87 300D heating system problem
 

I have had the HVAC dash control replaced last year. Over the winter I ended up with no heat. There was a clicking noise from a valve (?) near the battery. I disconnected the cable at the top and I have plenty of heat. (Too much on a less than cold day.) What is this unit and how does it affect the HVAC system? Is there a good chance that if I replace it my problem will be solved?

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RE: 87 300D heating system problem
 

monovalve

RE: 87 300D heating system problem
 

The Automatic Control System HVAC is faulty by the look of it. I doubt it is the monovalve. The valve is controlled by 12V. Fully open when NO voltage or disconnected, fully close with 12 volt. Anything voltage in between is to control the opening gap for the hot water to flow through.

If you have heat when cable is disconnected then it means it opens OK. The cable is stuck at 12v even when heat is called for. Put the car on full heat, remove the connector and measure the cable connector with a voltmeter. Wait a while for the unit to settle. If it reads 12v ALL the time then the HVAC is faulty. Also your axuiliary pump is OK as you have heat when you removed the connector.

Good luck.