Fan missing "low" speed

[gwolfski]

Get the pitchforks out, it's not an XM!

It's a 2006 Toyota Avensis with an oveheating issue. This model (from what the workshop manual says, anyway) shoud have a dual-speed fan. The wiring for low-speed operation is missing entirely, which leads to the car overheating in traffic. The engine itself is fine, but the starter, which I suspect is a cheap off-brand replacement, hates it, and will refuse to operate when too hot (I believe the solenoid on it is the cause, it does not "click", it does get power ok). In situations with airflow (speeds above 30mph) it is fine.

After looking at the diagram of how it "should" be, and relocating some components to minimise the extent of wire splicing, I have come up with this diagram. Does it look plausible?

fan1.png

I may wire "low speed" to a physical switch, or the headlights or something, as I do not want to further hack the AC wiring on this car, as it is poor. (the car cost 800€ and it's a stop-gap vehicle, I'm surprised it lasted this long)

[Dieselman]

Your low speed relay is in parallel with the high speed one, both being controlled by the Ecu, so irrespective of the A/c switch being open, or closed, the fan will run at high speed.

Instead of the A/c switch being in the load circuit, install it to the relay switched circuit so that whenever the A/c is on the fan runs at low speed and the high speed is controlled by the Ecu.

If there are twin fans you could do away with the fan resistor and have a changeover relay that switches the fans in series for low speed or parallel for high speed, the same as an Xm.
Ecu not calling for cooling, changeover relay switched to series connection, A/c switch switching the main power relay.
Ecu calling for cooling, relay switched to parallel connection and either A/c, or Ecu activating main power relay.

You may need to add some diodes to stop the A/c switch interfering with the Ecu, but maybe not as they are both Gnd shunts.

[gwolfski]

Some good points there!

From what I can understand of the current wiring, the ECU can only call for "high" speed. There may be an unused position for "low", but I'd rather not pull the ECU wiring apart while it still works fine. It only calls for "high" when things are pretty toasty.

What I intended for it to do, is that the first relay (fan on-off) would be pulled in both by the AC switch, and the ECU. The relay in parallel with the resistor is pulled in by the ECU when it calls for high speed. The last relay (right hand side) is to control a negative signal using a positive trigger

A dual fan setup might be a good idea, I know of a scrapyard full of toyotas, and the higher trim levels did have a dual fan setup that uses the same mounting. Thanks for the idea!

The temperature gauge is... of limited use. It will show if the engine is too cold, but once the engine temperature is in the "operational" range (which for some reason includes mild overheating) the needle will stay stuck solid in the center. I've never seen it move above that, even when the car got to the point of "overflow drain dripping boiling water". It feels like the function of the gauge is more decoration that utility.

[Dieselman]

I've roughed out a drawing of how you could control this with only two relays.

[gwolfski]

I've roughed out a drawing of how you could control this with only two relays.

That's much simpler! I think I'll go this route when I get the parts

[Dieselman]

If you want to place the relays in the positive power feed, turn the diagram upside down.
You would need to ensure the common contact of each relay is closest to the fan motor and the resistor is in the positive feed, either before or after the relay and create a second positive feed.

The trigger side of the relays would stay as depicted.