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Clutch Wiring Theories

Shepherd777 on Tue November 22, 2011 1:51 PM User is offlineView users profile

Year: 2003
Make: Kenworth
Model: T2000
Engine Size: ISX
Refrigerant Type: 134a
Ambient Temp: 45
Pressure Low: 15
Pressure High: 15
Country of Origin: United States

Hello all -

I am interested in when the clutch should be "hot" with voltage.

On my Kenworth, with the key off, there is no voltage to the clutch, which is what one would expect.

With the at key "on" or at "accessory", one side of the two leads at the clutch is hot.

I would assume the other lead at the clutch is ground, but it is not, because I cannot get the test light to light up by going to the two leads. I can only get the test light to light up when going from the hot lead and then to the engine ground.

The KW has one electric switch for the A/C on the dashboard. There is no A/C electric switch in the bunk, only a temp adjustment switch. One would assume by turning off that dash A/C switch, power is terminated to the clutch. But that switch makes no difference regarding power at the clutch. I have power to the clutch no matter where the temp dial is set, or if that A/C switch is on or off.

This system has very low pressure due to a leak, until we fix the leak. Are the pressure sensors in the system causing us problems with that clutch voltage?

I really need to stop the voltage to the clutch when heat is called for as this compressor is powered by an electric motor and not by a drive belt. And we use that clutch voltage to energize a soft-start on the compressor electric motor.

Please, I appreciate everyone's help, but answer with germane clutch electrical/wiring theory answers only. This post is not for discussing the merits, or lack of, or your personal opinions of electric drives for compressors, or issues regarding the compressor type.


Edited: Tue November 22, 2011 at 1:53 PM by Shepherd777

mk378 on Tue November 22, 2011 2:31 PM User is offline

It sounds like the one wire is just the ignition circuit, the other wire is pulled to ground by the controls when compressor operation is called for.

If you don't have a closed circuit to pull up the controlled wire when it is supposed to be disengaged, you'll never measure any voltage on that wire.

The simplest though not real elegant way to interface with the existing truck controls would be to connect the two clutch wires to the coil of a relay, then use the contacts of the relay to control your electric drive. When the controls want A/C the relay will pull in the same as pulling in a clutch coil on a mechanical compressor.

Shepherd777 on Tue November 22, 2011 9:08 PM User is offlineView users profile

Thanks for the reply mk378.

I have never heard of the term, "pulled to ground" before.

Do normal belt-driven A/C clutches electrically engage and disengage the clutch from thermostat inputs? In other words, when the cabin is cooled sufficiently, the thermostat senses that and stops the current to the clutch, and the compressor just stops?

If so, when the thermostat senses compressor activation is required, and both clutch wires are attached to the primary side of the relay, then the second wire will now have voltage?

And the secondary side of the relay will act as a switch to turn on and off the electric compressor?


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