Torque converter locking in and out

Revans381996

Original poster
Member
Dec 18, 2013
41
Well at 191,900 miles it seems that my 2003 trailblazer is having it's first major problem. When I get into 4th gear at around 45 mph the truck starts to stumble and my rpms rapidly fluctuate between about 400 rpms. I haven't needed to go faster than that yet because the problem just started last night but I'm scared to go above 45 because I don't want the shudder to get worse. It's not throwing any codes yet but I think my problem is the torque converter locking in and out. Is it possible a solenoid could fix this or am I most likely going to have to rebuild the whole transmission?
 

Reprise

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Jul 22, 2015
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It might be that the TCC valve in your valve body (inside the trans) has widened the bore through use (the VB is made of aluminum, as are several other parts, but the TCC valve tends to wear the bore in particular, as it's constantly engaging / disengaging, in stock form.

If that's the case, Sonnax makes a kit that has a reamer to allow an oversize valve to be fitted. Or, you can get a new VB online (but make sure you disable TCC / PWM, either through a tune, or via a kit (a lot of people here have put in the TransGo corrective shift kit (orange box), with excellent results). But if the bore is worn in the VB, the TransGo kit won't help, as the bore has to be within operating spec for their fix to work. And the Sonnax reamer kit is expensive, and can be tricky for the lay person to use. Thankfully, some vendors sell replacement VB with the Sonnax kit already installed for you (and some rebuilders already have the reamer; it's designed for jobbers to use it over and over.)

TransGo may make an oversized TCC valve (can't remember as I write this), but if your bore is that worn, I'd go with the Sonnax - it's considered a 'permanent' fix. Again, if your bore isn't (or very slightly) worn, the TransGo kit is the easier / cheaper way to go - I have it in mine, and as I mentioned above - many here have had it in their trucks (and cars) for many years w/o issue.

Depending on how mechanically inclined you are, you can replace or upgrade the VB with the trans in-vehicle (or have a knowledgeable friend put it in for you). Plan on a weekend to have the vehicle down, taking your time and being careful. You'll need a few additional parts (like new separator plate gaskets, at a minimum.)

Yes, the TC is designed to slip, both for gear-to-gear shifts (making them 'butter soft', as manufacturers like to do), and for lockup (which can happen in 3rd, not just 4th.) The way in which this is done is called 'PWM', or Pulse Width Modulation. If you eventually get a code on this, it'll likely be P1870; you may have others, but that's the one that is most relative to this issue.

Generally, a solenoid won't fix this, and there's only one main fuse that controls the trans behavior - if it blows, or you have a break/short in the wiring harness, you'll see a bunch of codes, and the trans will likely start in 3rd gear (as 3rd is the only gear in the 4L60e that doesn't require the 'e'lectronics to shift). That's 'limp home' mode.

If the TCC bore isn't worn, and a TransGo kit doesn't fix it, then your torque converter and/or your 2-4 band may have gone south (when the trans is slipping the converter, it does this by engaging the 2-4 band on/off to 'grab' the converter.) Both of those require the trans to be dropped; depending on how much longer you plan on keeping the vehicle, you may decide to look for a less expensive option (like a used one from a junkyard, or a rebuilt one from your local Craigslist).

If you decide to do a full rebuild (or put a rebuilt one in), get a new torque converter (don't reuse the old one, and don't buy a 'reman' TC). If you go the rebuild route, you may as well have the weak areas of the 4L60e addressed (HD 2-4 band, HD clutches / steels - especially the 3-4 pack, and the sunshell). Those, along with replacing any plastic accumulators with metal (or pinless - Sonnax makes them), and a HD separator plate should give you a trans that outlasts the engine (if you got 191K out of the stocker, you probably haven't abused yours, changed the fluid / filter at least once or twice, probably don't tow with any regularity, etc.) Your builder should advise you if the pump assy. should be replaced (might be worth doing anyway, since you have almost 200K on the stocker).

Oh, and one more thing - putting in an external trans cooler is one of the best things you can do for your trans. They're not terribly expensive, and will keep your fluid under 200F - above that temperature, the life of the trans is shortened (by half, for every 10 (?) degrees above 200F.) You can put one in fairly easily, or if you're having the thing done professionally, you can have the builder plumb one in for you (a good builder will likely recommend you put one in, regardless.)

Finally, for a rebuild, putting in a good synthetic Dexron VI fluid will be cheap insurance. Valvoline makes a good one that isn't too spendy (about $8.xx a quart, retail). I use it in mine. You'll need 12qts, perhaps a 13th if you put in an external cooler that's on the larger side - I have a small 10-row stacked plate Derale that bypasses the stock cooler in the radiator entirely, and it keeps the trans at 150-160 in the summer, by itself. While you don't need a super large one (unless you're regularly towing 'heavy' or racing the thing), generally, you can never 'overcool' a transmission.
 

Sparky

Member
Dec 4, 2011
12,927
FWIW I've used the Transgo kit successfully on a very worn TCC valve bore and it has continued to work flawlessly since the install many years ago. That's one of the things it is meant to address is a worn bore. The Sonnax probably can handle a bore that's worn further than the Transgo can, but mine was pretty bad yet it worked great (it would never hold lockup once the transmission was warmed up and was beating the ever living snot out of shifts until I installed that kit).

Maybe the Sonnax is better in theory, but, it is more involved and with the success I've had with the Transgo kit I'm not apt to try it or recommend it.
 
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Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
The Transgo kit uses a different TCC valve and spring that eliminates the PWM as well as use an unused part of the valve body so when it engages the TCC, it does so positively without any PWM slippage.

On another note, my son's Silvy had a slipping TCC like the OP's. Had the PCM tuned to eliminate the PWM so on engagement, it goes from 0 to 100% engagement, eliminated the problem without opening the tranny except for a normal fluid/filter replacement.
 

Torker

Member
Jan 2, 2017
36
Udall,Ks
Good timing for me... Mine has been taking a long time to lockup and seems varies a lot. I just had mine flushed at 148K The mechanic said it was very thick. He also added a can of BG additive. Still have not done the filter. If I have a shop do it what else can they do without a teardown. Shift kit? Corvette servo? Or is it the sungear? Or is that internal?
Also I am planning to get a tune. I had one on my 04 from pcmforless Loved it. Should I just get that first?
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
Depends on what "thick" means. If it's just brown'ish, it should still be salvageable. Should do the filter to be sure the fluid supply to the pump is good. A shift kit might work. If it was the sungear, it would be very evident with grinding and not moving right. You could try a tune first but if it's something more, like a bad TCC solenoid or the TCC itself, it might not work.
 

Torker

Member
Jan 2, 2017
36
Udall,Ks
Thanks Mooseman.. Mine was brownish , not terrible. It smelled ok too. Mine is doing better than the ops. Sorry to hijack. I love this place. You all have the best advice on the planet on these trucks, or any vehicle for that matter.
 
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Locksmith

Member
Nov 19, 2017
76
Columbia,SC
I learnt in old auto tech school, that a transmission cooler should be in series of the "factory" transmission cooler that is built into the radiator (I say radiator like gladiator cause I'm a New Yawker!" ) because if you don't let the coolant heat the fluid, you will create sludge. Just another useless point to ponder.
 
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Torker

Member
Jan 2, 2017
36
Udall,Ks
My dad taught me the same thing. :thumbsup: The quicker you warm it up the better.
 
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