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?

3

In 4 years of MX5 ownership and time spent on here, this is the first I’ve heard of any starter issues on the NC.

However, you’ve raised some very interesting points and facts, so I’ll be watching this thread to gather other peoples input.

4

It could simply be a fuse problem.  It is working too close to its rated current for too long.  Your analysis comparing other similar starter and engine combinations is a good one, and it might suggest the equivalent fuse in the NC is marginal.

However, if the solenoid pull-in is slow, long enough to pop the fuse, then maybe there is a problem. 

Also another possibility; once pulled-in, the contact in the solenoid takes over the power and much less current is supplied by the ST fuse, in theory.  However, it might be a good idea to check the connections of the thick black high current wire from battery to starter at both ends, just in case there is some voltage drop here and the ST fuse is now also supplying some of the starting current!

 

A quick word about fuses.

At first sight one might think a fuse will only blow if its rating is exceeded, and survives if that is not reached; ie a 20A fuse can handle 19.9A and blows at 20.1.  Alas, no, it is not that simple.

Different grades of wire fuse can have different times before failing for the same rated current. A standard fuse is usually rated to take twice its rated current to open in one second (eg a 13A plug fuse), for the same over-current a Quickblow fuse might blow in 0.1 seconds, and an Anti-surge fuse in tens of seconds (eg a Hifi Amplifier fuse).

Most ‘one-time’ fuses including the ones in cars are simple bits of resistance wire that will get hot and eventually melt. However there are a lot of variables in the design, and these are chosen to slow or hasten the rupture, also to withstand the rated applied voltage and prevent a current arc being sustained (eg BS mains fuse).

If the environment is very hot the melt happens sooner, if cold later, so also a well insulated fuse fails quicker because its heat is contained.

If the fuse is under tension, it will fail sooner despite expanding when heating.  These are usually labelled “Quickblow” and might also be stamped “F”

If the fuse incorporated a spring to absorb thermal shock/expansion it will fail later.  These are usually labelled “Delay” or “Anti-surge” and might have a “T”  stamped on them.

The car fuses have a curve in the element, which suggests they have room to flex physically as they expand, and suggests they are more like a delay fuse than a quick blow.  So that 20A fuse might be carrying closer to 40A for a short while!

 

5

Interested in this as the MX5 starter motor is usually bullet proof.

Like the 5 speed gearboxes(MK1 - MK2.5), they sell for more or less scrap value.

I don’t remember hearing of a previous MK3 starter motor issue on this forum either. Historically that may be because the cars were relatively new with only a low percentage of ownership but car now 13 years old and ownership numbers increased exponentially.

I see that the usual auction website has some cheap MK3 starters for sale.  

6

The only starter motor issues that I’ve heard about on the NC are when water enters the cabin through the windscreen grommet, affecting the wiring causing the starter motor to run constantly.

7

Thanks for the comments. When I hit a motoring technical problem I often refer to Google for help. Typing “MX5 NC Starter Motor” threw up a number of issues with starters (in hindsight many on the US Miata site). So perhaps it is not really a “common” problem - sorry.

My problem was that the 20 amp starter relay fuse kept blowing every time i tried to start it and looking at the wiring diagram the only obvious cause must be a fault with the starter.

It was only when I found the price of a starter (£499.99 from Euro Car Parts) and the difficulties in replacing the starter motor (one writer said it took him four difficult hours) that I looked around optimistically to make sure it wasn’t some other cause. My weekday car is a Mondeo Mk3 petrol with the same engine and starting arrangements but I noticed the starter relay has a 30 amp fuse. So I tried a 25 amp fuse and the NC has been OK since. Great relief that the cost of fixing it was 50p and not several hundred pounds.

But my message to NC owners is if you appear to have a starter motor failure try a slighty higher ampage fuse before writing the car off.

8

This should have gone with my previous posting but including it exceeded my number of characters.

9

Sorry.

The Forum won’t let me add a wiring diagram. It disappears when I post. Try this link.

https://forum.miata.net/vb/showthread.php?t=621900

 

Regards

10

 

One thing :slight_smile:

The starter solenoid on a pre engaged starter does 2 things 

  1. Engages the pinion gear in the ring gear 

  2. Pulls main battery live contacts in and spins starter motor 

theres no seperate winding to turn motor slowly the engagement is done purely in one hit with a leading edge on the pinion gear teeth and ring gear teeth.

 

 

11

The n/c starter motor is usually very good, however we have changed the odd one. 

We recently did one for a guy from York…remember they are now 12 yrs old