I am probably not doing this right but I am getting a 'check engine' light so I placed the OBD1 cable into socket 2, turned on the ignition, pressed the button once, thereby selecting mode 1 and I got a 3-1-1 code.

I pressed the button one more time and got the same code.

The interpretation table says that a signal is missing from the speedometer and that the speedometer wiring or the instrument cluster wiring is faulty and that it can cause idling problems.

The speedometer has been working fine--just reads a little high and the car seems to idle properly. I hesitate to get into the wiring. Lately the cruise control has not been working so well but otherwise can't see any problems.

Has anybody else experienced this?

One slight possibility I can think of. Has the instrument cluster been removed in recent history? The edge contacts along the very bottom of the main circuit board can get caught on the edge of the dash during removal and installation causing damage there. I'm not sure if any of those contacts have the VSS signal, but it's quite possible. I've had to carefully re-solder a broken connection I caused there. Additionally, make sure the electrical connectors on the back of the cluster are pushed in all the way and seated squarely.

Also, has the ECU or ICM (injection control module) connector been off? Those tiny pins in the cable connectors are fragile and can get bent if you're not half careful.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

3-1-1 does not seem to be linked to a CEL scenario. Are there any codes on socket #6?

Asumming LH Jetronic on your vehicle:
The speed (VSS) signal from the diff goes to the instrument cluster first. That cluster modifies it and hands it trough to the ECU. The fact that the speed seems to register more or less correct (a slight but steady deviation may be caused by something as simple as a different tyre size) means the sensor and wiring to the cluster are likely OK.

There may be several causes, one of which is a bad contact in the connectors to the ECU and EZK units. Should be fairly easy to check.

Agree with Grey245,

Your speedo is working because it still receives signal from VSS at differential. The speedo should next send the signal to LH fuel computer. Possibly some corrosion happened at the connectors. There's Volvo service bulletin (5 pages) adressing such issue ie. DTC has been recorded although no fault detected. Also do this at the LH fuel and EZK ignition ECUs.













Amarin

My Volvo 745 speedo was acting up. I fixed it by cleaning up frayed wiring at the rear differential. I disconnected the wiring harness and stripped the cover back around 6 inches and insulated the deteriorating wire insulation with heat shrink tubing. I had to carefully disconnect each wire from the plug housing. It works fine now.I also removed the sensor.pickup and cleaned the finite metal fuzz off of it. The speed sensor has an O ring sealing it into the housing.