Question about 1998 GMC Jimmy

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Heater is cold at slow speed warm at 40-45mph and hot at high spe

I changed to thermostat and it is still doing the same thing. Need help soon its getting cold!!!!!!!!!

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Check your heater core.

Some heater/defrost controls are mechanically controled and some are vacuum actuated. Look under the passenger side dash and see if you can locate the means by which the flapper is controlled. Normally the flapper just redirects the heated air from the rotating fan to eith go out the vents or up to the vents in the defrost. Most mechanic units (attached by cables from the dash control unit) do not break or wear out. The vacuum operated ones sometimes the small vacuum hoses that attach to a solenoid come off or get brittle and loose their seal against the plastic tips. If you can work the dash controls while you look under the dash and try to see how and why the flapper is not moving, then maybe you can ascertain how the flapper is actuated. Last resort os to try to remove the in dash control unit. It will either have the mechanical cables attached or vacuum lines attached to it so be careful when removing. Once you control the flapper in the air box, then you norally can redirect the air to where it needs to be. Look for a cracked vacuum line by the master cylinder. There are several that interconnect which feed through the firewall and into the dash that supply all of the dash vacuum.

Posted on Nov 19, 2008

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