300D heater is either off or full blast? - Page 3 - PeachParts Mercedes-Benz Forum

funola · #31 12-02-2009, 06:24 PM

You replaced the mono valve. You are getting voltage to it but it is either always on or always off and not going on and off like it should. Did you check and clean the temp dial potentionmeter as I suggested? Looks like it's the only thing left.

edge · #32 12-03-2009, 12:08 PM

Quote:

Originally Posted by funola

You replaced the mono valve. You are getting voltage to it but it is either always on or always off and not going on and off like it should. Did you check and clean the temp dial potentionmeter as I suggested? Looks like it's the only thing left.

Yeah I cleaned it. I've got my stereo out and I notice a 2-prong female receptor is unplugged but I can't find where it should be plugged in? Could this be the fault? Any ideas:

MBeige · #33 12-03-2009, 03:28 PM

Quote:

Originally Posted by edge

Yeah I cleaned it. I've got my stereo out and I notice a 2-prong female receptor is unplugged but I can't find where it should be plugged in? Could this be the fault? Any ideas:

The temperature sensor on the dashboard center has a plug to it, but I don't remember what color the leads are supposed to be. That one may be it, if it's not connected, but it's too far from the sensor.

funola · #34 12-03-2009, 04:13 PM

Quote:

Originally Posted by funola

You replaced the mono valve. You are getting voltage to it but it is either always on or always off and not going on and off like it should. Did you check and clean the temp dial potentionmeter as I suggested? Looks like it's the only thing left.

Did you unclip the 4 plastic clips that secures the green circuit board that has the potentionmeter behind it to clean it? After cleaning, did you verify with an ohmeter that there are no dropouts throughout its range? That pot is a wear item and being almost 20 years old problems can be expected with it. That plug does not go to the CCU as far as I can tell (I looked at mine). If that plug is suppose to goes to the temp sensor on the dash and is not connected that could explain your problems. Didn't you already check the dash temp sensor out already and it was OK? Maybe you need a new CCU?

Undo the plastic cover on the monovalve plug and clip a test light across the two leads (red/blk and brown/white IIRC) and run it out the hood so you can see it. Make sure nothing shorts out. With the temp dial calling for heat (not in either extreme detent positions, the light should go on and off with intervals of seconds (can be many seconds depending on your temp dial setting).

edge · #35 12-04-2009, 12:41 PM

Quote:

Originally Posted by funola

Did you unclip the 4 plastic clips that secures the green circuit board that has the potentionmeter behind it to clean it? After cleaning, did you verify with an ohmeter that there are no dropouts throughout its range? That pot is a wear item and being almost 20 years old problems can be expected with it. That plug does not go to the CCU as far as I can tell (I looked at mine). If that plug is suppose to goes to the temp sensor on the dash and is not connected that could explain your problems. Didn't you already check the dash temp sensor out already and it was OK? Maybe you need a new CCU?

Undo the plastic cover on the monovalve plug and clip a test light across the two leads (red/blk and brown/white IIRC) and run it out the hood so you can see it. Make sure nothing shorts out. With the temp dial calling for heat (not in either extreme detent positions, the light should go on and off with intervals of seconds (can be many seconds depending on your temp dial setting).

I took the circuit board off and cleaned the inside of the temp selector pot with brake cleaner. It looked like it was in good shape, how do you check resistance, there's like 6 prongs. The unplugged 2 prong plug has a blue and red wire connectors. The red is always on, the blue is switched on with 3.7v, 5.8v with lights on. The Temp sensor already has a 2 prong plug in it. I now assume that the 2 prong is for perhaps an better optioned radio than the Europa that was in there.

I also noticed there is a 6 prong vacuum wheel behind the CCM and I noticed that one of the prongs does not have a vacuum line attached like the others. I don't know if it is intentional or is a vacuum line is unattached but I could fine anything unattached under there:

funola · #36 12-04-2009, 05:40 PM

Can't help ya with the vacuum lines. TO check the pot, find the 3 terminals that go the the wiper (the part that rotates) and the 2 legs of the pot. Take an ohmmeter and go from the wiper to one leg at a time and slowly rotate the pot and watch the readings on the meter, It should increase or decrease continuously in resistance depending on direction you're rotating. An analog meter works better than a digital to watch for dropouts but it can be done with a digital meter if you go slow and watch the readings carefully. The other 3 of the 6 terminals is the SPDT switch for full heat or no heat. If you follow the wires you can buzz them out too.

MBeige · #37 12-04-2009, 08:37 PM

Quote:

Originally Posted by edge

I took the circuit board off and cleaned the inside of the temp selector pot with brake cleaner. It looked like it was in good shape, how do you check resistance, there's like 6 prongs. The unplugged 2 prong plug has a blue and red wire connectors. The red is always on, the blue is switched on with 3.7v, 5.8v with lights on. The Temp sensor already has a 2 prong plug in it. I now assume that the 2 prong is for perhaps an better optioned radio than the Europa that was in there.

I also noticed there is a 6 prong vacuum wheel behind the CCM and I noticed that one of the prongs does not have a vacuum line attached like the others. I don't know if it is intentional or is a vacuum line is unattached but I could fine anything unattached under there:

Those are not vacuum lines, but optic fiber lines for the top row of switches (dome light, antenna, sunroof and defrost from L to R). That "spider" thing (or octopus, whichever works

) has one bulb (blue/brown wire) then the optic fiber lines run to their respective switches.

Renntag · #38 01-06-2010, 10:18 AM

There is no mention here about the Electric water pump?

What is its function, how is it controlled, how can I test it?

I have an intermittent heat issue and am freezing my butt off. Brrr.

I have not tested the mono valve via jumping power to it or checking the internals. I have unplugged it to see if it will go to default "open" and stay there. This is thinking there is a controller issue making it close.

No luck.

The hose connecting the temp sensor to the dash is desintegrated. Powder and mush. Driving with the glove box out to aid the sensor in getting cabin air, no luck.

Sometimes the heat works great sitting still and around town. On the highway it works for about 20 minutes and then blows cold. If I turn the system off, wait, turn back on, there is a little bit of heat, then cold.

I have searched several times for more info and have not found my solution yet.

Thanks for any help.

edge · #39 01-06-2010, 10:33 AM

Quote:

Originally Posted by Renntag

There is no mention here about the Electric water pump?

What is its function, how is it controlled, how can I test it?

I have an intermittent heat issue and am freezing my butt off. Brrr.

I have not tested the mono valve via jumping power to it or checking the internals. I have unplugged it to see if it will go to default "open" and stay there. This is thinking there is a controller issue making it close.

No luck.

The hose connecting the temp sensor to the dash is desintegrated. Powder and mush. Driving with the glove box out to aid the sensor in getting cabin air, no luck.

Sometimes the heat works great sitting still and around town. On the highway it works for about 20 minutes and then blows cold. If I turn the system off, wait, turn back on, there is a little bit of heat, then cold.

I have searched several times for more info and have not found my solution yet.

Thanks for any help.

Yours sounds like a monovalve problem. I have a used one that's perfect, pm if you're interested.

diesel85 · #40 01-21-2010, 01:19 PM

Quote:

Originally Posted by MBeige

If the cold setting does the same (off or full blast) the foam hose that goes from the temp sensor to the sensor itself, behind the glove box, has disintegrated and must be replaced.

Can anyone supply me with a few pictures of what this set-up is supposed to look like?

MBeige · #41 01-21-2010, 04:08 PM

#290 is the hose that disintegrates over time

yuke · #42 01-21-2010, 04:46 PM

Quote:

Originally Posted by Jeremy5848

Remove the glove box and you'll be able to see both the temp sensor, its wiring, and the foam tube that is supposed to connect it to the blower but probably has disintegrated. I suspect Mercedes used foam as an anti-vibration measure; you can use foam pipe insulation or a solid piece of tubing as you prefer. The blower sucks air in from the top of the dash over the sensor and, in theory, makes the sensor respond faster to temperature changes in the cabin. The sensor has two wires.

Jeremy is the location the same for the sensor and foam tube the same for an 85 300SD as I have the same problem with my heat control and a new monovalve was put in less then a yr. ago.

yuke · #43 01-21-2010, 05:09 PM

Quote:

Originally Posted by funola

If the heat is full off or full hot nothing in between and the means of temp regulation is via the monovalve, doesn't that mean the monovalve is working fine? Anyway, I had this problem with my 300D plus clicking sounds in the CCU. I took apart the temp dial to expose the potentiometer and cleaned it with brake clean, checked full range of the pot with an ohmeter to make sure I have full range of continuity, reassembled and no more clicking and I have control over the temp. YMMV

funola is it hard to take the temp dial apart to get at the potentiometer to clean it as I have a problem controlling my heat on an 85 300SD.