No hot or cold air blowing in '84 380SE

I have an '84 380SE. I was trying to use the climate control the other day but whether at "Min", "Max", or any temperature between, the air blowing out of the vents seemed to be the same: roughly the outside temperature.

The fans were blowing and when I had it on "Min" temp it seemed to have high fan speed on, then after switching temp to "Max" temp the fan speed slowed down to a very low speed. So, I know in at least some way the temperature wheel is doing something, just not changing the temperature. (I had it on auto fan speed, but also tried min and max fan speed and the air temp seemed the same.)

The AC was previously retrofitted to 134a and had been working. The AC compressor was engaged when I put it on "Min" setting but none of the tubes were getting cold so I suspect the AC unit is not working properly. I'm not surprised about this because recently I believe all freon was lost when the engine of the car was taken out for repairs. I had put some freon back in with a recharge kit but probably not enough.

Anyways, right now I am more concerned about why hot air was not blowing. I am not going to worry about the AC problem unless evacuating the AC lines, to get rid of any air that got in, and then refilling with freon doesn't fix it. Can anyone suggest the likely culprits of this problem?

Thanks in advance for any suggestions.

[carnut]

I believe this 84 uses a MONO valve to circulate hot coolant. The MONO valve diaphram could be torn. Verify if it has one, then purchase a repair kit for the valve and install it.

I had been reading and saw that the monovalve was a seemingly common problem so I took a look at mine. I had the car on and had someone turning heat on / off while I touched the monovalve unit and I felt it clicking as the heat was being turned on and off. If the monovalve had a break, wouldn't it likely cause heat all of the time? In either case I will pull it out and take a look at it though if nothing else seems to solve the problem.

Thanks for the suggestion.

[carnut]

All you have done is verify voltage. You must inspect the diaphram. When mine tore, it would blow warm only in the max hot temp wheel setting, turned anywhere else the heat would quit and not return unless car was turned off and restarted. My experience is its best to leave the wheel somewhere in the middle and select auto center button for fan and center push button for mode. And leave it alone all year long! except for occasional defrost or econ. operation. Some folks operate it as if it were a manual system? which I personally think causes early failure of vacuum motors and switches. I owned my 81 300CD 10 years and never had to repair it except for burned out bulbs in the control head once and one MONO valve repair kit. I would also verify that the aux. electric water pump works as well. At idle or low car speed it turns on to provide extra push to the coolant flow to the heater core. Some have suggested adding an in line fuse to that pump and I think thats a good idea. They say a siezed pump can damage the control head.

Ok, thank you for replying again. I was under the impression that a tear in the monovalve would have meant heat all of the time rather than what you described with heat only when you start the car on the max heat setting and not changing the dial at all. I drove the car around and did some tests and what you described is what is happening to me. I'll definitely pull that thing out and see if I can notice any breaks or problems with it. BTW, I don't play around with the climate control dial when its working properly, I just start playing around with it when something isn't working and I'm trying to see what exactly the symptoms are. Hopefully thats the only problem with the CC, and now I will just have to fix the rough idle until she's warmed up!

[carnut]

I think we've made some headway! Be aware to remove the radiator cap prior to MONO valve removal. Do it on a cold engine! Dont know where yours is located, but on my 300CD I loosened the two bolts holding it to the bracket on the firewall pushed it forward a bit and removed the four screws holding the lid on. Gently lifted the diaphram out paying attention to the order of washers and spacers! The tear will be obvious as you stretch out the diaphram. Also make sure the fine mesh screen at its bottom is clean and un-restricted.