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  #1  
Old 01-04-2006, 12:01 AM
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Heat problem in 1980 300sd

Completely lost my heat, but not the blower. A/C didn't work when I got the car, but that is the compressor as I was told by previous owner (in Canada in September, you don't worry about your A/C for about 10 more months ).

Am I to assume that it is the monovalve, and if so, how do I diagnose it to make sure that is what it is?

Thanks,
Dunl

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  #2  
Old 01-04-2006, 02:03 AM
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Also, where is the best place to source these?

Thanks.
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  #3  
Old 01-04-2006, 02:05 AM
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Your model doesn't have a monovalve. You have the dreaded servo unit. I can't help you on that one, maybe someone else with a servo equipped MB diesel can.

-Joe
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  #4  
Old 01-04-2006, 03:37 AM
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Sounded like $500 to replace that stinkin servo in my car, so I just took two pieces of heater hose and bypassed it. Removed the non-functioning aux coolant pump at that time also. Now I either have no heat, or it cranks hot out the defrost. If I push defrost first then half push in bi-level, I get a mix. Works good enough for me and cost a couple bucks. I made a thread on that a couple years ago with pics, wonder if it is still here....
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Old 01-04-2006, 03:44 AM
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Sounds like the job I had done on mine. There is a bypass switch underneath the ignition right now, that turns the blower on or off, and I get heat (used to, anyway), out the defrost - all or nothing.

Guess I'll take a better look tomorrow at it, and check it out.

Thanks for the tip, and if you find that thread, I'd like to take a look at it.

Dunl
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  #6  
Old 01-04-2006, 07:31 AM
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The guys who restore 71-73 imperials know a bit about this system, they call it Auto-Temp II.

http://www.duricy.com/~imperialist/tips.html
http://www.imperialclub.com/Repair/Air/index.htm

George Murphy seems to be the MBZ expert on this, but I don't think he ever set up a web page.
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  #7  
Old 01-04-2006, 09:23 AM
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Quote:
Originally Posted by dunl
Completely lost my heat, but not the blower. A/C didn't work when I got the car, but that is the compressor as I was told by previous owner (in Canada in September, you don't worry about your A/C for about 10 more months ).

Am I to assume that it is the monovalve, and if so, how do I diagnose it to make sure that is what it is?

Thanks,
Dunl
See if you have a big black thing on the passenger side firewall with a lot of vacuum lines going into it...

otherwise known as the Evil vacuum servo. Thats your problem...as the 1980 300SD was a W116 chassis and never had a monovalve.
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Last edited by boneheaddoctor; 01-04-2006 at 09:34 AM.
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  #8  
Old 01-04-2006, 01:19 PM
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Thanks for the tips and pictures, boneheaddoctor. $495 for an aluminum servo would fix the problem, but also my wallet for a while.

I did find your thread Jimmy, thanks for the info.....I bumped it to the top in the hopes that someone will have some pics or other options....you never know what some inventive diesel-head will come up with.
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  #9  
Old 01-04-2006, 01:58 PM
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Quote:
Originally Posted by dunl
Thanks for the tips and pictures, boneheaddoctor. $495 for an aluminum servo would fix the problem, but also my wallet for a while.

I did find your thread Jimmy, thanks for the info.....I bumped it to the top in the hopes that someone will have some pics or other options....you never know what some inventive diesel-head will come up with.
Figure a servo amp too....behind the golve box...always replace that as a siezes Servo with burn up its servo amp.

Performance products sells a technologicly up to date replacement for that monstrocity......Fastlane carries it...

True it costs more but its not going to fail again....
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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche
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  #10  
Old 01-04-2006, 02:59 PM
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I agree with BHD (a rare occasion). The Performance high tech upgrade is probably worth it. I did put the aluminum bodied upgrade on my 77 a few years ago before Performance came out with their system. I got mine from another supplier at a discount because it had a crack in an unimportant piece of plastic. Check junkyards, there may be a good one out there. Ebay sometimes has used ones.
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  #11  
Old 01-06-2006, 03:08 AM
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I have the 1980 AC Servo unit

I also have the AC servo unit and have had trouble with it for quite a while.
Looking forward to forking over the money for a new one just to get the
whole package working like it is supposed to.
But... when it got cold I bypassed the unit and ran hot water straight
into the cabin (just like the other posts talk about).
I seem to remember having to replace the small electric auxillary pump too.
A salvage yard had the aux pump for $25, not too bad.

I tore apart that AC servo looking to fix anything I could find.
Darn confusing. Did have a broken arm that I tried to JB weld.
It did not work. Hence the search for a new unit.

Anyone ever replaced the AC servo in a 1980 with some other year
method ? I have a 1981 donor car, but just can't see what needs to be
switched to eliminate the 1980 AC servo unit.
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  #12  
Old 01-06-2006, 10:09 PM
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Okay, I removed my servo, and now I have heat. But I am guessing that I now have to plug all the vacuum lines going out of the (removed) servo, as the vehicle keeps running when the key is turned off and removed.

While waiting for the glow plugs to heat, I could hear the coolant running and the bubbles gurgling, only when the glow plugs were heating (checked 4 times). I'm guessing this means my auxillary water pump works, and it doesn't need replaced?

Plus, I remember hearing something about replacing a fuse with another to extend the life of said aux. water pump - can't find info though.

Thanks,
Dunl
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  #13  
Old 01-07-2006, 01:47 AM
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Anyone able to help here?

Thanks,
Dunl

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