So I was driving at night and it was raining so I had headlights on, wipers on and the fan running. Suddenly the ignition light came on and the ammeter indicated that we were now discharging. This remained the same all the way home even driving on the motorway at 2500rpm. This is not the first time this has happened. Usually the next morning all is fine. At one point I did replace the control box with an aftermarket RB340.

My car is positive earth and still has a dynamo. As it is the 3.8 should I have a 30 amp RB340 and are the regulators different for positive and negative earth?

 

I don't think so, they are just re-polarized for either Pos+.or Neg- ground.

 

Mk2 3.8 '64 Dynamo not charging when engine hot - Jaguar Forums - Jaguar Enthusiasts Forum https://share.google/iLpbLIc2HI37APwCO

Rather than repeat what's been discussed before, I'll point you to the link above. It's not an uncommon problem and usually not anything complicated to fix. However, it may be a long process of checking and cleaning connections and contacts. Also, modern mechanics don't have the first idea about adjusting a mechanical control box though it's well described in the workshop manuals

 

When I disconnected the D terminal the wire separated from the spade connector with very little effort. It was corroded through even though the wiring loom is still pretty new. Anyway there is a thick and a thin wire connected to the D terminal. I stripped them back, tined them properly, clamped them back in the spade connector and the soldered the wires to the connector. After that the ignition light went out and all seems well. We will see!