Diagnosing R12 System on 1988 560SEC - Page 4 - Mercedes-Benz Forum
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post #31 of 62 (permalink) Old 07-06-2018, 10:28 AM Thread Starter
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Block the flow from the monovalve to the heater. The monovalve failure mode is not usually electrical.
When my expansion valve acts up it shuts off the flow through the evaporator. The low side gets sucked down below normal. If your low side was normal when it cools and remains normal when it starts to warm up, the expansion valve is probably not the issue. In fact, that sounds a lot like the heater kicks in as the engine warms...
Can you elaborate on how to block the flow from the monovalve to the heater? I am unsure if I want to disconnect the hoses going to it. If I test for resistance at the connectors and there is none, should I just pony up and buy a MB repair kit?
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post #32 of 62 (permalink) Old 07-06-2018, 02:22 PM
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There are 3 hoses that go into the heater core , if you have one of those pinch clamp pliers pinch the hose in the middle of the three .

Or there are two off the back of the motor that are easier to get at , you can pinch one of those closed.

If your hoses are old and brittle and crack when pinched feel free to blame me.
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post #33 of 62 (permalink) Old 07-07-2018, 04:29 AM
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If I test for resistance at the connectors and there is none, should I just pony up and buy a MB repair kit?
No. the repair kit does not address electrical issues, only the valve itself. While it's certainly possible, I've not heard for a failure of the coil. Generally, the failure mode is lack of heat at road speeds.

That said, if the coil did fail, the heat would not shut off and that does match your symptoms. You could just put your hand on the hose going to the heater core. if the hose heats up as the ac cools down, Bob's your uncle.
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post #34 of 62 (permalink) Old 07-07-2018, 07:26 AM
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Fwiw, here is the oem monovalve from my car, replaced yesterday.

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post #35 of 62 (permalink) Old 07-08-2018, 12:56 PM Thread Starter
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I tested the resistance of the current monovalve in the car. With my digital tester set to 200 ohms it is reading 15.6. If I move the scale up to 2000 ohms it is 18. If I move it up beyond that it shows 0.01. Since the car has 103K miles do I just go ahead and purchase the repair kit from MB?
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post #36 of 62 (permalink) Old 07-09-2018, 07:18 PM
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I tested the resistance of the current monovalve in the car. With my digital tester set to 200 ohms it is reading 15.6.
Sounds about right.
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Since the car has 103K miles do I just go ahead and purchase the repair kit from MB?
Have you determined that the system is producing heat? If the heater hoses are getting hot then I think I'd pull the insert out and look it over. I'd want to be sure the problem was the valve before I spent that kind of money. Also, put your volt meter across the terminals of the valve with the plug connected and the system on cool. You should see about 13 volts, indicating the valve is being controlled off.

Another thing you can try is to put the system on economy. The AC wont run, but with the temp control on minimum there shouldn't be any heat, either. Just ambient air blowing out the vents.
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post #37 of 62 (permalink) Old 07-14-2018, 01:04 PM Thread Starter
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A quick update. The hoses coming out the firewall are getting hot. With the AC running max cool, aux fan running, monovalve connector plugged in, I am getting 12.7V at the two wires of the connector. If I unplug it and check the voltage at the connector, it is also 12.7V. This is at idle so I am sure it would come up to 13+V with some revs. I noticed with the monovalve unplugged, with the temp wheel still at max cool, it was blowing 120F at the center vents. When I plugged it back in and revved the engine up, it was able to bring the center vent temp down to 80F. It was 95F in my shop. For grins, I turned the wheel to max heat and the center temp went up a little to 90F but the fan speed turned down significantly even though I had the switch at Hi for fan speed. Based on the voltage and behavior of the heat, I think the monovalve is working. However, I think I have multiple issues as to why the AC is never getting cold. I am reaching a point that I feel I need to drop it off with an indie that actually wants to work on it. Any other thoughts?
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post #38 of 62 (permalink) Old 07-14-2018, 02:48 PM
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When I checked my monovalve after experiencing the same symptoms I found it was sticking in the coil and not always free to move as demanded by the CC. It did not matter what the voltage values were the heat was mostly on when I was using the AC.

I would definitely take the insert out and examine it before letting some other tech. look at it. When the monovalves went on sale a few months back some members bought two and Iím sure one of them will give you a good price for a proper Bosch unit if need be.
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post #39 of 62 (permalink) Old 07-15-2018, 06:42 AM
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The hoses coming out the firewall are getting hot. With the AC running max cool, aux fan running, monovalve connector plugged in, I am getting 12.7V at the two wires of the connector.
This should shut the heat off and apparently isn't. If the hoses are soft, you can try clamping the heater off with a set of pliers. There are special pliers made for this purpose, with round jaws, if you can find a set. Sounds to me like the monovalve is not shutting the heat off.

Did you try max cool and economy (no A/C)? If you're getting heat then, the heater is not shutting off.
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post #40 of 62 (permalink) Old 07-15-2018, 11:22 AM Thread Starter
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I went ahead and pulled the monovalve insert this morning. I was honestly shocked how clean it was compared to others I have seen in various posts. It is possible it has been replaced before but there were no marks on the flat screws of the valve. Since I could not find a part number on it, maybe it is a MTC replacement? I was also surprised by the large clear zinc plated washer that is part of the assembly. It looks like a generic washer from the hardware store. Is there any other way to identify if this is a MB part or MTC part?
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