I'm having problems starting my 1994 mk1 mx5 1.8 after making a lot of changes to the car. The starter motor won't engage and when I turn the key to the 'off' position there are dashboard lights that turn on and solenoids clicking in the engine bay (even with the key removed from the barrel)

What is the most likely cause of this? I've changed so much on the car recently and not sure where to start. One thing to note is that the engine is sat inside the car with no gearbox attached (I'm doing the Kmiata BMW gearbox conversion). So the PPF is not connecting the front and rear of the car.

Other things I've changed:

Converted to LS2 Coils
Did VVT swap (new cam and crank sensor wired in)

I've checked the earth strap from behind the dipstick to the car body and that looks ok. There is another earth which would usually connect to the back of the inlet manifold, but I've attached it to the back of the head instead (inlet manifold is powdercoated).

There is another ground at the front of the engine underneath the throttlebody, which is also connected. Am I missing an earth? Does the PPF need to be attached for the starter motor to turn?

 

The negative side of the battery connects to the body and then to the PPF as the main ground point. No transmission, no ppf, no ground.

 

The negative battery cable connects to the chassis maybe a foot or so away from the battery.

I would remove the ignition switch and use a flat screwdriver to crank the engine and use a test light to verify proper wires are energized, just to make sure nothing went out of whack mechanically in the key barrel to switch connection.

Can you crank the engine by supplying 12v to the starter trigger terminal?

 

Right - but the starter motor is going to be sucking amps, which means the pesky sensor ground under the brake booster (on LHD) that everyone says to check are going to be overwhelmed/hot.

It's the PPF ground. And possibly, since he didn't list it, that ground strap to the body.