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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

Reply to
zdilly2000
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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.

Reply to
T.G. Lambach

Reply to
zdilly2000

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.

Reply to
T.G. Lambach

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.

Reply to
Chas Hurst

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.

Reply to
Tiger

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.

Reply to
Richard Sexton

Reply to
zdilly2000

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.

Reply to
Tiger

Reply to
zdilly2000

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