Automatic Transmission Cooling

Taking the Heat…

‘New stacked-plate coolers offer maximum protection against premature failure of automatic transmissions.

Today’s automatic transmissions must accomplish a monumental task. Consider that they must handle the torque from 450-cid and larger engines driving 8-ton RVs over mountain passes, and it’s a wonder that any transmission can sustain so much punishment.

Automatic transmissions were born in the 1940s with names such as Hydra-Matic, Dynaflow, Merc-O-Matic, PowerFlite and Jetaway that sound like something out of a Buck Rogers story. Today’s models, with Star Wars-like monikers “727, 4L80E, 47RH and E40D” are similar to their predecessors, but the technology and precision that goes into the newest transmissions make the earlier ones look like crude antiques. Because they are more complex, with extensive electronic controls, they require the utmost in care, maintenance, cooling and flow control.

An automatic transmission’s Achilles heel is heat; high fluid temperatures can bring a transmission to its knees in short order. When oil temperatures climb, the expected life span of the transmission plummets. According to information provided by General Motors Corporation, when average temperatures are near 175 degrees F, transmission life is expected to approach 100,000 miles. When you increase the average temperature to 355 degrees F, life expectancy is 160 miles!

GM recommends that the maximum transmission-fluid temperature (measured in the sump) for short durations shall be no greater than 285 degrees F. At 300 degrees F, research shows, metal parts inside the transmission begin to warp and distort, seals begin to melt, and fluid life is extremely short because of heavy oxidation.

Without the use of a good-quality gauge, it’s impossible to accurately monitor transmission-fluid temperature. Under heavy loads while towing, for example, the transmission will often ?hunt? between direct and overdrive in an attempt to maintain the selected road speed. In addition, lockup-type torque converters cycle on and off. This continual back-and-forth shifting will send the tranny to an early grave. Although GM is the only company of the Big Three that warns against use of overdrive for towing, lock out overdrive on any brand if hunting between gears is observed.

That age-old question of where to install the pick-up for the gauge is debatable. I prefer the oil sump or pan, as it gives an accurate reading of overall transmission-heat level. However, installation can be a bit of a challenge for some do-it-your-selfer’s. The pan must be removed from the transmission, and a sensor fitting must be carefully located so that it will not contact internal transmission components. Drilling, brazing or welding may be required. The best time to install a gauge is when the transmission is getting a filter-and-fluid change.

With the gauge sending unit located in the converter outlet line, the readings obtained will be higher than a sump-mounted pick-up and will fluctuate greatly (100 degrees F or more), depending on load and speed. According to GM, the maximum peak temperature allowed at this location is 350 degrees F. If the temperature exceeds this level the vehicle should be stopped in a safe place, shifted to park or neutral, and run at a fast idle (1200 rpm) for several minutes until it cools down.

The advantage of locating the sender in the converter outlet line is ease of installation. An in-line gauge tee is generally all that’s needed. The disadvantage with this location is that you can’t determine sump temperature and monitor whether the transmission cooler is working effectively. The ideal setup would be to have sensors in both places with a selector switch at the gauge.

In flatland cruising with 75 degree F ambient temperature, normal sump readings should range between 190 and 210 degrees F. The highest temperatures will occur at slow speeds on grades with large throttle openings. A proper-size cooler should keep sump temperatures below 260 degrees F in the most severe conditions.

If your transmission is running hot, you should consider installing an auxiliary external cooler or upgrading the existing factory unit. Vehicles with automatic transmissions are equipped with a heat exchanger located in the radiator. Under normal circumstances this is adequate, but under the stress of heavy RVs, these internal heat exchangers may fall short.

Vehicle manufacturers are concerned that the addition of an auxiliary cooler may cause a flow restriction. Flow reductions can hamper lubrication, resulting in short transmission life. Transmission engineers are steadfast when discussing the subject of flow: ?Restrict flow through the cooling loop and you will have a failure? was a statement we hear repeatedly.

The latest technology utilizes stacked-plate construction. This method of cooler fabrication is similar to tube-and-fin radiators that are used for engine coolant. Stacked-plate coolers make a greater amount of cooling capacity available in a more compact package than with the older, single-tube and fin design, and flow rates can be as much as 15 times greater. Coolers are available from vehicle manufacturers through franchised dealers and after market firms, as well.

Some pre-1994 D-series Dodge Ram pickup trucks equipped with the Cummins diesel and automatic transmission have experienced high fluid temperatures under heavy load. Chrysler is addressing the problem with a super-heavy-duty cooler. Known to dealers as the NHD super-duty cooler kit, part no. 82400-994, it will fit all Dodge/Cummins automatics. This unit mounts on the frame rails near the transmission and uses 12 volt DC fans to provide airflow.

In cold weather, excessive cooling can increase the fluid viscosity to a point where flow to vital transmission components is restricted. RV’ers who travel in both hot and cold weather may experience premature transmission trouble if they have installed a large auxiliary cooler.

Ford has recently issued information on overcooling that relates to F-series and E-series vehicles equipped with the E40D transmission. The company reports that during operation in cold (below 0 degrees F) weather, excessive cooling increases the transmission-fluid viscosity to a point where inadequate lubrication may occur. This is due to reduced flow through the fluid return line. Ford has a by-pass system installed on 7.3 liter diesels and 7.5 liter gas engines that preheats the transmission heat exchanger in the radiator before the engine thermostat opens. This ensures that adequate transmission fluid flow is established soon after the engine is started. Ford has issued Technical Service Bulletin (TSB) 92-1A-4, dated January 26, 1992, which lists the correct heater by-pass parts and approve auxiliary coolers. This TSB is for 1989 and later Bronco, Econoline, F-series and motor home chassis with the E40D transmission.

Hayden manufactures a thermostat (part no. 163) that taps into the transmission’s cooler lines; it’s suitable for nearly every application. This device bypasses the cooler until temperature reaches 160 degrees F, when partial flow into the cooler is permitted. If you need cooling capacity for summer driving, but will also face the rigors of winter travel, you may wish to install such a device. As an alternative, bypass the add-on cooler in winter months when not towing.

Tekonsha Engineering markets the Defender SR (Self Regulating) series of coolers that are designed to handle heat as well as cold. The stacked-plate construction features large passageways in the bottom two plates, which provide full fluid flow under cold weather conditions. The rest of the plates have smaller passages that come into play when the
fluid warms and will pass freely through the openings, according to the company.

P&E Industries Incorporated, builder of automatic transmission accessories, including remote filters, temperature monitors, coolers and synthetic fluid, takes a different view of overcooling. According to a spokesperson for the company, the solution lies in the fluid itself. Conventional ATF has a wide range of viscosity, but it can turn thick is sub-zero weather. For this reason P&E recommends synthetic ATF to do the job. Teamed with the company’s copper cooling units (which are claimed to do a better job of heat transfer than aluminum coolers), heat is rapidly removed when outside temperatures soar. When the weather turns cold, they viscosity and lubrication provided by synthetic fluid is reportedly not affected as much.

High gross vehicle weights punish transmissions. A fluid and transmission filter change every 12,000 – 15,000 miles will help ensure reliability for RV duty. Refer to the owner’s manual for manufacturer’s recommendations on specific models.

Transmission-fluid level is not quite as easy to check as engine-oil level. The transmission must be at operating temperature (after driving about 10 miles at 50 degrees F or higher), with the shift selector in park and the engine idling. Pull the dipstick from its tube and wipe with a clean, lint-free cloth; replace it until it is fully seated. Remove it again and verify that the level is between the ADD and the FULL marks.

Correct fluid level is a necessity. Low fluid levels cause air to be drawn into the transmission’s hydraulic system, reducing the fluid pressure and volume, causing clutch slippage and erratic shifting. High fluid levels can cause foaming and aeration of the fluid, resulting in similar problems. The fluid will expand as the temperature increases, which, combined with foaming, can cause it to be forced out the vent or dipstick tube. This can cause fluid to boil out over hot engine parts, creating clouds of smoke and possibly fire.

All automatic-transmission fluid contains a red-colored dye, so it should appear to be reddish in color. The fluid should contain no visible particulate matter. A frothy milk-white or pinkish appearance is the indication of coolant in the fluid, usually caused by a leak in the radiator-mounted heat exchanger. Coolant contamination will wreak havoc in a transmission. Usually by the time it’s discovered, the damage has been done.

The appearance and odor of the fluid on the dipstick can be an indicator of the transmission’s condition. However, according to Chevrolet’s Motor Home Chassis Service Guide: These two tests are no longer satisfactory criteria for recommending a fluid drain and refill. With the Dexron II fluid, rapid loss of the red color and darkening of the new fluids are normal and do not affect their performance.

Ford’s Motorhome Chassis Service Guide states: Fluid used with the automatic transmission contains a detergent which retains in suspension particles generated during normal transmission use. This characteristic may result in a dark coloring of the fluid and does not of itself indicate malfunction or need for repair.

However, a dark-brown to black color may indicate trouble. Give the fluid on the dipstick a sniff test. If there is an odor similar to overheated brake linings or burned electrical wiring, overheating may have occurred. The transmission may seem to function normally, but damage may have been done. As a simple check, place a few drops on a clean tissue and look for any residue. If particles are evident on the paper, change the fluid and the filter. Change fluid anytime it is suspected to have been overheated or contaminated.

Keeping a handle on automatic-transmission cleanliness and temperature is the key to long life and smooth shifting performance.


About jimhadfield

Retired and enjoying it.
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