I am in the process of installing a Hayden 678 transmission cooler in line with the radiator. Will I need to add ATF fluid to make up for fluid lost and extra volume of new cooler and lines?



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I would definitely add fluid and then definitely check that it's at the proper level using the proper procedure, transmissions don't like being low on fluid nor do they like being overfilled.

The factory service manual (FSM) covers this. Once you install the cooler you need to park the truck on flat ground and open up the check level plug in the transmission pan (not to be confused with the drain plug). If fluid comes out you are over filled. If no fluid comes out you need to add more at the fill plug up on the side of the transmission.

I would suggest that you add 1-2 quarts when installing the cooler then open the fill plug and let out the extra.

Once you have this all done you need to check it at temperature 115-130°F then recheck the fluid level.

You can get the fluid from the dealer. Toyota P/N 00289-ATFWS (WS is 'world standard').

*Edited this post for accuracy after others below caught me on my misstep.

Just to clarify what you wrote, the "fill" plug is on the passenger's side of the transmission, not the pan, there are 2 plugs on the pan, one is a drain plug and the other is the "check fluid level" plug, this plug is recessed up into the pan a bit, the drain plug is flush with the bottom of the pan.

Also you need to get the fluid up to a very specific temperature to properly check the fluid level, which I believe is right at 125 degrees, but that needs to be confirmed to be sure, there are many topics on this forum that has the correct temperature for checking the level, I know for a fact 175 degrees is way to hot to properly check the fluid level, if you pull the "check fluid level" plug at that temperature you will drain out more fluid than you should and the transmission will be low on fluid because of the higher temperature.

There's no need to drive the car to get the transmission fluid up to temp.

Here's how to engage transmission temp check mode: How to Drain/Refill Automatic Transmission | Tacoma World

And a bonus tip: it'll take much longer for the transmission fluid to heat up now that you have additional cooling. Take a piece of cardboard and temporarily cover the external transmission cooler to get the oil up to temp faster.

Wow so much bad info above. Follow the manual procedure above, its not difficult.

The temp at which you open the hex head drain is 106*-113*F.

On your install, I would orient the nipples either down or to the side. Use the lower nipple as the return to the transmission. This allows the fluid to drain back to the transmission. Why is this important? During very cold days, the atf wont gel in the cooler and block flow or create back pressure.

I went full Amsoil synthetic so Im not worried about the fluid getting too cool, but my transmission only gets up to about 135* now with my cooler installed.

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Just curious about your comment, "Wow so much bad info above", could you be more specific and clarify so the OP can know what you referring too, I know there is a bit of bad info posted but I wouldn't say "...so much...", it seems that you're saying all the info posted before you posted is bad info?

The factory service manual (FSM) covers this. Once you install the cooler you need to park the truck on flat ground and open up the fill plug on the transmission pan. If fluid comes out you are over filled. If no fluid comes out you need to add more. Which drain plug, there are 2 on the pan, the fill is on the right side of tranny

I would suggest that you add 1-2 quarts when installing the cooler then open the fill plug and let out the extra. way too much that cooler may take 1/2qt at best

Once you have this all done you need to drive it for a day or at least up to 175°F (aka operating temperature) then recheck the fluid level. just wrong

And a bonus tip: it'll take much longer for the transmission fluid to heat up now that you have additional cooling. Take a piece of cardboard and temporarily cover the external transmission cooler to get the oil up to temp faster. not if you just stay parked and not driving all over town for the day

AuSeeker you are correct. I didnt say all bad info, just so much bad info.

Green is good info, red is bad info. You know who you are. LMAO

I just filled the cooler and hooked it all back up.

Thanks for the input. Will order some ATF WS fluid and probably do a drain and fill or two while under there. I am familiar with the temperature fluid level check so will follow that procedure. As for the orientation, according to B&M upright is ok, down is not recommended. The upright position makes for a very solid install without any extra brackets by drilling three holes. Very cold weather is not a big concern in the Southwest.

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You're calling out others for bad info but the info you gave about orienting the nipples is totally wrong. The nipples go up or to the side. And if you mount it with them on the side the lower nipple is the inlet to the cooler, not the return to the transmission.

I screwed up on the check temperature with the operating temperature, my bad.

Check Fluid temperature from FSM pg AT-306 is 115°F-130°F.

EDIT: Fill plug is on side of transmission, check plug and drain plug on pan.

Actually, inlet (HOT) is up top, and return (COLD) is on the bottom. This is the principle radiators. As fluid cools it condenses and settles. If the nipples are top, it doesn't really matter.

The instructions image shown in post #10 above contradicts that. The 3rd red bullet point says the inlet should be at the bottom in a sideways mount. Perhaps that orientation is specific to that model but that’s where the info came from about which goes where.

I see that now. That is an unusual statement. Not sure if it has anything to do with the cooler design.