Does anyone put the transmission in neutral when stopped for say more than 20 seconds. In the old days I was told this would extend the life of the transmission. Don't know if the gear shifting would cause more harm than good though
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I would not for 20 seconds. There would be no point.
However let's say you are waiting for 14 minutes at a train crossing. Your transmission is engaged in drive and that means your torque converter split halves are turning at different rates, CREATING HEAT ! (Transmission side is stopped, engine side is rotating) So I would argue that just waiting there in gear creates more heat and will RAISE THE FLUID TEMPERATURE.
In general, first monitor fluid temperature leaving the transmission and returning. You are shooting for no higher than 180 F. For every five degrees you go above that, fluid is proportionally breaking down. If allowed by the manufacturer, the extra automatic transmission cooler is your friend.
So I shut my engine off in neutral or park position if I have a 14 minute train.
But let's use a different example. You get stuck in mud or sand or snow and ice and try to drive your way out. That is not the way to do it ! If you can't get out in 120 seconds, put the transmission into park, let it idle for three minutes to allow the transmission fluid to circulate and to cool and lubricate clutches, bands, etc. ''somewhat'' , then shut the engine off, and get a tow truck and save your transmission.
I have worked in an automatic transmission shop. I can can tell you about the ones who Macho/Mano were going to drive their way out of sand, simply because they had a V-8. They replaced their burned out automatic instead.
One guy in Montreal, Canada had a rear
wheel drive GM product stuck in snow and ice. He put the peddle of his large V-8 to the floor, and the standard differential spun ONE
wheel at 80 mph until it ''caught'' and then blew up the spider gears which then went crunch in the ring and pinion gears and left 2 lbs of crunched metal in the left and right axle housings.
That didn't go well for him.
What I do recommend (other than changing fluid and automatic transmission filters), is adding a powerful inline transmission fluid/metal filter (powerful magnets) as one of the most cost effective things you could do. It can go in inlet or outlet, but I prefer outlet returning to the transmission.
Been there, done that.