I have a Corvette that keeps having the blower control module go bad, probably because the car is over 30 years old. This module is no longer available and hard to find. This module also controls the AC compressor. My problem is that I want to bypass this module and come up with my own relay. The AC module uses a 5V signal from the control panel inside the car. Is a reliable relay that I could use that is triggered by 5VDC but allows a 12VDC output to the AC compressor? Pardon my ignorance on this I’m much better at the mechanical side. If I need a buffer relay is that something similar to a diode that won’t let it damage the controller? I’m looking at this, could someone tell me if this is good option ? CG Solid State Relay SSR-25DD DC to DC Input 3-32VDC To Output 5-240VDC 25A Single Phase Plastic Cover
You should ignore the 5V signal from the control panel (which could indeed be something like a PWM, current loop or even bus signal) and instead measure the voltage out from the module, because that's what matters. Lets assume it is 12V.
The first thing I'd try here would be to use a rugged transient voltage suppressor (TVS) diode. Pick one with voltage margins above the expected nominal voltage. So if you have 12V nominal like a car battery, that might in reality go up towards 15V somewhere when charged. The purpose of the TVS is to kill any spikes above that voltage, so you would pick one rated around 18V to 20V somewhere - the exact voltage isn't critical. Automotive grade usually means >1600W "peak pulse power", so pick one rated for that as well.
The diode should be mounted so that the anode sits on the output from the module towards the compressor and the cathode against ground. This will protect both against reverse EMF from the coil as well as generic electrical noise from whatever other sources that might be causing it. Also these are no doubt dirt cheap ($1-2 or so) in comparison to replacing the module, but you might need to solder it in place.
Adding a TVS also won't affect the existing electronics design, it will just add extra protection (that should have been built-in in that AC module to begin with). So the worst thing that could happen is that it does nothing.
The reason why you probably can't use a relay is because relays only work with pure digital on/off voltages. All proportional signals that correspond to a certain level, like the position of a knob, are not on/off. They'll be modulated somehow. In case you aren't certain if the signal is on/off or proportional, you can't really measure it with a multimeter either - you'd have to use an oscilloscope. But in case you know for sure that you have a 5VDC signal, then it could probably be used to drive a 5DVC coil relay indeed. Though in case of relays, you should still add a diode across the coil (anode against coil+, cathode against coil-) similar to the TVS proposed above - this is known as a flyback or freewheel diode, where flyback/freewheel refers to the purpose of the diode rather than the type.
The AC module uses a 5V signal from the control panel inside the car.
The 5V signal probably uses PWM (Pulse Width Modulation), which is a fancy way of saying that it turns the blower motor on and off rapidly to control its speed. If you operate a mechanical relay with this signal it will buzz at the PWM frequency and wear its contacts out quickly. The signal might also not provide enough current to operate a relay with a 5 V coil.
A DC solid state relay may work provided that the 5V signal has enough current to operate it. However most solid state relays are not designed for PWM either, so the fast on/off switching may burn it out - and good ones aren't cheap. If you decide to try this, make sure that the motor has a diode across it to recirculate the 'back-emf' generated, otherwise it could blow up the relay.
If you want to continue using the variable speed control then you really should replace the control module with another compatible one, or get the original repaired. Alternatively you could replace it with a multi-position switch and resistor module, as is used in simpler air conditioning systems. however this will also bypass any automatic blower speed control.