electrical help
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From: Golden Colorado
electrical help
Ok, this is probably a silly question to those who know electrical (of which I am not an expert). I put dual electrical fans on my C2 and want to run the relay hot lead to the B+ on the alternator (new 100 amp one wire alternator) since it is really close to the relay and fans instead of going to the key or battery. Is this ok? Is this lead hot all the time or only when the key is on? (car not running yet). The fans draw 19.2 amps running and start up around 25 amps. Relay is rated at 50 amps so I was told running one relay was ok. I plan on running a switch and ground to the dash so I can turn on the fans anytime I want just by grounding the sensor. Any problems with setting it up this way?
Thanks for the help.
Thanks for the help.
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Ok, this is probably a silly question to those who know electrical (of which I am not an expert). I put dual electrical fans on my C2 and want to run the relay hot lead to the B+ on the alternator (new 100 amp one wire alternator) since it is really close to the relay and fans instead of going to the key or battery. Is this ok? Is this lead hot all the time or only when the key is on? (car not running yet). The fans draw 19.2 amps running and start up around 25 amps. Relay is rated at 50 amps so I was told running one relay was ok. I plan on running a switch and ground to the dash so I can turn on the fans anytime I want just by grounding the sensor. Any problems with setting it up this way?
Thanks for the help.
Thanks for the help.
Dan
Last edited by dplotkin; Aug 20, 2018 at 12:52 PM.
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Is your stated current draw for both fans or only one? If for both, your relay and CB are sized appropriately. Given you are going to be shutting off power via a battery cutoff switch (wired to the negative terminal and completely isolating the negative terminal, right?), I would wire it as follows:
The shortest heavy gauge wire possible from alternator B+ to CB
CB to relay pin 30 (heavy gauge power wire) and pin 86 (light gauge control power)
Relay pin 87 to fans (heavy gauge power)
Fan to ground (same gauge wire as power source)
Relay pin 85 to switch (light gauge control)
Switch to ground
The reason for this approach is your stated desire to control the fans regardless of ignition switch position and use of a battery cutoff switch, uses no switched B+, and is controlled by switching the relay coil ground light gauge wire.
The shortest heavy gauge wire possible from alternator B+ to CB
CB to relay pin 30 (heavy gauge power wire) and pin 86 (light gauge control power)
Relay pin 87 to fans (heavy gauge power)
Fan to ground (same gauge wire as power source)
Relay pin 85 to switch (light gauge control)
Switch to ground
The reason for this approach is your stated desire to control the fans regardless of ignition switch position and use of a battery cutoff switch, uses no switched B+, and is controlled by switching the relay coil ground light gauge wire.
Last edited by Factoid; Aug 20, 2018 at 01:54 PM.
Thread Starter
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From: Golden Colorado
Yes
Is your stated current draw for both fans or only one? If for both, your relay and CB are sized appropriately. Given you are going to be shutting off power via a battery cutoff switch (wired to the negative terminal and completely isolating the negative terminal, right?), I would wire it as follows:
The shortest heavy gauge wire possible from alternator B+ to CB
CB to relay pin 30 (heavy gauge power wire) and pin 86 (light gauge control power)
Relay pin 87 to fans (heavy gauge power)
Fan to ground (same gauge wire as power source)
Relay pin 85 to switch (light gauge control)
Switch to ground
The reason for this approach is your stated desire to control the fans regardless of ignition switch position and use of a battery cutoff switch, uses no switched B+, and is controlled by switching the relay coil ground light gauge wire.
The shortest heavy gauge wire possible from alternator B+ to CB
CB to relay pin 30 (heavy gauge power wire) and pin 86 (light gauge control power)
Relay pin 87 to fans (heavy gauge power)
Fan to ground (same gauge wire as power source)
Relay pin 85 to switch (light gauge control)
Switch to ground
The reason for this approach is your stated desire to control the fans regardless of ignition switch position and use of a battery cutoff switch, uses no switched B+, and is controlled by switching the relay coil ground light gauge wire.
The amps stated are for both fans as per Flex A Lite. I put one 50 amp relay, one 30 amp circuit breaker as shown. This is the way I wired it except I have a couple questions. I am not sure about the numbers by 85 I am guessing that is the brown wire. I thought it had to go to ign on hot to energize the relay. I was going to take it to a lug on the fuse box hot / on. Are you saying there is a different way to do this? 2nd question. I was going to put the battery disconnect on the positive post side of the battery. I have not problem with putting it on the negative instead, just wondering why this is better?
Thanks
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That diagram does not follow your description. It is using the thermostat to control the ground to the relay coil (relay pin 85) and either IGNITION ON or a switch to enable the circuit to work by providing 12v to the other side of the relay coil (relay pin 86). If you are going to use the thermostat, follow this diagram. If you really believe you will use the cutoff switch every time you shut off the car, you can skip the switch and connect the brown wire to the red wire connection on the CB. This means that every time the thermostat reaches its rated temperature, the fans will turn on unless the battery cutoff switch is off.
Always switch the negative side of the battery for a cutoff switch. There are a variety of reasons, but two of the easiest to grasp are that the negative cable is connected to the entire frame and every ground location providing a huge path for current flow and there are typically many 12v feeds on the positive cable creating more complexity when wiring in a cutoff switch. Isolate the frame.
Always switch the negative side of the battery for a cutoff switch. There are a variety of reasons, but two of the easiest to grasp are that the negative cable is connected to the entire frame and every ground location providing a huge path for current flow and there are typically many 12v feeds on the positive cable creating more complexity when wiring in a cutoff switch. Isolate the frame.
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Oh, the black wire is 85, the brown is 86, the heavy red to the fan is 87 and the heavy red to the CB is 30.
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From: Golden Colorado
ok, thanks
That diagram does not follow your description. It is using the thermostat to control the ground to the relay coil (relay pin 85) and either IGNITION ON or a switch to enable the circuit to work by providing 12v to the other side of the relay coil (relay pin 86). If you are going to use the thermostat, follow this diagram. If you really believe you will use the cutoff switch every time you shut off the car, you can skip the switch and connect the brown wire to the red wire connection on the CB. This means that every time the thermostat reaches its rated temperature, the fans will turn on unless the battery cutoff switch is off.
Always switch the negative side of the battery for a cutoff switch. There are a variety of reasons, but two of the easiest to grasp are that the negative cable is connected to the entire frame and every ground location providing a huge path for current flow and there are typically many 12v feeds on the positive cable creating more complexity when wiring in a cutoff switch. Isolate the frame.
Always switch the negative side of the battery for a cutoff switch. There are a variety of reasons, but two of the easiest to grasp are that the negative cable is connected to the entire frame and every ground location providing a huge path for current flow and there are typically many 12v feeds on the positive cable creating more complexity when wiring in a cutoff switch. Isolate the frame.
Thanks again.
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Irwiny's advice is well taken if you are using modern electronics in your car, otherwise I wouldn't worry too much about it. If the relays are not diode equipped you can easily put one in parallel with the relay coil, just BE SURE you observe polarity, otherwise its a dead short, and, use a diode rated for both correct current and peak inverse voltage (PIV). Guys here will probably be able to tell you which one and whereto find it.
Dan
Last edited by dplotkin; Aug 22, 2018 at 09:31 AM.
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Dan is correct, but one point of clarity. Free wheeling diodes are typically used in more modern vehicles where the ecu controls fuel injection and spark, using switching transistors to control the ground circuit to relays like the fuel pump and cooling fans. The reason is that when the transistor turns off, the relay field collapses, inducing a current flow into the circuit. This resulting current can create a momentary spike which can burn out the ecu transistor semi-conductor switch over time. For most of us, the only potential semi-conductor device is an aftermarket ignition box and most of these use an internal mechanical relay to control their power source. Long story short, don’t worry about it.