914's with A/C, how to kick-up idle with compressor on?

[jjs3rd914]

Just installed a DPD A/C system (134a upgrade) on a newly rebuilt stock 1.7L D-jet (1972). A/C seems to be working fine (took a while to get charged) but really adds a load at idle. Have a new Sanden 508 compressor. Read a while back that someone had installed an idle "kick-up" switch as part of the D-jet system when compressor is on and car is at idle. Cannot find any details in a search.

Question, can one (or has anyone) devised an electric equivalent of the AAR such that it boost the idle only when the compressor is on and the car is idling? Right now I am having to set the idle (compessor off) at 1200-1300 and with compressor on it drops to around 600. Car wants to stall when start to pull away, just to much load to get going.

OR is there a way to automatically kill the compressor when the car is at idle with a relay that opens when the rpm drops. This may be the better solution as it would not put as much strain on the engine. I can live with the A/C off when car is at idle for short period of time. I have read that the Sanden's efficiency does not come into its own until above 1500 rpm anyway.

There has to be a simple fix.

Thanks
jjs3rd914

[Jared at Pelican Parts]

You could jumper the FI throttle switch on the throttle body to cut out the compressor?

[jjs3rd914]

Thought about that (jumper throttle body), by taking that signal. But would that also kill the compressor when the car is at speed and the gas pedal is not depressed (i.e. coasting, believe thats called over-run condition). If so then the compressor would cut out under that condition as well, even though the rpm is up?

I am really leaning towards an rpm sensor that would kill the compressor below a certain rpm. Any ideas where to get such a gizmo? What I need is an rpm sensor that has a threshold level output that could be tied into a relay, hence tied to compressor.

Thanks
jjs3rd914

[rfuerst911sc]

I'm surprised you're getting such a large drop in RPM's. The Sanden is a efficient wobble plate design with not much drag. On my 911 I barely see a drop in rpm's maybe 15-25. Granted that is a larger engine are you sure you are not overcharged with refrigerant?

[jjs3rd914]

Actually I think I may be undercharged. LP is around 30, HP is about 180 which I believe is nominal. However, when charging the system initially (yesterday) it only took a little over one can (12oz) when it should take over 20 oz. I just cannot seem to get any more in. I am reluctant to invert or shake can. I used 2 cans but they are still about half full, they will not empty. The low side pipe is getting cool but not like it should. Could this be contributing to the issue?

Thanks
jjs3rd914

[Dave at Pelican Parts]

I am not very familiar with AC systems, so I don't know what amount of drag is normal.

However, there are a number of electrical solenoid valves that you can install; you want to let air go from "upstream" of the throttle plate to "downstream" of the throttle plate when the compressor is running. You can T into one of the hoses coming off the air cleaner, and T off one of the hoses going to the manifold. Run those two new lines to the valve, and set the valve up to open when the compressor kicks on.

Exactly which valve works, I do not know...

--DD

[rfuerst911sc]

Originally Posted by jjs3rd914

Actually I think I may be undercharged. LP is around 30, HP is about 180 which I believe is nominal. However, when charging the system initially (yesterday) it only took a little over one can (12oz) when it should take over 20 oz. I just cannot seem to get any more in. I am reluctant to invert or shake can. I used 2 cans but they are still about half full, they will not empty. The low side pipe is getting cool but not like it should. Could this be contributing to the issue?

Thanks
jjs3rd914

Did you draw a vacumn on the system for several hours?

[jjs3rd914]

Yes, vacuum was on system for almost 2 hours. I am wondering how much of a charge is acually needed?