# Can someone help me in understanding this wiring diagram?

#### gte

Joined Sep 18, 2009
357
I believe that the way this works is:

On my tractor, power comes in through a 10amp fuse to Defroster mode detection switch and the a/c switch
If foot/defrost or defrost is selected, power is bridged to acld and a/cs
If the a/c switch is enabled, power is bridged to acld and a/cs
If the THE or HP comes back with different values, the a/c is turned on or off depending on their values
The acmg output is what controls the a/c clutch

If I understand this correctly, based on the diode arrangement, it appears as if acld and a/cs are inputs, so I don't understand why you would need both of them? To me they appear to be doing the exact same thing? What would that resistor be there by the LED for?

#### shortbus

Joined Sep 30, 2009
10,045
I believe that the way this works is:

On my tractor, power comes in through a 10amp fuse to Defroster mode detection switch and the a/c switch
If foot/defrost or defrost is selected, power is bridged to acld and a/cs
If the a/c switch is enabled, power is bridged to acld and a/cs
If the THE or HP comes back with different values, the a/c is turned on or off depending on their values
The acmg output is what controls the a/c clutch

If I understand this correctly, based on the diode arrangement, it appears as if acld and a/cs are inputs, so I don't understand why you would need both of them? To me they appear to be doing the exact same thing? What would that resistor be there by the LED for?

The resistor in series with the led is the current limiter for the led.

The "HP" is the high pressure safety switch for the a/c system, it opens shutting it down when pressure is too high.

"acld" turns on the led indicator when the a/c is on.

"a/cs" is the compressor switching signal, turns on the magnetic clutch allowing the a/c to work. It is an output not an input.

"THE" is the thermistor(temp control sensor). Depending on inside temp it allows either cold or hot air to the defroster and controls a/c temperature.

gte

#### gte

Joined Sep 18, 2009
357
Thank you

#### faley

Joined Aug 30, 2014
88
Hello gte. I'll weigh in for ya. The resistor is there to set the operating voltage of the LED. (Ohm's Law 101, so to speak. I'm making an assumption but it's a safe one.) In this case, ACLD is an output. Beyond this, you really need a schematic of the unit to understand the logic. In production there can be numerous reasons for any given electrical configuration; some are based on pure control logic while others are determined by "what was on the shelf" etc., etc. I can give you tons of guesses but they would be meaningless. Your best bet is to speak with an OEM service rep or tech.

Clear as mud?

#### DerStrom8

Joined Feb 20, 2011
2,390
Hello gte. I'll weigh in for ya. The resistor is there to set the operating voltage of the LED.
This is not true. A resistor resists current, and does nothing to "set the operating voltage of the LED". The LED has a voltage drop, that's all. Let's suppose it's 2 volts. LEDs are current-controlled devices, not voltage-controlled. You could drive it with a hundred volts provided you limited the current to a value that the LED can withstand (and that the LED's insulation can take it). I generally assume 15-25mA. That's what this resistor is doing--it's limiting the current to something that the LED can handle.

Let's suppose the circuit runs on 12 volts. We assume the LED has no resistance, so using ohm's law we calculate the necessary resistance as follows:

R = (Vs - Vf) / If

where R is the required limiting resistor value, Vs is the supplied voltage, Vf is the forward voltage drop of the LED (2v in our case), and If is the forward current (let's say 25mA).

Therefore, the R would need to be:

(12V - 2V) / 0.025A = 400 ohms

You would need a 400 ohm resistor in order to not blow your LED.

#### faley

Joined Aug 30, 2014
88
This is not true. A resistor resists current, and does nothing to "set the operating voltage of the LED". The LED has a voltage drop, that's all. Let's suppose it's 2 volts. LEDs are current-controlled devices, not voltage-controlled. You could drive it with a hundred volts provided you limited the current to a value that the LED can withstand (and that the LED's insulation can take it). I generally assume 15-25mA. That's what this resistor is doing--it's limiting the current to something that the LED can handle.

Let's suppose the circuit runs on 12 volts. We assume the LED has no resistance, so using ohm's law we calculate the necessary resistance as follows:

R = (Vs - Vf) / If

where R is the required limiting resistor value, Vs is the supplied voltage, Vf is the forward voltage drop of the LED (2v in our case), and If is the forward current (let's say 25mA).

Therefore, the R would need to be:

(12V - 2V) / 0.025A = 400 ohms

You would need a 400 ohm resistor in order to not blow your LED.
You're absolutely right.