Q. I have a convertible that I put away for six months a year. Will I have a problem with rust on my brakes in May, when I bring the car out of the garage and put it on the road?
A. When you start up the car, it will make a grinding noise while decelerating, as that light coat of rust is scrubbed off by the pad. Ignore this. Within a few blocks it will all be gone.
If you start to feel a pulsating pedal, it's caused by a high spot where the discs were protected by the pad. If this happens, you will need to get the brakes turned.
Next winter, every six weeks or so, pick a dry day and drive around for 30 minutes or so to prevent rust, to keep the fuel injection from gumming up, and to keep the battery charged.
Q. I have a 1991 Mercedes-Benz 300 D Turbo still going strong at 250,000 miles. I've had some engine work done, including valve replacement.
The guy who did the work recommends that I add oil - only a few ounces - to my fuel mix when I fill up my tank, to compensate for the new diesel fuel. The Mercedes dealer has never heard of this suggestion, and does not agree. What's your take?
A. I've never heard of this, so it's no wonder that the Mercedes dealer hadn't either.
Here's a quote from the Clean Diesel Fuel Alliance's website: "Like low-sulfur diesel fuel, ULSD fuel" - that stands for "ultra-low-sulfur diesel" - "requires good lubricity and corrosion inhibitors to prevent unacceptable engine wear. As necessary, additives to increase lubricity and to inhibit corrosion are added to ULSD fuel prior to its retail sale. With these additives, ULSD fuel performs as well as low-sulfur diesel fuel."
Don't worry about it, in other words.
Q. My audio system has been emitting a loud, A/C-like buzzing ever since I used a charger/booster to jump-start the car after I installed a new battery in my 2000 Chrysler Concorde. I had left the positive terminal at the battery in a state of disconnection, due to a bad terminal end.
Do you have insight into what happened?
A. That buzz is not A/C but AC.
Alternators have diodes inside to convert the AC current from the stator into DC to operate the car and charge the battery. You've almost certainly blown a diode.
This can easily happen if the battery is disconnected when the engine is running. An electromotive force known as "back EMF" can spike the voltage in the stator windings to hundreds of volts, more than enough to fry one or more of the diodes.
What's "back EMF?" It's the same thing that lets an ignition coil generate the thousands of volts to fire the spark plugs: A coil of wire with current running through it has a magnetic field, and it's a pretty powerful one when the alternator is generating current. When the breaker points that feed 12 volts to the low-voltage windings in the ignition coil open, the magnetic field collapses, generating current as the lines of magnetic flux cross through the copper windings in the high-voltage windings. It has nowhere to go except into the high-voltage windings in the coil, which generate 20,000 or more volts.
Your alternator has no high-voltage windings, so there is nowhere for the current being induced into the stator windings to go if the battery isn't in the circuit. Instead, the diode gets fried.
Your intermittent positive connection probably is what did it. A new battery should have had enough charge on it to start the car after one night idle. That says to me that there was a bad connection at the positive terminal. Hooking up the battery charger and disturbing the clamp may have made enough connection to start the car, but it wasn't good enough to keep from blowing out the diode.
A decent technician can test and replace the blown diode or diodes, but it might be cheaper simply to replace the alternator.
Mike Allen is a senior editor for Popular Mechanics magazine. Questions should be sent via e-mail to driveit@nytimes.com.
