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.

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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.

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'72 280SE 4.5 - looking to breathe life into it
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'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.

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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.

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DJ


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

His old diaphram was shredded.
The new one seemed to have a binding issue and did not want to seat correctly.

Oh...

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DJ


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

I had a shreaded Monovalve that wasn't giving me ANY heat......if that diaphram is torn at all its junk.....

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Old mono valve was in bad shape ,so it needed to be done. But it still feels like a monvalve problem, just the other way. Will check power to it and go from there I guess, thanks for the help folks. Thats way we are the best forum on the net!

You sure you don't have a refrigerant leak? My 84 300D was like that and it turned out to be low on refrigerant. Try shorting the pressure switch on the evaporator for a second.

This is of course assuming your compressor isn't coming on at all before you short the switch.

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'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.

I tested the power to monovalve and its the opposite of stated. Power to on full heat and no power on cold. Puzzled now. Seems to work now,bad contacts I guess. Thanks all.

but I'm glad it's working...

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