AC cause lower vacuum readings???

[Module Unknown]

So it was a bit warm out today and I was cruising around with the AC on when I noticed something while sitting at a red light. My vacuum reading was at 15. Turn AC off, back up it goes to 18. AC on again, and down it goes. Is this normal? I don't recall noticing this before, but then again I haven't used the AC for a while.

If that is normal, could someone please explain to me why that happens. I'd like to understand what's going on.

Thanks.

 

[crankbender]

This is normal...

the vacume the engine pulls is directly related to the load on the engine. Hence when the ac is turned on the load increases and vacumme decreases.

 

[Defiant]

An air passage opens when the AC is on so that the idle increases for the additional load of the AC compressor. Ergo, less vacuum.

 

[bsbllfit7]

this was nice to know because i was in my car with the AC on today and noticed the same thing happening to my car. Thanks again everyone