Electrical problem
1987 Range Rover V8 Four Wheel Drive Manual

My RR won't start and I have traced the problem to the dizzy. There is no spark at the coil when engine is cranked over. I have replaced the electronic pickup in the dizzy and still no spark. I took the dizzy out of the engine and turned it over and what do you know I get a coil spark. Put the dizzy back into the engine and again its dead. How can the distributor earth out when back in the car? What am I missing? Have replaced leads,coil and amplifier previously.
Any help appreciated