Cooling Fan Wiring Schematic

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Cooling Fan Wiring Schematic

Postby zeke » Wed Apr 16, 2014 11:57 am

This is how I have decided to wire the fans on my Monza. Someone may find it useful.

Power flows from an ignition source through a diode wired in series with the fan temp switch. When the engine reaches temp. the switch closes and energizes two small Bosch style relays. The normally open contacts handle the fan motor current load. A second diode feeds power from the A/C system to bring on the fans if they are not yet running and the a/c is turned on. This ensures air flow across the condensor and prevents a hi pressure cut out situation.

The first two diodes isolate the two sources to prevent any backfeed.The other two diodes across the relay coils are for noise suppression. Any small silicon diodes rated for at least an amp will work.

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Last edited by zeke on Wed Apr 16, 2014 9:00 pm, edited 1 time in total.
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Re: Cooling Fan Wiring Schematic

Postby spencerforhire » Wed Apr 16, 2014 3:56 pm

That's not a bad way to run the fans in most situations- A couple of suggestions- a 30a fuse might not be quite enough for a pair of beefy fans; one 30a each or maybe a 40 or even 50a breaker might be required. Also, on most newer fuel injected vehicles, the fan and A/C requests are negative trigger, so it might require using ground to trip the relays instead of ignition. This way, the fan could be left to run after the vehicle has been shut off. Most of the OEM stuff is wired like this.
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Re: Cooling Fan Wiring Schematic

Postby zeke » Wed Apr 16, 2014 4:47 pm

spencerforhire wrote:That's not a bad way to run the fans in most situations- A couple of suggestions- a 30a fuse might not be quite enough for a pair of beefy fans; one 30a each or maybe a 40 or even 50a breaker might be required. Also, on most newer fuel injected vehicles, the fan and A/C requests are negative trigger, so it might require using ground to trip the relays instead of ignition. This way, the fan could be left to run after the vehicle has been shut off. Most of the OEM stuff is wired like this.


Yeah, I'm not sure that the 30 amp fuse will be large enough, I doesn't blow when testing though. The tbi system I'm using shows a positive a/c input (which I have not shown in the diagram) and has no control over the fan. I debated whether to use battery or ignition feed and decided I want the car to turn off completely when I turn the key off.
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Re: Cooling Fan Wiring Schematic

Postby dadhad1 » Wed Apr 16, 2014 8:25 pm

Nice little schematic Zeke. I did the exact same thing really with success all last year. Only deviation is I used a single relay to run both fans and I used a single fuse at a rating slightly more than both fans rated load together. I also used the ignition wire to drove the relay also with diode to prevent any potential engine run on.

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Re: Cooling Fan Wiring Schematic

Postby Monza Harry » Wed Apr 16, 2014 9:06 pm

Chris (Zeke) can you go over the two "extra" diodes from 86 >86 on both relays again please I'm not following their purpose in noise suppression. I wouldn't think that there would be much noise (unwanted AC component superimposed on the DC base, for those that aren't familiar with that term, can really wreak havoc on a computer) from the "motors" on the coil side of the relay. Please shed some light on this for me, I'm just not seeing this tonight, (and I am tired so it could just be me suffering from "OldTimers"). Thanx Chris, Harry
Last edited by Monza Harry on Wed Apr 16, 2014 9:33 pm, edited 1 time in total.
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Re: Cooling Fan Wiring Schematic

Postby zeke » Wed Apr 16, 2014 9:13 pm

Sure Harry, np.

A relay is basically an electro-magnet. When a coil is de-energized, the collapsing magnetic field induces a charge in the coil that is in reverse polarity. The diode is then forward biased and shorts out this energy to keep it from adding to the DC level. You here this as "pops" in your speakers and it can wreak havoc with electronics. This phenomenon is known as back EMF or electromotive force. How's that? :)
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