Background

The most vulnerable area  of a motorhome or heavy duty towing vehicle is typically the automatic transmission (followed by the engine). Usually the automatic transmission is the same as in a passenger car or pickup and was not designed to handle the kind of continuous heavy duty load imposed by these applications. Load equals heat and this heat must be dissipated or the transmission will suffer. Heat also means wasted energy and lower fuel economy.

Continuous temperatures above 225° F in the sump should be avoided. In general,  transmission life expectancy is halved with every 20° F rise in transmission-fluid temperature above 175° F. When temperatures rise above the 250° mark, fluid breakdown accelerates rapidly. One way to improve fluid durability is to install an auxiliary cooler and/or switch to synthetic transmission fluid. 

If there are any doubts as to how well your transmission is being cooled, installing an aftermarket temperature gauge will quickly dispel them. Compared to the cost of replacing a transmission, a gauge is cheap insurance. Unlike simply inspecting the transmission fluid for a burnt color or odor, a gauge will tell you if fluid temperatures are excessive before fluid breakdown occurs. After installing a gauge, many owners are surprised at how much the fluid temperature varies with different road conditions and between towing and driving solo.

Many motorhomes are factory-equipped with a heat-exchanger-type transmission cooler built into a radiator tank. However, since the reserve capacity of these coolers is limited, some chassis manufacturers require the installation of a supplemental cooler before towing. In fact, an auxiliary transmission cooler is a good idea on just about any heavy vehicle. Many owners install the largest auxiliary cooler that will fit, since fluid overcooling is rarely a problem. However, some coolers are more restrictive than others, so choose carefully. If you have a concern about overcooling get a Thermostat. They will help your transmission fluid come up to temperature quicker for cold weather use then maintain the temperature setting. Another thing to evaluate is where you will be using your vehicle. Unless you will be driving in subzero temperatures consider bypassing the radiator and just using a properly sized transmission cooler. Some transmission coolers even include a fan so it can be mounted elsewhere and take some heat load off the radiator so it can better cool the engine. There are transmission coolers that include an electric fan and a thermostat (see below) so they can be mounted almost anywhere. Aftermarket sump pans of aluminum or with cooling tubes are available. The extra fluid capacity will lower transmission temperatures somewhat, although they're no substitute for a large external cooler.

Driving technique also has a lot to do with transmission cooling. Reducing speed enough to downshift one gear can be a big help on long upgrades, since the lower gear reduces slippage in the torque converter and increases engine-fan speed. Using engine braking to control your speed down long inclines may also increase fluid temperatures. Again, slowing enough to downshift will usually cool things off.

Several companies manufacture aftermarket torque converters that are designed to reduce slippage (or "stall speed"), thereby reducing fluid temperatures. This tighter mechanical coupling also improves engine compression braking. However, since reducing the stall speed also tends to reduce the converter's ultimate torque multiplication, some reduction in pulling power from a standstill may be noticed. Engine cooling is another area that may suffer when towing heavy loads. Unfortunately, many owners are unaware of the problem due to the lack an accurate, well-marked temperature gauge. Generally, aftermarket mechanical-type gauges are preferred over the electrical type because the latter's extra mechanical-to-electrical conversion step tends to degrade accuracy. Also, mechanical gauges often have a "wide-sweep" dial that's marked in smaller temperature increments. If a mechanical gauge doesn't fit your situation get a quality electrical gauge.

For most gasoline engines, engine coolant temperatures should be kept below 230° F for best performance and durability. At 260 to 270° F, boil over is imminent. In rare cases, boil over can create an engine fire if the glycol-based coolant is allowed to come in contact with hot exhaust parts. Finally, high temperatures reduce the life expectancy of hoses, gaskets, wiring and electrical accessories -- not to mention hastening the demise of most internal engine components. Maximum coolant temperature for diesel engines is in the 220 to 230° F range. Naturally, the first step in correcting an engine-overheating condition should be to confirm that the factory cooling components are functioning properly. Pay particular attention to the radiator cap, which may open at lower pressures as the relief spring weakens. A partially stuck thermostat can also cause marginal cooling. When replacing it, avoid the temptation to install a lower-temperature version; it normally won't improve the engine's ultimate ability to cool, and could adversely affect drivability, fuel economy, emissions and even the lubricating ability of the motor oil.

Defective fan clutches are another common trouble spot. Most of today's clutches are designed to fully engage when the air temperature exiting the radiator is around 215 to 225° F. Interestingly, one manufacturer claims that a fan clutch typically develops an additional 100 rpm of slippage for every year of service.

Some Heavy duty vehicles are factory-equipped with electric auxiliary radiator fans, which are also available as add-on items. Although these usually won't take the place of the original fan, they're great for supplementing radiator airflow, particularly when the coach gets a lot of slow driving. By installing a thermostatic switch, the fans can also be wired to operate for awhile after the engine is shut off, thereby reducing the tendency for engine "heat soak."

On many motorhomes, engine cooling can be further improved by simply closing off all gaps around the radiator edges, thereby preventing any air from going around the radiator. On front-engine coaches, avoid the use of bug screens or rock protectors in front of the radiator air inlet, and refrain from securing bicycles or other obstructions on the front bumper.

Bidena Products makes an on-demand radiator watering system that periodically sprays the outside of the radiator with a jet of water -- especially effective in reducing temperature spikes, which occur while climbing grades. Red Line's Water Wetter is effective at lowering coolant temperature. Both gas and diesel blends are available.

Not all overheating problems are easy to correct. Radiator-core restrictions are quite common; they usually require "rodding out" or replacing the radiator, which is more laborious on a motorhome than on a car or a light truck. Head-gasket leaks and cracks in the engine block are also responsible for a number of overheating problems. Often difficult to diagnose, these problems usually require a leak detector to confirm. Of course, the "fun" is just beginning after you've identified this problem!

As a last thought, it's important not to overlook the exhaust system's contribution to engine cooling. The exhaust system is responsible for carrying away approximately  one-third of the engine's total heat output. Installing a low-restriction aftermarket exhaust system allows this heat to be removed more rapidly. Besides significantly reducing engine temperatures, it will almost always yield torque and horsepower increases, plus fuel-economy improvements.

THE WORST SCENARIO (excerpt from http://www.americanrvrentals.com/) "Fire in an RV is a terrifying thought. Perhaps that is why it falls into the category of happening to the "other people". There is complete devastation in some 6,500 reported RV fires a year. Of these fires, 63% were in motorhomes, 1500 had damage over $10,000, and 28 people died. (There were an estimated 10,000 more fires than this 6,500 number, unreported for insurance reasons). 70% of the reported fires were motorhome fires. Most often these were transmission fires. It takes only 5 to 7 minutes for a 40 foot RV to burn to the rails."

 Copyright 2003

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