So, I think I know the answer but I wanted to get some confirmation.

My 2018 Duramax has never had a transmission service and I've got 118K on the clock. Last time I had it in for oil change and rotate/balance the shop quoted me a trans service. $350-ish.

Now, my monkey brain got to thinking, can't I just drain, install new filter, and fill? I do my own fuel filter because they charge a stupid amount ($215) for that service.

All I have to do is capture the fluid and dispose.

Anyone do this? Thoughts? Warnings?

So long as you are diligent in the process, you should be OK.

In my 986 I drained the automatic transmission fluid... dropped the pan... inspected for debris... popped off the old and installed the new filter and, using a thermometer to measure the new transmission fluid temperature following very specific and carefully laid out instructions... saved myself ~$850.

There were anecdotes upon the forums I frequent of people having their transmisions fail shortly thereafter... but at at the time sitting on 120K and no service history... took the plunge.

2,000 miles later a transmission seal failed. All in, including tows and the transmission being rebuilt, I paid ~$5K.

My point being you might consider checking out some of your make/model forums and see how things unfolded for other owners. Some may say leave well enough alone. If you do go the maintenance route... follow to a "T" the process directions and instructions.

Have a great day.

Edit: If you do choose to replace the fluid... measure how much comes out then putting back in the same amount. That mitigates under/overfill. In any event, check the level acording to manufacturers specs.

Do it. They were just trying to make a sale.

dahouse said:

---

So, I think I know the answer but I wanted to get some confirmation.

My 2018 Duramax has never had a transmission service and I've got 118K on the clock. Last time I had it in for oil change and rotate/balance the shop quoted me a trans service. $350-ish.

Now, my monkey brain got to thinking, can't I just drain, install new filter, and fill? I do my own fuel filter because they charge a stupid amount ($215) for that service.

All I have to do is capture the fluid and dispose.

Anyone do this? Thoughts? Warnings?

---

I did my own transmission service back when I had a duramax. It is just the same as changing the engine oil, nothing to worry about. Just make sure you have the proper fluid to put back in. I would go to a GM dealership and purchase the fluid there just to make sure it is the correct stuff.

I did the drain and fill service at home a couple of times on my LMM with this kit:

https://dmaxstore.com/products/?route=product/product&product_id=61#:~:text=Remove%2015%20mm%20drain%20plug,to%20begin%2C%20reinstall%20drain%20plug.


Easy job. I never dropped the pan, just drained and replaced the external spin on filter. Never had any transmission issues with that truck. Sold it at 160,000 miles. Good luck.

TRIDENT said:

---

I did the drain and fill service at home a couple of times on my LMM with this kit:

https://dmaxstore.com/products/?route=product/product&product_id=61#:~:text=Remove%2015%20mm%20drain%20plug,to%20begin%2C%20reinstall%20drain%20plug.


Easy job. I never dropped the pan, just drained and replaced the external spin on filter. Never had any transmission issues with that truck. Sold it at 160,000 miles. Good luck.

---

Have a 2008 Duramax with 225ish thousand miles. I did this twice at around 100K each time. Have had no issues with mine. <knocking on wood>

This is one of the easiest services on the Allison.

Do it. Cheap insurance.

TRIDENT said:

---

I did the drain and fill service at home a couple of times on my LMM with this kit:

https://dmaxstore.com/products/?route=product/product&product_id=61#:~:text=Remove%2015%20mm%20drain%20plug,to%20begin%2C%20reinstall%20drain%20plug.


Easy job. I never dropped the pan, just drained and replaced the external spin on filter. Never had any transmission issues with that truck. Sold it at 160,000 miles. Good luck.

---

I did the same, except I also added a deep sump pan at the same time. Might have been a waste of money, but I never had a problem.

That was on a 8.1L gasser. Now have a 2002 duramax, haven't had time to do this yet. Am trying to decided if the 6th gear conversion is worth the extra $2000 to do at the same time.

I went down that rabbit hole looking into it last year and it seems like all I remember it doing was drop the rpm's to a level where the cab was a little more comfortable. Didn't seem to be any difference in towing or mileage, I usually drive 70 and it's comfortable enough without it.

Yeah, I'm trying to figure out how many miles I'd have to drive to get a return on that investment. And I bought the pickup to pull n RV, so it likely will spend little time in 5th, much less 6th.