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Trans service

what’s better for auto trans, flush service or drop the pan and drain fluid?

It depends on the car, the flush equipment, and the reason for doing it.

Pan drop, fluid and filter should be enough in most situations

If the torque converter has a drain plug, so much the better

If you’re going to do a flush, make sure the pan is removed and cleaned, and the filter is also replaced

Some shops do the flush INSTEAD of dropping the pan and replacing the filter

I’d have more peace of mind knowing the filter was also replaced at regular intervals

I agree 100% with @db4690 . You need a “perfect storm” in order to get a transmission flush that benefits your transmission. You would need the right flush equipment, a management that always insists on replacing the filter and a competent technician to do the job. Most of the shops have none of these “components” much less all three. A pan drop, replace the filter and fluid is the best transmission service out there. Some will disagree but I see most transmission flushes as scams more or less for the reasons that I have already stated.

It’s actually both. Flush, drop pan and replace filter if applicable.

Use correct fluid or for older vehicles upgrade fluid to full synthetic such as Mobil-1 or Wolf’s Head full synthetic.

Pan drop and not a flush in my view. Did the Acura last week. Pull the plug, easy and no mess.

There’s nothing especially wrong with the flush technology. I think they just tap into the transmission fluid tubes which normally go through the radiator (to cool the fluid) and let the pump in the transmission do the job of pumping the old fluid out and introducing fresh fluid in. It probably does exchange the fluid ok, maybe even better than a pan drop. But doing a “flush only” is like changing the engine oil but keeping the existing filter. Might work a time or two, but eventually will cause problems.

thank you for all of your valuable advice

Dropping trans pan is messy. No way around that. None can deny changing filter/fluid at 60-100k miles is not good for any trans.

I think there is a lot to be said for buying a car with a transmission drain plug and draining and replacing transmission fluid regularly. A significant amount is left but flushing is only warranted IMHO, if you delay changing or have other problems.

speaking of trans drain plugs . . .

Apparently Dorman makes trans pans with drain plugs, for popular applications that don’t have a drain plug . . . think domestic

This seems like a pretty good idea to me

Have any of you guys gone this route?

When I take a transmission pan off for any reason…it goes back on with a drain plug installed. If I had a damaged transmission pan…I would only replace it with a pan that had a drain plug. That way…I can drain the fluid 2 or 3 times after I replace the filter without having to remove the pan just to drain it. I use the B&M universal drain plug kit. They are inexpensive and very easy to install and the best part…they never leak.

I too am a fan of installing a drain plug, and the universal kits are quite easy and I haven’t had any leakage issues either.

I’ll go a little off topic . . .

Since I’m a fleet mechanic, I essentially see the same type of vehicles all day long

It’s frustrating when one 2003 GMC Sierra 2500 has a 4L80E trans with a factory drain plug

Yet 2 hours later, I’m working on another 2003 GMC Sierra 2500 with a 4L80E trans with NO drain plug. Yet the pan has the “indentation” where the plug would have been, if GM had decided not to save a few bucks

It’s as if GM flips a coin . . . or says all 4L80E transmissions will get a drain plug this week, but next week we’ll save a few bucks

Using that drain plug, doesn’t that still leave 1/4-1/2" of fluid in the bottom of the pan?



Guess it was too good an idea for the original maker to come up with.

I understand that except most of the pans are stamped steel, not cast like Honda. So for the manufacturer to put a drain plug in would require welding or crimping threads on the sheet metal. So the cost of adding it would likely be as much as the pan itself. Then of course you have to take the pan off to change the filter. So really all you save is the mess trying to get the fluid out by loosening the pan. At dealers and trans shops its no big deal. At home you need a big piece of cardboard. So unless you just aren’t going to service the filter, I guess its no big deal. Now in Honda/Acura, there is no readily serviceable filter so you just drain and refill.

For a Honda Accord it is easiest to drain from the drain plug (not a plug in the bottom of the trans, but a plug higher up). There is no mess — the drained fluid is easy to catch in a pan, much like draining engine oil.

The Accord (and many other Hondas) drain exactly 3 quarts before the fluid stops flowing from the drain plug. The fluid capacity of the Accord trans is 6.9 quarts. The procedure to follow is (1) drain 3 quarts from plug, (2) refill 3 quarts, (3) run engine to mix fluid. Repeat procedure 3 times, When finished, the trans will contain 82% fresh fluid. This method requires (9 - 6.9) ≈ 2 quarts more of trans fluid than a single complete drain and refill, but it’s fast and clean to do.

I do a three cycle drain/refill at 60,000 miles, followed by a 1 cycle (3 quarts) drain/refil at every 30,000 miles thereafter. Be sure to wipe off the metallic shavings from the magnetic drain plug at the first drain/refill.

For the mathematically inclined, here is an equation that gives the amount of fresh fluid (F) in the trans if D quarts are drained/added N times from a trans of capacity C:

F = C • (1 - (1 - D/C)^N), N = 0, 1, 2, 3, …

For three drain/refill cycles of the Accord,

F = 6.9 • (1 - (1 - 3/6.9)^3) = 6.9 • (1 - 0.5652^3) = 6.9 • (1 - 0.1806) = 5.65 qts

so that after 3 cycles (and 9 quarts added) the percent of fresh oil in the trans is 5.65/6.9 = 82%.

If 4 drain/refill cycles were used, the trans would contain 90% fresh fluid (and your wallet would contain about $25 less cash).

I found this video: Eric the Car Guy — How to Change Honda Automatic Tranmission Fluid

How to Change Honda Automatic Tranmission Fluid