This is the controller i have, It's not exactly low cost but it will do what you want even after you get the new rad. It does have a terminal for the ac control but i never hooked it up. In the summer i adjust the temp switch down a bit and it usually stays on when not moving with the ac on. This is a basic controller nothing fancy. If you want variable speed or two speed controller they have them also.

https://www.flex-a-lite.com/accessor...ol-module.html

 

Ok, sorry 'bout that, read the diagram backwards.... PCM sends the signal thru the pressure switches, and the HVAC control provides ground path for the circuit. So, according to theory, you could use that wire on the ground side of the relay control circuit. Test it out, see what ya get.

 

HeyYou. You are correct. Where I was measuring at the sensor plug on the tube on top of the compressor, around 2 MegOhms to ground with A/C on and open circuit with A/C shut off. This will take slightly more thought but indeed should be usable.Can probably make it work with a 50 cent SSR or similar off DigiKey. Thanks again buddy. Perfect!

To be fair. Yesterday I did note a a couple tenths of a volt of a volt change around ground, on that line, when A/C was on or off, which should have clued me something was up. But I just thought duh I can't use 2 tenths of a volt to reliably trigger a relay and moved on. I should have possibly investigated further!

Moparite. That controller does look really nice. I doubt I'd switch now since what I have now is working well, generally. Have to say, not a big fan of that style of temp probe, though. Would prefer dual relays for the fans. BTW what is the relay? Is it a commonly available and replaceable item? I doubt they allow you to use aftermarket fans do they? I already have fair investment in the equipment I have now Id rather not get rid of while it still works. The only thing I'll be jealous of now though is the speed regulation and I'm not super super jealous of that though a little bit. Do you know how they do it? ie do they regulate the voltage or is it a PWM?

 

You can use a relay to take a negative ground trigger to a positive trigger for your controller.

 

I'm probably going to end up using a p-channel mosfet to drive a relay. To keep it alive I'll use a snubber circuit to damp the relay coil fly back transient.

If you read my post to HeyYou you'd see that I measured open circuit with A./C off and 2 megohms or so, aka 2 million ohms, when A/C is on.

Automotive relays of the type one might use for a radiator fan, such as a typical Bosch/Tyco 12V/30A , has a coil resistance of somewhere around 85 ohms.
If you try to pull an 85 ohm relay coil to ground through an 2Mohm ground it will never turn on!

In this case some sort of signal conditioning will be necessary if I want to use the signal tapped where I measured it this morning to drive a relay. That is what I may try with the MOSFET circuit I described above. I haven't bothered simulating it yet, but I will before I plunk down cash and suffer Digikeys incredibly high shipping extortion rates.

 

You can get a standard low pull relay that will return on with that voltage. No matter what you do please reply with what you did and how you did it for future forum members wanting to do the same

 

I set my 87 up to run off of 2 electric fans. But the 2 parts I believe that will help you are below.


1. I used a Trinary switch. It will do 2 things. First, it will cycle your engine cooling fan based on condenser pressure. Second, if the fan fails to come on, it will break power to the compressor, shutting it off. Here is a link to the one I have. https://www.ebay.com/itm/New-A-C-Tri...IAAOSwr8RdlK7U

2 The second part is a tee to go in the line going from the condenser to the drier. https://www.ebay.com/itm/BEADLOCK-CR...53.m2749.l2649

I purchased a crimping tool to make my own lines, but for one job, you could take your orig. line to any tractor or ac shop and get them to cut it, and crimp if for you. You would then need to evacuate and recharge.
Once you have the switch installed, for the cycling side, you can just send power, or ground, whichever way you have the fan relay setup to come on, and turn on the fan relay...... The high pressure cutout, I just spliced in between the compressor coil.

 

It is not a matter of polarity, pull high or pull low that is the issue. It is a massive impedance mismatch issue. There isn't a relay in existence that could work under those conditions.

Think of it like this. I measured 2Mohms to ground. Imagine if one lead of your 85 ohm relay coil was tied to battery positive but the other lead of the relay coil gets tied to battery minus through this 2 Mohm resistor...We have 2 megs plus 85 ohms = 2,000,085 ohms with 12 V applied. That is 12 / 2,000,085 ~= half a microamp. That is literally thousands of times less current than what is needed to get that relay to close its contacts. We really want something closer to around 150mA or so.

On another note. I did simulate this thing last night. And I placed an order with Digikey tonight. 20 bucks to get less than a bucks wortth of parts. What a racket....

I'll hold off posting a schematic until I get parts and prove it works maybe next weekend if there is time.

Basically I ended up going with my P-Channel mosfet idea based on my simulation results.

 

What can I say but you are definitely the man Ragtop. That is quite clearly the most optimal solution I' have seen and probably the right way to do it. Very very impressive approach and one I never even considered. Way to think like someone with the last name of Mopar. That is factory if I ever saw it.

Though I think I may have come up with a workable solution based on my simulations last night. Maybe my approach isn't quite as cool but I think if it works as my simulation suggests, the casual observer could not even tell a difference. More than one way to skin a cat....but my cat skinning still has not proven itself....

 

I understand and you are wrong there are relays that will close with a quarter of a microamp. Heck a forum member here helped me setting it up.