Mercedes-Benz Forum banner

1 - 4 of 4 Posts

·
Registered
Joined
·
10 Posts
Discussion Starter #1
My 300D was smoking alt belts off after 2 or 3 weeks. I assumed it was a bad alt since the battery still starts the car just fine. I changed the alt out with a brand new one and it ran great for 2 or 3 weeks and the belt started squealing again. Belt dressing helped for about a day then it would start again.

I decided to check voltage on the battery with my very reliable and expensive multimeter. Got 12.4v with the car off. When running, got 13.1v. When revving, I got 13.8v. Turn on the heat and it drops to 12.8v and slowly eases down to 12.1v unless I rev it again. I never achieved more than 13.8v and I only achieved that with all electrics off and when revving the engine. Like I said, new BOSCH alt. Would a VRM cause this? I didn't change the VRM. I thought they tended to cause the alt to generate too much voltage?
 

·
Registered
Joined
·
513 Posts
My 300D was smoking alt belts off after 2 or 3 weeks. I assumed it was a bad alt since the battery still starts the car just fine. I changed the alt out with a brand new one and it ran great for 2 or 3 weeks and the belt started squealing again. Belt dressing helped for about a day then it would start again.

I decided to check voltage on the battery with my very reliable and expensive multimeter. Got 12.4v with the car off. When running, got 13.1v. When revving, I got 13.8v. Turn on the heat and it drops to 12.8v and slowly eases down to 12.1v unless I rev it again. I never achieved more than 13.8v and I only achieved that with all electrics off and when revving the engine. Like I said, new BOSCH alt. Would a VRM cause this? I didn't change the VRM. I thought they tended to cause the alt to generate too much voltage?
Hi

Sounds like a torque problem: that is, the belts and pullys are not transmitting the power correctly. Could be low quality or incorrect belts, worn out or misaligned pullys, incorrect tension - or something (like oil of coolant) getting on the belts causing slippage.

You may wish to carefully examine the pully grooves for condition.

Good Luck,

Joseph
 

·
Registered
Joined
·
10 Posts
Discussion Starter #3
Now that you mention it, the crank pulley looks a bit munged. If the pulleys are out of alignment, is that something I can do myself?
 

·
Registered
Joined
·
513 Posts
Now that you mention it, the crank pulley looks a bit munged. If the pulleys are out of alignment, is that something I can do myself?
Hi

It would be unusual for a crank pully to be misaligned unless someone beat on it with a hammer. On cars that sit for long periods (years) sometimes the crank pully becomes very rusty and pitted and when shined by the belt can no longer grip it well.. In which case it might require replacement

Other pullys can be misaligned due to bent or broken brackets - or someone installing them with incorrect bolts, washers or spacers. Or they can just be worn from use.

Did your new alternator come with a new pully - or did you need to reuse your old one?
If you reused the old one - that might be your first suspect.

You might try to observe how well your belt rides in the pully grooves. The belt should ride high on the groove - gripping both sides but not touching the bottom.

Good Luck,

Joseph
 
1 - 4 of 4 Posts
Top