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heater vacuum ?

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Defrost is where it defaults to when there is no vacuum. There are only two vacuum hoses running through the firewall to under the dashboard. One supplies vacuum to the controls, the other goes from the controls to the diverter valve under the hood. This valve serves no actual function other than being another part to break.


Your problem is somewhere in the vacuum line that goes from the main vacuum feed under the hood through the firewall to the controls. If it got disconnected or broke under the hood it's an easy fix. If not you may have to tear your dash apart.

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The vac lines youre looking for go through the firewall right with the heater hoses at the heater core tubes. Just follow the one to the heater control valve, and the other should lead you to the canister, and from the canister back to the manifold. I fised this on my bosses truck recently, all hoses looked fine, but still wasnt getting good vac, so I pulled all the hoses and lines out to inspect them and found a bad spot in the hard line where it goes through the rad core support. so I fixed the bad spot, and shoved the vac canister into a nice spot in the engine bay where it can be serviced easily. Good Luck.

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