Just what all you experienced wrenches wanted to read from a Barney like me...

Yet another Climate Control issue...

1983 300 D Turbo (123.133 617)

I recently had a MBZ independent tech troubleshoot the climate control system in my car. He said the push button unit is fine, but recommended a new ACC controller (“electronic switching unit” in the manual), monovalve, and aux. coolant pump in addition to the expected evacuation and recharge. I figured I’d blow off the AC side of things and concentrate on getting some heat--that is until I start sweating my @$$ off this summer, instead of freezing it off.

Thinking I could save a few $$$ I replaced the pump, rebuilt the monovalve, and replaced the ACC controller this afternoon. Only one problem... still no heat.

After exhaustive research and reading of old threads, I already found and tried the following advice from the forums:

“If no heat, you should check the voltage at the monovalve. If at 0 with far left button pushed, your climate control unit is working, at least on defrost”

Checked. 0 Volts found. What are the chances the controller is bad even though it passes this simple test?

“When electrical power to the monovalve is removed, it opens to a ‘full heat’ condition.”

“Disconnect the electrical plug on the monovalve. You should get full heat with the plug disconnected. See if that happens.”

I did, and it didn’t--still no heat--even with the car at operating temperature.

“I have been down the no heat road with a brand new monovalve repair kit. The plunger would intermittently stick in the monovalve body. I found the problem by bench testing it. Remove it from the car and very carefully wire it up so you can apply and remove power from a battery or other 12 volt power source. You should be able to blow air through it with no voltage applied, and it should seal completely when voltage [is] applied.”

It passed the blow test. I am still spitting out the coolant ...

“You can try bypassing the aux pump with copper pipe connecting the 2 hoses and you should get heat while driving if the pump has failed and indeed restricting flow.”

Pump worked fine when 12 volts were applied directly. When installed in the car, I can feel coolant running through the hose after the pump and into the monovalve. I can’t feel any coming out the other side, though. Why not?

“The temp sensor that measures the air temp inside the evap housing or the cabin air temp sensor is bad IF it is not the pushbutton unit. ...disconnect them and see what kind of resistance you get across them. If you don't get any then it is bad.”

Tried this one too. Reaching behind the vents, I could wrestle with the sensor, but couldn’t figure out how to unplug the connector so I could meter it. The manual shows the sensor, but not the connector, so I can’t tell what I’m doing wrong. Any ideas?

“...one way to crudely test the monovalve is to pull the electrical connection. ...touch the electrical connector to the pins - you should be able to feel the monovalve move as you make and break contact. For that matter, you should also be able to feel the pulses of the monovalve at any temperature setting by placing your finger on the top of the valve.”

I can feel the monovalve working--when energized directly by a battery--but can’t feel it working when installed in the car. Is this due to the fact that the interior temperature has not gotten up to a temperature where the sensor would close the valve? Another sign of a bad controller?

Which is/are the likely culprit/s?

Bad ACC controller? It’s a reconditioned one from ********AZ. Any ideas on how to test this bad boy without the plug in tester?

Air bubble in system? Should risk a major coolant spill by taking off the heater hose which comes through the firewall and goes into the engine block to see if there is some type of blockage, obstruction, or air bubble?

Bad heater core? Where is it? How do I get at it? How can I check it?

Possible thermostat problem? Even though the car operates at normal temperature while idling and at highway speed?

Please help, I’m not sure what to try or in what order to try it.

Thanks for reading my rant and offering any sympathetic suggestion,

RSCarey

Maybe the fan brushes on the blower are your problem.

A lot has been written on this site about worn brushes and a lack of heat. It only costs a couple of bucks and is an easy job, supposedly. I will be replacing them on my car sometime soon.

Search for fan brushes 123, or something like that.

Good luck

__________________
Ralph

1985 300D Turbo, CA model
248,650 miles and counting...

Thanks for the idea, but the blower motor seems to be working correctly. It blows, just not hot air.

RSCarey

I'm going to guess it has to do with the inside temp sensor. my 83 is really stubborn about delivering heat. after the engine is warm, i have to cycle the push buttons a couple of times before getting heat. now that being said, are you sure there's no bypassing of the heater core due to a leak? did the heat used to work?

__________________
1985 300D 197K - Semi-Daily Driver Diesel
1998 Volvo V70 AWD 226K - Daily Driver 2
1998 Volvo S70 140K - Wife's DD
2003 GMC Sonoma ZR2 Option - Rusty Truck
THE BABY 1958 220S Sedan 66K All original, never restored and never will be.

Maybe check this too?

What is this?

If it never reads hot, the heat would never come on.

Other than swapping a known-good sensor in there is there a way to test this?

More background: I just got this car a few months ago, and it has never produced heat.

Does anyone know any of the ACC controller output voltages for any of the other components than the monovalve? That way I could at least test the brain to see if it's any good.

RSCarey