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The supplemental cabin heater (viscous heater) uses an electro-mechanical clutch that receives a signal from the diesel engine control module (ECM) via the A/C-heater control and the cabin heater controller that energizes and engages the clutch. Once engaged, the clutch allows the viscous heater to increase the temperature of the engine coolant flowing to the heater core, which provides heat to the passenger compartment quicker than diesel engines without the supplemental cabin heater.
The supplemental cabin heater generates heat by means of friction which heats a special silicon oil within its housing. The heat is then transferred to the engine coolant when the coolant passes over the fins within the pump. When demand for passenger compartment heat decreases the supplemental cabin heater will receive an input from the cabin heater controller to disengage the clutch.
The supplemental cabin heater cannot be repaired and, if faulty or damaged, the entire cabin heater assembly must be replaced.
Are both heater hoses going to the firewall equally hot or is one colder than the other?
If the lines are cold then you likely have a bad pump, bit if both are hot then you have a HVAC issue meaning your blend door is likely bad..
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I have attached the location of the heater core lines..