RX-8 Manual Cooling Fan Switch Wiring
Manual low- and high-speed cooling-fan override wiring for the Series 1 Mazda RX-8 while retaining normal PCM control.
This note applies to the Series 1/pre-facelift RX-8, primarily the 2005 UK RHD high-power 231 PS car.
The cooling fans can start automatically. Disconnect the battery while making connections, and keep clear of the blades whenever the system is live.
Factory fan operation
| Fan mode | Relay No. 1 | Relay No. 2 | Relay No. 3 | PCM control |
|---|---|---|---|---|
| Off | Off | Off | Off | — |
| Low | On | Off | Off | 5X |
| High | On | On | On | 5X + 5AD |
Cooling-fan relay No. 1 runs both fans at low speed. High speed requires relay No. 1 and relays No. 2 and No. 3. The PCM controls relay No. 1 through output 5X, and controls relays No. 2 and No. 3 together through output 5AD.
These are low-side drivers: the PCM energises each relay coil by pulling its control circuit to ground. A manual request should reproduce that ground request through added isolated relay contacts. It must not feed the fan motors or Mazda control circuits from the dashboard switch.
| Condition | Factory fan state |
|---|---|
| Below low threshold, no A/C request | Off |
| Approximately 97 °C | Both fans low |
| Approximately 101 °C | Both fans high |
| Cooling from high | High releases around 98 °C |
| Cooling from low | Low releases around 94 °C |
| ECT failure | High-speed fail-safe |
The main cooling page includes the temperature and hysteresis graph.
Manual LOW-speed switch
The LOW switch manually energises cooling-fan relay No. 1. Its factory control path is:
relay No. 1 coil → relay terminal E/control side → PCM 5X → ground when requested
Leave the original PCM connection intact and add a parallel ground path.
LOW switch off: the added contact is open and the PCM operates the fans normally.
LOW switch on: the added contact grounds the relay No. 1 control node, relay No. 1 energises and both fans run at low speed.
Added control circuit
ignition-switched 12 V → approximately 1 A fuse → LOW dashboard switch
→ added relay coil → ground
Isolated contact
relay No. 1 terminal E / PCM 5X control node
→ added normally-open contact → ground
For a conventional numbered automotive relay, a typical arrangement is:
| Added relay terminal | Connection |
|---|---|
| 85 | Coil ground |
| 86 | Fused switched input from the LOW switch |
| 30 | Ground |
| 87 | Relay No. 1 terminal-E / 5X control node |
Check the pin diagram printed on the actual relay; numbering is not a substitute for identifying its coil and normally-open contact.
Do not apply 12 V to the Mazda relay control node or PCM 5X circuit. Do not cut the factory PCM connection.
Diagram still to add: Manual LOW-speed override. Show cooling-fan relay No. 1, its factory PCM
5Xconnection, the added isolated relay contact to ground, dashboard switch, ignition-switched fused supply and added relay coil.Placeholder path:
/assets/rx8/diagrams/manual-fan-low-speed.svg
Manual HIGH-speed switch
Grounding 5AD alone does not request the complete high-speed state. High speed requires both control circuits grounded:
5Xenergises relay No. 1.5ADenergises relays No. 2 and No. 3.- All three factory relays must be active for both fans to run at high speed.
Do not directly connect 5X and 5AD; they are separate PCM-controlled circuits. Use two electrically isolated normally-open contacts operated by the same HIGH switch.
HIGH switch control circuit
ignition-switched 12 V → approximately 1 A fuse → HIGH dashboard switch
→ added relay A coil and added relay B coil → ground
Separate isolated contacts
relay A: 5X / relay No. 1 control node → NO contact → ground
relay B: 5AD / relay No. 2 and No. 3 control node → NO contact → ground
HIGH switch off: both added contacts are open and factory PCM control is unchanged.
HIGH switch on: relay A grounds 5X, relay B independently grounds 5AD, all three factory fan relays energise and both fans run at high speed.
A suitable dual-pole normally-open relay can provide the two isolated grounding functions, but two ordinary automotive relays are generally easier to identify, source and test.
Diagram still to add: Manual HIGH-speed override. Show separate isolated ground contacts for the
5Xand5ADcontrol circuits. Both contacts are operated by the same HIGH switch. The two PCM control circuits must remain electrically isolated.Placeholder path:
/assets/rx8/diagrams/manual-fan-high-speed.svg
LOW and HIGH switches together
| LOW switch | HIGH switch | Result |
|---|---|---|
| Off | Off | Automatic PCM control |
| On | Off | Both fans low |
| Off | On | Both fans high |
| On | On | Both fans high |
The LOW circuit grounds 5X only. The HIGH circuit already includes the low-speed relay requirement, so it grounds 5X and 5AD through separate contacts. Never make the HIGH switch ground only 5AD.
Optional AUTO / LOW / HIGH selector
| Selector position | 5X manual request | 5AD manual request | Result |
|---|---|---|---|
| AUTO | None | None | Factory control |
| LOW | Grounded | None | Both fans low |
| HIGH | Grounded | Grounded | Both fans high |
A three-position selector is a tidy alternative to two switches. It must not have a position that disables automatic control: the original PCM connections remain intact in AUTO, LOW and HIGH.
P0480 and P0481
The PCM monitors the fan-relay control voltages in both commanded-on and commanded-off states. Manually grounding a circuit can therefore produce a low voltage when the PCM expected it to be inactive.
P0480: cooling-fan relay No. 1 / low-speed control monitoring on5X.P0481: cooling-fan relays No. 2 and No. 3 / high-speed control monitoring on5AD.
Because the PCM monitors these circuits, manually grounding 5X or 5AD can potentially store P0480 or P0481 depending on when the PCM performs its diagnostic checks. Scan for pending and stored codes after testing. Do not disable the monitoring.
Important cautions
- Cooling fans can start automatically; keep clear of the blades with the system live.
- Do not run fan-motor current through a dashboard switch.
- Do not apply 12 V to
5Xor5AD. - Do not directly connect
5Xand5ADtogether. - Do not cut or remove either factory PCM control connection.
- Do not create a manual mode that prevents automatic high-speed or fail-safe operation.
- Verify relay terminals against the Mazda diagram and the actual vehicle before splicing.