1991 Terrano Air Conditioning

[Cliff]

A/C does not operate. Problem appears to be with control circuit ECCS control module or Thermal Control Amp. The latter appears to be is a thermister Temp sensor device which I suspect sends a signal to the ECCS (ECM) but I can’t understand how the ECCS powers the  AC relay on and off. Is there mechanical switching in the ECCS or is accomplished electronically somehow? 

Edited May 3, 2020 by Cliff
Clarification

[Mr_Reverse]

The ECU controls the compressor relay to switch on and off the compressor. It achieves this by using a power transistor commonly called a driver to drive the relay. It also uses inputs from the pressure switch, ambiant temperature, evaporator temperature, throttle position, and the HVAC control head. 

 

At least that is how the North American Pathfinders are. I suspect the Terrano is similar.

 

Edited May 6, 2020 by Mr_Reverse