Way Outta Control
Join Date: Sep 1999
Location: The Palouse
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Re: 98 TJ Heater PRoblem
There could be many causes. They've got to be eliminated one, by one. So, as a first step, look at the Factory Service Manual, Section 24, page 24-14.
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MAXIMUM HEATER OUTPUT
Engine coolant is delivered to the heater core through two heater hoses. With the engine idling at normal operating temperature, set the temperature control knob in the full hot position, the mode control switch knob in the floor heat position, and the blower motor switch knob in the highest speed position. Using a test thermometer, check the temperature of the air being discharged at the heater-A/C housing floor outlets. Compare the test thermometer reading to the Temperature Reference chart.
If the floor outlet air temperature is too low, refer to Group 7 - Cooling System to check the engine coolant temperature specifications. Both of the heater hoses should be hot to the touch. The coolant return heater hose should be slightly cooler than the coolant supply heater hose. If the return hose is much cooler than the supply hose, locate and repair the engine coolant flow obstruction in the cooling system. Refer to Group 7 - Cooling System for the procedures.
OBSTRUCTED COOLANT FLOW
Possible locations or causes of obstructed coolant flow:
<ul type="square">[*]Pinched or kinked heater hoses.[*]Improper heater hose routing.[*]Plugged heater hoses or supply and return ports at the cooling system connections.[*]A plugged heater core.[/list]If proper coolant flow through the cooling system is verified, and heater outlet air temperature is still low, a mechanical problem may exist.
Possible locations or causes of insufficient heat:<ul type="square">[*]An obstructed cowl air intake.[*]Obstructed heater system outlets.[*]A blend-air door not functioning properly.[/list]
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As you can see, this can be a combination of electrical, vacuum and mechanical problem. Where would you like to start? I'd start by feeling the two heater hoses in the engine bay to see if they're both hot. That tells you that there is coolant flowing into the core, and back out.