In the cars it does, most do not have the temp switch that the trucks do. I do not think the trucks shut off the compressor under high power and not all cars do. Mits has wired the AC trough the ECU so it can but it is up to the firmware if they use this feature or not.
A purge valve or EGR may be about right for a kickup, I think an aquarium valve is not enough airflow, unless you guys have much bigger fishys than we do up here.

It may not flow enough. I don't do fish. But I bet one of the ind. supply houses has a needle valve that will flow enough.

I used to have a car with a vac switch that killed the a/c compressor at very low vac, like wot runs, but of course, that won't work on a forced induction motor. Maybe a Hobbs pressure switch to kill it at high boost?

In the 80s the Corvette used the TPS for control over the AC compressor. Very interesting when at top revs and you lift for a corner entry, It sounded like the compressor was going to break off it's mounts.
The 3000GT has a rev counter on the AC compressor to reduce issues with this.

Weird doin's today.... I've been driving the 225 miles each way between Columbia and Atlanta with the MS set on AutoTune and trying to maintain a steady rpm rate over the course of each trip to tune the VE table. Each trip I have saved the .msq so the resulting data is averaged over a long trip.

Today, near the 200 mile mark, I tried to accelerate from 75mph and got an immediate strong lean surge/bucking pattern, followed by a backfire that sounded like it came from the exhaust manifold. Several times of this and no change. I looked at the VE table graph and saw some mountainous peaks in the graph above 100 kpa where the ECU tried to call for more fuel by adjusting the VE value.

Needless to say, I did not save the .msq and thought perhaps there was an exhaust leak - maybe at the manifold flange.

When I restarted 6 hours later under the prior .msq, the engine ran fine - even after it warmed up to operating temperature.

Strange...... really strange. <img src="/forums/images/graemlins/confused.gif" alt="" />

Charlie - ever see this?

Frank

I have seen similar but with the extra code. I was driving my EVO/ Mirage and when squeezing the throttle to climb a grade it backfired and lost power. I could not restart it but since I was running an adapter harness I could plug the factory ECU in and get home. Luckily I had the airflow meter with me.
One other time driving along the engine cut off, this time the plugs were soaking wet with fuel. I changed them and all seemed fine. This was all on the MS1 processor and I do not consider it as stable as the MS2.

It's still running well on the previous .msq. I have a hunch it's a glitch in MegaTune rather than in the MS processor or firmware.

Frank

Jumpering the 2 wires did not produce the desired results - still no A/C compressor. So, I took a longer look at the schematic and once I figured out (I think) what does what, I can say I've never seen such a convoluted circuit as the A/C system. <img src="/forums/images/graemlins/scared.gif" alt="" />

It appears that I was half right about the A/C OFF engine coolant temperature switch controlling the compressor shutdown on overheat..... and Charlie was half right about the ECU providing the shutdown. Although I have no schematic for the ECU internals, it appears that both components are involved.

Here's how I "think" the A/C circuit works:

1) Power from the main 100A fuse is always available through Fuse 6 to the Heater Relay AND through C-24 20A fuse (behind the glove box) to the Condenser Blower Motor Relay AND to A/C Relay A.

2) When the Blower Switch is in any position other than OFF, power is made available to the A/C Switch on the dash.

3) When the A/C Switch is closed, power flows through it and through the Thermistor (behind the dash) and makes power available to A/C Relay B.

4) A/C Relay B is triggered by the Ignition Switch through the Cigarette Lighter circuit and is closed full-time whenever the key switch is ON.

5) Power from A/C Relay B is used to provide coil voltage on A/C Relay A and feeds the Dual Pressure Switch.

6) If the Dual Pressure Switch passes voltage, it triggers the Condenser Blower Motor Relay (which in turn activates the Condenser Blower Motor) AND passes power to the A/C Engine Coolant Temperature Switch.

7) If the A/C Engine Coolant Temperature Switch is closed, it passes voltage to the ECU at C-39 on the GR#1 wire. NOTE this is 12v(+).

8) The signal coming out of the ECU at C-40 on the BW wire is the coil trigger to close A/C Relay A. NOTE this is a 12v(-) trigger to complete the relay coil's ground path through the ECU.

9) Once A/C Relay A is triggered through the ECU, it passes the voltage supplied to it by the Key Switch (see Step #1 above) to the A/C Compressor magnetic clutch.

Now, the question remains - how does the ECU take a (+) input and use it to trigger the ground path of another component? The only answer I can fathom is that there is another relay inside the ECU that is triggered by the incoming signal on C-39... which when closed, completes the ground path trigger for A/C Relay A.

Simple, eh? <img src="/forums/images/graemlins/lol.gif" alt="" />

It appears I need to add a relay behind the glove box.... trigger it with the C-39 GR wire and when triggered, have it complete a ground path for the C-40 BW wire.

If anyone wants to review this and suggest a misread or an alternative method, I'm all ears. <img src="/forums/images/graemlins/lol.gif" alt="" />

Frank

Try it with above grounded.
I do not have the proper manual so I was looking at a 1990FSM and they do not go into the ECU that year.
I will try to find the proper diagram in the morning.

That would allow the compressor to turn on, but would not provide protection from high engine temperature..... probably not a big deal, though.... I should be able to see a boil-over. <img src="/forums/images/graemlins/lol.gif" alt="" />

That might indeed be a simple solution.

Frank

Won't you get high pressure in the a/c in an engine overheat? That should kill the compressor.