RPM drops when turning A/C on

[72Z/28]

Hi guys

I have the March Serpentine system on my 72 camaro. The RPM at idle is around 800. However, it drops down to 500 or so when I turn the A/C on. In fact, the battery voltage drops as well from 12 to 10.

I just want to know why this is happening...Hopefully I get some ideas from you guys, and really appreciate it..

[Samckitt]

Not sure about your setup. The Factory LT1 programming raises the idle speed when the AC is turned on. If it is a carb, then probably the drag of the compressor slows the engine which in turn slows the alt which lowers battery voltage.

[PonchoJohn]

You'll need an idle solenoid. You can buy them from Holley or Edelbrock, or go junkyardin' for a Quadrajet setup. Wire it up so it engages when the A/C is turned on.
When you engage the compressor, it loads your engine down, and the idle drops. This used to be a trick to help w/ emissions testing.

[72Z/28]

You'll need an idle solenoid. You can buy them from Holley or Edelbrock, or go junkyardin' for a Quadrajet setup. Wire it up so it engages when the A/C is turned on.
When you engage the compressor, it loads your engine down, and the idle drops. This used to be a trick to help w/ emissions testing.

I guess I need to get that idle solenoid then, but how is it going to be wired up? just wondering..

[PonchoJohn]

It's wired up so that when the A/C clutch is engaged, the positive feed also engages the solenoid. It seems like GM used a diode across the clutch to control feedback/spikes, but you can experiment. Look at a 70's/80's carbed GM for inspiration.
BTW- the solenoid grounds through the body, so it only needs a positive feed.

[red67camaro]

The A/C dragging the idle down is normal and is especially noticeable with a big cam.

I ran a setup exactly as PonchoJohn describes and it works like a charm. The solenoid is adjustable so you can set the idle to suit you best with the A/C on and use your regular carb settings with it off.

Since my LT1 swap, I haven't got the A/C plumbed in yet but I see a table in my tuning program that compensates idle speed as a function of A/C high side pressure. Should work even better.

[180SX_Faktory]

There usually is a drop, but a 300 RPM drop is excessive. Sounds like the idle solenoid is the ticket. Please post results if you decide on this route.