Heater Kills My Alternator!!!

[slimak9]

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.

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]

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]

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]

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]

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]

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.

[ytr1903]

I agree with all the others, but since your other MB is operating just fine. Why don't you trade batteries with it. Cheap test--$0. Minimum amount of time-- both cars are at your house. Some of us have enough MB around of the same vintage to just swap parts with known good ones until the problem is fixed.

Good luck,
Tom

PS Let us know the solution.

[Graminal95]

I had a 240D a few years back that was doing the same type of thing, only it was when I was on the highway. I found out after trying a lot of different things that my belt was slipping at higher RPM/high load. There was no change in noise and the light also stayed off. I changed the belt for a new one and made sure it was good and tight.

Maybe your belt is just slipping with the high load of the blower motor on it and thus voltage drop.

[slimak9]

Sorry for the delay.
I do have a brand new battery, and I even swaped it with my '87 300D incase I was sold a defective battery.

Some new observations:
Vehicle Off: 12.4 - 12.5V
Key Turned to Heat-Glow (All Electrical Items Off...Radio, Heater, Lights): 11.3 - 11.4V
Vehicle Idle: 11.8 - 11.9 V
Then:
20 - 30 Seconds after idling the reading jumps to 13.8V, even 14V.
OR
If I rev. up the throttle, it too will jump to the proper reading.

This delay before it jumps to an acceptable Voltage doesn't happen on my '87 300D when idling, instead it's instantanious. I believe this could be the root cause of my problems.

Any ideas???

Thanks.

[vstech]

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]

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]

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

[Tymbrymi]

Originally Posted by slimak9

This delay before it jumps to an acceptable Voltage doesn't happen on my '87 300D when idling, instead it's instantanious. I believe this could be the root cause of my problems.

At some point in time MB started running the glow plugs for a minute or so after the engine starts to smooth out cold idle and help emissions. I don't know if the '90 300D is in that or not, but I think its likely... If your car will start without glowing, disconnect the glow plug fuse and I bet you won't see that delay to proper voltage after starting.

[ytr1903]

I think you are onto something with the delayed glow plugs, John. Is there a memorial at the Starkville City Jail that commemorates Johnny Cash's "visit" there? Sorry, back to alternators.

Tom