Switch that deactivates when truck turns off?

[Chapap]

My Ram needs an extra fan on the condenser. I'd ask this on a Ram forum, but TRS is byfar the most active and knoledgable automotive forum I've come across.

I could wire the solenoid to the compressor, but that would just mean it's on all the time. I want to be able to turn it off at highway speed or whenever else I decide. I'm sure there's a "proper" way involving differentiating pressures, temps, and probably controlled by a simple computer, but that's just too much and I shan't be drilling into any AC lines. Best I can figure is to have a switch on switched power in the cabin to control it manually. Now the problem is I'll have an extra 10+ amp load during startup if I forget to turn it off, and starter noise would probably be bad for the fan motor. Is there a way to have an on/off switch that reverts to off when power is disconnected? I found this schematic either here or a Camaro forum for separate, momentary on and off buttons that would work, but I'd like to only have one button. Maybe a (terminology lacking here) 3-position switch that is spring loaded to the middle, NO up, and NC down, preferably lighted if activated? I bet that exists, I'll look around.

Also... Where does switched power even come from? Can a switched power circuit be overloaded? Should I have the fan run from a non-switched circuit? See any problems with my schematics below?

 

[dvdswan]

Are you wanting to cool your condenser down more while stopped or in city traffic? You could get a temp switch and use that to wire the fan on/off.

 

[Chapap]

Are you wanting to cool your condenser down more while stopped or in city traffic? You could get a temp switch and use that to wire the fan on/off.

Yes. It’s an old Cummins that needs barely any cooling at idle or in stop and go traffic. On top of that the condenser is 8” in front of the rad so there’s hardly any flow when the fan clutch is disengaged. I ruled out a single temp sensor cause the range when it’s needed will vary drastically.

 

[RonD]

You need a relay for sure
Relays only draw 0.5amp when on so can be added to any Key On circuit

Relays are rated by activation voltage, usually 12v for vehicles, and by LOAD capacity, usually 30 to 40amps

Best way to use a relay is to give it's coil 12v and then run a Ground wire into the cab, for a switch, switch grounds it to cab metal to activate it, you can get switch that lights up when "on"
This uses a smaller wire gauge only 1/2amp 18-22 is fine

The LOAD needs larger gauge wire and its own fuse, with full time power, fuse needs to be rated LOWER that relays load rating, fuse is located as close as practical to the 12volt source, usually the battery
Electric fan motor will have a WATT or AMP draw rating, use that for best fuse and wire gauge


You could just hook up the relay to compressor clutch coil, it has 12v when engaged, so fan comes on when compressor engages, and condenser is heating up
No wires into the cab, and no Key On source
Fan comes on when needed and goes off with AC off or not needed
Fan wouldn't draw much power at Speed because air flow is spinning it not the motor

 

[Roert42]

I like the second schematic, but the first will work ok too. I assume the Switched + is going to be Keyon voltage, if so, the relay should open and reset the circuit if the truck is turned off. So good on the self resetting.

For the reasons Rod stated I would avoid having the load run though the switch, much less chance of the switch failing/ over heating.

I've done a lot of industrial control wiring, and we have always used a separate start and stop button. wiring it with one button will be more complicated.

You would have to find a toggle with two contacts inside, one NO and one NC. Or if you can't find that, have two NO contacts in the toggle, and have a third relay to be controlled by the toggle simulating the NC contact.

@ericbphoto what do you think about using one switch to start and stop. you have something you used before?

 

[Chapap]

Of course now that I'm looking specifically for adding a condenser fan, I can't find anything.... but every thread I read said that hooking the fan to the compressor is no bueno for some reason. Something about it being bad (or un-ideal) to have the fan running at speed when it's basically free-spinning in the wind. Being able to shut the fan off might be a lot of trouble with no benefit tho. I really don't understand why... elec fans run 100% of the time on vehicles with them.

There was a good thread I found where a fella hooked the fan upstream of the compressor somewhere... to the module that requests the compressor to be on. That way the fan won't cycle with the compressor, but constantly cool the condenser when it's in demand.

 

[RonD]

Well you wouldn't want to hook up the fan directly to the compressor clutch, lol, but relay is fine

And condenser ONLY gets very hot when compressor is on, when compressor is off its cooling down
Point of condenser fan is to turn the Vapor from compression back to a Fluid, to cool it below "freon" vapor point, faster, so should be on when compressor is

As far as I have ever read or seen no vehicle electric cooling fans have a speed sensor or wind speed sensor, just temp sensors
And these types of fans don't "lock" when off, air flow would still make them spin on or off
Radiator electric cooling fans spin at speed when off, just spin one with your hand(when its off, lol) easy peasy


But you can for sure wire up a switch in the cab, just need the one wire to ground the relay at the cab switch

 

[Roert42]

I forgot about these for some reason last night. You want something like this for your switch, they are used for panel building. You can configure exactly What you need for a switch. Then wire it up like either of your schematics.

 

[Chapap]

I forgot about these for some reason last night. You want something like this for your switch, they are used for panel building. You can configure exactly What you need for a switch. Then wire it up like either of your schematics.


View attachment 79736View attachment 79737

What do you call that? I've found them online, but not sure how it works.

Anyone know if the compressor is deactivated during KOEO or cranking?

 

[RonD]

Computer doesn't turn on Compressor clutch until engine is running, i.e. RPMs show above 500
In a 1994 that means WOT relay is ON to disable clutch

 

[Chapap]

I added an NC solenoid so both the on and off momentary buttons are NO. That way a 3-position, momentary toggle will work.

 

[RonD]

You don't need 3 relays to do that, just 2

Key on 12v to both relay coils
Momentary switch Ground wire in cab to relay 1's coil
Full time ground on NO Relay 1, NO other connection goes to Relay 1 coil's ground, AND to relay 2's coil ground

So when key is on both relays get 12v on coil but neither coil is grounded
When you press the momentary button, relay 1 closes and grounds BOTH relays full time
So relay 1 stays closed until key is turned off
And then relay 2 is closed powering fan until key off


If you just want an on/off switch, which it sounds like, then just use the one relay grounded in the cab
Use a switch with built in ON light, so you remember to shut it off when starting engine
2 relays on one circuit adds a layer of complication
3 Relays on one circuit will not be reliable long term, in my opinion

 

[Roert42]

What do you call that? I've found them online, but not sure how it works.

Anyone know if the compressor is deactivated during KOEO or cranking?

Here is a brochure I found on Allen Bradley's website.

 

[Chapap]

You don't need 3 relays to do that, just 2

Key on 12v to both relay coils
Momentary switch Ground wire in cab to relay 1's coil
Full time ground on NO Relay 1, NO other connection goes to Relay 1 coil's ground, AND to relay 2's coil ground

So when key is on both relays get 12v on coil but neither coil is grounded
When you press the momentary button, relay 1 closes and grounds BOTH relays full time
So relay 1 stays closed until key is turned off
And then relay 2 is closed powering fan until key off


If you just want an on/off switch, which it sounds like, then just use the one relay grounded in the cab
Use a switch with built in ON light, so you remember to shut it off when starting engine
2 relays on one circuit adds a layer of complication
3 Relays on one circuit will not be reliable long term, in my opinion

It sounds you're describing fig 2 in my original post but without the off (NC) button (it'd sure be easier if I labeled everything)? I added that 3rd relay because I can't seem to find a 3-position, momentary toggle with NO and NC... only NO and NO. Really I think I'm going to nix the switch in the cab and let the fan run non stop via the compressor signal. I figure the worst that will happen is I'll have to replace it in 5 years instead of 10. I really haven't found a substantive explanation to have command over this fan.

I forget what thread I got this from, but the fella who informed me of it said that it was standard on whatever military vehicles he worked on. He agreed that it was a stupid over-complication.

 

[RonD]

Well if there was never a factory fan on the condenser then the car maker's engineers didn't think it was needed at all

So having it cycle on and off with compressor clutch would be all that's needed as an upgrade

Generally speaking car's AC compressor cycles on and off based on pressure, usually High side pressure, once compressor has built up enough heat/pressure it cycles off until pressure goes down enough, so will cycle on for about 10-15min then go off, and come back on once pressure drops enough and the on/off cycling is partially based on outside temp which changes internal pressure
If low side switch is causing the cycling it usually means system is low on refrigerant