School me on relays

[pressureangle]

I'll suggest that you won't find a fan that draws only 7 amps. The 8" fans I have on my classic car draw about 40 amps together. I had to study the problem carefully because I can only get 40 amps at idle and 80 max with the old alternator.
More importantly, I've found that in both that old Charger and in my '94, my problems were not airflow as much as poor condenser performance. An upgrade in my second '94 to a Classic Air condenser made the thing work right, even idling in 95* city standstills. The older '94 with a whatever brand condenser would overheat and drop out right before the fan clutch kicked in, giving 30 seconds to a minute of AC fail and was weak after that until you got some MPH. Fans would have helped but the airflow is an accommodation to insufficient condenser efficiency.

 

[Martin Evans]

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Let me know if this doesn’t make sense

Your diagram requires that BOTH the A/C and the toggle switch be on in order for the fan to operate. I believe that OP wanted to operate the fan with EITHER the A/C OR the toggle switch on. If that is the case, I would just use one relay, provided that it can can handle the current load and run the wire from the A/C, as well as the wire from the toggle switch to the 86 pin with a diode between each and the relay.

 

[Erik the Awful]

I believe that OP wanted to operate the fan with EITHER the A/C OR the toggle switch on.

I got that same impression. Put the switches in parallel instead of series.

 

[Supercharged111]

AC signal wire to pin 85/86 with a diode inline will fire the relay when the AC is on. Run the signal wire from the switch to the same relay terminal to fire the relay at will, and the diode will prevent that signal from backfeeding into the AC system. Depending on where you pull the signal within the AC system, that diode may end up being redundant. 1 relay, 1 switch, moar simpler, moar betterer.

 

[GoToGuy]

The hand drawn diagram reveals you really don't understand DC voltage. Both switches would have to be closed wired in series to activate the left rely only. The right relay has power in standby regardless of switch positions as it has no ground to complete the activation circuit.
Read up on Automotive Electrical, it will only help in any future projects, and any repairs.
Understanding " Ohm's Law " will help.
You've probably heard this before, don't guess with electricity, the automotive Amps may not kill you but they can start fires.
Don't be " that " guy. Good luck!

 

[rob249]

Wow, thanks for all the responses everyone, special thanks to TechNova for the knowledge bomb. My goal is to operate the fans only when the AC is on, and the switch is flipped on. So from what I think I understand here, without a diode between the switch and the relay, power will feedback through the switch and keep the compressor running even if i turn off the AC on the dash, assuming the switch is still on. Is that correct?

edit; On the Spal website, it calls for a 15amp fuse for one fan. https://www.spalusa.com/products/fans/7_5/va14-ap7-3-c-34s-12-7-5-7-s-3-12v-30100393

Im trying to not be "that guy".

 

[Macs Garage]

Doesn't the AC compressor also turn on when on defrost?

 

[kennythewelder]

Doesn't the AC compressor also turn on when on defrost?

Yes, but intermittently, depending on what year truck, you have.

 

[Orpedcrow]

@GoToGuy
Was it my hand drawn diagram you were referring?
My understanding was OP wanted auxiliary fans ONLY when a/c was activated at idle, then being able to turn them off at speed-running the switches in series would allow that.

I could have drawn the diagram better, the input for the toggle switch would just be T’ed off of the a/c switch, or compressor clutch, or pressure switch etc… also, terminal 86 is fed switched power on both relays. People keep saying there’s no ground so maybe it isn’t my diagram

 

[Supercharged111]

Wow, thanks for all the responses everyone, special thanks to TechNova for the knowledge bomb. My goal is to operate the fans only when the AC is on, and the switch is flipped on. So from what I think I understand here, without a diode between the switch and the relay, power will feedback through the switch and keep the compressor running even if i turn off the AC on the dash, assuming the switch is still on. Is that correct?

edit; On the Spal website, it calls for a 15amp fuse for one fan. https://www.spalusa.com/products/fans/7_5/va14-ap7-3-c-34s-12-7-5-7-s-3-12v-30100393

Im trying to not be "that guy".

So the logic I gave was AC on or switch on, if you want both conditions present let the AC signal be an input to your switch, and the output of that be the trigger for the relay. With the switch off in this scenario, there's nothing to backfeed.