Put my 300TD away last year for the winter and now only blows hot air. AC was weak last year but still working, now HOT on everything. Put new thermostat and heater selanoid dewhicky thing in. Anybody know where there is a heater troubleshooting page or can diagnose this. On the other hand, the wagon started first crank and runs like a top. Diesel purged it yesterday and ready for the summer yahoo!!!

Mono valve is bad.

__________________
ASE Master Mechanic
https://whunter.carrd.co/

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
Vehicle build.
Fleet Durability
Technical Quality Auditor.
Automotive Technical Writer

1985 300SD
1983 300D
2003 Volvo V70

https://www.boldegoist.com/

heres a link to what cured my "always have heat problem" HTH Mike
Fixed the "only have heat problem"

monovalve brand new, thats what I replaced couldnt think of name.

The default on these systems with a loss of power is constant heat. The default for loss of vacuum is full defrost. When power is applied the plunger inside the monovalve pushes down to restrict water flow through the heater core.

You may have a bad connection (no power) to the monovalve. You can test this by putting a test light on the plug to the monovalve and then switching between the defrost setting and then any other setting with the temp wheel on cool on the controller. No power on defrost and then power on the otehr setting.

BTW - when it is working properly you can actually hear it clicking on and off to maintain the temp in the car.

__________________
'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique

I often read these monovalve problem threads and most everyone is quick to condemn the monovalve. The most common thing to happen is the electrical connection gets weak. Pull the plug, pull the plug appart, clean the contacts on the plug and valve. Then slightly crush the plug's connectors. Reassemble the plug, put a little di-electric grease on all the contact points and reconnect.
I'm glad Fisherman chimed in and suggested the things he did. Maybe more members will try this instead of replacing the valve for nothing. The kit's about $20 now isn't it?
I checked it the very same way nearly two years ago when I had my heater problems. I heard the clicking noises but only when I jiggeled the plug. It's worked flawlessly ever since.

__________________
DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012

Member "rg2098" replaced his mono valve and it did not function correctly, on my advice he removed it and reinstalled it several times, the final time it began working correctly.

Since the car was in storage, I would replace all fuses with NEW, it can not hurt anything and may fix the issue.

The other suggestions are very good also.

Sounded like an electrical issue. Bet his original mono-valve was fine.

__________________
DJ


84 300D Turbodiesel 190K with 4 speed manual sold in 03/2012