Reader Question: Flush That Tranny?

Here’s the latest reader question, along with my reply!

Ryans asks: I likely have 180,000 miles on my tranny fluid in my 2006 Ford F150. Abhorrent, I know – go easy on me! That being said, the manufacturer recommended fluid change interval was 150k, giving me false confidence.  Anyway, feeling it’s time to get this done. I have surveyed the opinions of many mechanics/service technicians, and about 9 out of 10 say don’t do it. Based on the age-old “wisdom” that my transmission is “used-to” the current viscosity and if i make a drastic change now I am asking for premature failure.  General advice is “let ‘er buck.” My fluid is not burnt, nor am I experience any slipping.  Just getting nervous is all…I’d like to drop the pan, replace the filter and add new fluid (no crazy “flushing”). To finalize my global survey, I respectfully request your sage advice on this matter.

My reply: Assuming you were to replace the fluid in the transmission with the same type, the viscosity (flow rate, or thickness) would be the same. The reason those mechanics are advising against a fluid change is that the fresh fluid will have a detergent (or solvent, if you like) effect. The worry is that in an older, high-miles transmission, there is likely crud and varnish inside the thing – kind of like arterial plaque – and that the fresh fluid, with its fresh additive pack, will dislodge or loosen some of that and it will find its way into any of several of the very tiny passages inside the valve body and the result will be kind of like what  happens when a chunk of plaque breaks loose and ends up clogging a vital artery inside your body.

So, yeah, there is a risk.

But there is also the risk of the filter being clogged – and when that happens, the fluid bypasses it and circulates . . . unfiltered. Which is also a risk. And of course, the fluid is old and (as with engine oil) its additive package isn’t what it was.

Probably, the transmission’s life will be less than it would be if you had changed it per schedule. But, if you change it now, it could be even shorter.

So, it’s a gamble either way.

What would I do?

If I planned to keep the truck and if the transmission seemed to be operating normally, I would take the risk  . . . halfway.

I would drop the pan, to clean it out and replace the filter. I would then refill the transmission with the amount removed (to bring it back up to full normal) and probably leave it at that.

The new filter will last the life of the transmission and the partial fluid change should be be enough to renew the additive package, etc. enough to be a help without too much detergent/solvent effect.

The main thing, in my opinion, is the filter. Running any mechanical or hydraulic device that is supposed to have its lubricating/hydraulic fluid filtered without filtration is like expecting to have a long life without your kidneys…

Keep us posted!

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