Q: I own a 2004 Chevy Silverado 2500HD with the 6.0-liter V-8 engine and the factory towing package. I plan to buy a fifth-wheel RV, which weighs 12,300 pounds. Chevrolet's literature lists maximum fifth-wheel towing capacity as 10,100 pounds, yet the same truck with a diesel engine can tow a 16,100-pound fifth-wheel trailer. Is it safe for me to tow this fifth-wheel trailer with my truck? I understand the diesel has much more torque, which makes for easier towing. My Chevrolet dealer's chief mechanic said as long as I drive it sensibly, and it's a fifth-wheel trailer, it should be fine. Also, from a structural point of view (axle load, wheels, and tires), am I endangering anything by towing the heavier-than-approved fifth-wheel trailer?

A: Double-check your owner's manual for the towing capacity numbers. Looking at 2009 figures, there's about a 2900-pound difference in towing capacities between the same two 3/4-ton GM trucks pulling a fifth-wheel, one with a Duramax and one without. A truck's towing capacity is determined by a formula that takes into consideration a number of variables, including, but not limited to, chassis structure (frame strength, wheelbase, track, suspension, wheels, tires), engine, transmission, brakes, axle strength, axle ratio, and so on. The reasons behind these maximum ratings are to avoid excessive stress on the vehicle in question, and safety. According to the National Highway Traffic Safety Administration, each year there are approximately 59,000 crashes in the U.S. involving passenger vehicles towing trailers, resulting in about 28,000 people being injured. Do not tow above the vehicle's maximum capacity. It's best to keep trailer weight at a decent margin below the maximum and allow for extra weight, including cousins and cargo inside the truck, which increases the Gross Combined Weight Rating. As far as the diesel difference, when you see higher towing capacities on the same truck with a diesel engine as opposed to gas, there may be more to the equation than better towing ability due to the diesel's exemplary low-end torque. Diesel engines are also built stronger to withstand the non-spark high-compression combustion process: They not only perform well while towing, but are more durable and last longer than a gas engine under towing conditions. There may also be additional heavy-duty components attached to certain diesels, like a transmission upgrade.

Q: My 2005 Chevrolet Silverado's "Service 4WD" light comes on about once a week. When the light is on, four-wheel drive won't engage. If I turn the truck off and start it again, four-wheel drive works until the next time the light comes on. But if I turn off the truck with four-wheel drive engaged, when I restart it, 4WD is still engaged. I replaced the switch on the dash and GM replaced the electronic control module and updated the programming. It seems to happen mostly when I hit bumps, especially washboarded gravel roads. It seems it could be a bad connection or ground. Any ideas on where I could start looking?

A: Based on the VIN you submitted, your truck is a half-ton with the 5.3-liter V-8. When the Service 4WD indicator lights up, there's a problem related to the transfer-case shift-control module, the appropriate diagnostic trouble code has been stored in the control module's memory, and it's going to shut down 4x4 operation to prevent additional damage. When the failure is intermittent, every time you restart the engine (cycle the ignition), it's like it never happened until the fault recurs. Then the light comes back on, and again you're out of luck. According to your records, the transfer case shift control switch and the control module have been replaced, and the module's software was reflashed. But you didn't mention the diagnostic trouble code stored in the control module's memory bank--and that's the most important factor here. If it was DTC C0374, chances are there's a bad connection in the wiring harness between connector C151 (under the hood) and the transfer case. This is part of the circuit between the front and rear propshaft speed sensors (mounted on the transfer case) and the transfer case control module. Chevy technical service bulletin 05-04-21-003C, dated April 15, 2008, discusses a water intrusion condition affecting the harness and gives part numbers for modified harness assemblies and connectors (part numbers vary by vehicle option codes). This problem has been setting DTC C0374 and causing erratic 4x4 system operation on certain Silverados and additional 2003-2007 GM-built full- and midsize trucks and SUVs.