Background: the S90 was experiencing an occasional "no crank" which became more frequent until it would not crank at all. Strong battery, all the dash lights came on as they should, but no action at all from the starter. I'm thinking bad pnp switch or ignition switch at this point.

I used the starter service socket in the underhood fuse/relay box - same result...indicating the ignition switch is not the culprit.

I pulled the starter relay and shorted across its plug-in sockets 30 and 87. This bypasses the pnp switch on the transmission and puts battery voltage straight to the starter solenoid, Same result - nothing. Concluded bad solenoid at this point and ordered rebuilt starter assembly.

But later on, with the relay shorting wire in place I reached under the fuse/relay box and pushed-pulled the wiring harness that seems to go toward the starter and got a brief burst of life from the starter.

Under the car with a strong light, I could see the 1/4-inch female spade terminal on the Blue wire from the relay to the solenoid. It felt solidly connected, but I removed/reinserted it a couple of times on the solenoid's male spade. Eureka moment...the car starts fine now.

Conclusion: corrosion on the solenoid terminal and connector was preventing current from passing. Moral: check all the small things before ordering parts.
Anyone want to buy a genuine Bosch reman starter and solenoid?
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Bob: Son's XC70, daughter's 940, my 83 240, 89 745 (V8) and S90. Also '77 MGB and some old motorcycles