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Yes, the solenoids are one possibility. I'm sure there are other possibilities as well.

I thought the tech2 had the ability to manually command those solenoids? If so, did you try to see if you could manually command a downshift when it wouldn't downshift?

I'm no expert on electronic transmission diagnosis, but whenever I see a problem like this, the first thing I would look at is this: Is the computer not commanding the shift, or is the transmission on responding to the PCM's commands. I would hope that your technician would have been able to maybe identify which scenario while he had it hooked up to the scanner. Did he give you any indication as to the nature of the problem?
 

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It is kind of frustrating when a technician starts in on the guess and check method. Did you have a reason for choosing this technician or were you randomly assigned? It might be time to consider a different technician.

Clarify terms: TCCM=transfer case control module? ATC=automatic transfer case? Assuming this is what you meant, yes, there is a connection between the transfer case and the PCM. I'm not certain of all of it, but the PCM at least needs to know if the t-case is in high or low range so it can adjust the shift points for the 2.7 gear reduction of low range. The Haynes wiring diagram for my '98 shows both a 4wd input and 4low input. If the PCM erroneously thinks it's in low range, then I would expect it to wait longer to downshift. Again, though, I would expect your guy with the tech2 to be able to see that fairly quickly on the scanner. Or, with the proper knowledge of what those two signals are supposed to look like, one should be able tos see them with a DVOM.

Can you manually downshift with the lever?
 

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I wouldn't expect the TCCM to be bad, but I also won't say that it is impossible.

Will you have access to a scanner? If so, I think the first thing I would do would be to drive yours with the scanner hooked up, and do a "freeze frame" when the problem occurs. Then hook it up to the other truck and drive it, and see if you can see anything different.

If you don't have access to a scanner, I might backprobe the shift solenoid circuits (maybe the shift solenoid circuit for the 4-3 downshift since that seems to be a fairly consistent failure) and see what his truck is doing different from yours.

As far as parts swapping, I don't know. Would it be too much work to swap transmissions and see if the problem follows your transmission? I expect so.

Is your buddies truck exactly the same (year, engine, transmission, etc.)? You could try swapping PCM's if he'll trust you to do that and see if the problem follows the PCM.

I wish I knew what the lever did inside the valve body to effect the downshifts: is it electronic or not? If you can manually downshift the transmission, then what is happening differently than when the PCM should downshift?
 

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I'm sorry, but I haven't ever needed to know that much detail about the 4L60E, yet, so I don't know which solenoid is responsible for the 4-3 downshift. Does anybody know? Do you have access to any manuals (maybe at the library)?
 
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