1977 280E heater problems...yep you heard it (ACC)..edblock help!!. - PeachParts Mercedes-Benz Forum

hos280 · #1 10-09-2004, 12:25 PM

Hi everyone. 1st, what a great site. I have been reading for 2 days now tracking threads that help with problems similar to mine. This my first post so bear with me.

I believe the only thread with the exact problem is edblock's. edblock can you help me? Larry, can I get a copy of the heater troubleshooting guide you have graciusoly put together?

MB: 1977 280E
Problem: no fan when "auto low","auto hi" or "bilevel" is pressed. Fan blows hard when defrost is pressed, but no heat with temp wheel maxed at 85C. I can't get heat out in any format, except for 20s after defrost is pushed and engine is warm, then it quickly cools down. The AC blows cold when "compressor" and "defrost' are pushed. ACC control unit lights work.

Here is the weird part: I went to start my diagnosis and found the large tube (center) going into the servo had a pair of vise grips crimping it.!!!! The car is 1 month old for me, so the previous owner pulled some funny business on me.

Had he bypassed to get the "always on" hi heat in winter and then disabled it for the summer??

Mark DiSilvestro · #2 10-09-2004, 02:33 PM

The ACC servo is basically a Chrysler system. Check over on the Diesel Discussion for the most recent posting on the ACC and a link to the Chrysler Climate Control manual, which is (supposedly) more comprehensive than what's availible from Mercedes.

Happy Motoring, Mark

ctaylor738 · #3 10-09-2004, 07:12 PM

You almost certainly have a servo that has gone south and maybe other problems. The best thing you can do is call George Murphy at 865 482 9175. For five bucks he will send you a troubleshooting guide to the system. He also has reasonably-priced parts and excellent advice.

The reason you get fan on DEF is because that is the hardwired default position that works even without the servo.

hos280 · #4 10-09-2004, 08:23 PM

Well I decided that the dreaded servo has to be shot, or just a crappy piece of engineering anyway, so I took it apart today. after the gears and springs went flying, I noticed that there were several probelms. Corrosion for one, clogging for another and over crappy device.

I think I will try the bypass. I believe that i need to bypass the line nearest the engine...correct? not the line nears the wheel well.

I am picky about my heat, though so I was thinkin about the digital replacment.

Here is my main gripe!! I want to use the bi level, auto-hi and auto-lo to adjust my fan speed. I could care less if the temp is not regulated. I can do that with the cable.

Is this possible? will my heat only work on DEF only, if i do the bypass?

You guys have made this endeavor fun. Many people share my pain and that is very comforting. heat would be better though.

So here is my goal: I want a bypass, no servo, that allows me to use the vertical buttons as fan settings, while I adjust the temp with the pull cable?

The other soltuion is to give my wife the 280E and i take the 1994 c280 which is in mint condition!

Mark DiSilvestro · #5 10-09-2004, 09:50 PM

I bypassed the servo and installed a manual cable-operated heater valve on a friends '79 300D. He didn't care about the AC and just wanted some heat so his son could drive to school in winter. Resulting climate functions were HIGH DEFROST-COLD, HIGH DEFROST-HOT and ALL OFF.

Happy Motoring, Mark

hos280 · #6 10-09-2004, 10:35 PM

yes , that is what I thought. Servo Bypass means HIGH COLD and HIGH HOT!

I downloaded the factory 123 manual from the website. It has details of everything. http://skinnerbox.steaky.org/Service/W123/Index/110index.html

Here is what I think maybe could be configured.

Bypass the servo in the familiar format and use a cable. The vacuum lines could be simplified into 1 or 2 lines such that all vents come (or just Bi-level mode) on when any button (or just bi-level) is depressed other than OFF. It seems the only use of the vacuum lines are to actuate the different air vent valves to redirect the air from center vents to lateral vents to floor, etc.

This would simplify fixing all of those vacuum lines which looked cracked and old on the servo side and simplify trouble shooting the vaccum system. There are prob. a few tubes that are good, so I could use them. This can be done under the glove box, or at the servo, by just using barrel tube connectors.

The next problem is how to get the blower to run at different speeds. I found the blower under the glove box. I can access the power wires and use a power supply to see what voltages give what speeds. I can then maybe use the potetiometer of the temp wheel to vary a voltage straight from the battery to vary the blower speed.

So temp control comes from cable, vent control is simplified, and fan can be turned down by the wheel.

Sounds crazy but i think it can be done.

Mark DiSilvestro · #7 10-10-2004, 10:06 AM

Theory sounds OK, but actually, you'll need to add a manual heater valve in place of the bypassed servo or you'll have heat all the time. Also, the temperature wheel potentiometer probably won't be up to the load of the blower motor. You'll need another setup for that.
If someone could design a kit to fully bypass the servo, they'd have the gratitude of many afflicted Mercedes owners.

Happy Motoring, Mark

hos280 · #8 10-10-2004, 01:50 PM

Thanks Mark.

I decided against the temp wheel as a blower motor adjustment for just that reason.

I did put a bypass on the servo. I bypassed both lines and put a manual valve on the feed side. Still no heat, but i think my water pump is shot.
I will hook it directly to the battery to see if I can get it to flow.

I also took the whole servo out. The vacuum lines and electrical feeds are now tie wrapped out of the way. The defrost button still produces blowing, so if i can get the hot coolant to flow into the exchanger, I should have heat and lots of it. Obviously the vacuum lines won't work, but my idea of simplifying that is more complicated than I thought.

Here is my new plan. I searched for a variable valve that is actuated by 0 to 12 V (0V=closed,12V=open). I found one for water that is 250$. Not too bad. I was then going to put this in place of my manual valve and use the temp wheel potentiometer in series with the battery to adjust the valve position. This would be slick and almost like the original operation.

I still have the problem of the blower speed and vent control. Fixing the vacuum system without the servo would be tricky. I have to think this one out.

QUESTIONS:

Is there any downside to taking the servo out?

Everyone calls the aux pump a "water" pump. Am I missing something, or is it just pumping coolant into the heat exchanger? Are their two different fluid loops-one for water-one for coolant?

Thanks again. Cost so far is 6$ for manual valve and 9$ for tubing and clamps. no big loss yet, but aux pump will set me back about 120$.

Mark DiSilvestro · #9 10-10-2004, 02:55 PM

There's no separate water & coolant - whatever flows in the radiator should flow in the heater. When I did the 300D servo bypass, I also bypassed the electric coolant pump. As I recall I also changed one of the heater hoses at the firewall to make it more convenient to mount a cable-operated heater valve there. Then I installed a lawn-mower cable control under the dash. Only downside - without the electric coolant pump, heating was weak while idling, but it picked up fine once the car was moving.

Happy Motoring, Mark

hos280 · #10 10-10-2004, 05:34 PM

So that is why I do not feel heat!!

I started the car, let it warm up, and sat there feeling the vent and was disappointed. It seemed there was a little, but I think my hand was in a sunny spot.

I need to drive and run the heat to see if it is working!! Duh!! Thanks.

I ordered the aux pump so that should be nice to have. High class baby!!
Now I suddenly realized why they call it an "auxillary" pump. It is not really necessary, just handy in traffic and sitting still.

Mark , you have been more than helpful.