1991 MAZDA MIATA engine

Drain coolant. Disconnect heater hoses at engine compartment firewall. Remove grommets from holes (if
equipped). Remove instrument panel. Remove heater case. Disassemble case and remove heater core. See Fig.
1 . To install, reverse removal procedure. Fill cooling system.
INSTRUMENT PANEL
Removal & Installation
Obtain code number and deactivate audio anti-theft system. Disconnect negative battery cable. Remove all
components in order listed in illustration. See Fig. 3
. To install, reverse removal procedure.
Fig. 3: Exploded View of Instrument Panel

Courtesy of MAZDA MOTORS CORP.
WIRING DIAGRAM

1991 Mazda MX-5 Miata
HEATER SYSTEM 1990-92 Heater Systems
Microsoft
Sunday, July 05, 2009 2:12:14 PMPage 5 © 2005 Mitchell Repair Information Company, LLC.

A/C-HEATER SYSTEM - MANUAL
1990-92 Manual A/C-Heater Systems
A/C SYSTEM SPECIFICATIONS
A/C SYSTEM SPECIFICATIONS
DESCRIPTION
Blower case, mounted under right end of instrument panel, contains blower motor and fresh/recirculated air
door. Evaporator case, to left of blower case, contains evaporator, evaporator thermoswitch and expansion
valve. Heater case, to left of evaporator case, contains airflow mode door and air-mix (temperature blend) door.
ELECTRICAL COMPONENT LOCATIONS
OPERATION
All air control doors are controlled manually by cable from control panel. Top lever controls the air-mix door,
center lever controls the fresh/recirculated air door and bottom lever controls the airflow mode door.
ApplicationSpecification
Compressor TypeNippondenso Rotary Vane
Compressor Belt Deflection (1) (2)
New5/16-23/64" (8-9 mm)
Used23/64-25/64" (9-10 mm)
Compressor Oil Capacity2.7-3.3 oz.
Refrigerant (R-12) Capacity28 oz.
System Operating Pressures
High Side171-235 psi (12.0-16.5
kg/cm
2 )
Low Side21-43 psi (1.5-3.0 kg/cm2 )
(1)With 22 lbs. (10 kg) pressure applied to center of belt.
(2)On vehicles with P/S, measure midway between crankshaft pulley and P/S pump pulley. On vehicles
without P/S, measure midway between crankshaft pulley and A/C idler pulley.
ComponentLocation
A/C RelayIn Right Front Corner Of Engine Compartment
Blower Motor RelayIn Left Rear Corner Of Engine Compartment
Blower ResistorOn Bottom Of Blower Case
Evaporator Thermoswitch. On Left Side Of Evaporator Case
Pressure SwitchIn High-Pressure Line, Between Receiver-Drier
And Evaporator

1991 Mazda MX-5 Miata
A/C-HEATER SYSTEM - MANUAL 1990-92 Manual A/C-Heater Systems

1991 Mazda MX-5 Miata
A/C-HEATER SYSTEM - MANUAL 1990-92 Manual A/C-Heater Systems
Microsoft
Sunday, July 05, 2009 2:11:52 PMPage 1 © 2005 Mitchell Repair Information Company, LLC.
Microsoft
Sunday, July 05, 2009 2:11:56 PMPage 1 © 2005 Mitchell Repair Information Company, LLC.

Blower motor relay supplies power to blower motor. Blower resistor determines blower speed. With blower
switch in highest position, blower motor is grounded through blower switch. With blower switch in all other
positions, blower motor is grounded through blower resistor and blower switch.
Circuit through compressor clutch is completed when A/C relay is energized and pressure switch is closed. A/C
relay is energized when Engine Control Unit (ECU) unit grounds the solenoid circuit of the relay. The ECU
energizes A/C relay if evaporator thermoswitch is closed and A/C and blower switches are on. The ECU also
controls A/C relay operation according to engine load.
ADJUSTMENTS
AIRFLOW MODE DOOR
Set airflow mode control lever to vent position. Disconnect control cable at airflow mode door lever. Extend
airflow mode door lever until it stops, and hold it there. See Fig. 1
. Attach control cable to door lever and clip.
Ensure control lever moves freely between defrost and vent positions.
AIR-MIX (TEMPERATURE BLEND) DOOR
Set temperature control lever to maximum hot position. Disconnect control cable at air-mix door lever. See Fig.
1 . Extend air-mix door lever until it stops, and hold it there. Attach control cable to door lever and clip. Ensure
control lever moves freely between hot and cold positions.
RECIRCULATED/FRESH AIR DOOR
Set recirculated/fresh air control lever to fresh position. Disconnect control cable at recirculated/fresh air door
lever. Retract recirculated/fresh air door lever until it stops, and hold it there. Attach control cable to door lever
and clip. Ensure control lever moves freely between recirculated air and fresh air positions.
TESTING
AIR BAG WARNING
BLOWER MOTOR CIRCUIT
1. Check 30-amp HEATER circuit breaker in passenger compartment fuse panel. If Red button has not
popped out, go to next step. If Red button has popped out, repair short circuit and press Red button to
reset circuit breaker.
2. Turn ignition on. Turn blower switch to 4th position (high). Check voltage at Blue wire terminal of 2-
wire blower motor connector on bottom of blower case. If no voltage is present, repair wiring between
circuit breaker and blower motor. WARNING:To avoid injury from accidental air bag deployment, read and carefully
follow all SERVICE PRECAUTIONS and DISABLING & ACTIVATING AIR
BAG SYSTEM procedures in AIR BAG article in the
ACCESSORIES/SAFETY EQUIPMENT Section.

1991 Mazda MX-5 Miata
A/C-HEATER SYSTEM - MANUAL 1990-92 Manual A/C-Heater Systems
Microsoft
Sunday, July 05, 2009 2:11:52 PMPage 2 © 2005 Mitchell Repair Information Company, LLC.

3. If battery voltage is present, turn off blower switch and A/C switch (if equipped). Check voltage at
Blue/Red wire terminal of blower resistor 1-wire connector. If no voltage is present, replace blower
motor. If battery voltage is present, check voltage at Blue/White wire terminal of blower resistor
connector.
4. If no voltage is present, replace blower resistor. If battery voltage is present, check voltage at Blue/Red
wire terminal of blower resistor 4-wire connector. If no voltage is present, replace blower resistor.
5. If battery voltage is present, check voltage at Blue/Green wire terminal of blower resistor connector. If no
voltage is present, replace blower resistor. If battery voltage is present, check voltage at Blue/Yellow wire
terminal of blower resistor connector.
6. If no voltage is present, replace blower resistor. If battery voltage is present, turn ignition on and blower
switch to 4th position (high). Check voltage at Black wire terminal of blower switch connector. If battery
voltage is present, repair wiring between blower switch and ground.
7. If no voltage is present, turn ignition switch, blower switch and A/C switch off. Check voltage at
Blue/White wire terminal of blower switch connector. If no voltage is present, repair wiring between
blower resistor and blower switch.
8. If battery voltage is present, check voltage at Blue/Red wire terminal of blower switch connector. If no
voltage is present, repair wiring between blower resistor and blower switch. If battery voltage is present,
check voltage at Blue/Green wire terminal of blower switch connector.
9. If no voltage is present, repair wiring between blower resistor and blower switch. If battery voltage is
present, check voltage at Blue/Yellow wire terminal of blower switch connector. If battery voltage is
present, replace blower switch. If no voltage is present, repair wiring between blower resistor and blower
switch.
COMPRESSOR CLUTCH CIRCUIT
1. Turn ignition on. Check 20-amp WIPER fuse in passenger compartment fuse block. Check 20-amp AD
FAN (additional fan) fuse in engine compartment fuse block. If fuses are blown, repair short circuit.
2. If fuses are okay, run engine at idle. Turn A/C and blower switches on. Check voltage at compressor
clutch connector (Black/Red wire terminal). If no voltage is present, go to step 5).
3. If battery voltage is present, turn ignition off. Disconnect evaporator thermoswitch connector. Check
continuity between switch terminals. If there is no continuity, replace evaporator thermoswitch.
4. If there is continuity, disconnect compressor clutch connector. Set ohmmeter to X1000 scale. Check
continuity between compressor clutch connector and ground. If there is no continuity, replace compressor
clutch. If there is continuity, adjust compressor clutch air gap or check compressor for internal damage.
5. Run engine at idle. Turn A/C and blower switches on. Check voltage at Black/Blue wire terminal of A/C
relay connector. If battery voltage is present, go to step 9).
6. If no voltage is present, check voltage at Blue/Yellow wire terminal of A/C relay connector. If no voltage
is present, repair circuit between 20-amp AD FAN fuse and A/C relay.
7. If battery voltage is present, check voltage at Blue wire terminal of A/C relay connector. If no voltage is
present, repair circuit between 20-amp WIPER fuse and A/C rela
y. NOTE: Blower resistor uses a 1-wire connector and a 4-wire connector. The wire
in the 1-wire connector is Blue/Red, and one of the wires in the 4-wire
connector is Blue/Red. Check voltage at appropriate wire.

1991 Mazda MX-5 Miata
A/C-HEATER SYSTEM - MANUAL 1990-92 Manual A/C-Heater Systems
Microsoft
Sunday, July 05, 2009 2:11:52 PMPage 3 © 2005 Mitchell Repair Information Company, LLC.

8. If battery voltage is present, check voltage at Blue/Black wire terminal of A/C relay connector. If no
voltage is present, replace A/C relay. If battery voltage is present, check Engine Control Unit (ECU)
terminal voltage.
9. Turn ignition off. Disconnect pressure switch connector. Check continuity between switch terminals. If
there is continuity, repair circuit between A/C relay and compressor clutch.
10. If there is no continuity, connect manifold pressure gauges to system. If low pressure is 21-43 psi (1.5-3.0
kg/cm
2 ) and high pressure is 171-235 psi (12.0-16.5 kg/cm2 ), replace pressure switch. If pressures are
not as specified, check refrigerant system for proper charge.
CONDENSER (ADDITIONAL) FAN CIRCUIT
1. Turn ignition on. Turn A/C and blower switches on. Check voltage at Black/Blue wire terminal of
condenser fan connector. If no voltage is present, repair circuit between A/C relay and condenser fan.
2. If battery voltage is present, check voltage at Black wire terminal of condenser fan connector. If no
voltage is present, replace condenser fan motor. If battery voltage is present, repair circuit between
condenser fan and ground.
A/C & BLOWER SWITCH
With switches in positions indicated, check continuity between specified terminals of A/C and blower switch
connector. See Fig. 1
. If continuity is not as specified, replace A/C and blower switch.

1991 Mazda MX-5 Miata
A/C-HEATER SYSTEM - MANUAL 1990-92 Manual A/C-Heater Systems
Microsoft
Sunday, July 05, 2009 2:11:52 PMPage 4 © 2005 Mitchell Repair Information Company, LLC.

BLOWER MOTOR
Disconnect 2-wire connector from bottom of blower motor case. Apply battery voltage across blower motor
terminals. Replace blower motor if it does not operate.
BLOWER RESISTOR
Note 4-wire connector and 1-wire connector attached to seam where upper and lower blower cases meet. Wires
in these connectors lead to blower resistor on bottom of blower case. Disconnect both connectors. If continuity
does not exist between 1-wire connector terminal and each terminal of 4-wire connector, replace blower
resistor.
CONDENSER (ADDITIONAL) FAN MOTOR
Disconnect condenser fan connector. Apply battery voltage across condenser fan terminals. Replace condenser
fan motor if it does not operate.
EVAPORATOR THERMOSWITCH
1. Remove glove box. Run engine at idle. Turn off A/C switch. Turn blower switch to 4th position (high) for
a few minutes to ensure evaporator temperature is greater than 32°F (0°C). Turn off blower switch and
engine.
2. Disconnect evaporator thermoswitch connector. Check continuity between switch terminals. If there is
continuity, thermoswitch is okay. If there is no continuity, replace thermoswitch.
PRESSURE SWITCH
1. Turn ignition off. Connect manifold pressure gauges to system. If high-side pressure is less than 31.2 psi
(2.2 kg/cm
2 ), check system for proper refrigerant charge.
2. If high-side pressure is greater than 31.2 psi (2.2 kg/cm
2 ), disconnect pressure switch connector. Check
continuity between switch terminals. If there is continuity, switch is okay. If there is no continuity,
replace switch.
REMOVAL & INSTALLATION
AIR BAG WARNING
BLOWER CASE
Removal & Installation
WARNING:To avoid injury from accidental air bag deployment, read and carefully
follow all SERVICE PRECAUTIONS and DISABLING & ACTIVATING AIR
BAG SYSTEM procedures in AIR BAG article in the
ACCESSORIES/SAFETY EQUIPMENT Section.

1991 Mazda MX-5 Miata
A/C-HEATER SYSTEM - MANUAL 1990-92 Manual A/C-Heater Systems
Microsoft
Sunday, July 05, 2009 2:11:52 PMPage 6 © 2005 Mitchell Repair Information Company, LLC.

Disconnect electrical connectors from blower case. Remove glove box. Loosen seal plate between blower motor
case and evaporator case. Remove blower motor case nuts. Remove case and seal plate. To install, reverse
removal procedure.
COMPRESSOR
Removal & Installation
1. Disconnect negative battery cable. Discharge A/C system using approved refrigerant recovery/recycling
equipment. Disconnect compressor clutch connector. Disconnect refrigerant lines from compressor.
Remove drive belt.
2. Raise and support vehicle. Remove compressor mounting bolts and remove compressor. To install,
reverse removal procedure. Adjust drive belt to specified tension. Evacuate and charge system.
CONDENSER & RECEIVER-DRIER
Removal & Installation
Discharge A/C system using approved refrigerant recovery/recycling equipment. Raise and support vehicle.
Remove splash shield and air guide. Remove condenser and receiver-drier as an assembly. To install, reverse
removal procedure. Evacuate and charge system.
CONTROL PANEL
Removal & Installation
Remove rear console assembly. See Fig. 4 . Remove vent outlets from center panel assembly. Remove center
panel assembly. Remove control panel screws. Pull control panel from hole. Disconnect door control cables
from back of control panel and remove panel. To install, reverse removal procedure.
EVAPORATOR CORE & THERMOSWITCH
Removal & Installation
1. Discharge A/C system using approved refrigerant recovery/recycling equipment. Remove condenser and
receiver-drier as an assembly. Disconnect low-pressure and high-pressure pipes from evaporator tubes at
engine compartment firewall.
2. Remove right undercover and glove box. Disconnect thermoswitch electrical connector. Loosen left seal
plate (between heater case and evaporator case). Loosen right seal plate (between evaporator case and
blower motor case).
3. Remove evaporator case. Disassemble evaporator case, and remove evaporator core and thermoswitch.
See Fig. 3
. To install, reverse removal procedure. Evacuate and charge system.

1991 Mazda MX-5 Miata
A/C-HEATER SYSTEM - MANUAL 1990-92 Manual A/C-Heater Systems
Microsoft
Sunday, July 05, 2009 2:11:52 PMPage 7 © 2005 Mitchell Repair Information Company, LLC.

Fig. 3: Exploded View of Evaporator Case
Courtesy of MAZDA MOTORS CORP.
HEATER CORE
Removal & Installation
Drain coolant. Disconnect heater hoses at engine compartment firewall. Remove grommets from holes (if
equipped). Remove instrument panel. Remove heater case. Disassemble case and remove heater core. See Fig.
1 . To install, reverse removal procedure. Fill cooling system.
INSTRUMENT PANEL

1991 Mazda MX-5 Miata
A/C-HEATER SYSTEM - MANUAL 1990-92 Manual A/C-Heater Systems
Microsoft
Sunday, July 05, 2009 2:11:52 PMPage 8 © 2005 Mitchell Repair Information Company, LLC.