Hi, I came here looking for some guidance on troubleshooting a 4L60E in a 2004 Trailblazer. The vehicle belongs to my friend, and I’m trying to help him figure out if he needs a new transmission, or if this one is easily repairable. The transmission is evidently stuck in third gear. Shifting down manually to second does not produce a shift.

A month or two ago the vehicle abruptly, according to his recollection, stopped shifting. Shortly afterward, it threw P0756 (Shift solenoid performance code). He replaced shift solenoids 1-2 and 2-3. The code disappeared but the problem did not. The vehicle seems to be in third gear and will not shift down to first or second, even manually. It shifts to reverse fine. Here are the diagnostic actions I have taken:

Check for power at pink wire on transmission connector. Voltage present.

Operate 1-2 and 2-3 solenoids via bidirectional control scanner. Activating the solenoids produces an audible click.

Attempt to shift transmission using bidirectional control. I am not able to produce a shift.

Observe shift solenoid live data while driving the vehicle. Scanner shows that the PCM is activating solenoids according to desired shifts, but no shift is happening.

Check line pressure with manual gauge. About 60 psi at idle.

Manually command increase in line pressure using scan tool. My tool seems to only allow me to drop duty cycle to 10% or so for a brief moment. But this produces an increase in line pressure up to 170 psi or so (gauge maxes out at 150 so this is an estimate. I can adapt to a higher pressure gauge if an exact number here is necessary).

Observe line pressure while driving. PCM appears to modulate PC solenoid and line pressure increases to at times 150 psi under load. Gauge also flutters a lot under load. Please see included video. Is this normal and would it cause inability to shift?

Any thoughts on what happened here would be much appreciated!

Video can be seen .

 

Since you have AFL and pressure control yes only have 3rd even manually , My best guess is a collapse solenoid screen blocking oil to the solenoids or the solenoid orifice feed holes in plate have become blocked So both valves stay in the 3rd gear position.

In addition, fluttering pressure when high is the pump slide bouncing on dyno this was observed as low fluid levels or possibly badly worn AFL valve and bore..
AFL valve and bore issues used to not be to much of a problem but as these things age its become enough of an issue I just reem and replace on all now.

 

Thanks very much for your helpful response! It sounds like the AFL valve is a very likely culprit for my symptoms. Are there any other diagnostic tests to confirm this suspicion?

It sounds like the AFL is a relatively minor repair requiring removal of the valve body, possible to do with the transmission in the vehicle. I suppose the collapsed solenoid screen would also be repairable in this manner. I am not entirely sure how long my friend has been driving with this condition but certainly some….at least a couple of weeks of normal driving before he attempted to address the problem. What is the likelihood that the transmission will need a rebuild in addition to the AFL valve repair?

 

Repairing the AFL valve, likely replacing the solenoids and perhaps even the separator plate would be part of rebuilding that transmission anyway. With the pan off, I would also install a Sonnax boost valve as its o-rings can help seal another wear area.
Hopefully that fixes the problem; if not you have only spent the time of drop the pan and R&I the valve body. Everything else has to be done in any case. Just my 2-1/2 cents worth (inflation adjusted).

 

Just to clarify, you suggest replacing AFL, boost valve, solenoids, and maybe separator plate. If that doesn’t fix the problem, go for a full rebuild or transmission replacement?

Regarding the AFL, I’ve seen it said in a couple of places that what usually happens is AFL pressure starts to be too low, resulting in losing 4th and sometimes second gear starts. I suppose I have no way of knowing if any of these signs presented or not, but my friend says the stuck in 3rd condition began abruptly.

 

Another comment here is that I unplugged the transmission connector to force limp mode. Shifts into gear are firmer, so this suggests it is raising line pressure accordingly. However I am still unable to shift into second, even manually. Is this useful information? Is there a different mechanism than the shift solenoid to shift into second manually when in limp mode?

 

I am leaning towards the solenoid screen being blocked myself

 

Ok, another development. Thought I would take the car onto the highway to verify if it shifted into 4th. It did not. However, it fell out of gear on the highway and now has no gears at all. Very slight movement in reverse. I’d guess with my limited knowledge that it lost pressure completely.

 

Yes, I was suggesting what you concluded in your post #5. The AFL valve is constantly moving to regulate pressure and therefore wears out the bore after many miles.
But now it sounds like the problems have gotten worse. Hopefully Frank can give you feedback based on your latest post.

 

Based on your previous post and now, I would say you have a big failure, Probably converter clutch initially sending debris through that blocks solenoid screen or feeds but now larger failure and filter clogged with debris .
You need trans and converter rebuilt or replaced. ,
I do have one suitable ready with this build and price that could ship as early as Monday .
STAGE 2 4L60E BUILD UP TO 750 RWHP BASED ON A 4000LB CAR
New Frictions
New steels
All seals
29 element input sprag
Beast sunshell
Corvette second servo
Billet Dual servo overdrive 4h
New wide band
New reverse input drum.
8 friction 3-4 set carbon alto with custom made thick backing plate. Apply plate surfaced to be perfectly flat with specially modified steel plates to aid in cooling
Custom shift kit with moderate firm shifts that are progressive to aggressive as throttle increases.
Full time overrun mod to protect sprag keeps overrun clutch on in all forward gears except actual 4th gear (Engine braking 1-2-3 gears even in OD position.
New valve body plate
Modified but not eliminated PWM circuit custom.
AFL valve reamed and replaced if needed after testing
Custom 2-3 shift valve not sonnax for quicker upshifts on the 2-3
.500 boost valve set with PR set up for 200 psi max line based on factory tuning set up. Can go as high as 230psi with tune change to 99 percent max
New wiring harness and shift solenoids
New factory style pan with removable drain plug heavy duty not cheap tin.
New bushings throughout
(dual bushings in rear sungear)
This unit is suitable for anything everyday driver to all out performance beaten on
PRICE = $2400 WHICH INCLUDES SHIPPING NO CORE REQUIRED TO ANY PLACE IN MAINLAND USA
Price is for 1993 TO 2006 4L60E YEARS FORWARD OF THAT PLEASE ASK FOR PRICE AS I HAVE TO CHECK PRICE AND CORE AVAILABILITY. 700R4 ALSO AVAIABLE IN THIS BUILD CALL OR MESSAGE FOR PRICE.
NO RETURN CORE REQUIRED
BIG NAME BUILD AT AFFORDABLE PRICES, DELIVERY TIMES VARY BUT CAN LET YOU KNOW AT TIME OF ORDER, ALL BUILT BY ME WITH 40 YEARS EXPEREINCE AND KNOW NOW.

I don't have converters But do suggest FTI or Circle D for performance and https://www.rockauto.com/en/moreinfo...QHTse7iB2TKFZY ROCK AUTO for Stock.

This is a photo album of the HD PERFORMANCE BUILD https://www.facebook.com/media/set/?set=a.275195888831054&type=3I also have lower builds but they are not that much cheaper.