Choose Correct Diode

I had to do some creative wiring on my car’s A/C to get it working without a $1000 repair bill. Now the A/C works perfectly, but I need to isolate the current on one of the wires to so that it only flows one direction.

A manual switch is wired between the negative battery terminal and the A/C relay, with a second wire between the A/C relay and the Fan 2 relay. This is because the condenser coil would overheat and blow the high pressure valve unless I keep constant air-flow across the condenser coil while the A/C is running.

The downside is, when the A/C is off and the engine is hot enough for the fans to run, the short wire between the Fan 2 relay and A/C relay carries the Fan 2 current to the A/C relay and the A/C compressor kicks on too. If I add a diode inline in the short wire between the relays, then it will keep current from flowing backwards when only the fans should be running.

The wire carries 10 Amps at 12 Volts. Since there is a 10 Amp fuse on the circuit, is it okay to install a diode that is rated higher on Amps and Volts? For instance, a 15 or 20 Amp, 45 Volt diode? Or do I have to match the diode’s specs exactly to the circuit?

What are the consequences of using a 15 or 20 Amp, 45 Volt diode on a 10 Amp, 12 Volt circuit?

Edited – 2020.09.11 (added diagram and outcome)

The Amps pulled across the jumper wire between the A/C relay and the Fan 2 relay is only 0.4 Amps. So the power dissipation is insignificant. The DIY wiring is like this:

Diagram of New Wire                           ╔═════════╗
                                              ║dashboard║
┌───────────────────────┬─────────────────────║ switch  ║──────────[neg battery terminal]
|                       |                     ╚═════════╝
|       A/C Relay       ┴      Fan 2 Relay
|     ╔════════════╗    ▲     ╔════════════╗
└─────║→ 1─load─2  ║    |     ║  1─load─2  ║
      ║  3──/ ──5  ║    └─────║→ 3──/ ──5  ║
      ╚════════════╝          ╚════════════╝
                        ↑
                  Inline Diode
                  on wire with
                  0.4 Amp draw

A 20SQ045 diode, sinked to the car body with plumber’s tape, worked perfectly. The temperature increased from 90°F when the A/C was off, to 97.5°F after running the A/C constantly for 5 minutes.

When the A/C is on (via the new manual switch), the radiator fans also always run. When the A/C is off, and the engine temp is high enough to kick the fans on, it no longer kicks the A/C compressor on.