1983 300SD Mercedes Heater Weirdness - PeachParts Mercedes-Benz Forum

Adonis · #1 01-24-2015, 11:24 PM

I purchased a monovalve repair kit to replace the faulty unity with a torn rubber diaphragm. The new one I put in doesn't seem to work at all. All I get is cold air. I put the old one back in with no diaphragm on it and it delivers hot air full on. It only worked on low before that. What's going on? I have troubleshot this problem for days and all I have come up with is use the faulty monovalve for winter and the replacement for summer. I have already flushed the heater core, i have checked for blockages and the heat works perfectly with a modified unit with no rubber diaphragm but not with one that is brand new. Help!

toomany MBZ · #2 01-25-2015, 08:01 AM

May be a faulty new unit.

whunter · #3 01-25-2015, 04:20 PM

Quote:

Originally Posted by Adonis

I purchased a monovalve repair kit to replace the faulty unity with a torn rubber diaphragm. The new one I put in doesn't seem to work at all. All I get is cold air. I put the old one back in with no diaphragm on it and it delivers hot air full on. It only worked on low before that. What's going on? I have troubleshot this problem for days and all I have come up with is use the faulty monovalve for winter and the replacement for summer. I have already flushed the heater core, i have checked for blockages and the heat works perfectly with a modified unit with no rubber diaphragm but not with one that is brand new. Help!

#1. A faulty new unit?

#2. Electric fault supplying constant 12V to the mono valve?

If the new unit is OK, when installed - without the wire connected = you should have 100% heat.

Good luck.

Adonis · #4 01-25-2015, 10:58 PM

Quote:

Originally Posted by toomany MBZ

May be a faulty new unit.

Would there be a way to tell if that is the case? I have had the unit out and it responds appropriately to voltage and I can see it working when ac is switched on and when the heater is switched on.

If the new unit is faulty, what should it look like when I take it out if it is not under load. Should it be fully extended due to pressure from the fluid? The part that I have seems to have a suction issue when I pull it out due to the rubber would this also be an issue?

whunter · #5 01-25-2015, 11:34 PM

Quote:

Originally Posted by Adonis

Would there be a way to tell if that is the case? I have had the unit out and it responds appropriately to voltage and I can see it working when ac is switched on and when the heater is switched on.

If the new unit is faulty, what should it look like when I take it out if it is not under load. Should it be fully extended due to pressure from the fluid? The part that I have seems to have a suction issue when I pull it out due to the rubber would this also be an issue?

Diagnostic.
With the vehicle running at operating temperature.
When you disconnect the wire, you should have instant 100% heat.

If you do NOT have heat with the wire disconnected = the mono valve is defective or you have air trapped in the system..

.

Adonis · #6 01-25-2015, 11:42 PM

Quote:

Originally Posted by whunter

Diagnostic.
With the vehicle running at operating temperature.
When you disconnect the wire, you should have instant 100% heat.

If you do NOT have heat with the wire disconnected = the mono valve is defective or you have air trapped in the system..

.

Okay, this is what I did. I have been trying to figure this out for days. I have been driving for a week or two in 0 - 20 degree weather every morning with my boys and they hate me!

I put the old monovalve in and looked around the whole vehicle after talking with an older import mechanic who is a family friend with a 56 merc. He said the one thing he remembered about 300sd's was that vacuum was everything. I searched all over the engine area to make sure that everything was connected in the system and lo and behold right next to the fuel injectors was a tube out of place, which was causing the my engine to sound like someone was on and off the gas pedal. I had be perplexed about why I had an unsteady idle as of late and that seems to have been the culprit. With the old monovalve in I got up to about 90 degrees Celsius and then I heard a click and all of a sudden I got heat at full force. That had certainly never happened before. So now I am afraid to take the old one out and put the new one back in again.

whunter · #7 01-26-2015, 12:02 AM

Quote:

Originally Posted by Adonis

Okay, this is what I did. I have been trying to figure this out for days. I have been driving for a week or two in 0 - 20 degree weather every morning with my boys and they hate me!

I put the old monovalve in and looked around the whole vehicle after talking with an older import mechanic who is a family friend with a 56 merc. He said the one thing he remembered about 300sd's was that vacuum was everything. I searched all over the engine area to make sure that everything was connected in the system and lo and behold right next to the fuel injectors was a tube out of place, which was causing the my engine to sound like someone was on and off the gas pedal. I had be perplexed about why I had an unsteady idle as of late and that seems to have been the culprit. With the old monovalve in I got up to about 90 degrees Celsius and then I heard a click and all of a sudden I got heat at full force. That had certainly never happened before. So now I am afraid to take the old one out and put the new one back in again.

The 300SD climate control system has a ground control thermal sensor on the engine.
This sensor PREVENTS any function when the engine temperature is below 40° C.
When the sensor makes contact, you will hear system activation = the click you heard, followed by heat.

During severe cold weather:
I suggest cutting cardboard to block off 25% of the radiator air flow.
This will give better / faster heat response from the climate control system.

I have done this trick more times than I care to recall.
The trick is modified to cover larger radiator area as the temperature plummets = at -60° F and lower I cover 90% of the radiator while keeping a diligent watch on engine temperature.

.

whunter · #8 01-26-2015, 12:06 AM

Quote:

Originally Posted by Adonis

Okay, I have been driving for a week or two in 0 - 20 degree weather every morning with my boys and they hate me!

.

A block heater will eliminate the delay for heat, and make cold starting EASY.
http://www.peachparts.com/shopforum/diesel-discussion/85227-engine-block-heater-ideas.html

.

whunter · #9 01-26-2015, 01:43 AM

Go to EBAY.

Copy and paste this in the search: Kat's circulation heater

You simply splice it into the heater line.

.

Adonis · #10 02-04-2015, 01:57 PM

I do have a block heater. I am also wondering if I should by pass the pump that is between the heater core and the monovalve. I do feel it spinning though, so maybe not, perhaps just a pull and clean. I did contact the seller and have another monovalve shipped out which was completely different from the first one in terms of how it looked. It was slightly different, however, it arrived and already had a torn diaphragm, so I contacted the seller again and should have another shipped out soon. If I could take them apart I could have a good one possibly, but I hear it's nearly impossible to put back together.

Adonis · #11 02-23-2015, 03:27 PM

Okay, sorry for the delay. I got a new part after receiving damaged parts and here is the issue now. One day while it was warm out, I turned the heat on and it was strong. Sooo hot. When it's really cold, I am getting nothing unless I wait until she is warm, them turn the car off and start it again. I watch the thermostat go down a little and then the warm air disappears again. I have tested the voltage going to the monovalve and I am getting 12.3 to 12.9 volts and the ground is at -12.3. This seems as it should be to me. Is there anything else I should be checking? Could this be a thermostat issue?

SD Blue · #12 02-24-2015, 07:57 AM

That sounds more like air in the coolant system. With a cold system, and the nose of the car elevated on ramps, or similar, add coolant through the top radiator hose until it won't accept any more. Reconnect and run to bring the system back to operating temp and the temp control to high. (Keep the nose of the car elevated.)

CAUTION! Be careful disconnecting the upper hose as some have snapped the neck off while trying to free the hose from the radiator.

Adonis · #13 02-24-2015, 09:23 AM

Quote:

Originally Posted by SD Blue

That sounds more like air in the coolant system. With a cold system, and the nose of the car elevated on ramps, or similar, add coolant through the top radiator hose until it won't accept any more. Reconnect and run to bring the system back to operating temp and the temp control to high. (Keep the nose of the car elevated.)

CAUTION! Be careful disconnecting the upper hose as some have snapped the neck off while trying to free the hose from the radiator.

I drove this morning and there was another issue....it seems as though my vehicle is heating up to almost 120 c. Would that be cause to suspect that maybe my thermostat is stuck in the open position? If it was stuck open then wouldn't it just be circulating coolant and not really ever getting up to temperature properly?

SD Blue · #14 02-24-2015, 09:47 AM

It could be the thermostat but it still sounds like you have air trapped in the coolant system. The reason I point to this is that you are seeing major fluctuations. Usually, but not always, a failed thermostat goes one way or the other, either stuck open or stuck closed.

Stuck open and the temp never gets high enough. Stuck closed and you have danger of over-heating. You seem to be experiencing both.

Adonis · #15 02-24-2015, 01:05 PM

Quote:

Originally Posted by SD Blue

It could be the thermostat but it still sounds like you have air trapped in the coolant system. The reason I point to this is that you are seeing major fluctuations. Usually, but not always, a failed thermostat goes one way or the other, either stuck open or stuck closed.

Stuck open and the temp never gets high enough. Stuck closed and you have danger of over-heating. You seem to be experiencing both.

So, elevate and fill, re-attach hose and run? Is there anything in particular I am looking for when it's elevated to clue in that it has expelled air? I have never gotten the heat while moving and got it once sitting idle at a stop light for a few minutes. I do hear gurgling coming from the center console which I assume is coolant flowing into the heater coil.