Hi folks. Here’s a video showing how to disassemble and repair a Webasto U4840 coolant pump from a Webasto Thermo Pro 90 heater, the hydronic heater used in Garcia Exploration 45 yachts and many others. Regards, Rick.
Reassembly tips.
– apply a small amount of grease or Vaseline between the surfaces of the shaft, the magnetic rotor, the impeller and the two small washers, to minimise wear during startup before coolant reaches the pump and is providing long term lubrication.
– apply a small bead of sealant between the junction of the rotor cup and the outer housing. This will reduce the chance of coolant reaching the electronics if it leaks past the inner o-ring seal. Caution – applying sealant in this way will make future disassembly more difficult.
– if you can, use a new o-ring for the inner seal when reassembling. I measure the inner o-ring as 2mm cross section by 42mm inside diameter. If someone has a friend at Webasto, please confirm. An option would be to use a slightly thicker 2.4mm x 42.2mm o-ring, which should provide a better seal.
– if your repair involves replacing components on the circuit board, remember to apply a protective coating where needed. Clear, non-conductive silicon could be a good choice. 
– consider applying sealant to the rubber seal that sits on the two external, electrical connector terminals, before installing the motor into the outer housing. This should minimise the chance of liquids reaching the electronics through that opening in the housing.