How to choose the right tow vehicle transmission

Not so long ago, the manual transmission was considered, generally speaking, as the best transmission for this purpose. In 4WD applications, in particular, they were almost a truck transmission, intended to cope with the rigours of hard-core offroading and commercial use, where the manufacturers built in a high threshold for abuse.

Automatic transmissions were almost an afterthought – they were, in the main, considered a town vehicle choice, or fitted to a luxury vehicle. Not all were built to cope with the strain of constant high-load situations, such as towing a heavy caravan.

Automatics are not only more popular, they are the better transmission choice when towing. Towing a large caravan takes a lot of concentration and to have one less thing to worry about – changing gears – can only be a good thing. They are also much quicker to change gears, especially in a 4WD, where the engineers have given priority to gear strength over light, fast gear changes.

That translates to much better progress in those situations where you want to keep up momentum. When climbing a steep hill and losing momentum, you want to be able to get into a lower ratio quickly so that you can allow the engine to give its best in the peak torque band.

Downshifting in a manual vehicle can lose precious seconds – and, therefore, momentum – that you can’t afford to lose.

The increasingly sophisticated electronic supervision of hydraulic automatic transmissions has a lot to do with their better performance and ability to handle heavy-duty towing. Most autos can now sense heavy-hauling situations and hold lower gears for longer and downshift more quickly. Others have a ‘sport’ mode that does the same thing.

Manual-mode has become more common, too, allowing the driver to move the gear selector over to a spring-loaded toggle position and change gears manually by simply flicking the lever forwards or backwards. Others have taken this one step further, borrowing from F1 the steering-mounted paddle shifters, making manual gearshifts in an automatic even easier.

The biggest problem I hear caravanners worried about (with auto or manual) is if, or when, you can use overdrive when towing. Because the transmission input shaft is turning more slowly than the output shaft, there’s the concern that the transmission is under enormous load and more prone to failure.

This is true for a manual transmission or earlier autos that did not have the ‘electronic brain’ to sense heavy load conditions. You can still use overdrive, but it takes mechanical sympathy on the driver’s part. You have to only use it sparingly, in light load or no load conditions. So when touring on the plains, where you can feel there’s little accelerator input required to maintain speed, or when travelling downhill, overdrive is fine.

But on the plains with a heavy, full-size van behind most tow vehicles won’t be happy in overdrive – there’s simply too much load – and when heading downhill you have to be selective. There’s no point dropping into overdrive when it’s a blustery day, the road is uneven and pushing the rig around, or if you have traffic ahead. In those instances, a lower gear is always better, so that you can accelerate quickly out of trouble or use engine braking to help slow the rig down.

As for modern autos, they do this for you.

AUTO BENEFITS

The Jeep Cherokee and Grand Cherokee both have overdrives in their standard hydraulic automatic transmissions. The Cherokee has overdriven gears from sixth through to ninth gear, while the Grand Cherokee’s top two gears in its eight-speed auto are overdrives.

We asked Alan Swanson, senior manager of product strategy at Fiat Chrysler Automobiles Australia, how these two vehicles operate under heavy load conditions, such as towing. He said, “The gearbox in the Cherokee and Grand Cherokee will detect a load, and select a gear automatically. So if the most efficient gear is a higher gear, then it will select it, and if it is lower, it will select a lower gear. Most Jeep vehicles are also equipped with a transmission temperature read-out as standard, which is there to show you how the transmission is performing, and will warn you if it is ever in danger of overheating”.

This applies to most modern automatics and while many don’t have the real-time transmission temperature read-out the Jeeps have, most at least have a transmission temperature warning light which alerts you if they’re getting too hot.

If you’re towing offroad on uneven ground, having an automatic transmission is better, too. Automatic transmissions give the driver more control in precarious driving situations, such as in soft sand or mud – which is not a situation you want to be in with a heavy camper trailer, let alone with a full-size caravan! In these circumstances, you might find that the speedy downshift of an automatic transmission allows you to keep up momentum that would easily be lost if driving with a manual. An automatic is also easier to apply torque from a standstill on slippery surfaces than a manual transmission vehicle.

Generally speaking, there is a price to be paid for the automatic’s ease of use. Have a look at the official fuel consumption figures and you’ll often see that an automatic version will be thirstier than the manual model of the same car. Yet the growing number of ratios, smarter electronics and, in particular, the lock-up torque converter has made the gap between manual and auto fuel consumption much narrower. Power loss resulting from slippage of the clutches in the transmission and the torque converter are an inherent barrier for the hydraulic automatic matching or bettering the fuel figures of a manual.

Automatics also cost more to buy (where they’re an option) and acceleration can be more blunted than a manual, too.

TRANSMISSION TYPES

Muddying the waters is the clutch-less manual – also called an automated manual. Though rarely seen in heavy-duty tow vehicles (except cab/chassis and van applications such as the Fiat Ducato and Mercedes-Benz Sprinter), these transmissions change gears using an electro-mechanical setup. They are usually a conventional manual transmission, but with an electronic selector mechanism and clutch actuator. As with the manual-mode hydraulic autos, the driver can either shift gears ‘manually’ (but without the need for a clutch pedal) or let the transmission do the job.

Another variety of transmission is Constantly Variable Transmission (CVT). While quite dated in principle, manufacturers have taken to the CVT for its ability to make the most of an engine’s efficiency potential.

The guts of the CVT is two shafts, one conical in shape, with a band running from one to the other. As the band is made to change position on the conical shaft, the ratio between the input and output shafts changes. While CVTs are getting better, they’re still not used in heavy-duty towing vehicles, as they can’t quite handle these high load situations.

One item that many automatic transmissions benefit from is an additional air-to-oil transmission fluid cooler. Some manufacturers specify the fitment of such a cooler as part of their heavy-duty towing package, while other manufacturers have faith in the original design.

Even though some manufacturers claim their auto transmissions are ‘sealed for life’, don’t pay attention to that if you are towing regularly with your vehicle. A fluid and filter change every 50,000-60,000km is a good idea. Make sure that the mechanic uses the correct fluid for your vehicle, though, as some transmissions require a specific fluid and if you use something else you may find that your transmission fails because of it.

All in all, the modern automatic transmission is the way to go for heavy-duty towing. Most often, it’s the only way you can go, and you and I are better for it.

The full feature appeared in Caravan World #553 July 2016. Subscribe today for the latest caravan reviews and news every month!