Transmission service - Yes or No?

[whiteboyslo]

Hey all -

Just picked up my new-to-me 1993 C2500 Suburban and am curious what the conventional wisdom is on servicing the transmission. It has a 454 and 4L80E with 145K miles on it and shifts very very well. The 1/2 is a bit firm when cold but everything else is butter. The fluid is kinda more light brown than pink, but doesn't smell burnt.

I've heard some who say that, if the transmission is fine and the mileage on the fluid is unknown, that it may just be better to leave it alone. It sounds wrong to me, but I don't want to tempt fate on this truck, either. So what should I do? Should I just drop the pan and change fluids and the filter? Should I flush it? Or should I not touch a dang thing and just keep driving it as is?

Sorry if this is the transmission equivalent of 'what motor oil should I use?', but it's been a bit a LONG time since I bought something pre-owned with a slushbox. I typically row my own on my older rides, and the newer ones I keep up with the maintenance from Day 1 to avoid this kind of stuff.

Mike

 

[df2x4]

I'd never flush one under any circumstances, but a pan drop and fluid/filter change would be a great idea. ACDelco 24210956 is the factory filter and gasket kit for the 4L80E and Dexron 3 or 6 fluid will work fine.

 

[DerekTheGreat]

Since you say the fluid is on the light brown side I'd flush it, change the filter and fill it back up with fresh stuff. A flush or even a simple "drop the pan & change the filter" won't harm a good trans, will only help it. I've done this to three of my older vehicles and it's only made things better in terms of shift feel & quality. Most notably are two of our cars, one being my '89 Firebird which before I did anything (which was just a pan drop & filter change) the torque convertor stopped locking up. Once I changed the filter, shift quality improved and lock-up was back. The other was a '91 Mark VII, which shifted firm & harsh, especially out of over drive and fluid was brownish and stanky. That got the full flush & pan drop with filter change. Night and day, shifts much better and certainly feels much more reliable then it did. Have put ~2k miles on it since the flush and such.

 

[RedLeader289]

I would just drain it and change the fluid and filter. A flush on a high mileage transmission can cause issues if it has never had a flush before.

Sounds like the fluid is just old, if you change it and drive it and it stays red or close to it then you're fine. My truck has 185,000 on it and has never had a flush, only fluid/filter changes at the recommended intervals, shifts great and the fluid stays red.

 

[biggbiker]

I have 2 trucks a 1996 C1500 ECSB with 221000 miles and a 1999 Yukon with 57000 Both 5.7L. Both shift great and lock up converter. The high miler C1500 had a fluid change @200K Yukon unknown. Both are great dependable trucks that everything works on, or I would have it fixed. I would drop the pan and change fluid and filter. Hope this helps.

 

[DerekTheGreat]

"I would just drain it and change the fluid and filter. A flush on a high mileage transmission can cause issues if it has never had a flush before."
All folklore and mythos. If the trans grenades it was already on it's way out. If the trans has never had a service than I recommend flushing it and changing the filter. If it's been serviced like the manual states then it won't need a flush, just a new filter and replace the fluid that drained out.

 

[RedLeader289]

It's not a matter of it grenading, you're right in that will happen if it's going to happen regardless. What I was referring to was that when they flush a transmission it can cause knooks & crannies that were gunked up before to get blown clear and depending on where they were it can cause leaks.

 

[GarrettGmc]

I just did this the other month as my oil was dark dark red and had already started to smell burnt, only 30k on a rebuild, all I did was drop pan an replace filter and filled back up with new fluid and it's been fine. I have checked the fluid and no discoloration. The old filter looked clogged up so I made sure I wore gloves and wiped everything clean before putting it back together.

 

[Supercharged111]

Absolutely do a full flush, don't let the naysayers scare you. I've flushed more than one around 200k with no issues.

 

[Dr.Zoom]

It's a toss up. Going without a flush can keep one going that is in bad shape. You may have enough friction material in the fluid that once removed you notice a slip. Plus all the new fluid acting as a solvent can free up chunks that can clog passages.

Leaving old fluid in that's not doing its job can kill it.

Dropping the pan and changing the filter then refilling is the middle road.