So, I put that 300 Ohm resistance in and then it was time to check RPM. Fine at idle but what about higher speed/RPM.
In a Service Bulletin K-E Målberg found this. Here’s the gear/speed chart for an early P1800. B18D is the P120/PV sport versions of the P1800’s B18B, should be exactly the same in this context.
And it did. It follows the curve fine. I am not used to see the Tacho so stable and down at 1500-3000 but that’s where it is now. In all, a very simple and cheap fix for a problem I thought meant a lot of expense and the need to send the Tacho away.