I just saw an advertisement for an SUV that claimed to have “electronic four-wheel drive.” I know there have been many advances in electronics, but I am wondering how literally to take this. The last time I checked, there were still gears and shafts involved in spinning the wheels – am I missing something?

-Jim Paulson, San Jose, Calif.



You’re right, Jim. There’s more to one of these systems than a handful of silicon chips and some clever software.

A great example is Honda/Acura’s Variable Torque Management Four-Wheel Drive system, one of the most slick around. Vehicles equipped with VTM-4 operate in front-wheel-drive mode on the highway, while continuously spinning a front-to-rear drive shaft and a unique rear differential. The rear differential contains two magnetically operated wet-plate clutches, one on each side, which can be progressively engaged whenever the VTM-4 computer deems necessary.

During hard acceleration or when wheel slip is anticipated, the clutches are engaged, transferring power to both or either of the rear wheels. The system borrows wheel speed information from the ABS (anti-lock brake system) computer and engine load and other inputs from the engine-control computer to provide proactive traction control. Front-to-rear and side-to-side torque might be varied to provide optimum traction and vehicle stability.

The beauty of this system is that it does not contain a heavy and economy-robbing center differential, and it applies torque to needed wheels prior to, as well as after, wheel slip occurs. Like many mild four-wheel drive systems, it omits a transfer case containing low-range, stump-pulling gears.



Now that taxes have been paid, I’m making plans for some summer traveling and will soon have my Tahoe serviced. My question is: Do I need to have my air conditioning serviced, even if it seems to be working OK? My usual garage is running an early bird AC special for $69, and I’m not sure what to do.

-Jackie Conrad, Watsonville, Calif.

A. Wow! You’ll be packed and ready to go before most of us have even given summer a passing thought.

Here’s how to tell if your AC system is up to snuff: Acquire an instant-read digital thermometer (such as a wide-range kitchen model) and place it in the center AC vent register. During a warmer day (70-plus degrees) operate the AC system on normal mode, the coldest setting, and at medium blower speed while the Tahoe is rolling.

A sound AC system should discharge air in the mid-40s or below. If the temperature is 50 degrees or warmer, and/or frequent compressor cycling occurs, then service is probably necessary. (To check cycling, listen under the hood for a distinct click and a change in engine tone, from a safe distance.)

It’s also a good idea to check the AC system’s condenser, a large, flat radiator-like device just behind the Tahoe’s grille, for obstructions. If necessary, blast away leaves, bugs and debris, from the side or top, using the garden hose. Poor AC system performance might be due to refrigerant leakage, or a handful of other causes, and should be checked by a professional.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif. E-mail him at [email protected].