My AC was working great, even in the hot Sacramento area, until today, when all I could get was hot air blowing out all vents. After reading many posts which indicated the monovalve might be the problem, I pulled it out and found that the diaphragm was ruptured. Thanks to those who suggested that I start the problem-solving with the monovalve. Now my problem, which I'm embarrassed to say, is I dropped the valve and the 3 washers came off before I could see how they went on. There is a large flat washer, a small flat washer, and a waving washer. I'm not sure of the order they go on. Any help would be appreciated.

gdl40
84D turbo

As counterintuitive as it may be, a torn monovalve diaphragm usually is not a factor when uncommanded heat is involved. In most cases, a torn diaphragm causes the heater to not work, especially at higher engine speeds. Uncommanded heat is most likely an electrical issue.

tango I hate to take issue with you on this, especially considering my current status on this board, I'm afraid to say much of anything in the least bit negative. I have to offer this with all due respect since the member is having trouble and needs this information.

I have indeed seen ruptured diaphragms cause EXTREME uncommanded heat. I remember it vividly because of my wifes reaction when it happened on her 300TD.

I hope my disagreeing with you does not insult you or draw more criticism my way. I will apologize ahead of time if you are offended or insulted in any way.

In any event, the valve internals need to be replaced. If that doesn't fix the issue, more troubleshooting will be required.

Wow, how fesicious-feel free to criticize the spelling. I have the no heat at all problem which I attribute to a ruptured monovalve diaphram. I know my vacuum is all fudged up and the only blow I get out of the dash is through the drivers side vent and the defrost. The only other thing I can think of is a blocked heater core. Sorry gd; this doesn't really answer your question on how the monovalve goes back together. I may take mine apart when i get home and see if I find a ruptured diaphram. If I do I will post a pic or town before it gets taken apart.

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1983 300D TurboDiesel

Is it possible that repairing the monovalve also involved removing the electrical connector? And that, in the process, a bad connection was unknowingly restored, so that it appeared that the diaphragm was the culprit when the original problem was actually a poor electrical connection?

I never touch the electrical connector when putting in a repair kit. It seems to me that the diaphragm can fail leaking coolant past it allowing flow.

I wish I could help with the monovalve reassembly questions, but I don't have one at hand.

I did cure mine once by putting conductive "grease" on the contacts, it was periodically losing the connection and giving me a blast of full heat.

If you pull the electrical connector, you should be getting full heat. If you wire the monovalve to 12V and ground you should get no heat. The 12V signal energizes the monovalve and seals against flow through the heater core. The following statement is incorrect, but left here for the record. See below. If the diaphragm is torn, this is the same as de-energizing the monovalve since it allows unrestricted flow through the heater core whether energized or not.

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

...sometimes. In other cases folks have experienced a complete lose of heat due to a failed diaphragm. Either way, it needs to be replaced.

It's not that way at all. Flow is controlled by the position of the plunger.

If posts on this forum are any indication, the most common symptom of a torn diaphragm is, by far, no heat at highway speeds. And that situation usually resolves as soon as the vehicle slows down.

Small washer, wavy washer, large washer.

Goes to show you how I get into wierd situations. It was probably about 1992 or so when my wifes 300TD was frying her you know what. I pulled the top off the monovalve, found it flooded, got a kit for about $30 at the time, put it in and all was well. From that experience I've assumed ever since that a flooded monovalve meant full coolant flow.

You learn something new every day.

I've been thinking about this and I was incorrect in my earlier statement regarding the torn diaphragm. I said it allowed unrestricted fluid flow, but it doesn't have anything to do with fluid flow. I've edited my original post.

I don't believe the torn diaphragm itself causes the symptoms that have been described previously, but rather is the source of the problem.

The monovalve is an iron core inside a housing that sits inside an electomagnet. When the electromagnet is energized the core moves out. A spring returns it when de-energized. Attached to the core is a rod with a seal on the end. When the core moves out the rod extends the seal into a cavity where hot radiator fluid enters the valve body and blocks the flow of this fluid. As I recall, the diaphragm seals against radiator fluid entering the area that houses the iron core and return spring. If a diaphragm tears, coolant enter this area and probably washes away the grease that lubricates the iron core. Eventually, the rod binds either open, closed or somewhere in-between. So a torn diaphragm doesn't cause cold or hot air to blow. It causes the moving parts of the monovalve to deteriorate and eventually fail in a condition that is unpredictable. It's likely that one failure mode is more prevalent than another, e.g., failing in the energized position (the spring is not as strong as the electromagnet so the force to move it out is greater than the force returning it), but other failure modes are possible and not unlikely.

And tango, in normal operation the plunger is either fully extended, or fully retracted. Your post seemed to indicate there is an in-between state, and this is not the case. Flow is controlled by cycling the valve in intervals of several seconds, varying the number of seconds it remains on and off. For hotter air, the 'off' (de-energized) interval is increased in relation to the 'on' interval.

Any thoughts? Darts? Hand grenades?

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mjk

'84 300SD 119KMi (Liesl der Diesel)
'84 300D 326KMi when the oil left (former parts car)
'82 300SD 253KMi (new parts car)

I haven't had one apart in a couple of years, but I believe the entire area is normally wetted, even with the diaphragm intact. I'm really just guessing; but I suspect that a torn diaphragm affects the pressure difference across the valve "plug" and at higher engine speeds the spring does not have enough force to overcome the pressure and open the valve, resulting in a loss of heat. Just my thoughts.