[D. Newland]

2005 GMC Sierra 1500 w/102K mi.

I had the dealership do the trans service at 50k, and a local shop do it now at 102k.

The local shop used a reverse flush machine after adding a suspension additive, 18 qts, but I just learned that they didn't drop the pan and replace the filter.

Any thoughts on this reverse type flush that doesn't replace the filter?

[Crosley]

I do not like flushes.

Not replacing the filter in the trans and a reverse flush? Would you do that to an engine?

[crussell85]

wouldn't a reverse flush be like blowing out an air filter while in the air box of the vehicle?!? I hate when people talk about flushing a transmission, I hate shops even do it. Just change the fluid and filter that you can get out of the pan and call it a day.

[1930u]

I change every 50K always change filter and use synthetic fluid. Before synthetics fluid I changed every 25K and have never had a trans failure in any vehilce I've ever owned. Mostly GM.

[raybo]

I agree with 1930u synthetic a must after 100,000 miles

[D. Newland]

Thanks, guys.

I was going to do it myself, but procrastinated long enough to where I just took it in to have it done.

The truck isn't driven more than 5K/yr now, so I'll just sit on this for a year or two, then go at it again.

[302 Z28]

I do not like trans or engine flushes either. I go elsewhere if that is what the shop offers, it's just a quick gimmick. I have heard of lots of problems with valve bodies after a transmission flush.


Vince

[richard stewart 3rd]

Hi,
The type of flush you had don can cause serious problems.
Rich
The machine connects into the transmission cooler lines.
The line going from the transmission to the transmission cooler is disconnected and connected to the machine line in.
The line out from the machine carrying new fluid is connected to the line going to the cooler. There is a chamber on the machine that has a diaphragm in it. The top part of the chamber above the diaphragm is filed with new fluid. The engine is started which turns the torque converter and the input shaft on the transmission. The input shaft turns the transmission pump and it makes hydraulic pressure. This causes fluid to flow through the cooler line. As fluid leaves the cooler line it enters the chamber on the flush machine. As the old fluid side of the diaphragm fills it pushes the diaphragm up and forces new fresh fluid into the transmission. After a while the DILUTED fluid is collected in the machine.
When the fluid leaves the transmission pump it passes to two different pressure regulators. One regulator supplies fluid at one pressure to the transmission itself that operates the pistons and controls gear shifting. The other supplies the torque converter and the transmission cooler. So you can see that all the fluid leaving the pump does not go to the cooler. A bunch of it is cycled through the transmission and dumped back to the pan without going through the cooler. This type of flush machine does not remove all the old fluid, but it continuously dilutes it down with new fluid. It never really removes all of the old fluid.

Dropping the pan is very important. Looking in the pan is a fantastic diagnostic tool that can tell you if something is going wrong in your transmission. Now lets say some crud is flushed out of the trans with this flush method. Where does it go? It can go into the pan, and then sucked up into the filter that may clog the filter causing the pump to starve for fluid and a pressure loss. On the engine the filter is after the pump and if the filter gets clogged there is a bypass valve that opens and oil bypass the clogged filter so the engine is still supplied with oil. Unlike the engine oil pump and filter the filter is on the intake side of the pump. If it gets clogged, that is it, it is clogged and stuff does not get lubricated and the clutches do not get enough clamping pressure and they slip and burn up. In just a faction of a second you just bought a new transmission if the filter clogs.

[red65novawagon]

A word from the wise! Take your transmission fluid and filter to a QUALIFIED trans repair shop so they can diagnois what the pan material holds. Once you remove that material kind of a tough read after that.

[D. Newland]

Will do.

My judgment lapsed on this one and my gut was unable to get me back on track.

[Drewnashty]

Not to be an *** but many of the statements regarding the trans service may not be entirely accurate depending on the machine used. However as was stated earlier, the trans filter not being changed is definitely not ideal. Just like changing your engine oil without doing the filter isn't really doing all that can be done. Even though to this day some shops and dealerships only change the filter every other time you bring it in and have been doing so for decades now.

Back to the subject however; this type of service isn't a "flush", it's an exchange, and yes there is a difference, just like flushing the radiator or engine block out is different from exchanging the fluid in the entire coolant system. It also depends on the machine whether or not the trans cooler's fluids are exchanged. The machine I worked with would exchange the fluid from the transmission, the torque converter and out of the cooler as well and you could keep going until the trans cooler return line (with old fluid) was just as pink and clean as the sending line. Since the engine was still running the torque converter is still spinning and this is actually a more efficient way of getting as close to 100% new fluid in the system as opposed to just draining the pan and relying on gravity, as old fluid will puddle inside the converter, cooler and certain areas of the trans.

So obviously there is a compromise if you only do one or the other. In all honestly i think it depends on how often your filter needs changing. If you only change the filter once every 100k miles which i'm pretty sure is the recommendation for passenger car automatics by most manufacturers then you could exchange the fluid in between that time and that would help the filter from getting as dirty by the time you are supposed to get it done. It's all preventative maintenance no matter how you look at it.

The fluid exchanger I have worked with didn't have it's own pump and it relied on your vehicles normal operating pressure in the lines to flow and by doing this the system was not over pressurized. So depending on the vehicle, different pressures and volume were the main factors of how long it would take for the fluids to be exchanged. I've seen big 6 spd diesels take just as long as 3-4 spds for passenger cars just because of the pressure difference even though diesels tend to have have far more volume.

I personally prefer to do both methods on vehicles that have a big question mark on whether or not the trans service was done. But on my personal vehicles, one is a TH-350 the other 3T40 (TH125), I drop the pan, drain and change the filter and then exchange the fluid to get all the crap out that's left over in the system that gravity couldn't force out of the nooks and crannys. I know there's plenty of folks on here who have rebuilt transmissions who are aware of how many "nooks and crannys" there are especially in automajic transmissions.

[302 Z28]

I agree with the above statement, doing both a filter change and a fluid exchange is the ideal way to do it. Unfortunately I have only seen places advertise a transmission flush with no mention of a filter change, which is still a bad idea IMO.

Vince

[T-bucket23]

I think the transmission service machine is the biggest ripoff in recent automotive history. There is no point to changing the fluid without replacing the filter. If the fluid is bad, most likely the filer is at least partially plugged.

[DanielC]

Keep in mind everything done in a service shop has one main goal in mind. Beat the flat rate.

It is good to drop the pan, and change the transmission oil filter. On my Ford Aerostar, that get about 3 or so quarts of the around 12 quarts of transmission fluid in the system.

On another forum I sometimes visit. they recommend a do it at home flush procedure. Disconnect one of the oil cooler lines, and after filling the pan with new fluid, start the engine, and and let the engine pump new fluid through the converter, and transmission.. Keep filling the pan, and pumping the new fluid through until you get new fluid out.

To be honest, I have not done the disconnect the oil cooler line procedure. This is what I have done. I used a vacuum pump to suck the fluid out of the pan, and then I replace the fluid sucked out with new fluid, about two and a half quarts. I try to do this every other time I change the engine oil. I know that does not change all, or even half of the oil. But it is better than just ignoring the transmission oil.

I am at more than 175,000 miles on my Aerostar, and the transmission is still working good. I use my Aerostar for towing my boat, so is has worked a little more than you would think a "minivan" normally does.

[MouseFink]

My neighbor has owned and operated a transmission repair shop for about 15 years. He told me that he will not change the fluid in a transmission unless he knows the owner and the history of the transmission. If the transmission goes out after his shop changes the fluid, he will have a difficult time proving he is not liable. He felt like the minor benefits in changing the fluid is not worth the effort and expense. The filter should be changed but he would not do it for the same liability reason. All shops cover their *** when changin oil or transmission fluid. When some auto repair shops notices that your engine has a motor oil leak, they will not change the oil until the leak is fixed. They tell you the old common auto repair shop phrase...."Take it somewhere else" Especially if the auto repair shop is in a large metropolitan area where they can cherry pick the jobs they do. If you can take your car to an auto repair shop in a small town, or at least 150 miles from a large city, the shop will do anything.

I changed the fluid and filter in my 1991 S10 blazer 700R4 transmission just because I wanted to install a deeper transmission pan and no other reason. The old fluid was as red as the new fluid and afterwards I could not tell any difference in the function of the transmission. On the other hand, the transfer case on my S10 Blazer was noisy so I changed the transmission fluid in the transfer case while I was under the vehicle. The tranfser case was not low on fluid when I drained it but it was dark in color. After I refilled the transfer case with fresh fluid, the noise was gone . Apparently a 4x4 transfer case is more sensitive to the age of the transmission fluid than a transmission.