Joined: Wed Dec 21 2005, 07:34AM
Location: indiana
Posts: 791
I’ve heard that ‘leave the old fluid alone’ thing also and I’m a bit conflicted on it. Heard it many places but rarely does anybody explain why (for they may not really know). Maybe they only parrot what the local garage told them. Local garages (and even transmission shops) are not scientists or statisticians, and IMHO they'll vehemently criticize something if they get a warranty call-back on it. Whether to change fluid depends a lot on the vehicle’s use, remaining lifespan, how well the trans is operating, and whether the owner will roll the dice.
Most of us have heard that ATF is ‘more detergent’ than motor oil and that fresh ATF will start to clean a dirty environment. From that regard, a fresh-fill might dislodge crud and move it to someplace where it does some gradual harm or maybe causes a near-immediate malfunction (such as the valve body). There’s no guarantee the filter will catch it, as the valve body is submerged and is not oil-tight, and the filter only does its job on oil passing thru it (it can’t catch particles that move around downstream).
The newer flushing methods intrigue me a bit, as any dislodged junk might make it out of the trans and into the machine’s filter (they surely have one?), but the torque converter acts as a centrifuge so some crud might stay in there. I guess the question is -- does the fluid actually clean the trans fast enough that all the junk is dislodged and purged *before”* the trans flushing is done? If not, you’ll have new fluid working on whatever old dirt remains when the car leaves the shop.
As for leaving old fluid in, that depends on the color, which is the only criteria we have (short of laboratory fluid analysis, of course). Brown, oxidized fluid is doing the bare minimum – it doesn’t lubricate as well, transfer heat as well, it could have acid levels that are leaching the lead (the anti-wear component) out of all your bronze bushings, and so on. If it’s not removing as much heat, your clutches are slowly running hotter, which burns/glazes the friction material and prevents oil from entering. Clutch material, the paper type anyway, is actually a sponge-like matrix that allows/requires fluid to soak and flow thru it.
My personal suggestion on a dirty trans (although I’ve never actually tried it) would be to remove/replace 1 qt of fluid periodically (at a faster interval than the trans breaks it down). This would prevent a shock to the system and give a gradual improvement in the overall fluid condition. Yes, you’ll still be cleaning stuff and hoping it’s trapped by the filter, but you’ll be doing it at a reduced rate yet still improving the fluid’s overall chemistry. You could do this via a suction gun down the dipstick tube, or an even better idea would be a spin-on filter added into the cooler return line. Remove the filter, fill the new one with fresh, and reinstall. Might be costly to swap filters frequently, so going 3-4:1 on suction gun vs filter would be less wasteful. A new quart every few months is perhaps a reasonable/safe schedule?
For my disclaimer - Although I spent 7-ish years breathing transmissions on a daily basis, I can't recall the last time I did a preventive-maintenance ATF change. In the last 10-15 years I've had 1 rebuilt trans, replaced 1 pump bushing (man, what a leak that made!), and did an LA-RB swap from a donor car (there's your old-trans fresh-fill, I guess ). Other than that, if the car is still shifting good and doesn't leak it, I check it maybe 1/year when getting them out of winter storage.
And BTW, as our goal is to get better info here vs anywhere else... there's no 'T' in Dexron.
Joined: Tue Oct 06 2009, 09:37AM
Location: Paris, KY
Posts: 1496
I'm not a believer in the leave the old fluid alone. Eventually that stuff turns to varnish and starts causing real problems. I've changed transmission fluid in cars that had quite possibly original fluid and well over 150k miles and I have had no problems. The St. Regis has Dexron in it but the VIP has Mopar ATF+4 and the only problem I'm having with the VIP is a crappy pan gasket and not the fluid. I need to swap it out with a better quality gasket.
Joined: Sun Jul 11 2010, 09:35AM
Location: Boerne,Tx
Posts: 411
Patrick, I used to have issues with the cheap pan gaskets that come with the filter kits. I started going to advance auto and getting just the pan gasket by itself in the felpro brand. No sealing problems ever since. There about 6-9 bucks.
starshipconstellation wrote ...
I'm not a believer in the leave the old fluid alone. Eventually that stuff turns to varnish and starts causing real problems. I've changed transmission fluid in cars that had quite possibly original fluid and well over 150k miles and I have had no problems. The St. Regis has Dexron in it but the VIP has Mopar ATF+4 and the only problem I'm having with the VIP is a crappy pan gasket and not the fluid. I need to swap it out with a better quality gasket.
Joined: Fri May 08 2009, 10:41PM
Location: Finger Lakes, NY
Posts: 315
68plymouth383 wrote ...
Patrick, I used to have issues with the cheap pan gaskets that come with the filter kits. I started going to advance auto and getting just the pan gasket by itself in the felpro brand. No sealing problems ever since. There about 6-9 bucks.
Good to know as I am just about to replace one. They have always leaked after a while for me.
starshipconstellation wrote ...
I'm not a believer in the leave the old fluid alone. Eventually that stuff turns to varnish and starts causing real problems. I've changed transmission fluid in cars that had quite possibly original fluid and well over 150k miles and I have had no problems. The St. Regis has Dexron in it but the VIP has Mopar ATF+4 and the only problem I'm having with the VIP is a crappy pan gasket and not the fluid. I need to swap it out with a better quality gasket.
Joined: Tue Oct 06 2009, 09:37AM
Location: Paris, KY
Posts: 1496
68plymouth383 wrote ...
Patrick, I used to have issues with the cheap pan gaskets that come with the filter kits. I started going to advance auto and getting just the pan gasket by itself in the felpro brand. No sealing problems ever since. There about 6-9 bucks.
We are definitely thinking alike. I don't have that many miles on the fluid and filter but I'm going to go ahead and do the whole thing, new filter, fluid and the better gasket.