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Old 02-28-2011, 04:50 PM
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Join Date: Feb 2006
Posts: 56
Auxillary water pump not working?

On the 1980 300sd that we just purchased I had a shut off valve in between the block and the tube on the firewall which goes into the aux water pump. I took the shutoff valve off to see if that what was causing air in my lines. I have replaced the thermostat and gasket and did a flush as best as I could. There was no oil in the antifreez so I was happy. I started the car up and waited till it got to 80 on the gauge and then turned on the heater function. Nada in the way of heat out off the dash. Prior to the change of thermo and after the thermo change I did get hot air out of the dash. Could anyone tell me what this valve could have been put there for. I have not gotten to worry about the ac yet as I am trying things one step at a time. Thank you.

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Old 02-28-2011, 05:27 PM
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Join Date: Dec 2003
Location: Metro Detroit, Michigan
Posts: 17,370

I would guess:

#1. a bad mono valve.

Mono valve links thread

#2. A fried ACC Board = no power to the mono valve = maximum heat all the time.
It is possible the mono valve diaphragm has failed since the manual shutoff valve was installed = no heat.

Aux. Water Pump takes out CCU
Aux. Water Pump takes out CCU

Fried ACC Board (picture) due to frozen aux pump?
Fried ACC Board (picture) due to frozen aux pump?

I rebuilt my auxiliary water pump

Fixed Auxiliary Water Pump!!
Fixed Auxiliary Water Pump!!


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