My speedometer takes about 20 minutes to wake up.

No kidding.

I know, I know, we all have mornings like that.

I reads zero and the odometer doesn't move. Then, suddenly, they both come to life. When they're alive, they seem to be accurate.
The tachometer, fuel gauge, temperature gauge and clock all work normally. So does the automatic transmission.

A few years ago, I had a tire blowout that took out the vehicle-speed sensor cable, which lit up the ABS malfunction-indicator lamp until I repaired it. It's not currently lighting up, though I haven't disconnected the VSS recently to see if that will still happen.

If I turn on the ignition about twenty minutes before I start the engine, the speedometer and odometer behave normally from the moment I start driving.
If I turn it off for just a few minutes, (to run an errand) something resets and they sleep for another 20 minutes.

I don't know where to begin. None of the wiring diagrams I have depict what's between the VSS and the speedometer, I don't know what kind of signal the speedometer expects, and none of the documentation I have offers any suggestions about how to disassemble the dashboard. (including the Volvo Body-Interior book)

HELP! This is really killing my gas mileage!

 

sounds like the fault lies with the VSS. maybe try remove/clean/replace...

 

What year 940? Check the rear end sending unit wiring at the differential plug as an outside possibility but most likely your speedo's diodes are failing in some combination--they cause weird anomalies like you describe. '91-92 are notoriously bad, '93-'94 are a little better.

Another possibility is cracked solder joints at the back of the instrument cluster. You can refloat the solder points, that sometimes works to revive failed indicators but most likely your speedo is going south due to failing diodes.

 

1993.

Where are these diodes and what type should I replace them with? I'm perfectly comfortable with electronics, printed-circuit boards and a soldering iron -- about one step shy of NASA-level expertise -- but it doesn't help much if I can't find/reach them. Still need instructions to remove & replace the instrument cluster.

Also: The cruise control works when the speedometer wakes up, and doesn't work when the speedometer doesn't work. What do the two have in common? Does the speedo receive its vehicle-speed signal from the cruise control, or vice-versa?

 

https://www.brickboard.com/FAQ/700-9...ClusterRemoval

Google's your friend

 

No speedo, no cruise control. The speedo removal description link is good, the bezel can be a pain sometimes...

 

We have a 1993 940. Had erratic speedometer back in 2011. Sometimes would work and sometimes wouldn't. Finally gave it up and had not functioned for several years. Used the garmin to track speed. Purchased an instrument cluster with 200K miles and took to the dealer and had it installed. Cluster was $60 on eBay and labor was $150 at dealer. I am so pleased to report that so far all is well. I now have a working speedometer and odometer.