Cam;

Welcome to this Forum!

In order to ask for, and get any useful help from any forum, you must first give all relevant info on vehicle...such as model, 6V or 12V, symptoms of problems, any changes from factory stock (if they are known to you), etc...

Furthermore, how did you come to the conclusion of "It needs a coil"? We need details on this also! Ignition Coil is supplied Ign Power by way of the unique Armored Cable, so before wrongly troubleshooting and indicting the coil, I'd recommend you first familiarize yourself with this out-of-the-ordinary Ign Sys with Armored Cable theft deterrent, to be certain of your troubleshooting conclusion.

Finally: "Everywhere else seems to switch off when the starter is engaged." is absolutely normal!( ), and due to Loadshedding in Ign Key Pos 4.

See: http://www.sw-em.com/Volvo%20Ignition%20from%20Scratch.htm#Vintage%20Volvo%20Ignition%20system%20and%20the%20Armored%20Cable

Good Hunting!