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Auto Doctor: Battery change requires backup power

Because power is interrupted during a battery change

Because power is interrupted during a battery change on a 2008 Chevrolet Corvette, or any model-year 1996 or later vehicle, the auto doctor recommends hooking up a battery backup power supply to the ALDL connector under the dash on vehicles. Credit: General Motors

Dear Doctor: I have a 2008 Corvette. Do I need to be worried about keeping power on the computers during a battery change? -- Rancy

Dear Rancy: When power is interrupted during a battery change, the vehicle's computers will lose memory. Once the battery is reconnected the computers need to go through a relearning period. On some vehicles the radio and navigation systems need their codes re-entered. Other problems can include transmission shifting, as well as the power window automatic functions. It is always advisable to connect a battery backup power supply to the ALDL connector under the dash on vehicles 1996 and newer. Always replace the battery with one that meets or exceeds the factory battery specifications.

Dear Doctor: I have a 2006 Nissan Xterra. The "plus" side of the battery always builds corrosion. I clean the corrosion off every 3 months and have applied corrosion prevention, but I still have the same problem. Also, the fuel gauge does not move on start-up; it stays on "E" and the check engine light came on? What should I do? -- Peter

Dear Peter: The corrosive buildup is from the gases leaving the battery and two dissimilar metals with acid fumes causing corrosion. The first step is to check the charging voltage and make sure the alternator is not overcharging. Clean off all of the corrosion. Once dried, use a battery corrosion product, or grease, or Vaseline to help prevent corrosion buildup. As for the gas gauge not working, the technician can connect a scan tool to see if there is any signal coming out of the fuel level sender to the computer and then to the dash gauge. I've seen many rust and corroded gas float assemblies.

Dear Doctor: I'm having a problem with the automatic tire pressure indicator in my 2008 Cadillac DTS. The light indicated "low pressure." I checked with tire with a gauge and all had 30 pounds, but the door jamb sticker calls for 28 pounds cold. I have my own compressor and reset the indicator according to the instructions in the book, but the light didn't go out. Is there a way to clean the sensors? -- Bob

Dear Bob: There is no way to clean these tire monitors. Most tire pressure monitors have non-replaceable, small watch-like batteries located inside the tire. The metal valve cap cover becomes corroded and sometimes cannot be removed. Most car owners usually want to replace the tire valve with a regular rubber valve stem and not spend the $75 to $125 for the correct tire pressure monitor valve assembly. There are aftermarket companies that have introduced universal monitors at a lesser price. And, there are companies that have great tools to reset and reprogram tire pressure monitors. I use the Bartec 400 and the Pilot programmer and relearn tool.

Dear Doctor: I own a 2012 Hyundai Sonata. While coasting to a traffic light at 45 mph, it seems the car downshifts till 35 mph, and then upshifts at 25 mph. I have driven automatic transmission cars for many decades, but never experienced this downshifting problem. Any reason for this? -- Morgan

Dear Morgan: The transmissions of yesteryear were not controlled by a computer or electronics. In the old days when slowing some transmissions freewheeled and others stayed in second or third gears, until the gas pedal was pressed. Your car uses a computer to calculate when to downshift and upshift. It is normal for the transmission to downshift, as well as it is for the transmission not to upshift into third gear until the transmission and engine temperature are warm. This is designed to speed up the warming of the engine and lower emissions.

Dear Doctor: My question concerns replacement of Xenon headlights. The owner's manual gives no information on my Xenon bulb replacement, except to "see dealer." In the past, I always changed my Halogen bulbs, being careful not to transfer skin oils to the bulb. Is there any difference in replacing Xenon bulbs? -- Boothe

Dear Booth: The same installation instructions and handling of the bulbs hold true. Be advised, the $100-plus bulb is not always the problem. Some headlight replacements require the removal of the front bumper cover. Other vehicles require the removal of the inner wheel wells to access the bulb. If you are doing all the work yourself, then make sure to check the bulb operation before putting the car back together. -- Junior Damato

Junior Damato is an ASE-certified Master Technician. Email questions to Mail questions to: Motor Matters, PO Box 3305, Wilmington, DE 19804

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