W126 Heater Blower/Controller
 

After suffering through the last several weeks with little heat due to a
nonfunctioning blower, I finally had the time to look into the problem. I've
pulled the blower itself and there doesn't appear to be anything wrong with it. A quick connection to 12 VDC and it spins to life silently and the brushes look
fine. I connected a multimeter to the leads feeding the blower and never got
more than 0.05VDC regardless of the HVAC settings.

I then disconnected the blower controller harness and measured the voltage at that connector - 7.4 VDC at high speed, 1.2 VDC at low speed, and something around 4 volts in the auto position. This leads me to believe that the problem is actually the blower controller (126 820 5310). Since this is a rather pricey item ($400+ even from Rusty or Priceline) I want to be sure of what I'm doing before buying a new one. How can I be sure that the controller is the culprit? Could there still be problems at the HVAC control buttons?

Thanks
Jeff
'87 560SEL

Hi Jeff
I have the same problem no heat/blower/fan on my 1983 W126 300SD finally found it under the glove box, became a monkey, removed it, and there is nothing wrong with the unit, spins under 12V perfecto, so I re-installed. Now I am trying to remove the wood console dash panel to get at the ACC unit to remove, check and re-flow the solder joints, but cannot remove the wood panel yet. Removed/lifted all the switches, but panel will just not slip down. Any ideas?
This is not a daily driver (yet), and I am retired so it gives me something to do.

Did you checked the 30A strip fuse ? It is located in the engine compartment out side of the fuse box.
I had the similar issue, I did not measure teh voltage so can not help there, but my fan did not work and strip fuse was blown. It will not hurt to try, might save you a lot of money. And if it turns out to be the blower fan regulator a cheeper option is to buty a used one from online source or a from a junk yard.
Keep us posted.

Thanks

Benzmatic, just whereabouts is this "strip fuse"? can you give another clue? Sounds like I should check it before I remove the module. Have already checked and numbered all the other regular fuses but did not know of another.
Any help is appreciated, thanks

I am not sure about your car, but I would imagine it is around the same area. it is behind the firewall next to the big black fuse box. It is a 1 inch X 3 inches black box, had two philips screw on both ends, it opens up by flipping the top withouth unscrewing any thing. Here is the link with pictures of the location, go towards the end of the post.
http://www.peachparts.com/shopforum/tech-help/111631-c-heat.html?highlight=strip+fuse

Jeff
you have checked monovalve and temp relay from engine block?
If your fan is cycling then I doubt the blower control is the issue

Richard
2 screws on underside of center wood just above ashtray.
Must remove ashtray to access.

Hi Melted Panda
Those two screws are above the radio on my 126. I successfully remove the whole thing today... good job I am retired and have a warm garage to work!!
Removed and cleaned the whole ACC module, all the solder joints look good even under magnifying glass. Will check for voltage before I put her together again.
Dick W

BenzMatic,

Thank you so much for the help!! Sure enough, the strip fuse is the problem. I never realized that there was an additional fuse in the circuit.

I'm hoping that the fuse is the whole problem and not just a symptom. The break looks more like a fatigue break rather than burning as there is no discoloration on the metal nor any decrease in width which is what I'm used to seeing on burnt fuses.

I'm going to stop by the dealer for a replacement tomorrow and then cross my fingers after I install it.

jlc

Hi Benzmatic and Panda, thanks for the help. So far I have removed the ACC unit (without damaging the wood panel!!!) disconnected all the console switches (signal lights do not work because the 4 way switch is disconnected), but I can still drive her short trips to the Auto part store in second gear.

Looked high and low for the "strip fuse" you mentioned and this car just does not have one, that I and a friend can find!!!
She is a w126 1983 SD turbo, and that darn fuse is nowhere that I can see. I have Benzmatic's pics on file and mine of the fuse box area are here, they are not the same.

So far for the heater problem, in addition to the above, I have removed and cleaned the Monovalve, it is OK, diaphram intact. Have found the auxilliary pump under the Monovalve hoses, blower fan works on the bench AOK. Somewhere, there is a power problem to activate the whole thing and I cannot find it.
As to my Tranny problem: I have replaced one of the two 3/2 vacuum switches on the valve cover and replaced the broken "plastic flapper" on the second one (both were broken) and tranny still does not shift to 3rd. Any suggestions? Next step here is a tranny job and there is a local good one.

These two problems solved and I have a daily driver for the first time since 1972 (been without my own since then) for the grand price of US$810. + parts & time, and a trip to NY and back to haul her home.

I cannot seem to get the pics to load here but will e-mail to benzmatic direct
Thanks Dick

I believe fuse #11 is the heater blower fuse. Do a search in Diesel forum, lot of info about your car. If seems like fuse # 11 was most common cause for not heater fan not working beside the ACC module. Some of the posts talked about having fuse # 11 outside of the box modification, may be you want to look into that.

Sorry could not help much, do not know a lot about your car.

Thanks

Does your unit come on at all or does it have to be "jump started" by pushing the defrost button?
If this is the case you may want to look at the temp interlock circuit. On my 84 SD the sensor is just above the thermostat.
This senses the need to turn the system on, that is ,once the unit calls for heat then makes sure that the coolant is warm enough to allow the fan to come on

Thanks Melted Panda
Did not have a chance to work on the car today, but will get back at it tomorrow early, I think my problem is definately power or rather lack of. This car does not have the strip fuse shown by BenzMatic's pics that I can find so far at least. I think I solved the pic attachment problem, here is one showing the Monovalve and aux pump underneath, and another showing the firewall in front of the fuse box where BenzMatic's strip fuse is located.

I presently have the dash all apart, have cleaned all the switches and will re-flow the solders on the ACC unit. While the panel is out, I will also run the wiring for the 2 switches needed for WVO conversion, one for diesel to WVO and the second for "purge".. The switches will go in the blank (undrilled) ports on each side of the red 4 way flasher switch. Then later when everything is up and running, I can convert without tearing the dash down again.

Re the tranny: the Bowden cable and kick down switch will be first on the list in the AM. The outside temp is now up to -5C so heating the garage will be easier than last week when it was -30C

Richard,
I wish I had your energy :D

Success!!!
 

Hi BenzMatic and Melted Panda, Kudos to you guys and the forum, "Maxxie" my 300SD now has HEAT. Really not sure what the problem was, but I re-flowed the circuit board, cleaned out the dust balls in the temp control wheel, cleaned and re-installed the monovalve, changed the fuse and other things, the heater and blower now work, not sure about air conditioning yet. Again thanks!!!

Now my attention for the next week will be focused on the tranny problems. After that Maxxie will have to wait here, in a snowbank, until I return from the Rio Dulce in Guatemala where I have to replace the exhaust system in my Mercedes OM636 (180D) now there is a real old one!!! powered sailboat "Whitecap". The OM636 has worked fine since 1973, it is just all the stuff made in America bolted to it like alternators transmissions etc...

Again Thanks for the help
Dick (www.whitecap-pei.com)

I am glad for the happy ending, at least for the heat.
Did you got a chance to look at Bowden cable, kick down switch, etc as I mentioned before. One more thing you also want to check is your vaccum, if your vaccum pump is not producing enough vac. or there is a leak somewhere in your vaccum lines, you can experience all kind of problems. Most common ones are bad idle, AC ducts, tranny shifts etc. The vaccum line on the vac. modulator (it is on the tranny, on driver side) some time get blocked or unplugged due to aged rubber. When was the last tranny service, clogged tranny filter can also prduce **** problems. I guess this will keep you busy for few days. Hopefully you can fix the tranny before you go to your trip. keep us posted.
Happy sailing, white cap looks like a nice boat.
Thanks