Any 6L90 experts here?

[Big Moose]

Have a 6L90 that will not back up
Last time it went in reverse it gave a clunk and now it won't back up at all in reverse,
what could be the issue? Solenoid, perhaps?

Dropped oil pan to see what oil looks like and it is very clean and also no metal shavings either. trans has 160 000 kms on it.

Any ideas?

[YoungPup1977]

The 3/5 reverse drum is known for the welds to crack and loose pressure for reverse, also the snap ring could of broke.

How are the forward gears ? Any slipping ?

[Big Moose]

Thanks for the reply. Forward gears were not slipping.

Took it to a transmission shop and they tore it apart but did not find anything broken or cracked.

[YoungPup1977]

So why did the trans not have reverse ? Did they say. The cracked drums are not visual but found thru pressure checking the weld on the 3/5 reverse drum which is known to crack. Needs Pressure checked thru the pump/stator which is part of the bell housing.
Make sure they upgrade the PR valve in your trans. Common problem which needs this upgrade on most 6lxx’s

Good luck !

[Big Moose]

They did upgrade the PR valve.
They did run some tests and said everything checked out.
I will ask about the pressure testing. Thanks for bringing this up.
We will be getting the trans back tomorrow and install it, so we shall find out what gives

[YoungPup1977]

Hope you don’t have to R&R twice based on nothing found bad in trans by the builder. On the 6lxx’s trans it helps the builder test drive the vehicle prior to him rebuilding it and of course after it’s rebuilt. It does help me for sure. Plus hope the builder upgraded the VB along with testing the pressure switches at the time of rebuild. Lots of upgrades on this trans to make it work and last for time to come.

[YoungPup1977]

In the first pic shows the 3/5 reverse drum. The second pic shows the area where there is a machined surface which is welded just outside of the machined surface. This is the area in which the weld cracks.

[Big Moose]

We got the trans back and installed but we have the same problem.

They did the pressure test on the drum and it checked out.
They also did a bunch of upgrades, The VB is the only one they didn't.

We now can't even move it in neutral and it won't go in forward as well now.
I did check the line pressure and got
58 in neutral
150 in forward
200 in reverse
Disconnected the electric plug and got
250 in neutral
270 in forward
280 in reverse

I was thinking, is it possible for the sprag that holds it in park to break off? cause it fells like it's under a load in forward and reverse when you give it a bit of gas

[YoungPup1977]

Take it back to the shop who built the transmission. Let them troubleshoot their mistakes. There is no sprag for park. Performing the R&R yourself on this trans won’t save you anything as I mentioned in a previous post.

GOOD LUCK to u !!!!
Posted via Mobile Device

[Big Moose]

Yeah, we took it back to them.

it'll be interesting to see what they find

[YoungPup1977]

The upgrades to the VB are as important as all the other required upgrades on the 6L90’s.

[YoungPup1977]

The input carrier planetary can be installed upside down. I have had other shops bring their transmissions to me after they worked on it and had problems they could not fix.
Hopefully they can fix it for you. I have heard of a shop R&R a 6L90 six times before they just replaced the entire unit. These transmissions are not cheap to rebuild nor replace. The pump in these are part of the bell housing and are prone to be bad along with the pump stator.

[Big Moose]

It turns out the tip of the park pawl rod had broken off causing it to stay in Park all the time. They have fixed it and everything seems good now.

Thanks for the help YoungPup

[Big Moose]

Forgot to add. they also changed the VB now as well

[YoungPup1977]

Originally Posted by Big Moose

Forgot to add. they also changed the VB now as well

Do you know exactly what fixed the no reverse condition on your 6L90 ? Just curious to how much you had to pay for this fix

[Big Moose]

Originally Posted by YoungPup1977

Do you know exactly what fixed the no reverse condition on your 6L90 ? Just curious to how much you had to pay for this fix

It was the park rod, for some odd reason it was able to go in forward til the tip broke off completely and then it wouldn't go in forward as well.
I did some diagnosis on it before we took it back to the shop the last time.
I disconnected the electric plug but it still wouldn't move in reverse, my understanding is with the plug disconnected, it should move.
Since they told me it was sound mechanically, I took off the driveline and tried to run it freely like that in reverse, but it still wouldn't go. I then took the transfer case off to rule that out. With the transfer case off I was able to get it turning in reverse after shifting it from reverse into neutral and back into reverse, but there was a clicking sound coming from inside the trans. when the shop took it back apart they found the park rod issue.
Cost over a grand all told

[Daaaanz67]

Pup all the things you mentioned the issues with 6l90. Does this pertain to all years? Are some years better than othres?

[solidaxel]

Originally Posted by Daaaanz67

Pup all the things you mentioned the issues with 6l90. Does this pertain to all years? Are some years better than othres?

Following

[YoungPup1977]

In 2007 several upgrades were addressed on the 6LXX's transmissions by GM.

Park rod is a issue and GM provides a upgraded rod to fix the Park issue. Normally a no Park condition. 2008 and earlier 6LXX's transmissions.


Always check the filter on 6LXX's transmissions, I have seen filters develop a crack leading to air ingestion in the hydraulic circuits. This condition has been found on the OE as well as some aftermarket filter designs. Closely inspect the filter for cracks and separation in the seam area of the filter. This will cause a no reverse condition. Also this would harm the front pump over time.

[YoungPup1977]

If you plan to go the used trans approach, I would suggest to reuse the TECHM from the original trans. This way you eliminate the mismatch of information from PCM and the Trans TECHM. The TECHM is bolted to the VB, the pressure switches can be tested via ohm meter before installing the VB back onto the used Trans.
Good Luck !

[rustyrodknocker]

That's great to know, thanks. I will indeed do that if I purchase a used unit. This scenario has been a huge learning curve since a month ago when I drove it home thinking it had a 4l80e. I thought I had a good grip on it until I learned there is a thing called a 6l90. The side effects of having never owned a late model vehicle I guess.