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Topic: Single relay elec fan, a/c trigger with diode? (Read 1574 times) previous topic - next topic

Single relay elec fan, a/c trigger with diode?

I am just about to get my a/c going for the first time since the 5.0 swap and I need to make sure my elec fan comes on when the a/c is on.  I didn't really want to add another relay so, I was wondering if I could run a second 'trigger' to the one relay from the a/c using a diode so the temp sensor can't feed back into the a/c and turn it on.  Only other problem now is the temp sensor is currently wired as a ground, not 12v, but I'm assuming I could run 12v through it....?  Seems like this should all be fine but I must be missing something otherwise, why wouldn't it be that way from the factory?  Thoughts?
1988 Thunderbird Turbo Coupe
'89 5.0 w/ ported intakes, Mallory Adjustable FPR, BBK shorties and H with 2.5" Flows, 130 amp 3G, 89 Mustang comp/wiring, Aluminum radiator w/ elec fan, T5 trans, King Cobra clutch, 3.55 gears, 13" Cobra brakes (front), Wilwood prop valve, Mustang A-arms, Front Coil-overs, MM CC plates, Silver 17x9 R's, 03 Cobra IRS, Aluminum DS, 2002 Mustang dash/console etc..., custom leather seats

Single relay elec fan, a/c trigger with diode?

Reply #1 –
The easiest way (though you don't wanna) would be to just add another relay. Just put the contacts in parallel with the existing relay, that way either or both could safely trigger the fan.

CoogarXR
CoogarXR : 1985 Cougar XR-7

Single relay elec fan, a/c trigger with diode?

Reply #2 –
right, that would be just like the factory setup.  I was hoping to avoid a second relay, minimize the clutter I guess...  I do have a relay sitting around, I suppose I could use it.  hmm...  I'd just really prefer to add one wire/diode and be done!  :hick:  I think that would actually be easiest, if it could work
1988 Thunderbird Turbo Coupe
'89 5.0 w/ ported intakes, Mallory Adjustable FPR, BBK shorties and H with 2.5" Flows, 130 amp 3G, 89 Mustang comp/wiring, Aluminum radiator w/ elec fan, T5 trans, King Cobra clutch, 3.55 gears, 13" Cobra brakes (front), Wilwood prop valve, Mustang A-arms, Front Coil-overs, MM CC plates, Silver 17x9 R's, 03 Cobra IRS, Aluminum DS, 2002 Mustang dash/console etc..., custom leather seats

Single relay elec fan, a/c trigger with diode?

Reply #3 –
first off , which coolant device are we talking about?

are you still using the integrated controller module?

your sig says mustang harness,

Single relay elec fan, a/c trigger with diode?

Reply #4 –
No, that module is long gone, left with the 5.0 swap.  I am using an aftermarket cooling fan, relay and temp sensor.  It has no integration with the mustang EEC harness.  I was thinking, to get the fan to power on when the a/c is on, I could pull voltage from the a/c system (after the pressure switch) and use it to trigger the fan relay.  I'd also use a diode between the a/c system and the relay to prevent the fan relay from powering up the a/c everytime the temp sensor fired up the fan to cool the engine.
1988 Thunderbird Turbo Coupe
'89 5.0 w/ ported intakes, Mallory Adjustable FPR, BBK shorties and H with 2.5" Flows, 130 amp 3G, 89 Mustang comp/wiring, Aluminum radiator w/ elec fan, T5 trans, King Cobra clutch, 3.55 gears, 13" Cobra brakes (front), Wilwood prop valve, Mustang A-arms, Front Coil-overs, MM CC plates, Silver 17x9 R's, 03 Cobra IRS, Aluminum DS, 2002 Mustang dash/console etc..., custom leather seats

Single relay elec fan, a/c trigger with diode?

Reply #5 –
That should work.
Like you said, you will have move the temp sensor to the +12v side of the fan relay coil.
You will need to have some idea how much current the relay coil draws to pick the right size diode.
And hopefully the extra load on the A/C circuit won't be enough to matter.

Single relay elec fan, a/c trigger with diode?

Reply #6 –
How do I figure that out?  Can a diode be too big?

I'm thinking the a/c circuit won't notice because a relay coil must be pretty small draw (?).
1988 Thunderbird Turbo Coupe
'89 5.0 w/ ported intakes, Mallory Adjustable FPR, BBK shorties and H with 2.5" Flows, 130 amp 3G, 89 Mustang comp/wiring, Aluminum radiator w/ elec fan, T5 trans, King Cobra clutch, 3.55 gears, 13" Cobra brakes (front), Wilwood prop valve, Mustang A-arms, Front Coil-overs, MM CC plates, Silver 17x9 R's, 03 Cobra IRS, Aluminum DS, 2002 Mustang dash/console etc..., custom leather seats

Single relay elec fan, a/c trigger with diode?

Reply #7 –
Quote from: booksix;274547
How do I figure that out?  Can a diode be too big?

I'm thinking the a/c circuit won't notice because a relay coil must be pretty small draw (?).


I can interpret diagrams and trouble shoot but not much good at design.
If the relay has a manufacture number maybe you can find it's specs with google. Or someone on here my have a idea what they draw.

If you are going to put the diode under the hood it has to be designed to live in that environment.
They are rated by voltage (20v is probable enough) and current. They also have a Z voltage rating, which is the reverse voltage at which they breakdown (not an issue here).
Can they be too big? I guess they could get physically bigger than you would want and very expensive at high voltage and high current ratings.

Single relay elec fan, a/c trigger with diode?

Reply #8 –
Ok, thanks.  I have a diode from the ABS harness i pulled out.  It's one of those black ones you see on stock wiring harnesses; I was hoping to use that but now idk.  Once again, I can't imagine the coil on that relay will draw much current but maybe more than the ABS computer.  Then again, that diode may not even be 12v.  I say that because I believe some of these auto computers used 5v signals and such....?
1988 Thunderbird Turbo Coupe
'89 5.0 w/ ported intakes, Mallory Adjustable FPR, BBK shorties and H with 2.5" Flows, 130 amp 3G, 89 Mustang comp/wiring, Aluminum radiator w/ elec fan, T5 trans, King Cobra clutch, 3.55 gears, 13" Cobra brakes (front), Wilwood prop valve, Mustang A-arms, Front Coil-overs, MM CC plates, Silver 17x9 R's, 03 Cobra IRS, Aluminum DS, 2002 Mustang dash/console etc..., custom leather seats
 

Single relay elec fan, a/c trigger with diode?

Reply #9 –
A good relay to use in automotive circuits is the 1N540x style. It's good for 3 amps and has nice, heavy leads (1/16" dia) on it for crimping butt connectors to. Just replace the "x" in 1N540x with any number. The number is the voltage rating of the diode, but in your 12V application it does not matter, since the lowest rating for those diodes is 50 volts (1N5400). You should be able to pick up a 1N540x diode for under a half dollar at any electronics store.
2015 Mustang GT Premium - 5.0, 6-speed, Guard Green - too much awesome for one car

1988 5.0 Thunderbird :birdsmily: SOLD SEPT 11 2010: TC front clip/hood ♣ Body & paint completed Oct 2007 ♣ 3.55 TC rear end and front brakes ♣ TC interior ♣ CHE rear control arms (adjustable lowers) ♣ 2001 Bullitt springs ♣ Energy suspension poly busings ♣ Kenne Brown subframe connectors ♣ CWE engine mounts ♣ Thundercat sequential turn signals ♣ Explorer overhead console (temp/compass display) ♣ 2.25" off-road dual exhaust ♣ T-5 transmission swap completed Jan 2009 ♣

Single relay elec fan, a/c trigger with diode?

Reply #10 –
I'm assuming you meant diode rather than relay in that first sentence?  I'll look into these...  Radio shack have them or something bigger like fry's?
1988 Thunderbird Turbo Coupe
'89 5.0 w/ ported intakes, Mallory Adjustable FPR, BBK shorties and H with 2.5" Flows, 130 amp 3G, 89 Mustang comp/wiring, Aluminum radiator w/ elec fan, T5 trans, King Cobra clutch, 3.55 gears, 13" Cobra brakes (front), Wilwood prop valve, Mustang A-arms, Front Coil-overs, MM CC plates, Silver 17x9 R's, 03 Cobra IRS, Aluminum DS, 2002 Mustang dash/console etc..., custom leather seats

Single relay elec fan, a/c trigger with diode?

Reply #11 –
Yes, I meant diode.

Radio Shack should have it, it's a run-of-the-mill part
2015 Mustang GT Premium - 5.0, 6-speed, Guard Green - too much awesome for one car

1988 5.0 Thunderbird :birdsmily: SOLD SEPT 11 2010: TC front clip/hood ♣ Body & paint completed Oct 2007 ♣ 3.55 TC rear end and front brakes ♣ TC interior ♣ CHE rear control arms (adjustable lowers) ♣ 2001 Bullitt springs ♣ Energy suspension poly busings ♣ Kenne Brown subframe connectors ♣ CWE engine mounts ♣ Thundercat sequential turn signals ♣ Explorer overhead console (temp/compass display) ♣ 2.25" off-road dual exhaust ♣ T-5 transmission swap completed Jan 2009 ♣