Adding a missing trans dipstick to a 2021 GMC HD


$50 Site Donor 2023
Jan 14, 2011
in Az where the Deer and Antelope play
Something is missing in this photo. (Besides the plastic beauty valve covers)


When GM introduced the 6.6 gas engine in the HD trucks it decided to delete the trans dipstick in the 6L90. Instead to check fluid level you have to have the fluid to 122f, and with the truck running unscrew a small drain plug type bolt in the upper rear part of the pan. Correct fluid level is just seeping out. To add fluid you have to pull a rubber plug out of the hole where the dipstick used to go and add with a pump then recheck. Hot pipes and cats. Not ideal. Someone decided to check if the OEM dipstick tube would fit in the new body style and low and behold it fits perfectly. This is a no brainer addition and wasn't hard, just had to fiddle the tube into the right spot. Reaching up and getting the flange nut threaded and tightened was the hardest part. Parts were $62 shipped from Rock Auto.

This is better, OEM dipstick right where it should be.


Here is a link to a how-to post on that includes part numbers. dipstick install

There's also a company that's putting a drain plug in OEM pans: BET trans pan

The new pan will be going on when I do the first spill & fill @ 20k miles.
Just curious... When your trans is at 122°F, and the weir-tube is barely dribbling, what does your new dipstick read? Hopefully at the top /full mark? Or maybe smack in the middle of the range? If you know, then in future, with a thermal measuring device (to advise when you're at 122°F) you can feel confident that you're at the correct level.
I've got a 2005 Ford Explorer that also has the dipstick-less 5R55S tranny. Matter-of-fact, I'm going to change the transmission fluid in it this weekend. Not really looking forward to it....
Adding fluid to it also involves removing the plug from the bottom of the transmission and also adding fluid into the same opening. I've looked for aftermarket dipsticks, but to no avail, so I guess I have to do it the messy way. I was a Ford tech back in the day when these things were being sold brand new and I remember Ford having a recall on these transmissions (well, they probably officially called it something else so it didn't make the news....) where we had to add a bottle of secret sauce to each one of these transmissions that hadn't been sold yet. So, yea, you had to take out the "dipstick" from the bottom of the pan, add the fluid from the bottle, and try to get the plug back in without making a mess or losing a bunch of tranny fluid.
Now I can see their reasoning for going to a dipstick-less transmission. I do remember some idiot customer bring his vehicle in because he had dropped the transmission fluid and adding 9 quarts of red fluid into the engine. They did it to initially save money on assembly and to decrease the warranty work from the do-it-yourself crowd. It still doesn't make me feel any better.
BTW, good job on your dipstick install!
while i don't think you are doing any damage to the transmission, adding a dipstick. the lack of a dipstick does keep the fluid from oxidizing. when there is an opening to the atmosphere the air does age the fluid sooner than if the unit was sealed. just my .02.

@ Cdn17Sport6MT , I just installed the dipstick and haven't had the truck out. I'm going to use the traditional procedure before the dipstick was eliminated, 145f, level, up and down the gears and back to P. The 6L90 is unchanged and the parts are all 6L90 parts. I may confirm with the pan plug method buy lying under the truck with the hot exhaust pipes and cats directly above me is what I'm avoiding.

@ Snagglefoot , thanks, that book came with the truck. ;) Severe service is listed as the following:
(does anyone on BITOG believe a transmission is a non-service item?)

Right now 55% of my miles are towing a travel trailer in mountainous terrain in mostly hot weather. After the upcoming East coast trip I'll be around 70% towing miles. I'm going to do a spill & fill every 20k miles with a filter replacement every 40k miles as long as we're traveling.

@b_b , The dipstick tube is sealed with a nice rubber grommet where it enters the case and the handle has the rubber grommet that tightens when you flip it over. As trans fluid expands and contracts air has to be vented in and out somewhere.

@repairman54 , could be but the factory plug has a push-to-expand insert that seals it pretty tight. There must be a vent somewhere. There's a photo of it in the install link.

Again, all this is doing is installing the GM factory dipstick that was installed in every 6L90 trans in every truck until GM deleted it. The extended stud that the tube attaches to is on the housing with no other purpose.

Appreciate the thoughts and concerns but there's really nothing sketchy here. (y)
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I missed your name at the end, thanks for figuring this out. You get a nice discount at your dealer, MSRP on the parts is around $71, with our sales tax close to $80 OTD. $62 from RA included $14.95 shipping.
I doubt that the new owner of that truck will even realize it wasnt built with the dipstick. I sold it last week and ordered a 24. Now I have to figure out a way to simplify the 10 spd, I have a couple of ideas. Thanks for the heads up on the pan. I was going to have a drain TIG welded onto my original pan if I had kept the 2021.
My 2004 Caddy SRX with the 5L40 trans from France had that same lack of dipstick.
I've still got a burn scar on my right palm from checking the level after a fluid change. Very close proximity to the exhaust.
Smart move on your part to add the dipstick, I don't think they ever offered one on the trans I had.