Here is what I found at one site: 

The problem is, things could go the other way. In fact, changing the fluid on an older, worn 
transmission can actually cause the transmission to fail. There are a few reasons this can happen: 
• New fluid acts as a solvent, cleaning away built up dirt and varnish from worn valves and passages. The 
newly cleaned surfaces begin to leak, causing additional damage and, ultimately, transmission failure. 
• This cleaning process can also displace the dirt in the transmission, and move it to a different location … 
one where it's more likely to cause the transmission to fail. It can even clog the filter, preventing the 
transmission from moving at all. 
• New fluid is usually less viscous - thinner - than the worn fluid. So the new fluid is more likely to leak 
past worn sealing rings and valve passages than the old fluid. 

In one of my vehicles, a 1990 Ford F150 pick up I bought with 44K on it and no maintenance records. By the schedule it should have been changed at once by 44K miles. It had a 60K/6 year warranty, so at 57K I had the fluid changed. Within 3 days it started slipping, and on the 5-6 day I drove it into the Ford dealer at 5 mph because thats as fast as it would go. Luckily for me, a $50.00 deductible and I got a rebuilt trans. Jeff 

Jeff Stewart 

----- Original Message -----

I still haven't heard anything telling me *why* new fluid can be a bad 
thing. What is the physical or chemical mechanism at work here?? 

Larry A. 

On Mon, Jul 18, 2011 at 3:13 PM, Scott Daniel - Turbovans 
<scottdaniel@turbovans.com> wrote: 
> a compromise cheater method is to just change some of the ATF. 
> use the right type ATF of course.. 
> I have a sucker tool I made with an old fuel pump . 
> and I just pull maybe 2 qts of old ATF out of the dipstick tube.. 
> then top up with new fresh good ATF. 
> 
> If I do that on a properly working auto trans.. 
> and put in two qts of new Royal Purple synthetic ATF ..the trans 'goes' 
> noticeably better. 
> 
> and for sure .. 
> on any fluid system that is very shaky and old.. 
> especially brakes, clutch hydraulics and auto trans.. 
> if it's on the edge already............changing the fluid 'a lot' can 
> totally blow out the whole system ...like push it over he edge to complete 
> failure.. 
> 
> Scott 
> 
> 
> ----- Original Message ----- 
> From: "J Stewart" <fonman4277@COMCAST.NET> 
> To: <vanagon@GERRY.VANAGON.COM> 
> Sent: Monday, July 18, 2011 5:09 AM 
> Subject: Re: Automatic transmission question 
> 
> 
>> Larry, 
>> I was told at the time it was due to mixing old/new trans fluid. It was 
>> back before the power the power flushing like they do today, so only some 
>> of the trans fluid was changed, basically what was in the pan. I have also 
>> been told by a couple of mechanics that if you have a high mileage vehicle 
>> and don't know if/when the trans fluid has been changed it is best to 
>> leave it alone unless you are having problems. Jeff 
>> 
>> 
>> 
>> Jeff Stewart 
>> 
>> 
>> ----- Original Message ----- 
>> 
>> 
>> That is very interesting. I hope that someone out there can explain 
>> how and why this would happen. 
>> 
>> Larry A. 
>> 
>> 
>> On Sun, Jul 17, 2011 at 10:16 AM, J Stewart <fonman4277@comcast.net> 
>> wrote: 
>>> 
>>> Now, that brings up a question I have. I've now owned my '85 w/auto trans 
>>> for just over a year. Even though the PO gave me a ton of maintenance 
>>> records going back 10 years, nothing about the trans fluid ever being 
>>> changed-tho it could have been done at a Jiffy Lube, who knows. It has 
>>> been 
>>> my (unpleasant) experience on two used vehicles where trans fluid status 
>>> was 
>>> unknown that changing the fluid caused total transmission failure with a 
>>> few 
>>> days. Since my fluid looks OK and the trans is shifting fine, I've 
>>> decided 
>>> to just leave it alone. Jeff 
>>> 
>>> Jeff Stewart 
>>> 
>>> 
>>> ________________________________ 
>>> 
>>> Jim, 
>>> Change the fluid every two or three years. You don't get to drain 
>>> it all because of the torque converter, but that's the best you can do 
>>> without expensive flushing procedures. Check fluid warm, idling, with 
>>> van level. Do not overfill. Frequently lubricate the place where the 
>>> throttle cable from the front hooks to the lever on the side of the 
>>> trans. It's a ball that snaps into a spring loaded socket, a real 
>>> PITA if it rusts solid. 
>>> Leaks that can be tracked down visually: 
>>> Bottom pan gasket 
>>> Filler/check tube joint to pan 
>>> O-ring on the governor cover (up on the side of the differential, 
>>> held on with a wire bail) 
>>> Rusted thru governor cover 
>>> 
>>> Common and annoying leak: Seal behind the torque converter. This 
>>> is the usual suspect if you seem to be getting oil spray on the left 
>>> side of the engine compartment that may be coming out of the openings 
>>> in the bell housing and the torque converter looks oily. To get to 
>>> this seal you separate the engine and transmission. Don't let the 
>>> torque converter fall off the front shaft prematurely. Then carefully 
>>> pull the torque converter off the front shaft and you will see the 
>>> seal that it fits into. What fun! 
>>> 
>>> Larry A. 
>>> 
>>> 
>>> 
>>> 
>>> On Sat, Jul 16, 2011 at 7:36 PM, Jim Felder <jim.felder@gmail.com> wrote: 
>>>> 
>>>> I have a 91 Vanagon GL automatic that my daughter drives. I've done a 
>>>> little 
>>>> work on it over the last year or so and it is in pretty good, reliable 
>>>> shape. The one thing that is giving me some trouble is the automatic 
>>>> transmission. If you can help me with that, read on. 
>>>> 
>>>> I have driven Vanagons for 20 years or more, but I have never had an 
>>>> automatic transmission. I have done about everything you can do to a 
>>>> vanagon, almost, except mess with an automatic transmission and though 
>>>> my 
>>>> wife has had them on her non-vanagon cars, I have never had an automatic 
>>>> Vanagon. I just recently learned how to check the fluid level in them. I 
>>>> know virtually nothing about them. My daughter's car is leaking AT fluid 
>>>> so 
>>>> that it has to be topped up once a month or so. 
>>>> 
>>>> So I have two questions. One, what should I do, in general, to the 
>>>> automatic 
>>>> transmission just to keep it in good shape? What are the preventive 
>>>> maintenance best practices? Two, what should I do specifically about the 
>>>> loss of fluid; what is a "usual suspects" list of things to address? 
>>>> 
>>>> Thanks, 
>>>> 
>>>> Jim 
>>>> 
>>> 
>>> 
>