I've done an engine swap in a car and have finished everything but the Air Conditioning.
This particular vehicle uses a computer to control the AC. It's working and outputs a 4.6v signal on the AC control wire when you press the AC button on the climate control. The current on the signal wire is only about 4-5 mA.
The original engine computer looks for that signal and then raises the engine idle and grounds the relay for the AC compressors magnetic clutch. Simple.
However, the new engine computer is looking for a 12v signal!
I've tested this by supplying the ECU with a 12v on the signal wire (everything works)
So what I need is a relay that will sense the small (4.6v, 5mA) voltage/current and switch on a 12v signal to the engine computer.
I assume this would be some type of solid state relay, maybe inductive?
If someone could point me to a product or alternative solution I would be very grateful.
A couple of transistors and a couple of resistors is all you should need to level shift your 5v to 12v, no need for relays. I would doubt that the management computer can deliver the power required to pull-in and hold anything but the smallest of relays, nor will the output likely be protected against the back emf generated by them. If you are determined to use a relay, perhaps look for a small telecoms reed type relay and make sure that you put a fast protection diode across the coil to control the back emf.
actually, using a MOSFET to drive the relay will use the fewest components. you can also use a bipolar transistor to drive the relay. the diode can be either a 1N4148, or a 1N4004. L1 is the relay coil. if you use the 2N2222 , make sure the relay board isn't where it's subjected to high temperatures. the MOSFET can be used without a heatsink. but should be at least bolted to the board so it doesn't wobble with vibration and eventually break the leads
an SSR (solid state relay) might work. most of them are made to switch 120Vac, so you'll need to find one for low voltage DC, some of the 120V models might work, some might not, as they use zero crossing detectors to switch the mosfets on and off.