lumen8

Hey guys, so I have an LS swapped 2001 Australian Ford that has the factory fitted Air con 'ON' switch giving out a 25Hz 12V square wave , when what I need to work the AC relay low side is a simple smooth 12V DC.

Id like to be able to convert the 25Hz 12V square wave coming out of the factory AC controls, to a smooth constant 12V , so when i press the AC on button it can work the compressor relay.

Anyone know if there is a simple device that can do that? I dont want to mess with the inside of the factory AC controls as its a circuit board with a lot going on that still works all of the other functions like air vent position, 10 speed fan, and blend doors, and it still works well, plus they are a little difficult to find for these cars with the display pixels all working, which mine does... so id rather not go into it at all if possible.

I do have the air-con working with a stand alone simple on/off switch ive added into the AC relay low side circuit to signal the AC compressor, and I do have the A/C working (the circuit also has an aftermarket trinary switch and evaporator t-stat, so it works like it should).

But it would be cool to have the factory climate control unit be able to demand AC on, and eliminate the after market switch.

Ive added a pic of the factory manual describing the signal it gives out, which originally went to the factory Ford EEC-V ECU.

Jimbo1367

How about some pics of the swap vehicle?

lumen8

Pics: https://tinyurl.com/47ssy7b7

Vid: https://tinyurl.com/4nvetjbp

smog_exempt

Havent EE’d in a while so forgive me as I forget what this circuit is called, or if it even has a name beyond a filter with a diode.

Not sure if they’re saying it’s a 12vdc square wave, but that makes the most sense in automotive.

Looks good on paper, but will probably need more for your application.



You need a capacitor and a resistor to form an RC circuit, and a diode so the capacitor will not discharge when the square wave goes low.

I’ll try to design a circuit for that application once my login gets restored for this simulator (it’s decent enough for automotive). They limit the amount of devices you can put into a circuit, so you get the basic theory version for now.

Alternatively, there is such thing as an AC relay coil, but I’m unsure how they would respond to square wave DC. There may be an option out there, but I don’t know about it.

You will need a solid state relay to drive your AC signal, as typical automotive relays can take around an amp to operate. If you use a regular relay, the above circuit will most definitely fail as you are bleeding 1A of current continuously out of that capacitor. In other words you’d need a massive capacitor and the circuit would take some time to charge.

LSswap

This is a starting point. Maybe replace the transistor with a darlington. Values for components are based on the impedance of the square wave signal.


If the impedance of the square wave is low enough, meaning it can provide more current, then you can go with a solid state relay instead of the transistor and relay. Some SSRs need about 7ma or so your square wave will need to provide more than that to allow for keeping the capacitor charged. The transistor method can run at much lower input currents.

Jimbo1367

You have a nice looking car. I’d LS swap it too.