Mazda MX-5 Miata / Grand Touring. Manual - part 42

9. Turn the following electrical loads on and verify that the current reading increases more than the 

minimum value indicated below. 



If it is not as specified, go to the PCM AND GENERATOR SHEARING INSPECTION.  



Headlights (high-beam)  



Blower motor (HI)  



Rear window defroster  



Brake light  

Generator generated current minimum value  

70% of the nominal output current (nominal output current: 100 A)  

[ambient temp. 20°C {68°F}, voltage 13.0-15.0 V, both engine and generator are hot]  

PCM AND GENERATOR SHEARING INSPECTION 

1. Connect the M-MDS or equivalent to the DLC-2. 

will not be verified although the generator is normal. In this case, 
increase the electrical load (by turning on the headlight and leave it 
for a while, then discharge the battery or by similar methods) and 
recheck.  



When the generator itself or the ambient temperature are too high, 
the specified current cannot be verified. In this case, cool down the 
generator and recheck.  

 

2008 Mazda MX-5 Miata Grand Touring 

2008 ELECTRICAL Charging System - MX-5 Miata

  

Fig. 9: Locating DLC-2 Connector 
Courtesy of MAZDA MOTORS CORP. 

2. Inspect as follows:  

PCM AND GENERATOR SHEARING INSPECTION TABLE 

Step

Inspection

Action

1 Measure the generator terminal B voltage when the electrical loads 

(1)

 are 

on and off.

15 V 
or 
more

Go to Step 2.

13-15 
V

Normal 

(2)

13 V 
or less

Go to Step 3.

2

Monitor the ALTT V PID using the M-MDS or equivalent, or measure 
the voltage of PCM terminal 2AJ using a tester. Is the voltage between 13 
and 15 V?

Yes

Go to Step 4.

No

PCM input error.

3

Monitor the ALTT V PID using the M-MDS or equivalent, or measure 
the voltage of PCM terminal 2AJ using a tester. Is the voltage between 13 
and 15 V?

Yes

Go to Step 5.

No

PCM input error.

Monitor the ALTF PID using the M-MDS or equivalent, or calculate the 
duty value of the PCM terminal 2AI using an oscilloscope. Is the duty 

Yes

PCM input error.

 

2008 Mazda MX-5 Miata Grand Touring 

2008 ELECTRICAL Charging System - MX-5 Miata

  

GENERATOR INNER PARTS 

Rotor 

1. Measure the resistance between the slip rings using a tester. 



If not within the specification, replace the rotor.  

Generator rotor resistance (between slip rings) [20°C {68°F}]  

2.0-2.3 ohm  

4

value 100%?

No

PCM, generator, 
or both are not 
normal.

5

Monitor the ALTF PID using the M-MDS or equivalent, or calculate the 
duty value of the PCM terminal 2AI using an oscilloscope. Is the duty 

value 0%?

Yes

PCM input error.

No

PCM, generator, 
or both are not 
normal.

(1)

Headlights, blower motor, rear window defroster, and brake light.

(2)

If the generator field coil duty value does not change when electrical loads (such as headlights, 
blower motor, rear window defroster, brake light) are on or off, inspection with discharged battery is 
needed.

 

2008 Mazda MX-5 Miata Grand Touring 

2008 ELECTRICAL Charging System - MX-5 Miata

  

Fig. 10: Measuring Resistance Between Slip Rings Using Tester 
Courtesy of MAZDA MOTORS CORP. 

2. Verify that there is no continuity between the slip ring and core using a tester. 



If there is continuity, replace the rotor. 

 

2008 Mazda MX-5 Miata Grand Touring 

2008 ELECTRICAL Charging System - MX-5 Miata