Testing the regulator – part 2

Having tested the regulator and experienced the problem of the power being generated only hitting 13 volts at full revolutions and having replaced the batteries (they were faulty as a result of the previous electrical issues and the rapid discharge) there was only one option remaining: replacing the field generator coils.

Unfortunately to do that the only option is to source another Dynastart cover or have the coils re-wound by a specialist.

Luckily the chairman of the UK Bella club had another R204 cover lying around and he sent it through to use for testing (thanks Brian!!)

When the cover arrived, the first thing was to test the generator coils. My current coil’s resistance:


As you can see this is 1.0 Ohm. You can measure this by taking the yellow wire that leads to the field coils and putting another probe on the 61 connection on top.

A good value should be between 4.5 and 5.7 Ohm according to the Isetta club (see under documentation section in this site). So what does the new cover read:


A bit hard to see in the photo, but a very healthy 5.1 Ohms. This is promising. Next was moving the carbon brushes and the new points I had installed to the replacement cover and setting the ignition. Once done I tested the starter coils; working great. A quick start of the engine showed that the red light disappears much sooner with the new coils and seems ‘back to before the problem’:

However, the previous problem has now re-surfaced in that the 16 amp fuse blew quickly and voltage is too high at the regulator output. Here’s a video of where you can see when the fuse blows measured from 61 to earth 13 seconds into the video:

It’s clear that the fuse blows at 19 volts and the fuse holds at revolutions below that which generators 19 volts or more. Here’s another video measured at battery plus to earth:

So clearly the regulator is not working again, or something else is causing this problem.