Tom Colson

2006 Z71, 5.3, 4L60E, 175K on it. Quite abruptly, at 40-55 mph, on flat or slight uphill, when at "cruise" (4th gear, steady throttle), I get the infamous "Chevy Shake" which goes away if I stomp on it, let off the gas, or lightly tap the brake. ATF changed around 100k. It needed it anyways, so I did a front-rear fluid change of everything related to the drive train. U joints "look" and feel good. Changed the plugs and wires while I was at it. Fluid coming out looked like I would expect for 75k, not milkey,it's dark red, not a lot of material stuck to the magnet. Between what came out of the pan and what I pumped out of the dipstick probably changed 7-8 quarts. All other transmission function is silky smooth, can't notice any shifts. Changed both upstream 02 sensors too. Old plugs looked pretty good. New ones were perfectly gapped.

Same shudder, same conditions. Tube of Lubeguard shudder fix did nothing. And yeah, the tow-haul switch is broken, and I'm not willing to tear apart the dash board to fix a wire, as that seems to have little to do with causing the shudder. Noticeable "skip" (down) in RPMs when it happens (maybe? I can't be sure now). At first I thought the tranny cooler in the radiator was leaking but I'm pretty sure I ruled that out. Cannot detect any coolant in the old ATF.

What else do I need to look at? At the GM dealer in town, they have a notorious and well-known solution to every tranny problem: you need a new tranny we'll hook you up for 4500$. Local shops are worse. How does one diagnose coil misfires at cruise speed without a tech2? All the so-called Bluetooth OBD scanners won't do that, neither will the car parts store scanner I have. There are no codes. A Pulse Width Modulator solenoid is 60$, is that a pretty common thing to go wrong and not throw a code? Next day, spent more time looking at the tach than the road on the way to work, and the tach needle wasn't moving when the shudder (at times very violent) was occurring on a road with perfect pavement, not a single bump, crack, pothole, etc...about 5 miles, 2-5% uphill incline the whole way. Don't have a guage to check fuel pressure, which I can check only park anyways, and this is occurring in cruise.

Or is this for sure the TC on it's way out......

tech2

the symptoms you describe and no misfire codes make me think this is a tcc problem. see if there is no tcm module reprogram update to address this... if not, the fix probably is a new trans and tc.

tow haul switch wire will break at the gearshift lever under the steering column trim cover. no dash pull required. and yes, not related to the problem