Hi all,
I've got a 1993 940 wagon that has intermittent failures of the heating system. It has what I believe is called the manual climate control system. Just has a temperature knob on the right side of the control unit. No ability to set as specific temperature.

Three mornings ago, I drove it to work and had no heat. The next two days it worked fine. I haven't been able to catch it in failure mode when I have time to look at it. The heater control valve under the hood is relatively new and the vacuum line seems to be attached well. I think the controls between vents, defrost, etc. are working so that would seem to indicate I'm getting good vacuum to the control unit. On the days the heater works, it seems to operate normally.

Any thoughts on what is going wrong or what to look at? It seems most likely to me that it is an issue in the control unit given that when it works, everything seems to be fine. The FAQ talks about some control board/relay issues but these seem to be more AC related.

Thanks!

two possibilities come to mind.

1. there is an air bubble in the heater core not yet work our of the system.

2. the heater control valve is vacuum controlled and you maybe a poor vacuum connection

Thanks Trichard. I'll try to collect more observations if it continues to have issues. If it fails, I will confirm vacuum supply by changing from defrost to vent and/or recirculate. I haven't noticed any control issues with the AC system. The coolant system hasn't been touched for a while, at least months so I don't think an air bubble is very likely. I think the heater acted up like this a little last winter but it was very intermittent.

I'm trying to understand how the heating side of the control unit works. Are there any failure points in the control unit that could explain this?

the heater control is vacuumed operatede as you can see by looking for the tiny plastic line attached to the valve near the intake mnaifold.

then its either open, closed or partially open

from there on it is a matter of where the heat goes.

windshield, floor out the dash etc etc.

one other possibility comes to mind. a clogged heater core.

you might want to seriously flush your cooling system with the heater core accessible to the flushing process by leaving the heater control valve fully open

How does the control unit control the vacuum? Does the rotating heat control switch control the vacuum directly or are there electronics involved somewhere?

I'm driving it to work about every other day right now. It was fine on both trips yesterday. I'll continue to monitor to see if it acts up again and try to get some more information if it fails. I checked coolant level and it was fine. AC functioned well all summer.

The heater control valve is spring forced to be open without a vacuum. No vacuum and the coolant will flow. A disconnected (and plug) HCV vacuum hose should result in full heat if everything else is working. This is true for most cars. Also, the lever could be hitting a hose.

I had a Plymouth voyager that could not get much heat in the winter until I found out the thermostat was stuck open. Whats your temperature gauge say? May be worth a check.

Tom

That is really helpful information since it shows that not coming on is not a vacuum problem to the valve. It surprised me they worked that way- I had never thought of that and I confirmed it today with a spare valve I keep in the garage.

This problem has been very intermittent- it hasn't happened since I first posted a couple of weeks ago and I've been driving it every other day. The suggestion from Trichard about an air bubble is possible but I haven't had the system open for quite a while and the coolant level is good. The temp gauge indicates normal operation. I'm still suspicious of a control issue but I can't do any diagnosis until I have a problem.

I'm hoping for no more problems but it rarely works that way. :)

Thanks all! Sorry I've been slow responding but I've been traveling a lot.
Lyle