RPM/Vacuum change when heater on foot or windshield

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daver18qc

Member
:
Mazda 3 2.0L Stick 4-dr
Hi guys !
I'm quite mechanically inclined but have something puzzling me on my 2007 Mazda 3 2.0L.

If i place the heater selection knob to face or face+foot, i've got a low idle speed and high intake vacuum.
If i place the heater selection knob to foot, foot+window or window, i've got a sudden rise in idle speed with a lower intake vacuum (talking about -18 in.hg instead -20)

At first i thought "HA! I've got a simple vacuum leak on one of the heater control "door" inside the heater duct"

WRONG.

It's electric in the Mazda 3 ! (I believe ?!)

So what could cause it to have a higher RPM while on some HVAC settings and lower RPM on others ?

Here's a picture to explain better if that's confusing to some :

2007-2008-Mazda-3-Ac-Climate-Control-Oem_zpsrurfhbu3.jpg


If someone can shine a light on this that'd be great !
 
Last edited:
When you say "low RPM" do you mean lower than normal (heater blower = off)? Lower vacuum pretty much has to indicate a vacuum leak. Since idle speed is in lockstep, an idle speed regulator failure can be ruled out, I would think. I wonder if a smoke machine can be used to establish where the leak is or whether there is no leak.
.....puzzling, to say the least.
 
The AC is AUTOMATICALLY turned on to prevent fogging in all of the settings you have circled in green.

(kiss)
 
Yep its the AC compressor cycling on and off, you should be able to hear the clutch engage
 
But I believe the idle speed should be regulated so that it doesn't drop when the AC kicks in.
 
Ok sorry guys for the late reply, we had a thunderstorm 3 days ago that killed my internet modem and was replaced only today.

So let's begin !

When you say "low RPM" do you mean lower than normal (heater blower = off)? Lower vacuum pretty much has to indicate a vacuum leak. Since idle speed is in lockstep, an idle speed regulator failure can be ruled out, I would think. I wonder if a smoke machine can be used to establish where the leak is or whether there is no leak.
.....puzzling, to say the least.
Click to expand...

The vacuum thing could be a symptom of higher/lower RPM and not the actual cause. I'm not sure.

The AC is AUTOMATICALLY turned on to prevent fogging in all of the settings you have circled in green.
Click to expand...
Yep its the AC compressor cycling on and off, you should be able to hear the clutch engage
Click to expand...

Good guess, unfortunatly i have cut the belt because the compressor had a leak. Without a belt to drive it, i don't think the RPM should rise when it "engages" electrically.
BUT maybe if it drains alot of current when engaging that could cause a surge in RPM maybe ?
I'm gonna try to remove the AC fuse entirely and i'll come back with the results in a couple of days !

Thanks guys !
 
Well then I would try turning your knobs to the area you have highlighted in blue and then turn your AC switch on and off to see if you have an RPM drop
 
If the knob is turned to the positions where the A/C compressor woukd engage, even if the belt is cut the car would likely idle up. An A/C request signal from the heater control to the BCM, will result in a idle step up command to the ECU when the BCM has the A/C request signal.

Sent from my Moto G Play using Tapatalk
 
UPDATE: Some of you guys were bang on, if i put the knob in a "low rpm" area and then push the AC button, it revs up to the same level that the "high rpm" area runs at.
So the ECU receives a signal telling it to raise RPM because the AC will kick on.

Thanks guys !
 
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