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blowing out of top vents goes to defrost whenever speeding up and at +60mph


adam83094

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okay so i have a 1999 mazda b3000 and this has been going on for a while now and has me seriously stumped. whenever i cut my ac or heat on it works fine when sitting still but once i speed up it goes to the defroster only and if i stay at about 45 or 50mph it works fine but once i get any faster it goes right back to defrost. it is so annoying any help would be greatly appreciated. thanks!
 


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Thed

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Check all of your vacuum lines under the hood, primarily more so on the passenger side.
 

adsm08

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You are losing vacuum to the vac motor and so it is reverting to it's default position.
 

adam83094

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alright hopefully ill get around to checking those tomorrow. do they go out often in rangers?
 

Thed

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It isn't uncommon. The reapir is very, very cheap so it may often go unsaid. Went out on mine...
 

budro

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get a cheap hand held vac pump and you can add vacuum without the motor running so you can hear it also you can break it down in sections and add vaccum manually and see where you are loosing or holding a vaccuum
 

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Check the vacuum canister (reservoir) carefully for any leaks. Also, IIRC, there is a check valve inline on the hose to the canister, check that.
 

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My 02 Ranger acted very similar to yours. Mine has a check valve built into the vacuum reservoir. That check valve had broken and would lose vacuum in the reservoir when the engine was under load. I installed a new reservoir and that solved the problem. I don't know if a 99 Mazda has the same system but it sounds like the check valve is not functioning properly where ever it is.
 

Earl43P

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It's under the airbox.
Should be a black plastic sphere.
Check its tubes.
 


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