ATF changing proceedure question - Diesel Forum

yardape1 · 09-05-2009, 03:21 PM

After reading the procedure for changing the transmission fluid and seeing some of the comments about the bypass value engaging causing ATF to spill out I was wondering if it makes sense to just dump the fluid from the output cooler line instead of the return line.

If you connected the hose to the output line before the coolers, then there should be absolutely no back-pressure on the bypass value. Assuming that your valve is working properly and that the unit is at operating temp, then this should be a easy run.

Also, how do we know that the bypass valve is pushing all of the fluid toward the coolers? I thought that this was a temperature operated switch and that it slowly opens depending upon temp. Seems to me that there will be some mixing of old and new fluid in the pan because of this.

just some observations.

tx
Rich

**SmokeyWren** · 09-05-2009, 05:13 PM

One of the benefits to following Mark's procedures to the "t" is that the coolers and lines get flushed during the process.

Quote:

I thought that this was a temperature operated switch and that it slowly opens depending upon temp. Seems to me that there will be some mixing of old and new fluid in the pan because of this.

No, the bypass valve is operated by pressure. In case the coolers or lines get stopped up, the tranny pump will cause the pressure to build up and eventually cause the bypass valve to open and relieve the pressure. But ice cold ATF is thicker than warmer ATF, so with cold enough ATF the pressure of the cold ATF trying to flow through the lines and coolers might be enough to open the bypass valve.

So if you remove the tranny cooler return line from the back of the tranny, that opens the bypass line at the banjo connection. So if old ATF gets past the bypass valve, then the worst that can happen is you'll have ATF from the bypass valve all over your driveway. But that should happen only with a very cold tranny or with a defective bypass valve.

yardape1 · 09-05-2009, 09:42 PM

Interesting,

do you know where the bypass mechanism is withing the bypass valve assembly? Is it in the front where the fluid exits the tranie or in the back where the return fluid comes in?

tx
Rich

**SmokeyWren** · 09-06-2009, 08:41 AM

Quote:

Originally Posted by yardape1

do you know where the bypass mechanism is withing the bypass valve assembly? Is it in the front where the fluid exits the tranie or in the back where the return fluid comes in?

It's at the 2-arrow on the following sketch. There is a "banjo" (two-way) connection there, and the valve is in the bypass line right next to the banjo connection. By design, the bypass valve is closed, so the hot ATF heads to the coolers. But if for some reason the ATF cannot go down the line to the coolers, the bypass valve will open and then the hot ATF heads past the banjo connection on the back of the tranny where it meets the return line from the coolers before going into the rear of the tranny.

yardape1 · 09-06-2009, 08:54 AM

Thanks

444-4D · 09-06-2009, 12:05 PM

The valve isn't at the banjo connection at the 4 arrow. It is at the banjo connection at the 2 arrow. If you crawl under there and look at it, it is easy to tell where it is located.

yardape1 · 09-08-2009, 06:55 AM

Just a followup.

I ran this procedure yesterday and it was easy. With the mag-hytec pan, I only had to run the TC dry only once. Dump the pan, add 13 qts, pump 13qts, add 13 qts, top off.

The only problem was that the Mobil ATF comes in qt bottles and this was time consuming to pour all of them in.

Thanks for your help.

Rich