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Erratic heater 81 300sd

527 views 8 replies 4 participants last post by  carnut62 
#1 ·
My 81 300SD heater and ac work but the heater will alternate between being too cold and the current setting. It is a us car but was delivered in Europe and has Celsius controls . How do you get the setting to stay consistent ? ???

Thanks Zeke
 
#2 ·
I see you are in Oregon. Being "too cold" may actually be outside temperature (heater does not heat up the air going through the core). What kind of monovalve do you have? Is it the cheap version off the market (less than $30)? Do you have heat at lower speeds (translated in lower RPM, let's say less than 2000 RPM), but you loose it at higher speeds?
 
#3 ·
Rpm does not seem to have an effect going over the cascades the temp would be fine and without changing any settings would go cold / hot on its own . uphill or down did not seem to be a factor but I was less than critically observant ?
 
#7 ·
Yep. eng temp is very constant the airflow from the fan is as expected, just for some reason it will fluctuate. heat right on the setting and then if you have turned the thermostat up because the setting was too cool it will either quit heating altogether or delay and come roaring on at the higher setting. Hmmmmm.
 
#8 · (Edited)
Dang.. that must be a pain!

If you disconnect the monovalve electric feed you should get full heat..... That's what I did to diagnose whether or not the monovalve was functioning properly...

Then I replaced the monovalve insert with one from MB directly and have had excellent results since. As has been mentioned above, the cheap MTC branded inserts are proven poison.. definitely to be avoided....

With an intermittent condition as you describe, its really a crapshoot as far as remote diagnosis goes.... I wish there was something more definitive to grab on to.

As far as common heater issues go.. if it is the heated water flow the monovalve [insert] is most often the cause.. Easy to diagnose as above..

Airflow problems seem most often to be caused by vacuum problems.. and those mostly the vacuum actuator pods having ripped diaphrams over time.

I will get off this iPad and upload a pdf on the hvac for you.. Maybe you have a mityvac tool to use to check for proper vacuum operation..

Others will chime in for sure on this... This time of year its a popular theme.

Brb,

MBL
 

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#9 ·
In my experience a heater temp fluctuation can be as simple as trapped air in the system. You need to first verify the cooling system is completely full then attempt to burp or bleed the system. Secondly the MONO valve should be removed and the rubber insert inspected for a tear. If both are OK then the vacuum pods need to checked for holding vacuum with a MityVac tool.
 
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