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Prevent current backfeeding from aux to compressor

tomibriggs on Mon June 16, 2008 2:31 PM User is offline

Year: 1997
Make: Toyota
Model: Corolla
Engine Size: 1.8L
Refrigerant Type: 134
Country of Origin: United States

This is in reference to a previous post regarding the aux fan back-feeding current to the compressor. Previous Post

How do I prevent this? Is there a device I can attach between the fax & compressor to allow current to only flow in one direction?

TIA!

Edited: Tue June 17, 2008 at 7:04 PM by tomibriggs

GM Tech on Mon June 16, 2008 3:11 PM User is offline

Yes-- a 12 volt relay is the only way to go........you don't want to pull hi amp fan current through your a/c circuit- or you will blow fuses and/or melt wires....

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

TRB on Mon June 16, 2008 3:22 PM User is offlineView users profile

We have a relay & wring kit available. We send this with all the electric fans we ship. If you like give Jack a call and he can set you up with thee needed items.

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When considering your next auto A/C purchase, please consider the site that supports you: ACkits.com
Contact: ACKits.com

bearing01 on Mon June 16, 2008 3:50 PM User is offline

I'm not familiar with your circuit in question. There is an electrical device that only lets current flow in one direction, like a 1-way electrical check valve. It's called a Diode. You'll need a power diode that can handle the current that flows through it to feed the circuit. The voltage drop across the diode is typically like 0.7V and the power it must handle is 0.7V * I in watts, where I is the current drawn by the fan or whatever is drawing power through the diode. The alternative is to split up the circuits to not share a common power feed wire.

tomibriggs on Tue June 17, 2008 7:03 PM User is offline

Quote
Originally posted by: bearing01
I'm not familiar with your circuit in question. There is an electrical device that only lets current flow in one direction, like a 1-way electrical check valve. It's called a Diode. You'll need a power diode that can handle the current that flows through it to feed the circuit. The voltage drop across the diode is typically like 0.7V and the power it must handle is 0.7V * I in watts, where I is the current drawn by the fan or whatever is drawing power through the diode. The alternative is to split up the circuits to not share a common power feed wire.

I found several types of diodes from Radioshack
Which one do I need to allow 12V to only flow in one direction?

Matt L on Tue June 17, 2008 7:10 PM User is offline

You want a relay instead. A diode will reduce the voltage at the fan motor and won't take kindly to the induced spike when you cut the current.

Edited: Tue June 17, 2008 at 7:11 PM by Matt L

mk378 on Tue June 17, 2008 7:16 PM User is offline

OK if you really really want to do this, use a relay. You would need a considerably big diode to carry the 15 amps or so that the fan uses. The voltage drop in the diode would also reduce fan performance.

Another alternative mentioned by bearing01 is a double pole switch. One pole for the compressor, one for the fan.

It would be really good to have your pressure switch wired in to protect from having a hose blow out from overpressure. The pressure switch may not be designed for full compressor current. Thus a relay on the compressor would be necessary.

tomibriggs on Fri June 27, 2008 7:21 PM User is offline

This is what I did. Its temporary yet it works so I'm assuming its correct unless you guys think otherwise. Feel free to make suggestions.

Matt L on Fri June 27, 2008 7:58 PM User is offline

That's not quite right. When the compressor feed drops off, the fan may produce enough voltage to keep the relay terminals 87 and 85 energized.

Connect lead 87 to another fused line from the battery, which will completely eliminate any chance of feed from the fan to the compressor.

Also install a diode between leads 30 and 86. You want it to be reverse-biased in normal operation (no current flowing). That diode will take the back-emf generated when the relay contacts open, preventing an arc which will eventually destroy the relay.

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