RPM Drop When turning on AC

[lazabby]

How much should RPMs drop when turning on the AC? I was at a friends and his RPMs droped maybe by 50. When I turn on my AC the RPMs dropped 200-300. What could be causing such a larger drop in my car? The AC on my car blows cold so I don't think it's low on R12. I'm planning on checking/cleaning all electrical contacts. I have the Joe Cool setup in the fuse box to control the fans.

[DMCMW Dave]

How much should RPMs drop when turning on the AC? I was at a friends and his RPMs droped maybe by 50. When I turn on my AC the RPMs dropped 200-300. What could be causing such a larger drop in my car? The AC on my car blows cold so I don't think it's low on R12. I'm planning on checking/cleaning all electrical contacts. I have the Joe Cool setup in the fuse box to control the fans.

Not that much, the idle control system should compensate. I believe your car is EFI although you don't mention it here. Is your friends's car EFI or are you comparing yours to a stock KJet?

Assuming EFI - which I'm not hands on particularly familiar with but certain things have to be there -

What are you using to control idle speed? Do you still have a stock Idle Speed Motor (aka Idle Control Valve aka ICV)? What is controlling it? How did you calibrate it? What does your EFI use to determing fuel flow? I suspect that your EFI is not increasing fuel flow enough with the A/C on even if you are letting more air in. What input does the EFI use to increase fuel flow?

If the idle motor is almost wide open at based idle i.e. no load, A/C off, lights off, hot), it can't open any more to compensate. This means that throttle blades are too far closed at base idle. So all you need to do is bump up the throttle stop screw.

[dmc6960]

I believe he has abandoned the original idle motor/ECU, something I left alone on my EFI just for this reason. The basic stepper setup you have only allows for the most basic compensation, warmup. With no feedback and logic behind the idle controller, your engine is at the mercy of whatever load is placed upon it. Be it your alternator, AC compressor, or Automatic Transmission.

The original idle motor and ECU may be 31 years old, but its a very well designed system and works excellent, even with other fuel systems (its not tied into K-jet).

[lazabby]

Both cars are EFI. Similar setup.

[DMCMW Dave]

Both cars are EFI. Similar setup.

OK - but what IS the setup. Are the original ICV and ECU still there or are you, as Jim mentioned, at the mercy of MS.


(is that enough acronyms in one sentence for everyone?)

[lazabby]

OK - but what IS the setup. Are the original ICV and ECU still there or are you, as Jim mentioned, at the mercy of MS.


(is that enough acronyms in one sentence for everyone?)

Both cars do not have the original ECU or ICV.

[Spittybug]

I am the friend referenced.

Both cars running Megasquirt for fuel control and spark. Dave S., to answer your question, the fueling quantity is set by a user defined table of values. One parameter is the engine load (manifold vacuum level as sensed by the sensor in the MS unit), and the car's RPMs sensed from the distributor's VR signal input. The table can be autotuned by Megasquirt as it uses AFR or Lambda to optimize the table.

My car is running open loop for fuel (no feedback for AFR), open loop for advance and open loop for idle air. The idle air is controlled by a GM stepper valve that moves as a function of engine temperature. Wide open when cold allows more air in, fully closed when warm, a curve in between. The base idle is therefore determined by the amount of bleed around the throttle plates. While I have a non-stock manifold and throttle, the functions are the same with Shannon's. Let's ignore idle speed when warming up for a minute and talk steady state. When my A/C system kicks on and off, I experience 50-100 RPM drop. Not more. I have a new Saturn alternator, a good battery, Dave M.'s fan controller and known clean contacts and grounds. A/C compressor is ~5 years old, pulley bearings newer than that.

Shannon's car was also running open loop on all of the above. His fueling and spark tables are reasonable and similar to mine. Once warm we ensured that the idle air valve was closed and proceeded to adjust his throttle plate screw to set the idle. We've done this before. If we set it for ~ 900 RPM with A/C off, the car dies when the A/C comes on. If we set it to ~900 RPM with the A/C on, it jumps to about 1200 RPM when it is turned off (and remains there). To be clear, the amount of idle air isn't changing and I made the spark table consistent in that range. The fueling values give us acceptable AFR readings, maybe a little lean.

Something associated with the A/C system kicking on is putting a big load on his engine. Under normal circumstances THIS IS NOT A PROBLEM FOR MEGASQUIRT TO HANDLE as we can enact all the closed loop things mentioned. However, something is not right and trying to "catch" a 300 RPM drop is too big and is only masking an underlying problem. I told Shannon to try a couple of things:



Remove A/C compressor belt and see if he gets the same drop. If so = electrical load, if not, mechanical load of compressor or pulleys.
Check idle pulleys for resistance.
Compressor clutch


I'm betting on a combination of both. Electrically speaking, this car appears to have a few challenges. The battery ground cable appears to have been rerouted to tap into the frame where the trailing arm aluminum shield clips to it. He says that DMCH did it. They actually cut a flap in the shield to give access. I couldn't really tell, but it didn't look like any epoxy had been removed before the eye ring was screwed down and oxidization was obvious too. The engine grounding strap doesn't look like it has been cleaned ever. He has Joe Cool fan controller. Is it supposed to turn on the fans (his are original) even if the car has been turned off???? His does. The alternator is believed to be original.....etc.

I suggested to Shannon that rather than guessing, the best thing to do was to attack every connection in the car he can lay his hands on and clean them. While his bearings may indeed be a part of the problem, I think he experiences massive electrical load, strain on the alternator, drop in voltage and the unpredictable effect that has on the MS box, injector opening times, dwell and whatever else.

I've said it countless times, if your car has issues, MS will find them!

[dmc6960]

Remove A/C compressor belt and see if he gets the same drop. If so = electrical load, if not, mechanical load of compressor or pulleys.
Check idle pulleys for resistance.
Compressor clutch



Thats a good quick list of the possible reasons for the RPM load drop. Check them out.

With that said, I'm curious to how your car behaved when the AC kicked on with a stock idle system. Was there a drop followed by recovery? There should have been. Its also very possible the stock idle controller compensated for the drop before it was even largely noticed, and kept the engine at about the same speed. Thats one of the very nice things about the stock idle setup which I made sure to leave intact when converting my fuel injection over to Megasquirt. Like many of us say, going to a carburetor is going backwards with fuel delivery. Well I also say going to a non-feedback controlled stepper IAC is going backwards with idle control with respect to the original full feedback PWM idle motor/ECU.

[Bitsyncmaster]

Just pull the clutch wire off the compressor to determine if it's the compressor load.

When I get home, (I'm in South Carolina now) I will run a test on how much the idle drops with the AC engaged. I can switch my idle ECU to open loop. I will probably set the idle speed to 1100 RPM before I switch it to open loop.

[mluder]

Just to clarify... Not certain but I may bne having a simmilar problem.

While driving, especially with the AC running but not always, as I come to a stop at a red light the RPMs drop suddenly when I step on the clutch. Occasionally it even kills the engine and I have to restart. Is this an issue with the idle motor> Like I said, it mostly happens with the AC running but soemtimes under normal operations.

Cheers.
Steve

[Spittybug]

Jim, the newer code variants now include closed loop idle that is supposed to be very good. I haven't had much time of late to play with it, but reading a lot of posts on the MS boards I get the overwhelming impression that it works well.

Dave, yes unplugging it would certainly work, but I had the ulterior motive of checking the pulley bearings while he was at it.

Shannon has his homework! I've volunteered the use of the lift and some assistance................

[lazabby]

The pulleys are spinning quite nicely. No resistance there. I didn't try starting the car with the belt off as I had the battery disconnected. I'm going though the car cleaning fuses, relays, grounds, etc. No major corrosion anywhere.

[Bitsyncmaster]

On my car the compressor going off at 875 RPM open loop changes RPM to 975 RPM. Then when the cooling fans cut off the RPM goes to 1000 RPM.

I have the lower power AC compressor made for R-134a and John's low power cooling fans.

[lazabby]

On my car the compressor going off at 875 RPM open loop changes RPM to 975 RPM. Then when the cooling fans cut off the RPM goes to 1000 RPM.

I have the lower power AC compressor made for R-134a and John's low power cooling fans.

I've been wondering if I should go to closed loop. I had it on closed loop for a while. Ca you send me you Tuner Studio Tune on closed loop so I can see you settings?

[Spittybug]

Shannon, Dave M. isn't Megasquirted. You SHOULD ultimately go to closed loop as this is the ideal situation. You SHOULDN'T try to get a 300 rpm gap to be solved by this alone however. Find why you have such a big drop.

[Bitsyncmaster]

Shannon, Dave M. isn't Megasquirted. You SHOULD ultimately go to closed loop as this is the ideal situation. You SHOULDN'T try to get a 300 rpm gap to be solved by this alone however. Find why you have such a big drop.

My results show you must have a problem because your drop is much more than expected. Maybe your AC system is overcharged.

The closed loop idle control should handle that much drop but it will push it to it's limit.

[lazabby]

I have the Joe Cool module running the radiator fan setup. Before that it was the spade wire set up in relays 6 and 7 to fix the fan fail issue. In order to rule out a problem with Joe Cool, can someone send me a photo of how the wire set up is suppose to look in the relay compartment so I can make sure I have the wires plugged in correctly?

[stevedmc]

How much should RPMs drop when turning on the AC?

Mine only drops somewhere between 50-100 RPM. I'm carbureted and have nothing fancy to compensate. I just set my idle a little high (900 ish) that way it can safely idle between 800-850 when the compressor kicks on.

Anything higher than a 100 RPM drop makes it sound like you've got some sort of other problem going on. For what its worth, I'm running stock cooling fans and a Saturn alternator. My compressor is still original.

[stevedmc]

I just thought of something else. Could there be a vacuum leak somewhere in the system that would only occur when the compressor engages? A vacuum leak could certainly make the engine RPM drop.

[Bitsyncmaster]

I just thought of something else. Could there be a vacuum leak somewhere in the system that would only occur when the compressor engages? A vacuum leak could certainly make the engine RPM drop.

Good idea. Maybe someone hooded the hose from the heater valve to the distributor. Or would that work the other way (AC on would bump the idle up)?

[lazabby]

A vacuum leak would cause the RPMs to go up.