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Electrical components

ScotY on Tue November 13, 2007 2:10 AM User is offlineView users profile

Year: 1988
Make: Suzuki
Model: Samurai
Engine Size: 1.3L
Refrigerant Type: 134a
Ambient Temp: 80
Country of Origin: United States

Can someone tell me how the electrical components in my a/c system interact with each other? I guess I should be more specific...

There's a switch of some sort on the drier. What does it do?

How does the thermostat work? Does it turn off the compressor? If so, when does it turn it off? Is it adjustable?

I'm thinking the little blue a/c button turns on the compressor and electric fan for the condenser. The expansion valve, being non-electrical, simply lets refrigerant flow to the evaporator. It senses, via the bulb, when the evap has enough refrigerant. How does it know this simply by being attached to the evap outlet line? The thermostat, if it senses the evap is too cold, shuts off the compressor. But doesn't the expansion valve already take care of this? The switch on the drier must be some kind of high pressure compressor shut off?

Thanks, Scot

Edited: Tue November 13, 2007 at 2:14 AM by ScotY

2POINTautO on Tue November 13, 2007 3:44 AM User is offlineView users profile

I will start with the switch on the drier, without looking it up to see if it is a single pressure or a dual pressure switch, it is just that, a pressure switch, either a LO pressure cutoff switch or a HI pressure cutoff switch or both, a DUAL pressure cutoff switch.

It protects the compressor from being able to run when system pressure is too LO either from a bad compressor, clog in the system or low freon in the system. Freon flowing thoughout the system carries the oil in the system to assist with lubricating the compressor, so low flow could result in underlubrication and resulting damage to the compressor. Too much pressure can damage the compressor from overheating and can allow an explosion at some weak point somewhere throughout the entire system. In the MVAC world (and HVAC), pressure and temperature go hand in hand, the higher the pressure, the higher the temperature, high temps will burn up the compressor and high pressure may cause a rupture anywhere in the system.

On a Honda, the pressure switch is in a circuit that feeds info to the ECU, if the ECU does not see a closed circuit then the ECU will not allow the Main Control Panel to start the compressor. The pressure switch can be momentarily bypassed (jumpered) to see if the switch is bad, once pressure has been evaluated as being in the good region. Jumpering the pressure switch and still not having a compressor activation means there is another problem, not forgetting that there can be more than one malfunction at the same time.

Many of these switches and circuits that you are refering to act as failsafes for the compressor, it is a type of Intelligent system, long before Intelligent systems were called that. AC Systems are relatively simple to understand and require a little more knowledge and experience to diagnose. Until only a few years ago, you only needed a simple multimeter or test light to diagnose the electrical system and a pressure gauge for the rest.

I will leave your other concerns for someone else to explain or add to my comments on pressure switches.

Give all the dirty details
and dont forget the LO & HI pressures
Year, Make & Model would be nice too

NickD on Tue November 13, 2007 8:56 AM User is offline

Nothing like having a circuit diagram, Japanese cars in that era were pretty much the same using a receiver with a sight glass with the dual function switch mounted. It had closed contacts if the pressure was between 40 to about 400 psi, lower or higher than that, an open circuit that would disable the compressor circuit. I liked the idea of this switch, our domestic cars used a high pressure relief valve to blow out refrigerant if you got one too many bugs in your condenser. Sure, R-12 was only a half a buck a can, but it was one more thing to do. I never was in the position where I needed one more thing to do. In a properly charged system, the dual function switch would kill the compressor if the ambient dropped below 40*F, but domestic cars would to the same thing with the cycling switch.

Key compressor control was with the evaporator thermal switch, either a thermistor or a capillary tube thing that would work contacts. Function was the same, kill the compressor if the evaporator temperature dropped below 33*F and turn it on again once it hit 39*F. Some cars using the thermistor with an electronic amplifier circuit had an econ position, where the compressor was killed about 7-9*F warmer, big deal.

Key control element is the clutch relay, dual function switch, blower in the off position, evaporator thermal switch, and that nice AC switch would all kill the compressor circuit. Either used multiple switch contacts, isolating diodes, or electronic circuits for the relay control. Simple in theory, working on these things was something else as all these parts are hidden.

One feature I did not like on some Japanese cars was a compressor underspeed sensor that would switch off the compressor if it's speed was around 20% lower than the crankshaft. If it slipped that much, you would have to be stone deaf not to hear belt squeal and just hit the AC switch, but these were a pain in the butt to repair with super way overpriced parts.

Radiator fan control that also cools the condenser are also part of the AC system but part of the engine circuit as well. I liked the Jap idea of using a radiator sensor to activate the fan if the coolant temperature exceeded 160*F rather than having the system ready to blows it's cork like so many domestic cars before the fan comes on. And your temperature gauge was always at a constant 195*F.

Get an subscription, very little money for the information you are getting.

TRB on Tue November 13, 2007 10:53 AM User is offlineView users profile

This is an AMA after market system. OEM diagrams will not be accurate for our wiring system. This a straight forward wiring system all isolated from the rest of the vehicle. Push button switch to preset thermostat. Thermostat to HPCO on drier. HPCO to compressor or a set of relays which then activate the clutch and or electric fan.


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ScotY on Wed November 14, 2007 3:41 AM User is offlineView users profile

2Point and Nick,

Thanks for the info. Even though I was referring to an AMA kit, all your typing is much appreciated...after I learn on the AMA kit, I will try and tackle refurbishing an ancient OEM Suzuki system so your info will come in handy when I try to figure that one out.


Okay, so does this sound right for my AMA system...

The switch on the drier turns off the compressor when a predetermined pressure is reached. Is this a safety overload type switch?

The thermostat also turns off the compressor when the evap gets too cold. So the thermostat is what tries to maintain a relatively constant high side pressure to feed the expansion valve/evap?

mk378 on Wed November 14, 2007 2:01 PM User is offline

The switch on the drier is for safety only. It should never turn off during normal operation.

The temperature of the evaporator is what is controlled during normal operation. If the evaporator became too cold, it would cover with ice and block the airflow. The thermostat is there to stop the compressor before that happens. After it warms up a little the compressor will restart.

The temperature knob on the dash has nothing to do with the compressor, even on a stock system. When the knob is set other than full cold, some of the cold air leaving the evaporator will pass thru the heater and be reheated before it goes out the vents.

Note also that a TXV does not regulate to any particular temperature. It controls the refrigerant flow relative to pressure and temperature to attain optimal cooling. But the actual evaporator temperature depends on ambient temperature, engine speed, etc.

ScotY on Wed November 14, 2007 5:39 PM User is offlineView users profile

Originally posted by: mk378
Note also that a TXV does not regulate to any particular temperature. It controls the refrigerant flow relative to pressure and temperature to attain optimal cooling. But the actual evaporator temperature depends on ambient temperature, engine speed, etc.

Can you explain further how the expansion valve controls the refrigerant flow? I saw a diagram or article about expansion valves somewhere before but am having a hard time grasping the concept. Since it only has the bulb connected to the evaporator outlet, I can see how it "thinks" by reading temperature. When it gets too cold, it shuts off the flow of refrigerant? Temperature and pressure must be related but I still can't quite wrap my brain around this.

Thanks also for clarifying my other questions!

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