Trans failure sprag vs shift solenoid

Bow_Tied

Active Member
Hi All,

I am trying to troubleshoot my trans failure and the symptoms I find on the net seems to fit both a failed forward sprag or a failed Shift Solenoid B (or communication to it).

I believe symptoms for both are that 3rd/4th are only available after the vehicle is moving, must start out in 1 st/2nd. Since 3rd/4th work after the vehicle is moving I have ruled out 3/4 clutch pack (hopefully that is correct).

Details/background/symptoms if it matters:
2006 Envoy Denali, 119k km reasonably well maintained.
Trans oil on dipstick appears excellent in colour/clarity, no burnt odour
Trans fluid/filter changed @ 105k km last summer by dealer
Good quality trans cooler installed just before fluid change
Tows 4800lb loaded travel trailer a few times a year. Trans temp monitored during towing (only in 3rd) by ScangaugeII typically 165-185°F ambient temp/hills dependant
No prior symptoms. Today truck made a lurch after turning a corner and seemed a little sluggish (according to wife) but drove fine rest of way to work. At lunch, she pulled to edge of parking lot, waited for traffic then no forward gear (rev like neutral). Selecting 2nd gets you going thereafter 3rd/OD work. Reverse works.
No CEL.


Any assistance is appreciated.
 
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MRRSM

Lifetime VIP Supporter
It is probably worth starting with the Pressure Control Solenoid as a failure there will cause erratic shifting. You will find it described on the attached diagram:

At 1 Hour & 14 Minutes into this 4L60E Complete Rebuild... The technician installs ALL of the individual solenoids into the Valve Body of the Transmission... the Power Control Solenoid install is described at around 1hr & 16-17 Minutes:


Here is an explanation of how the solenoid functions:

 

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Mooseman

Master Blaster
Moderator
And I would imagine a sprag failure is mechanical so if you do a pan drop, you should see some bits of metal.
 

Bow_Tied

Active Member
Thread Starter
Thanks for the reply gents.

The trucks seems a little sluggish to start from a dead stop (but could be because I was driving it gingerly and being hypersensitive) but otherwise drives fine if I start out manually selecting 1 or 2 and shifting manually.
Could that still be the PCS then?

Haven't dropped the pan. With little kids I don't have time to mess with this myself so I will likely drop it off somewhere this week. This should overheat the credit card. :/
 

MRRSM

Lifetime VIP Supporter
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Bow_Tied

Active Member
Thread Starter
Thanks again. The vid is very helpful for identification of parts and installation.


My original post was to intended to seek verification of symptoms for the solenoid failure or more specifically, is there a difference in symptoms between the solenoid failure and the sprag failure. Still hoping someone can chime in with that.
 

AtlWrk

Hobbyist
If I'm reading it right you basically just have no first gear and everything else is more or less functioning properly once you're moving, correct?

If that's the case it's not going to be a solenoid or the sprag clutch. The 1-2 solenoid is also used to command 4th so if it was malfunctioning you wouldn't have 4th/OD. If you're sure you do have 4th (and not just 3rd + TCC lockup) then the 1-2 solenoid is working. Doesn't necessarily rule out other parts of that hydraulic circuit though (see below). Double check this, because if you don't have 4th it's almost certainly the shift solenoid and you're looking at a (relatively) easy and inexpensive repair.

The sprag clutch is active in all forward gears (except 4th while in 4th/OD) so I'm curious what led you in that direction? The Low Roller clutch, however, is unique to first gear.

I would search around for "4L60e no first gear" keeping in mind that it is distinct from the common issue of "starting in 2nd". Some quick hits point to a cracked accumulator piston, a broken shift valve spring, etc. The valve body separator plate also takes a beating from the steel check balls and its not unheard of for them to punch through (this usually manifests as losing 3rd gear, but anything is possible--there's a dozen or so of them in there).

All that being said: there's no harm in dropping the pan, replacing the shift solenoids and inspecting for signs of serious trouble like metallic flakes or clutch material. :thumbsup:
 

AtlWrk

Hobbyist
When you're in 4th/OD will it shift back down to 3rd normally (as speed drops or you gas it)? Will it go down to 2nd normally when in manual 3rd or does it start revving again (i.e. you can ONLY get second in manual 2nd?).

After a little more digging it looks like it might be the input sprag after all. Seems to be fairly common (as far as what fails, not in that it fails often). In manual 1st or 2nd the overrun clutch is active and would override a failed sprag in those gears and get the symptoms you're experiencing.
 

Bow_Tied

Active Member
Thread Starter
Thanks for the comments and suggestions. To be honest I haven't done enough exploratory driving to cover all of your questions as when the sprag failure seemed to fit, and the on-line troubleshooting guide also mentioned to not drive it as further damage may occur so I limped home and left it.

I think I had OD locked based on the RPMs. I can't remember if I tried 1st or just second. I know I had second manually and could upshift from there. I didn't let it down shift to 3rd, as I was slowing I manually went back to 2nd. In doing so I felt no engine braking in 3rd; however, I was not going very fast. I think I did note some over rev when I pushed it hard in 4th - only did that once
so I am not really certain.

Do you think I'd be risking much to do some test driving? to get a more detailed analysis or should I not do this to avoid further damage? I could try to do some troubleshooting tomorrow evening.

Thanks for the help.
 

AtlWrk

Hobbyist
If your forward sprag is toast then you're relying entirely on the overrun clutch in manual 1st or 2nd. It can do it in the short term but I wouldn't plan on daily driving it like that :thumbsup:

The more I dig into it the less and less it looks like it would just be something simple like a servo. Just a failed servo A would manifest as starting in 2nd gear, not revving at a stand still like you have.

I hate to say it but you're probably looking at some major work--either something wrong in the valve body or more likely a mechanical failure of the transmission itself. Sorry.:cry:

(To be clear, I'm hardly an expert in these transmissions, just a weekend mechanic with access to a lot of technical info.)
 

Bow_Tied

Active Member
Thread Starter
Ya, I think you are correct.

I am taking it in tomorrow to a local trans shop for analysis and likely a full rebuild. I'll start a separate post on specific rebuild advice.

Thanks all for the input.
 

Bow_Tied

Active Member
Thread Starter
Bow_tied did you ever find what your issue was?
Yes, forward sprag fail. Did a full rebuild with upgraded parts and a vette 1-2 servo. Man it worked great after that, crisp shifts even towing. That was ~$3k all done. :duh:

However in the fall '19 a college kid rear-ended the rig. Though the damage appeared not terrible, the evil insurance company give little value to a then 13 year old rig and it was written off. Replace it with a 2015 Xterra Pro-4X Limited (and why I don't visit here much anymore). So unfortunately I cannot offer any long term perspective on the repair.

Are you experiencing trans issues?
 

Eadsch

Newbie
yeah I’m actually having the same symptoms you did, however, if I get my transmission into overdrive, the truck the drives and shifts normally, even down to first gear and stopped, and back up to speed. But if I shift to neutral or reverse, I have to start back off in 1st and shift up into overdrive.
 

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