An Editorial by dieselmann
Volume 2; June 99: Towing
With the start of the summer vacation season, I thought it would be a good idea to discuss towing related problems. Two areas of concern come to mind: brakes and automatic transmission.
The brakes on most vehicles are designed specifically for that vehicle based on it's Gross Vehicle Weight--the weight of the vehicle plus it's intended total cargo weight. In a F-250 Heavy Duty or Super Duty, the GVW is normally 8800 pounds, 1500 lbs of which would be cargo. Most loaded trailers pulled by this type of vehicle exceed this intended cargo weight. The result is increased wear of the brake linings and warpage, or cracking, of the discs or drums. As a general rule, trailers which exceed 1500 pounds when loaded should be equiped with their own braking system. In fact, if you intend to fully load the the towing vehicle to it's GVW in addition to towing a trailer, the trailer should have it's own braking system.
Most trailer brake systems are electrically actuated. The control head for electric brakes has sensitivity, or gain, and sometimes level adjustments. These must be set as per the manufacture's instructions to to ensure proper function. The trailer plug and harness should include a ground wire; the triler ball is unreliable and may not provide enough of a circuit for the brakes to work. If the trailer sets for most of the year, the brakes should be inspected and tested for operation before the trailer is loaded for your trip. Rust, leaking axle seals, loose wheel bearings and "rodent damage" can all conspire to prematurely end your vacation in the worse possible way. Once the trailer is loaded, the brake controller sensitivity should be checked and adjusted. You want the trailer to assit in stopping the truck and trailer, but not to lock up, which would cause a "jack-knife" as the trailer skids past the truck.
Many brake failures can also be attributed to down-grade braking while in automatic overdrive. Ford's automatic overdrive transmission provide no engine braking while in overdrive--they free-wheel when coasting. By cancelling overdrive, a coast clutch is applied in the transmission which helps slow the vehicle. While this only partially helps on a diesel--diesels do not have a throttle plate for the engine back pressure to work against--it's better than trying to slow with just brakes. And with the addition of a professionally installed exhaust brake, a diesel can utillize the full benefit of transmission/engine braking.
Overdrive automatic transmissions themselves have had problems when a trailer is added. Increased weight causes increased slippage of the friction plates, which results in increased wear and generated heat. Increasingly, computer controlled transmissions are programed for "controlled slippage" to soften shifts. Factory transmission coolers are usually not large enough to effectivly disapate the increased heat from towing. In addition, the transmission fluid will brake down faster when towing, requiring more frequent service. The simplest remedy is to cancel the overdrive when towing, especially up hill. The lower gear will increase torque to the wheels, and will help keep the torque converter from unlocking--fluid flow through the cooler is increased with the converter locked up, plus heat is generated from an unlocked/slipping converter--and keep shift hunting to a minimum. The next step would be to install a larger, aftermarket transmission cooler. Ensure the hoses are routed so they don't kink or chafe, or the cooler doesn't rub on and damage the radiator or other components. To reduce transmission wear through fluid break down, concider switching to a synthetic. Synthetic lubricants are more heat tolerant than convertional ones, so the increased cost is off-set by an increased service interval.
Finnally, there are aftermarket control devices which can default the overdrive to the cancelled mode each time the key is cycled instead of active overdrive--such as the InterMotive Overdrive Inverter--or even change the quality of the shifts, making them more firm--Banks TransCommand.
One thing I must state about modifing control systems: sometimes installing these can cause unfavorable operation when not towing.
With a little preventive maintenance and planning you and your truck can survive this summer's adventures.
Charles David Ledger; dieselmann©1999
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