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Sourcing a Boost Pressure switch to turn off A/C?

Anybody point me in the right direction for a boost pressure switch i can tie in with my mechanical boost gauge boost signal and A/C compressor clutch.

Let's say it detects 5psi, then the switch would "open" [turn off] and a/c compressor would disengage?

thanks,
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LIVN80S - - Red '79 Porsche 930 Steel Slant Nose Conversion [in 1987] w. 46k miles 3.3L; 964 Cams; K27HF @ 1.0 BAR, with Garrettson Intercooler; Rarly Zork; CIS Flowtech Fuel Head & BL-WUR.
Old 06-23-2011, 12:37 PM
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Ok, so here's where I'm at:

This boost switch adjusted to "open" at 3 or 4 psi boost:
Coolingmist Boost Pressure Switch - Modern Automotive Performance

After the switch, a relay with "delay back to on" set at about 10-15 seconds:
Automotive Time-Delay Relay

But would use the TRD-PO on this page:
TDR-O: Normally On Time-Delay Relay

From there, on to the Compressor Clutch wire...

What say you? Thanks,
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LIVN80S - - Red '79 Porsche 930 Steel Slant Nose Conversion [in 1987] w. 46k miles 3.3L; 964 Cams; K27HF @ 1.0 BAR, with Garrettson Intercooler; Rarly Zork; CIS Flowtech Fuel Head & BL-WUR.

Last edited by mooney265; 06-23-2011 at 07:01 PM..
Old 06-23-2011, 03:21 PM
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I can't add any input other to say that you have a great idea there. I'd think that I'd want the compressor to kick back on pretty quick unless running up through the gears. Maybe a 4-5 second delay would be about right.
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Old 06-23-2011, 04:23 PM
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Lots of cars have this. Would you consider talking to a friendly GM or Ford parts man?

Mark
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Old 06-23-2011, 04:57 PM
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maybe you could figure out a way to use the ignition delay relay already in the car for this?
Old 06-23-2011, 05:28 PM
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Quote:
Originally Posted by lucittm View Post
Lots of cars have this. Would you consider talking to a friendly GM or Ford parts man?

Mark
tons of cars have which? the boost switch or the "delay to back on" relay?
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Old 06-23-2011, 07:02 PM
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You can go a MUCH simpler route. Just have a full throttle switch that activates a relay. I did this on a car I built a long time ago and to my surprise, it was done by the factory on my Mazda GTX.
Old 06-24-2011, 05:12 AM
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Originally Posted by IMR-Merlin View Post
You can go a MUCH simpler route. Just have a full throttle switch that activates a relay. I did this on a car I built a long time ago and to my surprise, it was done by the factory on my Mazda GTX.
yes, it was on my Mark VIII, too... Initially, thought I broke the A/C when I accelerated hard...

my issue with that is, I'd like to disengage the a/c clutch at say 1/2 throttle or 3/4 throttle, too. So, I think it best to go with the adjustable boost switch - - gives me some flexibility...
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Old 06-24-2011, 05:28 AM
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What's funny is - - before I went with Griffiths and changed out all and added a third condenser [amongst other things], I never used my A/C - - So, this wasn't a problem...

Now, it's comfortable to drive the car around town, but a PIA and frustrating to gas it with the added compressor load - - feels like i'm towing a trailer

So, gotta get this fixed. Looks like the cost will be under $75 to get it done. Money well spent
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Old 06-24-2011, 05:32 AM
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you can do what you want with microswitch and a little thought behind it. all you need to do is have lever or something spring loaded on the throttle body, so when it rotates to 75% throttle the lever hits the switch, but if you push past it, the lever is spring loaded and will let the throttle keep rotating past it. We did for a nitrous system once too. It was a complete fail safe system. It was a timed injection with an RPM on and off level that only come on when almost at full throttle. The thought was if you needed to feather the gas a bit, it wouldn't cut the nitrous and give you a line surge.


I got the idea from the old CIS Rabbits. The has a full throttle enrichment switch on the throttle body.
Old 06-24-2011, 05:43 AM
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Quote:
Originally Posted by lucittm View Post
Lots of cars have this.
Yup. Been on lots of cars as OE for a long time now. However, the Germans didn't think it was necessary on our cars.
Too bad there was no way the factory could increase idle speed on CIS when the a/c is turned on either.
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Old 06-25-2011, 05:31 AM
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I took a bosch vacuum solenoid used for this purpose from a 3.4 liter bosch L jetronic efi 6 cylinder BMW in a junkyard and installed it on the back side of the CIS air flow meter housing and plumbed it in with a T fitting in the hoses on each side of the aux air valve so when this solenoid valve opens it is bypassing a small amount of air around the throttle body when the AC compressor is running.
Hooked the solenoid up in parallel to the AC compressor power wire so it opens when the AC compressor clutch engages.

With this there is no drop in idle speed when the AC compressor kicks on. I also used a small spray gun ball valve in the bypass hose to adjust the idle speed exactly the same when the AC compressor is running.

easy and satisfying little modification.
Old 06-25-2011, 06:25 AM
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Awesome - - now i know what happend to Macgyver when he retired - - he turned in to JFairman!
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Old 06-25-2011, 07:32 AM
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Quote:
Originally Posted by mooney265 View Post
Awesome - - now i know what happend to Macgyver when he retired - - he turned in to JFairman!
+1. I thought the solution was to go to EFI! Silly me....Jim that is brilliant! Can you post a photo?
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Old 06-26-2011, 04:13 AM
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Because I removed the AAV, my idle tends to be a little low on initial startup - maybe 300 rpm's. but, it will idle without me adding any pedal. as the car warms fully up, the idle goes to about 1,050 [without the A/C]. i believe it drops to about 900 with the A/C.

My point being - - i have the idle adjustment screw turned all the way in, and the lowest i can get my idle is 1,050 when fully warmed... I'm sure I have a small vacuum leak somewhere, but have not been able to track it down...

I think it was JFairman that posted a while back a device that he hooked to the rubber 90* elbow right after the FD. It basically plugged the elbow along with injecting compressed air downstream... I tried to pull this off, but didn't have any luck. Maybe I'll try again - - I'd like to get my idle down to say 900...
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Old 06-26-2011, 05:16 AM
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Quote:
Originally Posted by 911nut View Post
+1. I thought the solution was to go to EFI! Silly me....Jim that is brilliant! Can you post a photo?
I have pics of it mounted on the back side of the CIS air flow meter housing from a few years ago but they are on another sata harddrive I'd have to plug in and restart the computer to find them.
The on/off solenoid was used by BMW on the big six pre motoronic 2.8 through 3.5 liter L-jetronic efi cars from about '77 through about '82 for just this purpose. The inner diameter is about 8mm so it only lets enough air through to raise idle about 2-300rpms and it was mounted up top next to the throttle body and plenum chamber.
3/8" carburator fuel line fits it fine.

Bosch part number on it is 0 280 141 009. I'd only look in salvage yards for a used one because they are cheap that way.

I have a spare one and took these pics so you can see what it looks like.

Old 06-26-2011, 07:27 AM
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Hobbs switch,Google it.
Old 06-26-2011, 10:40 AM
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JFairman - Mine had one of those installed (not wired up) when I got my car. I removed it when I removed my A/C as it never really worked.

Now that I am putting back my A/C I may consider reinstalling the solenoid if I have idle issues with the A/C on. Does the idle drop that much with the A/C switched on?
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Old 06-26-2011, 04:18 PM
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Old 06-26-2011, 04:25 PM
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a fully charged SC system will slow the motor arond 200rpms when it engages. all cars will be different and if the charge is low idle won't drop as much.
i like my idle at 800rpms.

check and see if your solenoid is still working, it's looks like it's getting wet sometimes installed there. you hear a click when they open.
Old 06-26-2011, 05:28 PM
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