Transmission issue with a twist

[AudioHead]

So, the transmission on my 2001 S80 2.9L Automatic is starting to act up.
However, not in the way I'd suspect after reading through everything I could find about the issue on this and other forums.
I can't find a single reference to the issue I'm having.

First off, the car drives fine. No hard shifts, and no slip when the lockup is engaged.
However, when cruising down the highway at 65 or just going a steady 35, the transmission seems to "click" every 4 seconds or so. The interval is steady, and the period is way too long for it to be a mechanical clicking.
The clicking is audible, and can also be felt in the shifter.
Every time the transmission clicks, the "D" light next to the shifter goes off for a fraction of a second, and then comes back.
There's no other indication something has happened. The engine noise is constant, and the RPM doesn't change when it happens.
If I'd have to venture a guess, it's a solenoid activating. Why it (and which on it is) would activate every 4 seconds when cruising with lockup engaged, I have no idea.

Here's the twist though. When pressing the Wet/Winter button to go into W mode, the clicking disappears completely. Going out of W mode brings it back. 100% reproducibility.

I've looked at the ATF fluid, and while it's not completely out-of-the-can colored, it's not bad at all. Especially considering the car has 135K on it.
I've also scanned for codes using a regular handheld scanner, but nothing shows up. No warning-lights or messages in the dash either.

I'm stumped. Anyone here got an idea ?

[jimmy57]

Sounds like a faulty Gear Selector Module (GSM). It is the control module housed in the shifter selector gate trim and it runs the shiftlock solenoid, the winter button, and has the illumination for selected gear and backlighting.

[AudioHead]

I think you're on to something there.
I didn't even know there was such a thing (coming from the GM world), but it does make sense.
Measuring the shiftlock solenoid during driving definitely tells me that it's the solenoid I could her, and that pressing W "resolves" the problem.
I've already got my local Volvo guy trying to find a replacement.

Thanks!

[AudioHead]

Quick update in case someone stumbles over this thread with the same issue.

I replaced the GSM today, and the clicking seems to be gone. Big thank to Jimmy for his crucial clue.

Some clues for other people with the same issue:
The GSM is hard to find for some Volvo shops. It's actually in their system as a "panel". I.e, when you want the part, what you're essentially asking for is the piece of plastic with the PRND321 indicators on it. The GSM is inside that panel.

Replacing the GSM takes about 30 minutes. To do a complete replacement, you'll need to remove the shift-knob, but you can get away without having to do that with the associated risk of breaking it (and your face) in the process. The key to this is to disassemble the GSM (four screws on the bottom) that's in the car. You'll need to cut the PCB in the bottom (where no PCB traces are) as well as the plastic "sheet" that sits underneath it, making it into a horse-shoe shape instead of an O-shape. That way you can slide it out instead of sliding it over the shift-knob.
Putting in the new GSM is done the same way.

No reprogramming or flashing was needed either.