Dayton Daily News

Wheels: An e-mail from Vaughn says, "Please give me your opinion on this matter. Recently I was told that I needed a new alternator because I was having problems with my battery going down. A new alternator was installed and now I am getting a "hum" from the alternator area. I took the car back to the mechanic and was told that the alternator was okay, and the humming sound might be coming from the AC compressor. He also had installed a new drive belt. I did not have the "hum" problem before the alternator and belt were installed. If not the alternator, could it be that the belt has the incorrect tension?"

Halderman: The hum could be the brushes wearing in and the noise may stop. However, it sounds like the alternator has a bad diode and this will cause a hum and not affect the output. It can be checked by measuring the AC voltage ripple at the output terminal of the alternator. Use a digital multimeter and select AC volts. Attach one meter lead to the output terminal of the alternator and the other lead to the case of the alternator. Start the engine and turn the headlights on bright to give an electrical load. The reading should be less than 0.4 volt. If 0.5 or higher is read, the alternator is bad. I hope this helps.

Update: Vaughn wrote saying he tested the AC voltage and read just 0.06 volts AC indicating that the diodes are okay. He mentioned that the hum has decreased and assumes that the brushes are wearing in.

Wheels: An e-mail from Mark says, "I just purchased a new Jeep 2 months ago and just took it to get its first oil change (yes, I drive a lot). I had a plan that I would buy some synthetic oil and see if the dealer would still give me the free oil changes, but one of my co-workers suggested that until I break in the engine, I should wait and add synthetic oil when I get to 20,000 or 30,000 miles. When do you recommend adding synthetic oil? Should I wait until the engine breaks in more or should I wait until a few thousand miles? I have been reading some articles online but I cannot find one unbiased to a synthetic product. Please advise."

Halderman: I, too, use synthetic (Mobil 1) and I waited until about 7,000 miles (the second oil change). While many companies use Mobil 1 at the factory in new engines, most of the experts I have talked to recommend using conventional oil for at least 5,000 miles. The same is recommended for lawn and garden equipment. Wait until the first or second scheduled oil change before switching over to synthetic.

James D. Halderman is an ASE-certified master technician, a member of the Society of Automotive Engineers and the author of 12 textbooks. Submit your automotive care questions to Halderman via e-mail at jhalderman2@juno.com, or write him in care of: Wheels, Marketing Publications Department, Dayton Daily News, 1611 S. Main St., Dayton OH 45409.