GMC Transmission -- Failure at Speed!
 

The transmission in my 2006 GMC Sierra failed while I was driving to visit my daughter at college. On the highway, two hours from home, cruise control set at 65+/-, the truck suddenly seemed to be stuck in 3rd gear and running at 4,000 RPM. I hadn’t even noticed the downshift.

I got off the highway, made it to a garage and checked the fluid level. It was down a quart. And it was leaking fluid. The mechanic came out and determined that I’d blown a transmission cooler hose. He said it would take a day to get the parts and replace the hoses. He gave me an estimate and I said go ahead.

I managed to rent a car, the next town over, and made it to my daughter’s in time for the music concert we were scheduled to attend. Weekend saved! (At great expense).

Saturday, I return the rental and pick up my truck. The transmission doesn’t leak, but it won’t shift higher than 3rd gear. The mechanic explained that what’s wrong is beyond his capability. So I proceeded home at 60mph/4,000RPM. Now I’m home, with a broken truck and a disassembled Porsche.

My reason for writing is simply to ask, “What do I do next?”

I’m a decent wrench but I know nothing about automatic transmissions. And if I wanted to work on it, it would have to be outside, because my shop is filled with disassembled SC Targa. I have friends at a local shop that I like and trust. I will ask their advice, but I’m hoping you guys can give me some parameters. Such as,

Does anyone have a clue what’s wrong with this transmission?

Does one rebuild an automatic transmission, or replace it?

And (inevitably) – Is this an opportunity to upgrade my transmission? Or install a stronger one? Six speed?

The truck is a 2006 GMC Sierra Crew Cab, 5.3 liter V8, and I really love it. I’m hoping to keep it forever. The engine is clean as a whistle. It has a fresh paint job. Brand new leather interior. And, I coated the frame with POR-15.

Any help in where to go with it from here, will be most appreciated!

I blew one on my 01' Tahoe. Sun gear is a weak point on those trannies.

Call around to tranny shops, you should be able to find a pre rebuilt unit sitting on a shelf.

They'll just swap it in and use yours as a core.

They'll also upgrade the weak points when they do the rebuild so it should last longer than the original.

FWIW, I bought the 01' off the lot brand new and that is the only major repair I've had in 18 years so the $1,300.00 it cost for the rebuild was money well spent.

I've more than gotten my moneys worth out of it.

I like them so much I also have an 06' and a 16'

Sounds like a great truck. No wonder you want to keep it.
I'm not sure what transmission you have. My experience is only with Powerglides, TH 350s, and 700R4s.
I would opt for a new transmission - or at the very least a new valve body. Sounds like there is either something wrong in the electronics ( way beyond my experience) or dirt or broken transmission parts in the valve body. There are so many small oil passages in the valve body that it takes a patient and dedicated mechanic to really clean it out. Not worth the effort.

I worked at GM in a friction materials test lab. We ran the crap out of a lot of 700R4s. They failed by twisting a shell drive shaft in two. It was catastrophic - like "BANG - engine revs out of control, emergency test cell shut down" sort of catastrophic. Obviously not your problem.

Thanks guys! I believe my transmission is an 4L60E or 4L70E, which I just read are offshoots of the 700R4 that WD used to blow up!

If you can, hook up a scan tool and check codes. A good scan tool can give you full diagnosics. No 4th generally means low pressure. A flush and filter change may help but a rebuild may be needed. If you do not have a scan tool, It is easy to install a pressure gauge and check. 1/8" pipe fitting on the driver side comes out. The gauge is just like any oil pressure gauge but needs to read 300 psi. Normal pressures can go up to 180 psi or so running in gear. Idle in park around 60. These are ballpark numbers. Different year trans have different specs.

A huge factor on your decision would be mileage...... How much is on the clock?

Jack, thank you! "No 4th generally means low pressure" -- that's interesting. I'm so ignorant of automatic transmissions, that's it's embarrassing! And, I actually own a tech 2 scanner. I used the tech 2 to bleed my brakes, but haven't learned how to use it for much more than that.

I'll probably turn this over to my local mechanic friend who is also a muscle car guy. It will be expensive, but he's good and I just have too much on my plate putting the Porsche back together. My friend likes to use a local transmission rebuild shop that has a longstanding reputation with the drag-strip community. I have a feeling my transmission is heading there.

If my transmission needs rebuilding (as I suspect it will) are there modifications that will make the gearbox faster/better? Later GMC Sierras had a 6 speed trans. I can't help but wonder if this is a possible or beneficial retrofit. If the answer to that is obvious, I apologize for my ignorance!

I totally understand that. Do what you know. I just dumped a major lawnmower repair job because I need to spend the time on my cars. There is a thread here about being "the guy" you call. Time to call "the guy."

Asphalt, there's about 100m 'on the clock' -- if you're referring to whether the transmission is due for rebuilding. If you're referring to trading the truck in; well, that's not gonna happen! I searched high and low for this exact truck and shipped it in from Minnesota. I don't think it's 'better' than other trucks, but I like it! I like the looks of the earlier body style and appreciate the dearth of electronic gizmos!

There are upgrades that are normally done on a rebuild that will make it better. Your friend will be aware of these. A 4l80e is a bolt in that is near bulletproof but I am not sure if programming needs to be done . Probably so. 6L80e would give you 2 more gears but that complicates things even more.

Pick a part? Transmission on our 2000 X5 went out. I found one for $1400 and had it installed for $700. I expect a GM tranny will be less expensive.

Stomach, forgot to thank you! An off the shelf rebuild might be the thing to do.

Jack, thank-you! You're right, the guys I'm talking about will know their stuff. I suspected that 2 more gears might "complicate(s) things even more" -- but I had to ask. Driving this truck on long trips, I do notice that it often seems like it 'wants' a fifth gear.

I don't tow my lobster-boat often, but when I do it's usually long distances. With that load, on uphills, this transmission sometimes has trouble deciding on 3rd or 4th gear. On these occasions, I wonder whether a six speed would be an improvement -- but then, as you say, that comes with further complications.

Same here the sun gear went out trying to reverse uphill. It happend on a plow truck at work too. The hard part was getting the tranny out. Rebuilding it was easy in comparrison. Its all on u tube. I dont know about the 06 models but auto trannys are actually quite simple to work on compared to manuals. BTW if the cooler line blew out you could have just bypassed it with a piece of hose and a some clamps

You need a good transmission shop. Do you live in a populated area? Search for a great transmission only shop, that's all they do is repair and rebuild transmissions all day. It may just need a valve body or selenoid and it may need a rebuild. Either way, you write a check and it's done.

Sometimes I think people here really over-complicate things. I'm a mechanic and I don't rebuild my own automatic transmissions. Nobody does. You send it to the pros and it's usually not even all that expensive. Good shops warranty their work as well. Done deal.

Porsche930, I love this truck, but to spend time with my daughter, I'd leave it on the side of the road and hitch! No way I was going to crawl under there and try to jury rig a gooey mess of something I know nothing about!

Denis, thanks for your professional advice! The rebuild shop I mentioned has an excellent 40 year reputation. But I don't think they take the transmission out of the vehicle. If I didn't have my Porsche in pieces, spread out all over my shop, I'd probably try to drop the transmission myself. Under the circumstances, I'll probably have to take it to my mechanic friend to drop the AT or find a full service shop.

Shop around. There are the routine shops in Akron that charge the traditional $2500 and you get a rebuild unit.

But there is one that charges $1,200 and yours gets rebuilt. Their lot is packed. It ain't pretty. Not one big screen tv in their office.

Pulled my pals Windstar trans. Said there was some spring in the valve body that was not strong enough. Slips out of overdrive. Incremental cost of rebuild parts and labor is not that huge once on the bench so they just rebuild them as a rule.

Thanks Bob!

This is funny, "It ain't pretty. Not one big screen tv in their office."

Around here the shops do not seem to actually rebuild on site. They install Jasper factory rebuilt units. They get around $1500 + labor for the 4l60E. I just found this out back in early winter when I was looking for one for my truck.

Thanks Jack, The place I know of does rebuild in house. I also found this company written up in several of the hot-rod magazines:

https://www.performabuilt.com/product/level-1-street-cruiser-4l60e/

https://www.hotrod.com/articles/building-the-ultimate-4l60e-with-better-gear-ratios/

How to Build a 4L60E Transmission That Can Handle 1,000+ HP

Looks like you will end up with a solid rebuild. Since the truck is a keeper, spending the extra coin on it is not a bad thing. I tend to keep cars a lot of years myself .