So: i did. Interesting purchase!
With help of your suggestions and the interwebs have learned the following:
- battery is okay: 12,4V
- large terminal on solenoid is permanent 12,4 V
- engine earth is okay: 0V
- white/yellow connector on the solenoid is the one that becomes live. Only: between battery-earth and white/yellow connector only 8,9V when ignition is turned completely. But also: the blue one also does read a bit: 4,7V with ignition turned.
- startermotor-earth reads 0,6V when connected to battery-earth and ignition turned completely: should be 0 or close to 0 right? I'll take off the small earth-lead and clean the terminals tomorrow, see if this makes a difference.
- at the starterrelay i measure 9,9V between W1 and W2 in stead of 12,4 V with, again, ignition turned completely: possibly it ís a problem with the earth of the relais? Is it possible to, by way of test, replace the earth of the relay and let the connection to the brake-fluid reservoir be, or does something terrible happen then?
If solved earth of relais and solenoid don't solve the starting issues, i'll replace the startermotor with solenoid and see what happens.
Thanks again!