Cruise control is managed by an computer up under the driver's footwell. When the cruise in my son's 95 940 failed, I performed the diagnostics thanks to the OBD unit under the hood. It returned "no signal from speedometer". The speedometer and odometer were working fine. I tested the output from the speedo using a multimeter and it "appeared" to be working. After making sure everything else was working, I finally replaced the speedometer and cruise returned to normal.
It so happens that the speedometer receives a sine wave signal from the rear axle sending unit and converts this into a square wave, with frequency proportional to rear wheel velocity, to serve as a the input into the cruise control computer. If this does not happen, no cruise control. I do not have an oscilloscope which would have made quick work of this diagnostic.
Anyway, if your cruise dies and you get this diagnostic report while your speedometer appears to be working fine, consider that the internal wave conversion might have failed. This means a replacement speedo.
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you are correct in you analysis but you did not check the most common cause. this is breakage (total or partial) of the wires at the connector that fits into the sensor in the differential. they are easily (sorta) repaired. (awkward- not hard). the wires break just at the top of the connector. when the wire break, you get the same error codes and responses you saw.

the most common cause of cruise control failure is failure of the switch that is at the top of the brake pedal. that switch has a electric and vacuum component. Typically, the vacuum part fails (easily determined). the switch cannot be repaired. just replace it

I completely agree, but checked all those. The rear axle sensor was new (and the speedo/odometer worked perfectly); the failed vacuum switch was replaced. All other components were checked and worked flawlessly. The system still had the code "no speed signal from speedometer" which indicates the speedometer did not correctly convert the sine wave from the rear axle into the square wave for the cruise control. I have yet to break into the old speedo to see if anything obvious caused this problem.
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See the 700/900 "FAQ" at the menu bar top screen left side.

Hi Steve. I'm having the same problem on my 1994 945T. I get the 3-1-1 code, but speedo works fine. Swapped it out with another working speedo and still getting 3-1-1 OBD code and no cruise control activation.
Should I just replace the module on the left lower A-Pillar? I have an extra one.

John

I tried the same and it did not solve the problem but your results may vary. I'd suggest first making sure you have continuity between the two little tangs on the back of the speedo, where the wire connects, and the inputs to the cruise module at connector position 13 using a continuity tester. Also make sure your cruise module is grounding by testing continuity at position 10 to known ground.
Keep us posted on your investigation. Mine took a lot of time to pin down. It now works.
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Dear Steve,

Hope you're well. At the risk of laboring the obvious, your insights belong in the FAQs, where they will surely prevent head contusions, as cruise-less Brickboarders batter heads against wall....

Yours faithfully,

Spook

Yep, you got it and it went into the FAQ.

Steve
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See the 700/900 "FAQ" at the menu bar top screen left side.