MORE Climate Control Trouble!! - PeachParts Mercedes-Benz Forum

dieseldaddy · #1 02-04-2003, 07:46 PM

OK, so now I have been working on this thing for almost 3 months.
1981 300TDT

The ONLY thing wrong with this wagon is that the heater/blower only come on when they decide they want to. (I guess I was that stubborn when I was 22 also...)

Well to make a long story boring, The temps have been down VERY low the last few months and it would be very nice to have some consistent heat.

1. The blower works (when it is on) in all settings. It is not the brushes. I just replaced the brushes.

2. The heat is on...I can usually stick my fingers in the vents and feel the heat oozing out.

3. The pushbuttons unit SEEMS to be working because all the buttons work (duh), again...WHEN IT IS WORKING.

4. The thermal couple just to the right of the center console just behind the dash cover pass. side has two connectors running to it. If I disconnect either one of them while the heater IS working and blowing and the A/C compressor is running the A/C comp. cuts out. Someone please tell me where these wires go!? I cannot trace them. Do they go directly to the A/C comp? I cannot figure out the wiring diagrams in the haynes manual. I think they are wrong!! !@%!@#%!

5. 4 out of 5 times I can get the heat and blower to come on by doing this:
If I start the car and no heat; wait 5 minutes or so and turn the car off. Wait about another 5 minutes and turn it back on. BINGO...HEAT!
This is not a very good solution when driving somewhere.

6. If the heat/blower are working and I shut off the car for less than about 6 hours the heat will guaranteed come back on next time. BUT if I leave it off longer (i.e. overnight) than guaranteed no heat/blower in the morning.

SO, it must be electrical short somewhere...right? How can I check?
Without totally replacing all the parts one by one can someone tell me what may be happening?

Ambient temp sensor?
A/C relay?
ETR switch? (found this one whilst searching the forum...but cannot find it in my car!? HELP!!)

My wife and chilly kids thank you in advance!
-Brian

kerry · #2 02-04-2003, 07:53 PM

Do you have the dreaded ACC servo or a monovalve. I had similar symptoms on my 77 300d and they disappeared when I replaced the ACC servo.
Have you tried the defrost position first and then another setting?

Erikir · #3 02-04-2003, 09:13 PM

A similar problem was resolved in my 81 300D by repairing the monovalve, which is located on the firewall, passenger side and has 2 coolant hoses attached. There are 2 12mm? bolts and a pair of flat head screws and a couple of hose clamps loosen to remove the unit. Open it up and have a look at the diaphram inside to see if it's torn.

A new monovalve is suprisingly expensive. Fortunatly the repair kit is cheap

dieseldaddy · #4 02-05-2003, 01:46 PM

I have not checked the monovalve for tears.
I thought the monovalve only regulated the temp of the heat, not the flow/blow or turning the heat on, etc.

I was out today and just one block away from my favorite salvage yard Petes Auto Salvage so I stopped in.
Walked out with an entire monovalve assembly (and a couple relays) for $12.

The diaphragm appears intact. I will replace it and see what it does.
Thanks guys!!

fredjj · #5 02-05-2003, 02:56 PM

Have you inspected or replaced the insulation between the sensor and the hose that runs to the blower? I replaced the insulation and cleaned up the contacts and switches on my acc box and temp. wheel. That has fixed my ACC woes which was also a will it or won't it blow heat situtation. Not a one day job but a very satisfying fix.

jcd · #6 02-05-2003, 04:17 PM

I'll make an offer I have made many times. Larry Bible gave me a great tech bulletin on diagnosing ACC issues. Send me a Private Message with our fax number and I'll shoot it to you.

I don't have an electronic copy, but will be glad to send it to you via fax.....it helps a bunch.

JCD

Inter911 · #7 02-05-2003, 04:21 PM

When you get it fixed let me know. I have the same symptoms on my 83 300SD. If I start the car with the heater on then I get heat. If I start the car and then switch on the heater I get no heat. If I have heat and turn it down then I get no heat until I stop the car then restart with heater on. The fan goes when and if it feels like it....new brushes too.

To at least have the fan working when I wanted I hooked it up to a separate on/off switch...not the best solution but at least the fan works when I want it to work.

Brian

leathermang · #8 02-05-2003, 04:44 PM

My 81 wagon did the same thing... I never knew when it was going to bless me with cooperating... and sometimes it even threw the button back at me... so I sold it to TxBill.....

dieseldaddy · #9 02-05-2003, 08:57 PM

Tonight is my worknight...now that the kids are in bed.

I have noticed the tube behind the glove box that you speak of, but I don't remember any insulation around it. Actulaly, I KNOW that there isn't any. Just a tube with a sheath over it. kinda odd.
Would plain ol' pipe insulation be a good replacement?
And what exactly is the insulation for/what does it do?

The bad part about this is that it works so erradically that when I *THINK* it is fixed...because the heat is working again...two days later it is not working again so I am back to square one.

At leaset these are so easy to work on and get to stuff! I couldn't imagine attempting any of this on a 1990 or newer car!

Thanks again!
-Brian

lrg · #10 02-05-2003, 09:09 PM

Foam pipe insulation works great, the kind of stuff you can buy at Home Depot for $1.00 for 6 feet. I used 3/8" which didn't fit perfectly but was easier to get in than bigger stuff. I think the idea is that the insulation helps the heat sensor work better when the dash is hot from being directly in the sun. On the later 124 model they moved the sensor up to the overhead light above the mirror to avoid the dash heating problem. The foam won't make the system work perfectly, just a little better. If your system is really erratic then it may be your CCU getting ready to pack it in. That's what happened on my 124. The system was erratic and got progressively worse until the circulation pump siezed and completely cooked my CCU. Since I put in a new/rebuilt CCU the temp control has been almost perfect.

dieseldaddy · #11 02-06-2003, 12:06 AM

Irg: CCU? Are you talking about the whole pushbutton unit or something else?

So now I replaced the entire monovalve unit including housing just to be sure (used unit).
The diaphragm on both seemed in good shape, the screen on my original one looked pretty gunked up...

So I hooked it back up and I do have heat/blower. However I am not going to cheer just yet, because as I said, sometimes it just comes on and sometimes it doesn't.
I will let it cool overnite and try it in the morning.

I will also try to get the insulation tomorrow and see but it sounds like it is for when it gets hot...not cold like it is now. I will try it anyway. Can't hurt.

lrg · #12 02-06-2003, 01:04 AM

The CCU (Climate Control Unit) is the pushbutton module in the center console. It has all the electronics to control interior cabin temperature. There is also another module in the engine compartment on some of the later cars that controls just the air conditioning but failure of that won't effect heat.

#13 02-06-2003, 07:26 AM

If I read your symptoms correctly, the problem is that the blower motor only runs intermittently.

Here's what you need to do: Start the car. If the blower motor is not running, wiggle the ignition switch. If the blower motor comes on, then you need to replace the ignition switch. This is a common problem.

Good luck,

jcd · #14 02-06-2003, 08:32 AM

Faxed the bulletin to Dieseldaddy, engatwork and IRG this AM at around 8AM.

JCD

dieseldaddy · #15 02-06-2003, 08:42 AM

Thanks for the fax Irg.

Larry, I have tried jiggling the key, and wiggling components under the dash at the ignition. when the heat/blower is off when I start the car I have not yet been able to get it to come on by wiggling, but I will keep trying. Like I mentioned, this is so erradic that I have to try everything.
LEt me throw this one out:
I checked to see if maybe the ambient temp sensor in the dash had come loose or something. It was connected, so I disconnected it. same results. I was wondering if a bad amb sensor may cause the whle system to not perform correctly. I disconnected it and let it run...heat still works. I will get the fax and run thru stuff with that before I try anything else...
Thanks!