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Originally Posted by First c10
Any luck with the diagram?
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Sorry for the delay. I couldn't find any diagrams online, so I rolled my own. I drew them in MS Paint, so no jokes about the quality please
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Here's the basic function of a trinary switch:
The dark green A/C clutch wires are easy. I assume that you have a power wire going to your A/C clutch already. For a GM vehicle, it's usualy dark green (ironic, I know). If you cut the wire going to your clutch, you have 2 ends. One end goes to one green wire on the trinary and the other end goes to the other green wire from the trinary, Easy and no magic required.
The fan control wires on the trinary switch are not quite as easy when you throw multiple fans, dual speeds, and PCMs into the mix. I know you didn't ask for all this, but I'm long winded
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To get started, here's a really dumbed down picture of the PCM control of a fan relay. It doesn't even show the fan, because what the relay is controling is irrelevant at this point:
Here's the same picture, but I added the trinary switch.
The trinary switch will control the fan as intended, but the PCM is not happy. You can literally see the frown in the picture
. Why is that? Well, the PCM periodically runs diagnostics on the fan control circuit. Since all that is between the fan control line(s) and 12V is the coil of a relay, the PCM should see a voltage on the fan control line because, electrically, the relay coils is just like a resistor. If the trinary switch closes, and forces that line to ground, the PCM sees the ground (0 Volts) and turns on your check engine light. To make things even more interesting, the default behavior when the PCM sees a fault with the fan control is to turn both fans on high speed all the time! This is not what we want.
How do we stop the trinary switch from upsetting the PCM? We can use a diode which is the electrical equivalent of a one-way valve. The band on the diode (the cathode side) needs to point to the more negative part of the circuit if you want electricity to flow through it. It looks like this:
In the picture above, the PCM is still not happy even though we added the diode to block the trinary switch's ground signal from getting to it. The problem now is that the diode also blocks the PCM's abilty to "see" the relay coil (even though the PCM can still control the relay by switch ing ground on the control line). The fix for that is a pull-up resistor.
The picture above works because the PCM sees the pull up resistor and thinks it is the fan relay coil. So, the diagnostics pass.
I didn't actually draw what it would look like to add in the additonal high speed fan relays, but it would require a total of 4 diodes and 2 resistors to make it all work. It's not that big of a deal, but I'm picky about such things and there is a "better" way.
On to the next diagram:
The picture above shows 1 resistor in series with the trinary switch. It's value of 47 ohms was selected for a couple of reasons. First, 47 Ohms is a common value for a resistor and its available at Radio Shack which almost everyone has nearby. 50 (or 40) would work just as well, but you would have to special order those. Second, and probably more importantly, most 12 volt relay coils are going to have a resistance of about 80-90 ohms. 47 is about half of the relay coil resistance (or 1 third of the total resistance of the coil plus the resistor). Since voltage, current, and resistance are all proportional to each other, we can calculate that the resistor will eat up about 4 of the 12 volts leaving 8 for the relay. 8 volts is still enough to operate the relay and, at the same time, hold the control line above ground potential so that the pesky fan control diagnostic test passes!
For anybody that's still awake, here's what you get when you add the high speed relays back to the picture:
In the picture above, I drew the PCM as unhappy. I've never tried that particular combination, though. In theory, the A/C trinary switch will turn both fans on high and the PCM will pass the diagnostic as expected. But, the possibility is there that the when the PCM tries to turn on low speed, both fans will come on at high speed. The reason is that the trinary switch ties together the low speed and high speed control wires together. Since the low control would have to go through all 3 resistors to get to the high speed relays, the voltage drop might be enough to prevent that from happing. It's just to close to call and intermittent at best.
To fix the problem, we need our diodes again to direct the signal from the trinary switch to the high and low controls without tying them together, like this:
So, the diagram above makes everyone happy. The PCM passes its diagnostics and the trinary switch can do it's thing. It is the most "correct" way to do it that I've presented here. It works like this:
1) When the trinary switch closes (A/C line pressure is high) both fans run at high speed (engine temp is irrelevant).
2) Both fans run at half speed when engine temp is between low speed fan temp and high speed fan temp and A/C line pressure is low.
3) Both fans run at full speed when engine temp is above high speed fan temp (A/C pressure is irrelevant).
Here's one last diagram to show the way that I personally wire the trinary switch on my conversions.
It is really simple as far as the actual wiring goes and it keeps the PCM happy. But, it only kicks on one fan (at full speed) if the engine coolant temp is below the low speed threshold and the trinary switch closes. You aren't likely to encounter this scenario outside of when you first crank the truck (with the A/C on) before it gets to operating temp. It doesn't hurt anything, it's just a little different. It's a price I've always been willing to pay for simplicity in the wiring.
Let me know if you have any questions.