start and run fine without the belt for a few minutes.
John

slimak9 · #11 03-02-2007, 11:15 AM

Thanks John…for the tip.

Well….I swapped alternators with my ’87 and got no change in readings. Still 11.8V at idle, then 10 – 30 seconds later it will jump to 13.8 – 14.0V (time is dependent on how it feels, maybe temp???)

To correct my earlier post, revving up the engine during the initial waiting period, will only bring the Voltage up to 12.2, then drop if let go, and eventually 13.8 – 14.0V after waiting 10 – 30 seconds.

Who knows, maybe the Voltage also drops while driving and therefore causing the battery not to charge, resulting in the dead battery.

I can only think, it’s a ground issue somewhere???

I noticed that after a day of driving, then after starting the idle Voltage was higher about 12.6V and the time is took to reach 13.8 – 14.0V was under 10 seconds???

This will be a challenge…but we’ll find it…Thanks to all.

Side Notes:
I noticed there was no cover on the rear of the alternator in my ’87…did this become available in later models???

Tymbrymi · #12 03-02-2007, 12:35 PM

Quote:

Originally Posted by slimak9

Well….I swapped alternators with my ’87 and got no change in readings. Still 11.8V at idle, then 10 – 30 seconds later it will jump to 13.8 – 14.0V (time is dependent on how it feels, maybe temp???)

To correct my earlier post, revving up the engine during the initial waiting period, will only bring the Voltage up to 12.2, then drop if let go, and eventually 13.8 – 14.0V after waiting 10 – 30 seconds.

I noticed that after a day of driving ... the time is took to reach 13.8 – 14.0V was under 10 seconds???

Forgot to mention the name of the glow plug feature I was talking about: "Afterglow" Just so you can search on it if you'd like...

It really really sounds like you have afterglow. The glow plug relay varies the duration of afterglow based on the ambient temperature and the temperature of the vehicle (I take it the engine was warm when the voltage drop lasted for less than ten seconds?).

I thought of a better way to test to see if the glow plugs running are causing your voltage droop. Get a simple electrical test light at your favorite McParts and connect the clip to a ground somewhere and touch the tip of a glowplug with the probe end... if you have afterglow the light should stay lit after the engine is running. I also think it should stay lit while you are experiencing that voltage drop and go off when it comes back up.

Either way, none of this explains why you are draining your battery. What is your drive cycle? IE, if you never drive the car for more than 10-15 minutes at a time, always have the lights on (especially with upgraded lights), have the blower motor at a high setting, maybe have your stereo turned up, and are driving relatively slow (since our wonderful MB trannies like to go into 4th so early... 45mph at 1400rpm doesn't create much charging power) you could be near the capacity of the alternator for the majority of your drive and the alternator could not be able to charge the battery (re-charging from cranking and all that afterglow).

I take it your 87 300D does the same drive cycle? Do you drive it around town in S? Any usage habits that are different? I'm presuming that it doesn't have this problem...

G'luck with this... little electrical gremlins like this can be sooo much fun

slimak9 · #13 03-04-2007, 05:55 PM

Yes...after speaking with an electrical mechanic (genius at a price) on Saturday, "Afterglow" is exactly what the '90 has and that is causing the voltage to drop while idling after cold starts...this "Afterglow" causes the vehicle to idle much more smoothly compared to my '87. I kind of like this feature and want it in my '87. So I can stop worring about this initial drop.

He's never heard of this feature causing problems after it shuts off and sugests this battery drain issue could be a ground fault inside the alternator which is hard to detect as it may spark problems 5% of the time and not happen again for some time.

I currently have the alternators swapped and he sugested leaving them like this. If the battery drains in the "87 we've found the problem, if it doesn't continue then, no use in spending money on testing, new parts, until this happens again.

As for the driving habbits, both M/V's are used quite frequently, 20-30 min. one way to work. '90 about 25,000 Miles a year. '87 about 20,000 Miles a year.

Some new readings:
All components off:
'87 with '90's alternator at idle in -5 Celcius: 14.2V
'90 with '87's alternator at idle after "Afterglow" shuts off (same temp.): 13.8V

I boggles my head, in the '87 I had 13.8V with stock alternator, now .4V higher with '90's alternator. And the '90's readings stayed the same when swaped. Exact same part #'s on alternators.

We'll have to wait this one out.
I'l keep you posted.

jeff-abalone · #14 01-20-2008, 01:25 PM

May be the battery problem.

Recently, my 1991 190E 2.3 had the same problem. Put new battery one (may be this one is defect)

Idle : 12.14V
2000rpm: 12.85V

Replaced 3 alternators within one week, still the same.

Put a 7 years old battery (old one from my wrecked 190E)

Idle: 12.14V
1500rpm: 13.34V

I will put the factory new battery on tomorrow (since today is sunday) and see what happened!!! I think that the aftermarket battery, which is not good for 90+ MB.

barry123400 · #15 01-20-2008, 02:22 PM

Very frustrating for the poster. Since the blower motor is not taking it's fuse out I would also forget it for now. What I would do is run a test light on a couple of wires back into the cab temporarily from a glow plug. I suspect they might be coming back on and taking your battery down. Possibly a defective relay.

You have more overall drain than the alternators output perhaps. So the method of reading battery voltage may not be really good in your case. Or I should say in comparison to the other car. Also not a good senario for the alternator. Does it get good and warm ever check?
Remember to drain the battery in such a short interval takes a real heavy load. Nothing I can think of with a heavy enough drain elsewhere on your car could manage this without other symptoms in my opinion.

If you change your viewpoint from a charging problem to a very heavy drain problem it might help. Another monitor would be a cheap digital meter connected to the cigarette lighter plug. I still would run the wires back off those glow plugs though as the lighter plug will just verify the drain by showing reducing voltage.
I am fairly suprised the hot shot mechanic did not advise you of this possibility. To be fair though it tends to be diesel specific in nature.
One last perhaps easier for you approach is to start the car and disconnect the main glow plug fuse or glow harness plug and see how things go.
Keep in mind I can be wrong it's just where I would logically look by the description of your problem combined with what you have done so far. Also takes very little to eliminate it as the problem and gets it out of the way as a possibility. Then go on to other things after if your problem is still there. I almost bet it will not be. Remember I stated almost bet.