November 2011: In recent weeks Blake Thornton and Michael Field have contacted me about coil-boost starting systems - Blake with a problem with the factory starter relay on a Jaguar, and Michael with an after-market geared starter that doesn't have the coil boost contact that his V8 should have, and for various reasons was causing starting problems.
Many of these after-market starters don't have the coil-boost contact on the solenoid as fitted originally to rubber-bumper cars and all V8s (the one I had for a while didn't). Whilst starting should be OK under most normal conditions, under marginal conditions it can make the difference between starting and not starting, and it's possible to reprovide the coil boost in a number of ways.
One way is to provide an additional auxiliary starter relay which operates in tandem with the original relay and connects 12v to the coil +ve (white/light-green on 4-cylinder cars, white/light-blue on V8), and a readily available accessories relay from all the usual suspects will do this as shown here:
Alternatively you can be a bit more creative and replace the existing starter relay with one with dual make contacts. However these are less readily available and you must ensure the contacts operate as described before connecting and using the relay. The modern 'cube' relay type that can be used must have an 87b terminal, in addition to the usual 85, 86, 30 and 87 terminals. However there is also a Lucas 22RA type in a metal can that would look more in keeping as a replacement for the 6RA type, and these have a C4 terminal in addition to the W1, W2, C1 and C2 terminals. This type of relay was used as standard equipment on some Jaguars and Triumphs for exactly this purpose. However like the 6RA type used on the MGB the 22RA is a generic designation and a five-digit number is used to describe the exact winding and contact specification. From what I've been able to discover the required relay is a 22RA 33336, sources for which can be found on Google, but as I say you must be sure the contact functions are correct before using the relay. With the relay released C1, C2 and C4 should all be isolated from each other. When the relay is operated all three contacts should be connected together. Nothing else will do. Connect the cube-type relay as shown here, for the 22RA type white/red goes to W1, black to W2, brown to C2, white/brown to C1, and white/light-green (white/light-blue for V8s) to C4:
Finally you can get quite exotic and use a 10 amp diode between the solenoid operate wire and the coil boost wire down by the solenoid, but really need to know what you are doing with semiconductor diodes for this option. Basically you want current to flow from the solenoid operate wire to the coil boost wire, but not the other way. For that, typically the marked end of an axial diode, is the positive end and would be connected to the coil boost wire, the negative end to the somenoid wire, as shown here:
