Truck Transmissions and You!

November 6, 2008 06:00 PM
 
Pickup truck transmissions have come a long way since the 80's. With the invention of the lock up torque converter in automatic transmissions, the better-made transmissions today can stand up to the torque put out by diesel engines and helps slow you down. Practically all of the buses and more medium duty big trucks are going to automatic transmissions. For 2009, you can't buy a manual transmission for a ½ ton truck. Folks who drive mostly in the mountains like the engine braking they get with manual transmissions. Manufactures in the past generally did give you around 3000 pounds more towing capacity on the automatic tranny on ½ ton pickups, SUV's and mini-trucks . This is because of the automatic transmission lock up torque converter, which locks up mechanically like a clutch and pressure plate in a manual, the absence of asbestos, (heat resistant,) from clutch discs, the torque converter doubling the torque coming from the engine flywheel from the stator, and the manufacture having more faith in the computer knowing when to shift verses us human types. Some auto trannies coming out this year will lockup the torque converter early and stay locked longer for better fuel mileage. When an auto tranny TC doesn't lock up and is in fluid coupling, looses 30% of its power. Older auto trannies only locked up the TC in direct drive (1-1.)


 Diesels are generally harder to shift with a manual transmission than are gas-powered trucks. Because of the power and compression of a diesel, (usually twice the compression of a gas engine,) this compression, when you let off the gas pedal and engage the clutch pedal, causes the truck to decelerate quicker than the same process in a gas powered truck. So you have to shift a diesel quicker to avoid the jerk between gears. An automatic transmission on the other hand requires no such skill.  I work in the city, with all of those stop signs and traffic lights; you know I like to drive an automatic. If I pulled a heavy trailer, (at the limits of the trucks capacity,) in the mountains, I use to opt for a manual transmission. The newer auto trannies behind diesels in ¾ and 1 ton pickup trucks, in tow mode, have grade shifting to down shift automatically when you apply the brakes and most can be locked in a gear like a manual transmission.

 That compression in the diesel mentioned earlier helps slow you down on level ground. It doesn't quite work the same way, slowing you down on a steep grade. Diesels didn't have a throttle plate like a gas engine so engine braking a diesel under load won't hold you as well as a gas engine. Lately diesels have a throttle plate to control engine hear for better controlling emissions. An actual engine brake or exhaust brake can be very effective with a diesel, but it s a gray area with manufacture warranties presently. I m hoping they all see the light. Now just Dodge has a factory exhaust brake in ¾ and one ton trucks. They keep giving us more power but the only thing they have given us lately for stopping is 4-wheel disc brakes.
 
Each gear you shift with a manual tranny, will hold you there. And I like all the gears I can find, when I m going down a mountain with a heavy trailer chasing me! Now most auto trannies have 6 gears.Even an experienced driver can get a little nervous if they miss a gear on a manual tranny using the fast side of the mountain. But again in the mountains the more gears the better and you can leave it each gear longer when you are down shifting a manual transmission or auto with grade shifting. The rest of the time manual trannies in diesels are a pain to shift all day if you are using it as a car.

Automatic transmissions overdrive saves you fuel. The newer auto trannies have 2 overdrives to improve fuel economy and slow the engine rpm's. And on fairly flat roads with the right engine combinations you can use overdrive with a load. Generally overdrive has no engine braking ability even on level ground. I'm near the mountains and here you don't use the overdrive on an auto tranny when you are loaded.  Or in case of double overdrives, lock out the last one. Coming down the mountain loaded in overdrive can be freewheeling life threatening experience. Overdrive as I said earlier is designed to save fuel not slow you down. The transmissions will also generally run cooler under a heavy load if you shift down one gear out of overdrive. This allows the pump in the tranny to increase line pressure and provide better cooling. Synthetic oil can also help auto and manual transmissions run cooler. If you pull heavy loads, having synthetic oil in the differentials can add life to them. Synthetic oil doesn't tend to thin out in the cold as much as mineral based oil. Synthetic oil will build oil pressure in the engine sooner also. With a quality synthetic oil, the pressure is instant and quiet. I'm glad to see the improvements that have come to pickup truck automatics and diesels, the only problem is this means they can pull bigger trailers, oh my look out below.
 
Author H. Kent Sundling writes for AgWeb.com via a special agreement withMrTruck.com.

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