Originally Posted by quadancer
My GM mechanic told me that once a tranny has a lot of miles on it, it's better to just run the existing fluid if it's not showing any funny smells or metals. He said changing it usually causes problems more often than not, so I'm wondering if that's true for these Foards too - especially the taurasses that seem to drop trannies like eggs.
The one I just bought has now 135,000 on it and shifts perfectly. I'd hate to mess it up, although I've become a believer in syn oil after putting it in the engine. Huge difference.
Changing fluid won't cause problems - flushing the transmission will; where all fluid is evacuated under pressure and replaced with new fluid.
If you break it down, when a vehicle is new it has, say, 12quarts of total capacity including what's in the torque converter. After 60K, a fluid change only, drains about 4-6quarts at the most. rarely does a fluid change require 50% total capacity. So I think the manufacturers are wrong here when they say transmission fluid change every 60K. After the first 60K, it should be changed every 30K at the maximum, as you'll only be changing 4quarts of fluid, or roughly 1/3 total capacity. This allows the fluid to degrade faster over time unless you shorten the fluid change intervals.
This is where flushing comes in. Flushing removes 90-99% of the current fluid in the system depending on how efficient the machine is that the shop uses. This also means that if any dirt buildup, or pieces that have worn off, can get lodged into the valvebody causing fluid flow problems, and ultimately cost you in transmission work, or repair. My rule of thumb is if the vehicle has more than 75K on it, don't flush, change only from then on. If you flushed it at say 50K (10K earlier than OEM recommend), it'll cost you more, but then you should be fine if you continue to flush it every 35-40K thereafter for best performance.
Running the same fluid in a transmission until it breaks down and causes complete transmission failure is NOT the way to go. Transmission fluid can go from great, to worn in as little as a thousand miles. Why? It all has to do with heat. Heat breaks down the fluid more than wear & tear does. After fluid temperature exceeds 240F, consider buying a new transmission, but most shops will try to make a few bucks and get you to get them to do a service first! Then when that goes south, they'll reuse the fluid on a rebuilt transmission and pocket the fluid change money.
If you tow, haul, or speed on a regular basis (cruising AT 75mph or above) it's best to get the transmission serviced regularly. If you live in hilly, or mountainous areas, it's especially important to change it every 30K. Changing the fluid also includes a new transmission filter, to help keep debris at a minimum.
If you're capable of doing your own maintenance, a transmission fluid change is a relatively easier job to do for the do-it-yourself-er. In most cases, you have to drop the pan, as there is no drain. I wipe down the pan each time, as some manufacturers coat the inside with a corrosion inhibitor to help prevent rust through. This is common in Dodge transmissions. I haven't seen it on my Taurus, and I my Pontiac isn't due for a change yet. At 100K on my Durango, this coating was starting to peel up in areas. It didn't come loose from the pan at least, but was starting to peel up. I ordered a new pan instead of cleaning off the coating, as I didn't want to take a chance of that crap missing the filter and clogging up fluid flow.
Amsoil ATF is a great all around product to use. They have a new 'fuel efficient' ATF, but unless you have a 2009+ vehicle, I'd stick to the original stuff. I know several people that have done a transmission service after even 150K miles and the fluid was still good, and the filter wasn't even moderately dirty. I can't say I'd be comfortable going that long, but at least you know it's possible.