I ask only because I'm interested...
Why does there seem to be a presumption against towing with automatics? I regularly tow a horse & trailer (total about 1.75 tonne), previously with an auto Rangey classic, and now with my X5. I've never had a problem, but when I tell people, I get the pursed lips and shaking head treatment. So far as I can see, there shouldn't be a problem so long as the transmission oil has a cooler.
Is there something I don't know?
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I think it stems from the old days when autos were prone to failure and overheating, and not particularly suited to small engined uk cars.
They pretty much all come with coolers of some description now, so shouldn't be a problem. Plus, when they lock-up in top gear there's no slippage at all, hence no extra heat generated. Therefore the extra slippage induced heat only occurs when pulling away. I believe all cars are tested at extreme temperatures towing max weight to ensure they can cope.
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See the section on the Federation of Automatic Transmission Engineers website, which I recently located via HJ's FAQs.
www.fedauto.co.uk
Their section says it's to do with dissipating the heat from the transmission fluid, which can become a problem when towing and when the OE cooling equipment has become less effective with age.
They recommend fitting additional coolers.
Does anyone know if there's an optimum temperature for the transmission fluid, or is it just a matter of keeping it as cool (and as viscous) as possible?
The auto gearbox on SWMBO's Mondeo recently expired on 100k miles. The man reckoned this was about an average mileage for the brake bands in the box to wear out. At £1,200 + VAT, it seems to be a very expensive 'consumable'. I thought I'd avoided the £500+ clutch replacements by getting an automatic.
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Having been involved in AT applications in new vehicle design, here are the facts as I remember them.
Optimum ATF operating temperature (Mercon, Dexron) is about 80 deg. C. Typical modern FWD applications tend to run hotter (space limitations) - typically 110 - 125 deg. C. This is really quite hot, and towing will lead to a further rise in temperature resulting in fluid oxidation.
Ninety percent of premature AT failures are due to fluid overheating.
Some transmissions have inherent design weaknesses (e.g. front clutch on ZF4HP22, diff. bearings on ZF4HP14, most things on Ford CD4E!).
The Ford CD4E (Mondeo, Explorer) is not a good auto, it has been bedevilled with problems since it was introduced and if you get to 100k mi. you are doing well.
If you are towing with any auto then consider a cooler (esp. if in hilly country) and change fluid frequently. If you have a Ford auto then change fluid very frequently (e.g. annually) and make sure you use the correct Mercon fluid from a Ford dealer. Personally I wouldn't want to tow with a Mondeo (CD4E) auto.
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