1994 850 not turbo with 275,000 km. Problem started yesterday normal city driving conditions. Speedo occasionally drops to zero for a short period then pops back up to the correct speed. When at zero no other warning lights (eg ABS) and odometer and trip odometer keeps working. No codes from OBD 1. Checked electrical connectors at ABS, pulled and cleaned speed sensor on transmission, checked wiring under dash at block 24/13 and in particular pins 33 and 34. Also, reracked the transmission and EFI modules in the box at passenger side front under the hood, but couldn't find block 25/15 which is supposed to be in the same vicinity (to check pins 5 and 6). Have yet to pull the combined instrument from the dash (have done it before to change light bulbs) (it's a Yasaki). I get the feeling is might be something on the printed board to do with only the speedometer and nothing else. Alternatively, I have seen other posts which indicate it might be in the ABS module (although no warning lights come on when the speedo goes to zero). Any help would be appreciated.
(my other car is a 61 P1800 and much easier to work on!)

 

yes the signal does go through the ABS computer, which are known to develop cold solder joints that open up. Check out Victor Rocha's web page/repair service (there are others but he's very helpful on this whole topic). The ABS computer bolts onto the pump so you can try swapping out without removing brake lines etc. take a look:

RochaTek-ABS Repair

 

Highly doubt it is the ABS unit itself on a 94. 96 and up is where they switched to a different ABS manufacturer, and that is when all the problems started (including other models that were later made).

I would look at the wheel speed sensors above the wheel hub on each wheel. Since these cars are so old, the wires will break or the sensor itself will disintegrate from age. My ABS/Trac light was on due to one of my wires on my front right wheel that was snapped (95 T-5R).

 

I still stand by my comment on your ABS unit, but I would ignore the wheel speed sensor comment . That really is only related to the ABS unit.

Form what I have read the problem is mostly related to the sensor on the transmission as that is what controls the speedometer, and maybe the actual cluster. I would check that sensor again, cleaning it probably wouldn't tell you much. It is fairly cheap too, if you want to just go ahead and rule that out.

 

I have been looking at the wiring diagrams for the ABS and the instrument cluster on a 94 850 from the Volvo Service Manual, Section 3(39) ((TP3903202). The ABS diagram does not have a speed sensor (item 7/33) anywhere on it. The Instrument cluster diagram shows the two wires from points 1 and 2 (Y-BN and GN-Y) going through junctions 24/13 and 24/15 to the speed sensor with no offshoots or connections anywhere near the ABS systems. As in my first post, there is no ABS light on at any time, no DTCs and the ABS is working normally. also - when the speedo needle drops to zero, the odometer and trip odometer keep working - still getting speed sensor inputs? I'm going to pull the instrument cluster and check inside.

 

To close the loop. Took the instrument cluster out of the dash. The wire positions in the A block in our car are nothing like the wiring diagrams but I was able to find the two for the speedo. Took the thing apart and remove the speedometer assembly. Two little motors take input from the speed sensor, one drives the odometer/trip odometer and one drives the speed indicator which explains why the odometers kept working while the speedo dropped to zero. The printed board looked good except for some discoloration near the top where two capacitors attach. Took their numbers just in case. Sprayed contact cleaner on all connections. Put it all back together. It has been running fine for four days. If it happens again I'll replace those two capacitors. End of story.
Bob