Heat..but no BLOWER! - PeachParts Mercedes-Benz Forum

dieseldaddy · #1 10-27-2002, 11:02 PM

1981 300TDT:
Symptom: Heat, but no blower. (A/C works great)

This summer when we used the A/C the blower worked great at all speeds. (low, auto, high)

Last winter the HEAT worked great as well. Now, we have heat barely trickling through the vents regardless of the blower setting.

Strange thing is that if I turn the dial to cold the blower will kick on just fine. Doesn't really help us for heat though.

I spent a fair amount of time yesterday attempting to diagnose and repair...but all I ended up doing is taking a lot of things off only to have to put them back together (dash related crap) with nothing to show for it but a couple bloody fingers.

So I have been online doing several searches for the last two hours HOPING that this is a very common problem and I can solve it with $0.37 rubber vacuum line and about 1/2 hour labor. (just kidding...but that DOES seem to apply to most of my repairs thus far!)

Well, nothing came up in my searches on the 123's but some things caught my attention:

1. Are there TWO SEPARATE blower motors for heat and A/C?
2. Does this appear to be a symptom of a bad monovalve?
3. What the heck is a "blower regulator switch" and does my car have one?
4. Yes, I have checked all fuses in the box...but are there other sneaky ones hidden somewhere?

Seems strange that this would be happening all of a sudden. Need help fairly quickly as old man winter is quickly approaching!
(Man, do I live in the wrong state or what?!)

#2 10-28-2002, 12:34 PM

Dieseldaddy,

I've discovered, as the mercury drops, that my fan is very slow to come on. Sometimes, even when I manually select the "high" or "low" fan speed setting, I get no response. This includes times when the car should be warm enough to provide heat at the temperature I've selected. One way I've gotten around this is by selecting the other climate controls modes (forgive me for forgetting their technical names) - the buttons to the left of the "economy" setting. Sometimes when I select these more "serious" modes of climate control, the system wakes-up and does it's job. I don't know if the automatic climate control is overriding your settings and won't blow heat because it thinks it is already at the selected temp in the cabin? Sounds like that could be the case if the system begins working when you select a lower temp. Anyhow, hopefully someone has better advice than to "press something until it works".

#3 10-28-2002, 01:45 PM

I suggest removing the passenger side underdash panel and slapping the motor. If it comes on, then it is probably the motor brushes. If it doesn't, remove it and jump 12 volts across the motor to see if it comes on. Be careful when you do this, it will jump out of your hand from the sudden torque.

If it does come on with jumped voltage, see if the connector is making voltage, then trace it back from there.

Good luck,

lrg · #4 10-28-2002, 01:49 PM

I'm not sure I have the answer but maybe I can at least help you eliminate a couple of possibilities.

If you are getting hot air in the vents, I mean really hot air, it's not likely the monovalve is bad. The monovalve regulates the amount of hot water to the heater matrix and when it fails it usually just means it won't let enough hot water through and the heater can't produce enough heat. The rest of the system shouldn't be affected. You should definitely check the blower motor. There should be only one and it's located in the passenger footwell below the glovebox. Remove the kickpanel to get access and you can remove the base of the blower housing which contains the motor and fan assembly. Check to see that the brushes on the motor are good. On my car the brushes wore out and made the blower run erratically. It drove me nuts trying to figure out what was wrong. If I had to guess I'd bet this is your problem. I replaced my blower motor assembly (I could only buy it as a unit which includes the base and the fan) with a used one since my old motor was ruined by the brush springs and the used cost 25% of the dealer price. It has worked perfectly since. The last possibility is that you need a new electronic switch unit (the board with all the buttons). I have no direct experience here but I know these do on rare occasion fail. If that is the case you can buy rebuilt ones that aren't too horribly expensive. Others here may know ways to check those to be sure that's the problem before you spring for a new one. My guess is that the problem is with the blower motor. Good luck and let us know what happens.

dieseldaddy · #5 10-28-2002, 02:01 PM

Thanks everyone!

Sorry for the long first post, but I wanted to clearly state the problem.

One thing that is confusing is why does the fan work fine when the dial is set to COLD but turns off immediately when I move it into the 'red zone'?

If I keep the HI-speed blower button pressed and rotate the temp dial I can get the fan to kick on when in BLUE but not in RED.

I will try to smack the blower around. I am getting very proficient in removing the under-dash components!

dieseldaddy · #6 10-28-2002, 03:19 PM

TXBill,
I have waited 1/2 hour at one point. (my commute to work) and still nothing. Cold will blow as soon as I turn the key.

Yes, the defrost will instantly kick on the blower but ONLY WHEN dial is set to COLD.

All buttons/vents work as designed.

With the dial set to anywhere in the blue zone, the blower comes on with any buttons selected. (including hi and low and auto)

Once the dial is turned to below blue (white or red) there is a "CLICK" and the blower turns off. If I turn the dial all the way to "MIN" (or is it MAX?) heat I can feel the resistance, and another "CLICK".

kerry · #7 10-28-2002, 06:45 PM

I may be thinking too simplistically, but I would remove the kickpanel and start testing the blower wiring with a multimeter. I'd figure out what wires are getting power to run the blower when in the cold mode and then switch the wheel to hot and see if there is still power coming to the blower. If not, I'd assume the problem is in the control panel.
The other possibility I would consider is whether there is some kind of blower interface switch related to the fresh air/recirculated air vacuum servos. Since the cool and heat systems are probably using different ratios of fresh and recirculated, there may be a problem in that circuit if there is such a thing.
I do know that as I watch the vacuum servos behind the glovebox in my 85td, the blower kicks in once the servo directly behind the glovebox opens.

dieseldaddy · #8 11-04-2002, 10:30 AM

Well I was doing some lazy mans diagnostics and found that the thumbdial was not sending a signal thru all of the pins going to the blower.
I narrowed it down to only TWO of the pins (out of 8 if I remember correctly) actually worked.
With just those two pins connected, I have full blower range (low/auto/hi) and still had control of temp. and venting options...

SO, I took some 'liquid electrical tape' and goobered the apparently non-working pins (both male and female ends) and reconnected it to the pushbutton unit.

Now, I have a seemingly fully operational heater/blower and AC/blower combo.

I know that this is only temporary because those extra pins MUST be doing something... But I cannot find a 123 pushbutton unit anywhere! (waiting for ebay...)

Thanks for all of your help!

lrg · #9 11-04-2002, 04:18 PM

This may not help but I saw a used unit from an '84 300D at **************.com. Yours may be an older design. Also check adds in Hemmings Motor news. I have seen several adds under the "Mercedes Parts" section of places that sell rebuilt units. I think you may be able to access them on a website too. Good Luck.

Benz300 · #10 11-22-2004, 01:37 AM

I'm having the exact same problem with my 87 260E. Is there a cure to the problem, till last year the 'low' setting produced heat with ample blow but now with the setting at 'low' i barely get any warm air coming out of the vents.
I just replaced the burnt out lights in the climate control, and for that I had to take the unit out. Could that have created internal problems ?

lrg · #11 11-22-2004, 01:50 AM

If the problem is no heat but plenty of air coming through the vents it's probably the monovalve. If the air is getting really hot (at full heat) but the fan won't blow that could be a fan motor problem (likely needs new brushes) or more difficult to diagnose electrical problem. Since you just had the CCU out if it's an electrical problem the connections behind the CCU are the place to start looking.

Benz300 · #12 11-22-2004, 02:04 AM

With the climate control on the "auto" or "hi" mode the blower works and so does the heating. but at the 'low' mode I can feel the heat slowly coming out of the vents but the blower goes into super low mode where it's barely pushing the heat out of the vents. It never did this before.

8threesd · #13 11-22-2004, 01:24 PM

I recently had this same problem.

The blower would come on when the dail was set to AC, however when on 'heat' the blower would not come on at all.

I took apart the climate control module and resoldered all the points, (allot of them were cracked, and i figured i'd just do all of them).

This repair resolved all the problems.

It realy is very easy, and only took about 20 minutes. Sure beats paying hudreds of $$ for a new unit.

Theres a few threads i found on this repair.

this site has allot of detail, and is a big help.

http://207.210.95.34/~boostd/Climate/

Hope this helps someone as much as all of you here have helped me.

sam

tangofox007 · #14 11-22-2004, 08:35 PM

Quote:

Originally Posted by dieseldaddy

With the dial set to anywhere in the blue zone, the blower comes on with any buttons selected. (including hi and low and auto)

Once the dial is turned to below blue (white or red) there is a "CLICK" and the blower turns off. "

That pretty much eliminates the motor as the culprit.

Lycoming-8 · #15 11-23-2004, 02:26 AM

Shortly after getting the 300D, I noticed that a wire had broken off near the thermostat. Some searching here on the board seemed to indicate that it was to provide a temperature delay for the blower, so that cold air was not blown into the cabin before the engine got warm. After replacing this wire with one from a parts car, the whole heat side seemed to work fine. By the way, the wire probably broke because the insulation at that end was very hard and stiff. You may want to check if this wire is hooked up at both ends and that the temperature switch is functioning.