1989 240sx: Standalone Air conditioning?

[BlackHat]

I'm in the middle of implementing a MSEFI standalone (msefi.com) and am struggling with finding a way to keep the A/C. Is there anyway to have the A/C run off of the stock ECU without any input from the engine?

As far as Idle is concerned, I'm probably going to ditch the setup they have and fabricate my own PWM valve to handle all of idling. The A/C won't have any problem running at regular idle speeds (750rpm) will it? Or freak out if it can't use the FICD?

Thanks in advance.

[pmkls2]

The only thing you need for the ac to work would be to have a method of compensating the idle for the extra load the compressor places on the engine. I havent looked at the factory schematics lately but I know I am planning a similar change only I am going to use a GM ecu to control everything and had looked to see what all needed to be done. What I am planning on doing is completely deleting all of the ridiculous idle controls and using a gm style IAC motor and control it with the ecu. There is a company that makes an adapter to use the screw in style IAC motor on anything. It simply uses 3/8 vaccum lines to tee into the intake and airbox and bypass the throttle body. The company is called electromotive and there are plenty of distributors that sell their parts. I dont know the specifics of how the megasquirt works but I would think that you could use it to control the GM IAC motor as it isnt PWM and is much easier to control.

[benemorius]

Did you mean is there a way to run the a/c on the engine without any input from the ecu? That sounds like it makes more sense.

Actually, the only thing the a/c needs to work is for the ecu to cycle the compressor based on the temperature of the evaporator core. This is a vital function and you can't do without it. You could, however, design a simple circuit to manage it in place of the ecu.

[nukeone]

I believe the thermo control amp provides the signal to cycle the compressor; it just happens to input to the ECU where a decision can be made based on other factors (such as load, throttle valve wide open) as to whether or not the a/c relay gets a signal to kick on. So the output signal from the thermo control amp could feed any other controlling device that needs a temperature input. Lots to consider because the ECU also sends a signal to the condenser fan relay as well, with input from the ambient temp. switch. A complete stand alone unit will have to consider all of this. And, yes, an input back to the ECU for compressor load would be an important consideration.

[benemorius]

I can't tell just from the wiring diagram whether the thermo control amp can function alone or not, but that would make it much more convenient if that's the case.

As for the condensor fan, it is run from the same relay as the compressor clutch and has it's thermo sensor wired in line.

So I guess if you're certain about the thermo control amp then it's settled that the a/c can be run entirely by itself, with the one exception of the 3 second WOT compressor cut which technically isn't required.

[pmkls2]

like i said i havent looked at the schematics in a while but from what i remember the wot cut out isnt really necessary and neither is the thermo switch for the condenser fan. on my car now the thermo switch has been bad for as long as i have owned it so i just simply bypassed it to run anytime the compressor is running. also i think that the thermo control amp does function just fine without the ecu. i believe it inputs to the ecu to compensate for the compressor load for idle etc.... but again this is what i thought, im not sure of anything. whenever i get around to my ecu swap i can give a definate answer but that will be some time

[BlackHat]

The extra load at idle is what I was curious on. The new ecu will be able to keep idle at 750rpm. I don't think I'm going to be able to raise the rpm like would normally happen when the A/C kicks on during idle (jumps up ~150rpm). Hopefully I'll be able to setup some fast idle for the A/C, but if I can't I was wondering if there were going to be any probs....

I'm also GREATLY interested in a way to getting the A/C to run withou the ecu. Although, I thought the ECU controlled when the clutch for the compressor kicked on so it wouldn't burn out on high rpms? Is that incorrect?

[benemorius]

The FICD is what provides extra air to raise the idle when the a/c compressor is engaged, and it is controlled by the same relay as the compressor clutch. As long as you keep that then your idle will be taken care of.

We have pretty much collectively decided that running the a/c without the ecu won't be a problem.

The ecu doesn't cut the compressor at high rpm. It cuts the compresor for 3 seconds when you floor the gas pedal. After 3 seconds it re-engages the compressor regardless of rpm. This feature isn't required. It only serves to give you maximum power when you floor it.

[BlackHat]

Heh... That really doesn't sound like a problem... I don't see many people drag racing while the A/C is running

To be honest I was going to rip out the IAA and have a fabricated plate with a 3/8" nipple, then run a hose to that and where the air reg is and have both controled by one PWM valve. The ECU will take care of idle, but it won't spike idle during a/c operation.

I would keep the IAA and air reg, but MSEFI to my knowledge won't control two idle air devices at the same time.

[benemorius]

I'm sure you'll figure something out. The only problem I see is that your setup probably won't react very quickly when the compressor kicks on. There may be a brief (1 second maybe) time during which the engine would stall very easily every time your compressor engages. It might not be a big deal or it might turn out to be a major problem. I guess you won't know until you know.

[BlackHat]

I'm beginning to see the merit in your idea of keeping the FICD (sorry, I'm a little slow to come around sometimes). See how this idea strikes you...

Where the idle air pipe T's put a PWM valve where the Air Reg is (removing the air reg and that should be a big enough opening to allow idling under any conditions). Remove the AAC and weld up the little opening on the inside of the IAA for the AAC. Leave the FICD hooked up to the A/C system and I even get to keep an idle screw! =D

To my understanding, the A/C would open the FICD valve whenever it wants to turn on the compressor and should take some of the sudden load to the idle away. Thereby helping the MSEFI handle the fluctuation in idle.

Sound good? (if it does I'll give you all the credit for the idea =P)

[BlackHat]

... the output signal from the thermo control amp could feed any other controlling device that needs a temperature input.

So what you're proposing is feeding the temp output to the other A/C devices directly? By-passing the ECU... Can everything in the A/C system work off of this signal?

Lots to consider because the ECU also sends a signal to the condenser fan relay as well, with input from the ambient temp. switch. A complete stand alone unit will have to consider all of this.

Are you suggesting having whatever replaces the stock ECU manage the A/C system too?

And, yes, an input back to the ECU for compressor load would be an important consideration.

I was debating pulling a signal from the wire going to the FICD (from what I could tell from the wiring diagram the FICD kicks on from a relay at the same time as the compressor) and sending it to the MSEFI, but that's something for msefi.com to help figure out

Assuming I've got the Idle controls taking care of at the moment. What modifications am I looking at to get the A/C to be a standalone? Sounds like just running some wires to bypass the ECU.