Sunday, Oct 24, 2004 at 08:57
It's a while since we've towed with an auto (and it was a Toyota Cressida 3.0L
sedan, not a 4WD) but I always made sure the converter was in 'lock-up' when cruising. Usually we towed in 3rd (Overdrive off) but when conditions were favourable (tail wind), it would lock in 4th (OD on).
You can see on the tacho when it locks - the rpm drops a hundred or so while the speed remains constant. You can also tell if you're in 'lock-up' when cruising: very gentle apply a little more throttle - if the tacho needle immediately moves up before the speed changes, it's not locked; if both needles move slowly together (like a manual), the converter is locked.
This not only improves economy but helps to keep the transmission fluid temperature lower. When the converter is slipping, energy is being put into the fluid, heating it up. This heat has to be removed by the oil cooler in the radiator, adding load to the engine cooling system - all things that it is good to avoid.
As you might imagine, I'm pretty interested in all sorts of operating temperatures and I fitted a simple bulb/capillary coolant temperature gauge into a tee-piece in the transmission oil cooler return line. The difference running locked Vs unlocked was quite noticeable.
Also used the gauge to good effect to monitor fluid temp when towing up mountain ranges.
Bellingen to
Dorrigo comes to mind (sp), where we pulled over part way up and let the fluid cool for a while after the temp approached 130 C. Just as EGT is a killer of turbos, excessive fluid temperature kills autos.
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