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TheWeekendWagon

'78 Factory A/C Relay Wiring Questions

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CLIFFS NOTES:

Has anyone replaced the factory A/C relay with an aftermarket relay? If so, what specs did you get and how did you wire it up? 

 

LONG VERSION:

Almost done returning my 280Z to functioning factory A/C after a previous owner's questionable retrofit/repair job. One of the last pieces I am missing is the factory A/C relay. It is missing completely and it is super hard to find without spending $80+.

 

Question #1 - Can it be replaced with a standard off-the-shelf relay? I would assume so, but I am not sure the specs on the factory one.

 

Question #2 - How would it be wired? It's not super straight forward on the factory wiring diagram side. I have stared at it for hours and I can't confidently figure it out.

 

There are 4 wires from the factory, 2x LY, L, and Y

 

Usually on a relay you have constant power, power from the switch, power to the component you're trying to turn on, and ground. I am no wiring expert but I cannot for the life of me figure out how the factory wiring diagram gets all those things to the relay.

 

The LY wires appear to be the power wires from the switch. It can be traced from the fuse box to the blower motor and a/c resistor (switch), then it runs to the relay where it splits into 3. Two of the LY wires go to the relay, the other goes to the F.I.C.D. Why are there two LY wires going to the relay when the originate from the same main wire?

 

The L wire, this one is pretty clearly the power to the compressor and it also branches off to the F.I.C.D. The A/C is switched on, the relay triggers and sends power to the compressor and F.I.C.D. Got it. That's the only easy one.

 

The Y wire goes from the relay to the Pressure Switch.

 

Which one is ground? Which one is 12V constant? It's driving me crazy!

 

Thanks in advance for any help!

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Four pin relays have two grounds and two positive, not 3 positive and 1 ground. I could guess at what the wires are based only on that, but you really should be able to figure it out based on the wiring diagram. Trace LY as far as you can and see if it runs to ground somewhere. 

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12 minutes ago, calZ said:

Four pin relays have two grounds and two positive, not 3 positive and 1 ground. I could guess at what the wires are based only on that, but you really should be able to figure it out based on the wiring diagram. Trace LY as far as you can and see if it runs to ground somewhere. 

 

Well you would think I would be able to figure it out, but I am either reading the wiring diagram wrong or not understanding how that particular factory relay works.

 

From what I can tell LY originates at the fuse block so I would think it was power. Here's the wiring diagram for the A/C circuit...

 

30923624388_f3e9541253_o.png

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The two LY's are +12v power from the fuse block - one is power to the relay switch and one is power to the relay coil, the L is power out of the relay switch (the opposite side of the switch to your LY) to the compressor clutch magnet, Y is obviously the ground side of the relay coil (again, opposite side of the coil than your 2nd LY).  That's how you wire up your new relay...

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14 hours ago, calZ said:

Four pin relays have two grounds and two positive, not 3 positive and 1 ground. 

????

A simple relay has a switch and a coil (electromagnet).  The coil requires power and a ground to operate.  The switch is just a "break" in a strand of wire.  In automobiles it can be a battery voltage wire or a ground wire.

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Sorry, I should have said two pins that go to +12V and 2 that are routed to ground, with switches or a component in each circuit. Those are their states when the relay is not triggered. I didn't communicate that very well, so that's my fault. Once the relay is triggered, then the output pin has 12V like you said. 

 

I'm glad you got it figured out. 

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10 hours ago, cgsheen said:

The two LY's are +12v power from the fuse block - one is power to the relay switch and one is power to the relay coil, the L is power out of the relay switch (the opposite side of the switch to your LY) to the compressor clutch magnet, Y is obviously the ground side of the relay coil (again, opposite side of the coil than your 2nd LY).  That's how you wire up your new relay...

 

Thanks! I must admit I still don't fully understand how that is the conclusion but I will take your word for it! I would like to learn more about this part of things as it is admittedly one of my weakest areas of automotive knowledge so if you don't mind a few more questions I would appreciate the lesson!

 

My confusion lies with both LY 12V power lines being constantly fed power from the fuse box. if the power to the coil is constant wouldn't that mean the relay is always triggered? In it's normal state shouldn't it not be getting power until the switch sends power to it triggering the relay? Clearly I am not understanding something here as I don't think Datsun would have some sort of weird voodoo relay, but I would like to understand. From what I gather you have the power and ground to the coil in the relay. You hit the switch, power goes to the coil and creates the magnetic field that makes the connection between the constant power source and the thing you're trying to turn on. In this particular case is the F.I.C.D. or the Pressure Switch the key to how this all works? Does the coil not get ground unless the pressure switch connection is made?

Edited by TheWeekendWagon

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20 hours ago, TheWeekendWagon said:

My confusion lies with both LY 12V power lines being constantly fed power from the fuse box. if the power to the coil is constant wouldn't that mean the relay is always triggered?

 

No (or not necessarily...).  

 

A relay is just an electrically operated switch.  It's almost always activated by another switch (switch working a switch - weird huh?).  Many of the switches in a Z are "switching" (breaking) the ground wire.  Thats the case here.  So, the relay coil is powered by the wire from the fuse panel (which is activated by the ACC (accessories) circuit I believe (it's not "hot" continually)) BUT, it's not active (the electromagnet doesn't "work") until it has a ground applied (has a path to ground).  The ground comes from the blower switch and passes through all the other control switches in the A/C circuitry (microswitch, thermostat switch, etc.)  If any of these switches in line are not "made" (closed), the relay coil is inactive (no path to ground) and it's switch is "open" which simply means no voltage is being passed to the compressor clutch magnet.

 

The relay is a way to control the switching of the compressor ON/OFF with lower amperage wiring and switches while the relay switch provides a higher amperage wiring path to the compressor.

 

The relay itself is VERY simple, the multitude of additional switches and safeties and controls in the A/C circuitry make understanding the circuit more difficult.  You probably got lost at the blower switch...

 

(The ground path is: Earth (shown in diagram above as an attachment to the body of the Ignition Relay) B -> Blower Switch -> LW -> LY MicroSwitch -> Y Thermostat Switch -> BG to Connector -> YR Pressure Switch -> Y to Relay.  (If you look closely at the Blower Switch, it shows that B -> LW is closed in all fan speed positions and open when in the OFF position.) 

 

OK, all the wire color changes don't help either...

 

The Pressure Switch in these cars is a "high pressure cutout".  That switch should ordinarily be closed unless the high pressure side (liquid line) of the refrigeration tubing is overly high pressure.  That doesn't happen normally if the A/C refrigerant charge is correct, but it's a "safety" that "shuts off" the compressor in an over-pressure situation.  It does that by removing the ground connection to the A/C Relay.  It resets itself when the pressure drops.

 

Edited by cgsheen

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13 hours ago, cgsheen said:

 

No (or not necessarily...).  

 

A relay is just an electrically operated switch.  It's almost always activated by another switch (switch working a switch - weird huh?).  Many of the switches in a Z are "switching" (breaking) the ground wire.  Thats the case here.  So, the relay coil is powered by the wire from the fuse panel (which is activated by the ACC (accessories) circuit I believe (it's not "hot" continually)) BUT, it's not active (the electromagnet doesn't "work") until it has a ground applied (has a path to ground).  The ground comes from the blower switch and passes through all the other control switches in the A/C circuitry (microswitch, thermostat switch, etc.)  If any of these switches in line are not "made" (closed), the relay coil is inactive (no path to ground) and it's switch is "open" which simply means no voltage is being passed to the compressor clutch magnet.

 

The relay is a way to control the switching of the compressor ON/OFF with lower amperage wiring and switches while the relay switch provides a higher amperage wiring path to the compressor.

 

The relay itself is VERY simple, the multitude of additional switches and safeties and controls in the A/C circuitry make understanding the circuit more difficult.  You probably got lost at the blower switch...

 

(The ground path is: Earth (shown in diagram above as an attachment to the body of the Ignition Relay) B -> Blower Switch -> LW -> LY MicroSwitch -> Y Thermostat Switch -> BG to Connector -> YR Pressure Switch -> Y to Relay.  (If you look closely at the Blower Switch, it shows that B -> LW is closed in all fan speed positions and open when in the OFF position.) 

 

OK, all the wire color changes don't help either...

 

The Pressure Switch in these cars is a "high pressure cutout".  That switch should ordinarily be closed unless the high pressure side (liquid line) of the refrigeration tubing is overly high pressure.  That doesn't happen normally if the A/C refrigerant charge is correct, but it's a "safety" that "shuts off" the compressor in an over-pressure situation.  It does that by removing the ground connection to the A/C Relay.  It resets itself when the pressure drops.

 

 

Awesome, thanks! It's all making sense now!

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