Hot air only sometimes, but monovalve seems fine

[Machiel]

Hi,

My name is Machiel and I am from the Netherlands. Since 2013 I have owned a 380 SEC from 1985. I hope my English is good enough to explain the issue I have since a couple of months. I have learned a lot already from this forum, yet I still have some questions left.

Let's start with the symptom. Sometimes, my heater only blows hot air, regardless of the temperature or push button setting. With the AC on, the hot air is cooled a little, which is the only way to prevent you from a heat stroke. I can unfortunately not force the heater to generate its problem. It appears (and dissappears) randomly.

I have removed and checked the monovalve and it seems really fine. You can manually move it up and down smoothly and the rubber has no cracks or something. Also the resistance level through the coils is ok (15 Ohm). My (preliminary) conclusion was that the monovalve is unlikely to be the culprit.

I also measured the voltage to the monovalve which was 12 volt. Unfortunately, when I did the measurements the heater was working fine. I could feel it vibrate / pulsate at intermediate temperature settings.

After studying the (attached) diagram, I concluded that it cannot be the temperature control unit that causes the problem. If the problem occurs and I set the temperature dial to MIN, the climate control unit (CCU) should bypass the control unit by making a direct ground connection. But the air remains hot in that case.

• 1. Is this correct? Can I ignore the temperature control unit?

If indeed the connection to ground in MIN is not well established, I reason that it is either the CCU that is corrupt or the temperature dial. I have read about bad soldering in the CCU. But before attempting to remove and opening up the CCU, I was wondering whether the temperature dial perhaps directly connects the monovalve to ground (G102 in the diagram).

• 2. Does anybody know if this is the case? Should the temperature dial in the MIN position establish a direct ground to G102, or is that signal processed through the CCU?

If so, I can perhaps first check the temperature wheel only. If this occasionally sends MAX regardless of the actual setting, this may also explain why I got hot air at all settings if the problem occurs.

All help is highly appreciated!