On The Road

Overworked alternator gives BMW something to whine about

Let me start by thanking all my customers and readers. Because we are an essential business, we are fortunate enough to remain open during these trying times, and I’ve enjoyed engaging with customers who have come into the shop these past few weeks. 

With so much happening all around us these days, it was difficult for me to think about what to write this month. This crisis has made me modify the way I operate my small business and forces us to evolve daily.

I want to share an interesting problem I ran into last week. We had customers bring in their 2001 BMW 530I. The customers brought the car in for normal service but also told me that they were intermittently hearing a whining noise. After checking the car, we found that the power-steering fluid reservoir and the two hoses going to the reservoir were leaking. If the power steering fluid gets too low in the reservoir, it will start to pull in air and not fluid. If it does that, the power-steering pump will aerate and make a whining sound. But in this case, we could not hear the whining noise, even when the fluid was low.

We kept the car overnight so that I could listen for that sound when it was dead-cold in the morning and listen to the front belt idlers and tensioners before they warmed up. Once the belt idler pulley or tensioner pulleys get warm, they usually don’t make a sound; when they are dead-cold, they will.

Diagnosing the problem

The next morning, we finished the maintenance and were ready to start the engine. That’s when we immediately heard a whining sound from the front of the engine. We used the stethoscope and heard the accessory belt tensioner pulley, the accessory belt idler pulley and the alternator bearing making noise. While it’s common for BMW belt idler pulleys and tensioner pulleys to make noise as they age, it’s rare for the alternator to do so.

We removed the belts and checked the pulleys and alternator bearing. We discovered that both belt pulleys were sticking and making noise, but the alternator bearing also was sticking and making noise. On further examination, we found that the inside of the alternator had become overheated and burnt. We then checked all the connections to the alternator and found that the main power cable that leads back to the battery was cracked – almost all the way through – at the connector.

Because the main battery cable at the alternator post was so broken, it caused a significant voltage drop in that cable. The alternator was working overtime and overheating because of the nearly fully broken connector. Because the idler pulley and tensioner pulley are clustered around the alternator, the heat from the alternator sped up the demise of the two belt pulleys.

After repairing the cable connector and replacing the alternator, the idler pulley and tensioner pulley, all was well. It’s not like I have not seen a bad accessory belt tensioner pulley or idler pulley on a BMW, but I had never seen them go bad because of an overheating alternator.

Matt Pataky owns Sunnyvale Foreign Car Service, 15 Pioneer Way, Mountain View. For more information, call 960-6988, email [email protected] or visit sunnyvaleforeigncar.com.

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