A/C Amplifier Location

Both fans should always run together. The relays switch the circuit around in a clever way, so both fans either run at low speed (they're electrically in series) or both at high speed (they're in parallel).

Roughly, they run in low any time the A/C is on, or if the engine needs some cooling, and in high if the engine needs more cooling or the A/C line pressure goes over a threshold.

 

If both fans don't run at the same time, that's a problem. It could be a wiring problem, a fuse problem, a fan motor problem, or a relay problem. One thing it can't be, if you look at the wiring diagram, is an A/C amplifier problem. The A/C amplifier doesn't have any way to make one fan run without the other one, even if it were messed up and wanted to.

 

Salved title infers the wiring is messed up to the fan if the car hook a hard front end hit and everything was crushed in.

Fans run off relays.

Or maybe the fan is dead. I would jump 12 volts into the fan if you are able to check it. Both fans must be on if ac on.

The ac ecus I discussed earlier turn those fan relays on and off.

 

The A/C amplifier. (One ECU, two wire harness plugs that connect to it.)

I remember for sure in Gen 1, both the A/C amplifier and the engine ECU got votes on switching the fan relays, and I think one or both of the engine coolant temp switch and the A/C pressure switch had direct representation too. But I haven't looked closely at a Gen 2 diagram to see what might be the same or different.

 

I t seems to me if you find an actual problem with the amplifier, it might be the first I've ever seen in years of reading PriusChat. Like most car microcontrollers, it's all solid state, no moving parts, just sits there and does its thing, year after year. Unless you're in a flood and drown it.

Intermittent steering wheel controls very often turn out to result from wear in the spiral cable behind the steering wheel. That thing moves every time you steer, so it can definitely wear out.

Returning a little to #24, if you've got one fan running and the other not, there are various things that could cause that, but the A/C amplifier isn't one of them—at least not if the engine is up to temp. Looking at the 2006 wiring diagram (pages 346 and 347), the A/C amplifier has two outputs, RF and CF (pins 14 and 15 of connector A8, respectively). It pulls CF low whenever you're using A/C, and it pulls RF low when you're using A/C and it needs more cooling. You might guess CF and RF control two different fans, but no, that's not how the relays and wiring are arranged. When CF is pulled low, the result is both fans run at low speed. Later, pulling RF also low switches both fans to high speed.

The engine water temp switch is in parallel with the CF output, so when the engine is hot, both fans will run on low, the same effect as CF from the A/C amplifier. That's a straight old-fashioned switch, so that'll happen no matter what any ECU is doing.

The A/C high pressure switch is in parallel across RF, so if high pressure develops, both fans will switch to high speed. Again, that's a straight, no-nonsense switch, so high pressure should kick both fans to high even if the ECU is out to lunch.

The ECU for the engine also has an output, FAN (pin 8 of connector E7) that's in parallel with that pressure switch and the A/C RF signal. So if the engine ECU wants to kick both fans to high, it can, regardless of the A/C pressure or what the A/C amplifier wants.

So you can think of "fans on" (initially at low speed) as the wired-OR of (engine water temp switch, A/C CF output).

Then, "bump both fans to high" is the wired-OR of (A/C pressure switch, A/C RF output, engine ECU FAN output).

By design, "fans on" is supposed to happen first, then "bump both fans to high" happens later. But I can see what would happen if somehow "bump both fans to high" happened without "fans on" first. That would be if any of (A/C pressure switch, A/C RF output, or engine ECU FAN output) was pulled low, but without either of (engine water temp switch, A/C CF output). The result would be just one of the fans running (the one with a blue and a white-with-black-stripe wire), but on high speed.

Other failure modes could cause only the other fan (the one with a green and a white wire) to run, and only on high speed.

It will take you some legwork to pin down what's going on. Maybe you could start by identifying which fan it is that is running. Also, what happens if you warm up the engine to 95°C or so? Does that change the fans-running situation?

A multimeter and the wiring diagram are your best tools for something like this. There's a problem in there somewhere, but a lot of things it could be, things more likely to wear out or corrode, further up the suspicion list than an ECU.

 

Look again at what the SPDT "Fan no. 2 relay" does. In its de-energized state, terminals 3 and 4 connect, and the two fan motors are in series. 12 volts from Fan no. 3 relay, through the "condenser" fan, through no. 2 relay, through the "radiator" fan, to ground. Each fan is seeing about six volts. Low speed.

You can see that the wired-OR for high speed energizes both Fan no. 1 relay and Fan no. 2 relay, which now connects terminal 3 to 5, not 4. Now both fans are getting the full 12 volts, in parallel; one from No. 3 relay and to ground through No. 2 relay, the other one from No. 1 relay and directly to ground.

 

Going in reverse order:

I don't have that open any more, but isn't that the one that just crosses to the next page and the FAN output of the engine ECU?

I'm not sure I can untangle all of the 'not's in that.

Oh, a wise guy, eh?

Sure, in metal wires, current flow is by movement of electrons, but at the same time, in comparing the charges on protons and electrons, the (arbitrary) decision to call electrons the negative ones got made long ago. So positive current flow is in the direction the electrons are not moving.

And there are other situations, like in battery electrolytes, where the charge carriers are positive ions, so they're moving in the same direction as the nominal current flow. In circuits that have both wires and batteries (like this one!), if you wanted to always say current 'flow' was in the direction the charge carriers were moving regardless of whether they carried positive or negative charge, you'd have your arrows bonking into each other.

 

Perhaps I’m stupid but if both fans are wired to go on at the same time but only one goes on it’s either the non working fan or the wiring. Stop making it so complicated. Check the fan and see if you have power to the non working fan.

 

With this series/parallel thing could something like the ac amplifier detect this and turn the AC off? Like the fans can’t work in series because of the bad fan, but the good fan can work in parallel?

 

Well, in a car where everything works right, you will see both fans running when you first start up the car with the AC on, regardless of engine temp. The A/C amplifier is responsible for asserting CF whenever A/C is turned on, and CF (if the fuses/wiring/relays/fans are all ok) causes both fans to spin at low speed. As would the water temp switch. They are wired together as an OR, so either one will have that effect.

You will probably make more progress approaching it from the other direction. You know the system is designed to never have one fan running without the other. Its only design operating conditions are both fans low, or both fans high. If you are seeing one fan without the other, you have a diagnosis task in front of you. Instead of deciding the A/C amplifier is the culprit a priori and then asking yourself how could a fritzy amplifier cause this?, I recommend asking what is causing this?, using your meter and observational skills to find out, and then you'll know if the cause is the amplifier or something else. As a statistical-variety hint, it's more likely to be something else.

Well, electrons attract protons too. Wouldn't that mean the protons should have been called negative? It was an arbitrary choice. Somebody made it, and there it is.

This is one of those areas where the definitions were all chosen in a way that is nice and consistent if you just step back and let yourself use algebra, rather than distracting yourself with what particular tiny particles might be doing. A "current flow" is just defined as the direction positive charges would be moving. In a substance where the actual mobile charge carriers are negative, they happen to be moving the other way, but that doesn't change how the "current flow" is defined.

Now, if you design semiconductors for a living or something, then yes, you have to care what the particles are doing.

Well, it's a smidge more complicated—there are relays and fuses in there—but yeah, that's the idea,

What is the sound of one fan in parallel?

I don't think the system has any direct monitoring of the fans. It will tell you if the coolant overheats, and stop the A/C if the pressure goes too high. Short of that, you'll just have cooling less effective than you expect.

 

And perhaps the R134A is getting overheated because only 1 fan is working? The OP is convinced it’s the AC amplifier when he has a known broken part in front of him. A cheap and easy to replace part no less. I wonder if he’s a college professor?

 

In the life of a college professor, there might be a need now and then to help some student see how to sort through the evidence before jumping to a conclusion, so a professor had better be at least able to pretend to be doing that.

The conclusion is not quite ready to be jumped to yet. There are a few relays, a couple fuses, two fan motors, assorted connectors and/or wiring (mouse damage?), all still in the lineup waiting to be eliminated.

A person who wanted to get to the bottom of the problem would be out there with a multimeter and the diagram making the tests that would eliminate some of those candidates.