I would only advise the disable procedure as a last resort. Perhaps a comparison would be that such a procedure be done only if the sub was leaking and no one could find the corks. Your writings are correct in every way and well done. I have 2 comments:

Whilst our Volvos will perform perfectly well without the 4th gear disable feature I prefer to have it available. I frequently drive in the mountains of North Carolina where you are always climbing or descending at less than freeway speeds. It is better to select 3rd gear (2nd gear is too low) under these conditions to make use of the power and braking effect that 3rd offers.

By performing some diagnostics it is easy to know where the problem lies. Transmission, solonoid, switch, wiring or relay. If I know it is the relay (by far the most common failure) I would much rather slip in a replacement than get grimey under the car. Many Bricksters fail to understand how the OD/light circuit actually works and get fooled when attempting to diagnose the real fault.

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David Hunter