Strange Heater Behavior?? - PeachParts Mercedes-Benz Forum

Chad300tdt · #1 01-04-2009, 02:47 PM

I just recently replaced the monovalve in the CD. Once replaced the heat was fantastic and very hot.

Just within the last couple days, it's been acting like a bad monovalve again. I just removed the monovalve about an hour ago and checked it out. No tears, and it seems fine.

I put it back in and went for a test drive and discovered its behavior pattern.

Heat is fine until I come to a stop and then pull away again. Then the heat fades away until it's completely cold. If I turn off the CCU for about 10 seconds and then turn it back on, the heat works perfectly until I come to a stop again.

Any ideas of what the problem is?

I'll be turning the CCU off and on after every stop light until I figure it out.

sixto · #2 01-04-2009, 02:57 PM

Can you temporarily wire an analog meter to the monovalve to check the signal? IIRC it gets a constant +12V and a pulsed ground. Digital meter might not respond quickly enough to the pulsing, but mabye you have one with a frequency display.

There are a lot of suspects but none that would be triggered by replacing the monovalve. What come to mind are cabin temp sensor and heater core temp sensor if there is no signal to the monovalve. Might be a faulty auxiliary pump if there is a signal to the monovalve.

Sixto
87 300D

Jeremy5848 · #3 01-04-2009, 02:58 PM

Aux water pump? Partially clogged heater core or crud in lines?

sixto · #4 01-04-2009, 03:02 PM

BTW, when 86-up ACC starts acting that way, it's time for a new PBU. Maybe it's time to reflow the solder joints in your PBU.

Sixto
87 300D

Chad300tdt · #5 01-04-2009, 04:03 PM

OK I looked into your suggestions. I don't think the Aux water pump is the issue, since from what I've read, the symptoms (No heat at idle and heat at driving RPMs) are the opposite of mine.

So far, re-soldering the PBU joints makes the most sense to me. I'd also doubt I have a clogged heater core since I'm getting awesome heat when it does blow.

Hopefully I'm explaining my problem clearly.

Read below just in case I'm not being clear.

1. driving along and heat is great. Come to a red light and stop car, heat is great.

2. Light turns green and I pull away. Heat begins to fade and goes completely cold.

3. I turn off the PBU and wait about 10 seconds. I turn the heat back on and it fades back to hot.

(return to #1)

daw_two · #6 01-04-2009, 04:30 PM

Chad.

Take out you monovalve and check the "screen" underneath it. Make sure it is not partially blocked. Coda's heat seems to be very sensitive to any changes in the ACC temperature adjustment......I often have to turn it OFF and then to AC to cool down the cabin because the heat just keeps on blowing.....

I'm also thinking I need to investigate what my air temperature sampler hose/blower is doing.

fdanielson · #7 01-04-2009, 04:38 PM

I had a similar mysterious problem and finally fixed it by performing the degrease/citric acid flush procedure. If you do not know when it was done last that is a definite indicator it should be. There are two things you can try. First as a test when the heat goes out at cruising speed, floor the accelerator to make the car downshift and hold it there until the car upshifts at 75mph or so. You should be rewarded with a blas of warm air as the added circulation moves coolant through the system. If that happens you definitely have a circulation problem. For temporary relief try disconnecting the heater hose that goes to the cylinder head and then the one at the monovalve that goes to the heater core. Backflush the heater from the monovalve end to get out any chunks that may be in the heater core.

When flushing my system I used one of those Prestone flush and fill kits and hooked the flushing tee to the hose going to the back of the head using a scrap piece of 3/4 heater hose I had lying around . This let me backflush the system with a garden hose instead of goingthrough the fill/drain iterations.

Johnhef · #8 01-04-2009, 05:02 PM

Chad, does it do it on the economy setting with the dial "clicked" in the full heat position and also does it do it with the defrost button pushed?

Chad300tdt · #9 01-04-2009, 07:11 PM

Quote:

Originally Posted by Johnhef

Chad, does it do it on the economy setting with the dial "clicked" in the full heat position and also does it do it with the defrost button pushed?

Yes. One thing I noticed though. When I clicked it into the economy setting or the foot only setting, it still blew through the top vents. But once I got back in my driveway, the settings all operated normally.

During this test drive, it also seemed that the AC was on at first until I flipped the PBU on and off a few times (waiting about 10 seconds between button pushes).

CANDIDE · #10 01-04-2009, 09:06 PM

Quote:

Originally Posted by Chad300tdt

I just recently replaced the monovalve in the CD. Once replaced the heat was fantastic and very hot.

Just within the last couple days, it's been acting like a bad monovalve again. I just removed the monovalve about an hour ago and checked it out. No tears, and it seems fine.

I put it back in and went for a test drive and discovered its behavior pattern.

Heat is fine until I come to a stop and then pull away again. Then the heat fades away until it's completely cold. If I turn off the CCU for about 10 seconds and then turn it back on, the heat works perfectly until I come to a stop again.

Any ideas of what the problem is?

I'll be turning the CCU off and on after every stop light until I figure it out.

Sounds like the controller. As a test; unplug the connector to the monovalve, which will cause it to be open all the time. If your heater then blows hot air all the time, its the signal coming from the controller. I believe its a transistor array that drives the monovalve with a PWM signal. Signal is about a few seconds on then off... or so, which would be read on a DC scale as pulsating 0Volts to about 12Volts. If the signal reads 12 Volts steady, your controller is kaput.

Johnhef · #11 01-04-2009, 10:16 PM

Yeah, swap units with the TD.

Motorhead · #12 01-04-2009, 10:40 PM

I just fixed a mind stumper on an '81SD with help from this site. The guy told me to check the foam tube that goes from the small square vent on TOP of the dash to the heater unit. It had deteriorated and disconnected. Made up a new piece with some foam pipe insulation and the climate control works great. You have to pull the glove box out and move a vacuum slave out of the way to see and replace the foam tube.
I DO keep a "known good" push button assembly and a temp controler to test systems when I have a problem. If you have access to some known good assemblies from another car, use them to test your car.

Chad300tdt · #13 01-05-2009, 07:23 PM

I swapped the CCUs and I have the same symptoms.

Is it possible that there's some crap in the coolant that clogs up the monovalve screen until I turn off the heat and let it settle. Then turning back on the heat it takes a bit to collect again.

If I keep the car under 60 mph, the heat stays on. As soon as I go faster or raise the rpms dramatically, the heat fades out. If I stay over 60 mph, turning the CCU off and on doesn't bring the heat back.

I won't get a chance to flush the coolant and back flush the heater core until Saturday.

EDIT: This was with defrost on and the temp wheel to max.

Cr from Texas · #14 01-05-2009, 07:40 PM

Did you replace the entire monovalve or just the core?

I bought and returned some of the cores (not Bosch) that were defective. They looked good but this is what I observed in chasing the problem. I removed the top of the assembly and applied 12V directly to the valve. The defective cores would not move as far as a good Bosch core (one actually pulsed under a constant 12V). I made a deal with the supplier and kept the rubber parts for use on some old Bosch pieces.

Test as above and that will confirm the proper action of the electromagnetic top as well as the valve insert.