NC Starter Motor

Technical Area Engine, Transmission, Exhaust etc
2

Reported starter motor problems seem to be common on the MX5 NC and are typically shown up by the starter relay fuse blowing.

The starter solenoid does three things:-

  • Push the starter gear into mesh with the starter ring.
  • Rotate the motor slowly through a separate winding (thereby assisting meshing of the gear).
  • Closes contacts within the starter to connect the battery directly to the starter motor.
A higher than normal current through the starter solenoid part of the circuit could be caused by:
  • Low battery voltage to the solenoid.
  • Stiffness in the gear sliding engagement
  • Stiffness in the rotation of the motor
  • Failure of the insulation (or a short to earth) within the motor secondary winding (i.e. that fed via the solenoid).
  • A short to chassis in the cable between the relay and the motor solenoid.
There are many vehicles on the road with the same Mazda-Ford MZR-L engine fitted to the NC (and some with the same starter motors) but a trawl through their respective Owners Forums did not reveal any particular problems. From Wikipedia they are:-
  • 2002-2012 Mazda6 for Europe
  • 2004-2018 Mazda Premacy/Mazda5
  • 2000-2007 Ford Mondeo
  • 1998-2010 Ford Focus
Looking at the wiring diagrams for some of these the MX5 is the only one with a 20 amp fuse in the starter relay circuit. Ford Mondeo and Focus cars with the MZR-L engine and a similar (1.4kW) starter motor have a dedicated 30 amp fuse in the starter relay circuit. The Mazda 6 (which has the MZR-L engine and an identical starter to the NC) has a 40 amp fuse (although this protects some other circuits as well).

My meter only goes up to 10 amps but I have measured the solenoid resistance at about 0.9 ohms which would indicate 12 - 15 amps at 12 volts. Peak current could higher.

In my case after blowing several 20 amp fuses and not looking forward to changing the starter I inserted a 25 amp fuse and the car has been OK since!

So perhaps the starter motor problems on the NC are due to the 20 amp fuse being a bit too low for the Mazda MX5 – particularly as our cars get a bit older?