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Babying battery might not be worth it

| Saturday, Feb. 15, 2014, 12:01 a.m.

Question: I am a very old man and use my car, a 1998 Lexus 300ES, once a week for a short run. I installed a disconnect switch on the negative side of the battery. Every time I come back to my garage, I dis-connect the battery. Does it make sense to do this? — Alex

Answer: It sounds like you are trying to be careful not to allow the battery to become discharged between trips, but it might end up being more trouble than it's worth. Since you aren't having problems starting, your short trips appear to be sufficient to replenish energy used during startup. Disconnecting and reconnecting the battery erases memory settings for the clock, radio and memory seats, as well as learned or adaptive values for engine management. Your emissions-readiness monitors also are erased, which could be inconvenient when testing is required. Some vehicles are fussy about running their diagnostic monitors, requiring several days or even weeks of driving before completing them. (They're required for emissions test compliance.)

If you were parking for perhaps three or four weeks at a time, I'd consider mounting a float/trickle charger under the hood and plugging it into an outlet at the front of the garage. Carefully mounting the charger cord in the grille could allow a tolerable “oops” situation, should you forget to disconnect before driving away.

Your short trips are of some concern, as you may not be bringing the engine up to temperature long enough to clean up your motor oil. A jaunt of a half-hour or longer, at least occasionally, would help with this as well as fully charge the battery.

Q: I have a 2008 F-150 with the 5.4-liter engine. The heater suddenly stopped blowing hot air. The temperature gauge starts at cold and after a few minutes goes to mid-scale normal. We have had subzero temps lately. Is there a water line from engine to heater that might have frozen? Any ideas? I am freezing.

— Randy Horvat

A: I'd start by checking the temperature of the two quarter-dollar-diameter heater hoses that lead from the engine to the truck cab. With the engine at operating temperature, both hoses should be fairly hot and close to the same temperature: within 6-17 degrees F. If one hose is noticeably warmer than the other, the heater core may be restricted. If neither hose is hot, the engine may not be reaching operating temperature or is low on coolant. If the hoses seem OK, the fault lies within the heater box in the instrument panel. Most modern vehicles use a temperature blend door to regulate ventilation temperature. When heating is desired, air is directed through the heater core. Air can be diverted around the heater core to keep it cold, or a blend of cool and hot can be made.

Your blend door is controlled by an electric servomotor. It's possible that the servomotor isn't functioning or the blend door is stuck or uncoupled. A check with a scan tool might yield diagnostic codes and door position information that will lead to an accurate diagnosis. Ford's pinpoint test H takes a technician through all these checks.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.

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