81 300sd no heat

been working on this problem and reading posts [1998-now]in this group for the last few days. have tried all i solutions i thought could be
prob. on def, a/c comp is not running, [not serviced] fan runs, aux pump is running. mono valve looks good, clicks with external power, but has no power to it when on def. i took off the hoses, and could pour water in one outlet and out the other. so i assume it is in open position. i have taken all 3 hoses off nipples on firewall and supplied light flow by garden hose [i suppose they are attached directly to core, right?] all flow very freely. end nipples seem connected inside, put water in one, it comes out the other. put water in the middle it comes out both sides. put back together, with engine at 80c and system pressure built i get hot pressurized hose from rear block to center nipple. but both outside hoses soft, and somewhat hot. squeeze either and i can hear some flow, but not much. zero warm air at dash. which way is the water circulation going? from rear left side block to center nipple and then to mono valve and then to aux pump then to water pump as i assume? can my heater core be hot and i have some valve closed to air flow under the dash? i appreciate any help, suggestion on this, i really want this heater/ ac to work. havent tackled a/c yet. -gary
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Mono valve's default is full open so no power in DEF is just what's appropriate for that valve.
(but both outside hoses soft, and somewhat hot. squeeze either and i can hear some flow, but not much.)
Your "hearing" some flow suggests an air bound condition. Try to purge the air.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
thanks for your quick response, i remember someone in previous posts mentioning purgeing, but HOW? can you help me? thanks
T.G. Lambach wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
An air pocket will be at the high point(s) of the plumbing. You may need to temporarily loosen a hose clamp or two to allow any air to escape.
I don't know this piping layout, and don't recall air bounding being a common problem; others here may have more incisive ideas and / or methods.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

It's been a while since I had my head under the hood of an old SD. I recall a temp sensor in an aluminum part that could be loosed/removed to assit in bleeding the coolant.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Are you getting some hot coolant to your monovalve when engine is running and hot? If not, then it is possible your aux water pump is bad. Follow that hose from the water pump area into your monovalve.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
BINGO! tiger, you won't believe this. all i had left was to check flow through that metal tube to water pump. not a likely prob huh? well, some bottom-feeding s.o.b had put a plastic plug in the rubber hose from this steel tube to the aux pump. &**^^%$#%&_! prob trying to eliminate any hot air mix with a/c on. then when heater don't work in the winter he takes out the plug again!! what a hero! thanks to you all for your help. what a source for a 'tinkerer' such as myself. -gary Tiger wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Whoa... can't believe that. We'll now that you know the likely problem... it is possible that your monovalve insert have a torn rubber diaphragm... which let coolant leaks into the heater core... so if that is so, just change the insert.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
told you it is hard to believe.... if i knew how to put a pix on here i'd shhow it to you. it's an actual plastic plug, not a part of the diaphram, it is good and monovave is working. i put a window reg in rt rr door, fixed the elect vac pump connection, new door striker... with heater fix, i am near a hero to my wife at this point.... but i gave you the credit on the heater :) -gary Tiger wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
If when you push the drefrost button nothinh happens - that is the fan doesn't spin and it will when +12 and gnd are applied then it is almost certainly your pushbutton assembly that's failed, they do seem to go first and it's about $50 for a good used one off ebay.
--
Need Mercedes parts? http://parts.mbz.org
Richard Sexton | Mercedes stuff: http://mbz.org
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.