My SDL developed an intermittent heating problem this Winter as in there would intermittently be no heat.... Turning the car off and then back on will sometimes result in coolant flow, sometimes. Simply turning the CCU off and then back on will never resolve the problem.

Voltage to the mono valve seemed to be correct but pulling the connector allowed coolant flow and heat galore. Did some credit card troubleshooting and installed a mono valve repair kit. No change. Hmmm, gotta be a flaky CCU, right? So I performed some more credit card troubleshooting and swapped out the CCU with a new rebuilt from Programa. Nuttin. Same intermittent issue.

Well, high time I did some real investigation and find out what's going on.

Thought I may have an aux water pump issue, but the fact that pulling the valve electrical connector results in getting coolant flow would seem to eliminate that I think. I noticed today that when in heat mode the voltage to the connector doesn't quite drop to 0 volts but about .6 volts. I surmise that this is probably correct as that would be about the voltage drop across a transistor to ground and certainly enough to release the valve. I have not been able to capture what that reading is during one of the times when the heat works normally to see if it is different.

So I have some data but I still can't put together an explanation for the intermittent nature of this problem. If anyone has ideas or experience with this type of issue please weigh in and tell me what I'm missing.

__________________
Joe

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1980 300SD - 495k miles - 'The Ambassador'
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Former Family Members
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