Hey everyone.
I've had an issue with my 1996 940 2.3 since I've owned it. (Almost a year)
Kind of erratic idle when it's warm and it starts hunting after full throttle.
After loads of faffing about and changing parts plus taking it too 2 different classic car garages who just took money and did fuck all. I finally deciding to make my own diagnostic tool for it and i think I'm onto something. Error 3-1-1 which I believe means no VSS

I did pull the dash out today and the wire is connected to the back of the instrument panel . I haven't checked for continuity at the ECU yet though but knowing how much of an arse this Volvo has been. I bet it has it.

Any advice on tackling this problem for when I inevitably pull the ECU out and find out that the pins are fine and I have continuity.
75k miles and in fantastic condition but it's about to be scrapped if I can't get to the bottom of this problem.

Idling problems are listed for a missing VSS signal (DTC 3-1-1), so it seems you're on the right track. I assume if you were to reset the OBD codes that 3-1-1 would soon come back?

Question 1: When you said you checked the wire on the back of the cluster, I assume you meant the single violet/white wire on the double spade lug (connector D)? That's the VSS signal from the speedometer going to the ECU.

Question 2: Does the speedometer work when driving? If okay then that eliminates the speed sensor and half the speedometer section. If not then check a) for damaged wiring at the rear speedometer sensor (not that uncommon) and then check b) the back of the cluster for connector A (behind the tach) being fully seated and that the brown-white and brown-gray wires (pins 3 and 5 respectively) from the speed sensor look okay and are as seated and fully pushed in from the back as the rest of the pins.

Question 3: In recent history when this problem first happened, had you had the instrument cluster out? Been doing work behind the instrument cluster? Or have the ECU connector off? VSS just happens to be pin 34 on the ECU connector, the very end of the connector, and may be more subject to being bent or damage during attempts to reseat the connector after removal, also if the connector doesn't happen to be fully seated. Inspect the end pins (pin 34) in the ECU connector carefully with a magnifying glass to make sure none of the pin fingers are bent.
--
Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now

Dear oneeyeblind,

Hope you're well and stay so! According to the Volvo pocket data booklet - Cars (700,850,900), 1991-95 - a 3-1-1 code means the speedometer signal is absent.

Just because the wire is present, doesn't mean current flows. A layer of corrosion invisible to the unaided eye - just a few molecules thick - can disrupt current flow. Use an aerosol corrosion-removed on the contacts at both ends (cluster and ECU). Check any connectors - especially those in the engine bay - for corrosion.

Further, the blue-green flexible circuit card can develop micro-cracks. These can be found with use of volt-ohm-multimeter. Plainly great care must be taken when reflowing solder to close a micro-crack or when using a copper wire bridge over a break.

Hope this helps.

Yours faithfully,

Spook