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Hi again,
The solenoid itself can indeed be temperature sensitive, as I've experienced myself. Internally it is a coil of fine wire under the stress of its encapsulation, housing rust, and wide swings in temperature. The winding can open up at terminals inside, or like I proved in this post mortem open up somewhere in the middle.
The idea that automotive electrical circuits are somehow exempt from the laws of physics as circumstantial evidence or evidential circumstance is unique. Theory suggests a break in an electrical conductor might be closed as the conductor expands with increased temperature. Even Click and Clack lean on that old explanation for temperature dependent car maladies.
The relay doesn't inherit any more immunity from the waveline solder defects that affects them all by having Volvo's name on it, though its aftermarket imitators have no incentive to meet Volvo's incoming inspection standards. Aside from solder crack defects, they (new or not) are exposed to damage caused when the wire under the car loses its insulation and shorts the load power to ground. Again, many posts have described this which you'll read in the search link.
You were asking for some other ideas, but I saw no responses to those valid ones presented from some very experienced folks. Verification of the electrical portion is only slightly more work than armchair supposition about the cause. And yes, there isn't much pressure in that hydraulic circuit, so a clog or restriction could put it on the edge of function, changed only by the thinning of warmed up fluid.
So in summary, I can only tell you what I would do, and have done, with your symptoms.
--
Art Benstein near Baltimore
My father said there are two kinds of people in the world: givers and takers.
The takers may eat better, but the givers sleep better.
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