Been having a drivability issue with my 99 4x4 4.3L Blazer. Light shuddering/hesitation, mainly between 40-50 mph. Getting worse the last couple of weeks. Was thinking it was misfires but my mechanic tells me he sees no codes. So did some research, asked my brother, who's a decent shadetree mechanic and used to have a 96; now the hypothesis is that the OD is engaging/disengaging erratically. My brother suggested a test consisting of driving it in 3, thus locking out OD, and see if I still see the problem. I tried that and the problem didn't recur.

Questions:
1. Does this diagnosis make sense?
2. Is the test valid?
3. Are there any further simple diagnostics I can attempt to confirm or disprove the theory?

Assuming # 1 above is true, my research indicates that potential causes of the OD hunting would be:
1. Throttle Position switch/sensor
2. Solenoid(s?) that actuates the OD
3. Vacuum leak
4. Torque Converter
5. Transmission else, aka worst case scenario

Does that list look correct, is is there something else I should be looking for?

Lastly, if the diagnosis and the list of potential problems is correct, what's the best/most cost effective order in which to work through the list? For instance, I should simply be able to test the TPS with a multimeter, correct?

I'm not a mechanic, so I'm basically regurgitating stuff I've heard/read, so apologies if my terminology is wrong or I've completely missed the mark.

 

torque converter shudder is common although not with gms or this unit. happens when friction modifiers in fluid get used up, usually cured by a thourough service. the 4l60e does have a common valve body issue that does cause the converter to slip but this doesnt sound like ur case plus u would get a code. the fast easy test is go to a auto parts, ask for trans additive called lube guard, u want the red bottle. pour it in and drive within a mile if its shudder this will fix it dead, no bs been around for a long time its not solvent or goo just additive replacement. i would also be suspect of a rear u joint. u can try to explain ur test results better, as to in 3rd were u at the ''happy'' speed'' of problem and is it load sensitive at that speed

 

I would also suspect/include in diagnosis, the torque converter lock up as a possibility. I had my 4l60e around 45-50 mph seemingly "popping" in & out (back & forth) of gear('s) while in "OD". I put in a transgo HD 2 shift kit, sonnax .500 over boost valve, corvette 2nd servo & a sonnax 3-4 servo & pin, pinless 1-2 & 2-3 actuators, pressure regulator & wiring harness (combined). I have had no problems since. As a precautionary procedure, I also put in "shutter guard", comes in a packaged long skinny red tube. Oh, I also used a new valve body plate (did the performance drilling) & used the nylon check ***** instead of re-using the metal ones as too guard against premature plate wear as the metal ***** do wear on the plate! My first shift kit install, took my time, paid attention, "hated" fumbling with the check *****, did the recommended shift stem (internal part) grind as recommended to releive some of the "reverse/drive slide bump". All has been well for over a year now, & second gear hits like a camaro, easier on clutch wear!