The Series 4 Spider instrument cluster is all electric.
On my 1991 Spider, after pulling the cluster, inspecting and cleaning harnesses and connectors fuses and solder joints and pins and removing hot glue from variable resistor pots and turning adjuster screws, I put it all back together and implemented the referenced SB in an attempt to fix my speedometer, which has randomly varied from smooth to erratic operation to nonoperational since I bought the car several weeks ago. For about six miles on the first test drive after complying with the SB, the speedometer worked as designed, then abruptly went dead.
The tach however, completely inoperative since I bought the car, decided to briefly work shortly after the speedo went back to sleep. Then it too became tired and fell lifelessly to the resting peg. I suppose it's one or the other, but not both. If I allow the car time to rest, the speedometer will operate for a few minutes. Or a few miles. Whichever comes first. Then it takes a nap again. The tach remains listless, immobile only until I give it a few solid pod whacks. It jumps a bit in fright with each whack, then cowers against its rest peg, trembling and wondering why I'm abusing it.
Jaeger, a decidedly Teutonic name, does not live up to the myth of German excellence in engineering. The car has been well cared for and the pod, connectors, and harnesses all appear nearly new. The Jaeger family obviously lived in Italy for too many generations prior to deciding to manufacture electronic components for the automotive industry.