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Marty
Marty, Mercedes Technician
Category: Mercedes
Satisfied Customers: 54027
Experience:  Bosch Certified Master Tech. Over 35 years experience.
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Did this issue ever get resolved? I have the same symptoms..

Customer Question

Did this issue ever get resolved? I have the same symptoms..
Submitted: 1 year ago.
Category: Mercedes
Customer: replied 1 year ago.
I thought I was replying to a conversation where another person had intermittent no heat in a 1988 Mercedes 560SL.
Expert:  Marty replied 1 year ago.

Hi, I do not see a reference to a previous question. Can you explain the problem that you are having and exactly what has been done so far?

Customer: replied 1 year ago.
No heat with mini valve installed, plugged in or not. Installed 2 different rebuild kits in mono valve. If it is removed and by passed using brass fittings heater blow warm.
Customer: replied 1 year ago.
Mono valve, not mini.
Expert:  Marty replied 1 year ago.

I'd replace the complete mono valve assembly. It sounds like you are not getting flow through the valve. There is a check valve in the bottom of the valve that may be stuck closed.

Customer: replied 1 year ago.
Water and air blow flow through valve.
Customer: replied 1 year ago.
Both flow through I meant
Expert:  Marty replied 1 year ago.

Does the auxiliary water pump work?

Customer: replied 1 year ago.
I removed outlet hose and not much flow out of it, but if I seal outlet w finger it pressurizes and will spray out if I release my finger just part way. Almost like it needs some restriction to flow.. Make any sense?
Also I was able to drill/break check ball out of bottom of mono valve thinking the same as you. Still no change.
Expert:  Marty replied 1 year ago.

Sorry for the wait I got pulled away from my computer.

Expert:  Marty replied 1 year ago.

Have you tried to bypass the aux pump with a fitting?

Customer: replied 1 year ago.
I have not by passed aux pump yet, only due to the fact it blows warm air with mono valve bypassed and pump still in place.
Expert:  Marty replied 1 year ago.

I know what you mean but I'd bypass it and see if it works with the mono valve connected.

Customer: replied 1 year ago.
Ok, Removed aux pump and reconnected lines together with 1/2" fitting. Got warm, not hot, air at idle. Raised RPM to 1500-2000 and it started blowing cold again. Returned to idle, warm air. Drove vehicle and cold until coming to a stop and lowing engine RPM.
Also at towards end of drive I pressed defrost and blower fan stopped working and would not come back on...
One thing after another..
Expert:  Marty replied 1 year ago.

You're going to need to get the blower going before we can go any further. I'd check and clean all of the fuses first. If the fuses aren't the problem you will need to check power and ground to the blower motor. The motor might have gone bad. In regards ***** ***** heat it sounds like you are losing the flow through the core or the core is partially plugged. All of your tests should be done with the defrost button pressed as that is a default for full heat to the windshield.

Customer: replied 1 year ago.
Blower fan is back working after cleaning fuses last night.
I don't see how it could be a plugged core, if it blows heat with the mono by passed.
Expert:  Marty replied 1 year ago.

I thought you were losing the heat even with the valve bypassed. If it blows great heat with the valve bypassed I'd have to try another valve.

Customer: replied 1 year ago.
Valve is only thing that makes sense.
By pass valve, aux pump still in place= good heat
Valve back in place, aux pump by passed = heat at idle only
Expert:  Marty replied 1 year ago.

Replacing the valve would be my next step. You should be able to find used ones on ebay.