Car stalling when AC unit on.

[watchdogd]

I was out driving this morning and noticed that as I would come to a stop at a light that the car would idle real low then die. This appears to only happen when the AC is on. As soon as I turn off the AC unit the car no longer stalls.

I have only had this car for a short time and not real familiar with what the ideal idle should be with the air on or off. Does this sound like a relay problem or something with the load on the engine when the AC is on ?

Thanks,

 

[goldNSX]

Sounds like your A/C compressor is dying and getting stuck. Does A/C work? How does the belt/pulley look like?

Idle is controlled and stabilized by the ECU whenever the A/C compressor kicks in and puts a load on the engine. So there might be sometime fishy there too. At least it will be less expensive to fix.

But my guess is a dying A/C compressor.

 

[watchdogd]

I went back out driving around with the ac lasting nicely. I notice if i put the car in neutral as i come to the stop slowly then it does not stall. Ive noticed its on more abrupt stops with ac on and clutch engaged. But if ac is off even if stopping abruptly then no problem stopping.

 

[goldNSX]

Any mods on the car or completely stock?

Sounds more like a idle control issue.

 

[watchdogd]

Everything is stock. It just had the timing belt service and valve adjustment. What should the idle be when ac is off?

It looks about 800 or 700 when im at a stop. But i do notice it diplong to like 500 before bouncing back up.

 

[Old Guy]

At the risk of becoming repetitive in my response to threads, I repeat my mantra that when responding to requests for diagnosis we need the model year. In this case at least whether it is a pre drive by wire or post drive by wire car because the idle control systems are different. When the AC is running the ECU implements an 'idle up' function to handle the additional load from the AC at idle.

The ECU on both pre and post DBW cars get the air conditioning request from the climate control unit and the ECU actually controls the compressor clutch. If your AC is working this means that the ECU is getting the air conditioning request, is energizing the AC clutch and presumably should be implementing the AC related idle up signal. The idle control system also gets signals from the clutch and brake switches that put the ECU into idle control mode. If a particular combination of applying the brake first then the clutch (or vice versa) causes a stall and reversing the order doesn't, this could be an indication that the ECU may not be getting one of the brake or clutch signals and the ECU is not going into idle control mode immediately or at all. The AC compressor running makes this a bigger problem because it puts a load on the engine and the base idle setting isn't enough to maintain reliable idle operation. As @goldNSX suggests the idle control system is a good place to start looking. The service manual contains a fairly extensive section on the idle control system including the AC, clutch and brake connections with the appropriate test procedures. There are links to downloadable copies of the early and later service manuals on Prime.

If you have a pre drive by wire car I would also be inclined to point a finger at the electronic air control valve (EACV) just because they do tend to get gummed up with age. The EACV might not be the cause of your problem; but, given the age of a pre drive by wire car, cleaning the EACV can't hurt (providing you don't break anything) and might eliminate the problem. Use the search function on Prime and you will be able to find a number of threads discussing EACV cleaning.

 

[Old Guy]

Everything is stock. It just had the timing belt service and valve adjustment. What should ie be when ac is off. It looks about 800 or 700 when im at a stop. But i do notice it diplong to like 500 before bouncing back up.

On my na2, I think the base idle is 800 +/- 50 RPM. When warm my idle is somewhere between 800-850 RPM according to the tach which is not exactly a super accurate measurement.

If your idle is dipping to 500 RPM I would definitely be looking at those brake and clutch inputs and if your car is pre DBW, checking out the threads on cleaning the EACV.

 

[watchdogd]

Thanks for the detailed input. Sorry for leaving out the year. It is a 2005 model year.

I will go look at the documentation for idle control system. This will be very helpful if I need to discuss with a shop.

 

[Old Guy]

2005 is DBW so you get to skip the joy of cleaning the EACV.

 

[watchdogd]

Hey OldGuy your comment on Clutch and Brake inputs makes me think I might have caused my own problem. Yesterday I noticed the brake pedal stopper needed to be replaced since my rear lights would not go off. Which was on another thread I had started.

Until I could get to the dealer for the part, I "JimmyRigged" a plastic fender clip to push against the switch to keep the lights off. Would something like this potentially interfere with stalling out ?

I got to the dealer and put in the new part on this evening its number is: 46505-SA5-000. I tried driving around the last hour and the car will not stall.
I feel dumb for not thinking that would cause it first and maybe it has no relationship but since putting the correct stopper in the car hasn't stalled at all with the ac on. Im either lucky so far or that jimmy rigged clip I had in there was affecting the input as you mentioned.

Either way you've been very helpful and even on a previous post I had so thanks again. I haven't even driven my car a full week yet and had some unexpected things go bad. Thanks to the Prime forums being a great resource.:smile:

 

[goldNSX]

Glad to know you've got it solved. If only you had mentioned the rear lights problem too... Bad idle can be causes by a lot more things as you also mentioned a recent TB service like a TB one tooth off and other horror stories hard to find and solve. But let's hope all is fine now. You have a rare car with your 2005. Have fun with it.

 

[watchdogd]

Thanks. I'm hoping the proper clip solved it. I plan on daily driving this for a while

 

[Old Guy]

Hey OldGuy your comment on Clutch and Brake inputs makes me think I might have caused my own problem. Yesterday I noticed the brake pedal stopper needed to be replaced since my rear lights would not go off. Which was on another thread I had started.

Until I could get to the dealer for the part, I "JimmyRigged" a plastic fender clip to push against the switch to keep the lights off. Would something like this potentially interfere with stalling out ?

Most likely the cause of the problem. By defeating the brake light switch the ECU never got the message that the brakes have been applied. The brake light input for the idle control system is a DBW car 'thing'. It was not on the pre DBW cars and it is not clear to me what role it has; however, you appear to have discovered the circumstances under which it operates (or doesn't). The limited number of ECUs that I am familiar with do not have a brake input for idle control. They just look at a RPM < xx, throttle position < xx and perhaps MAP < xx and if those conditions are satisfied put the engine into idle control. Since the brake switch input to the ECU showed up with DBW perhaps it is part of a back-up or safety system related to idle control on DBW cars.

 

[scratch90]

This thread is getting technical. Is there any real way to check if the inputs from the clutch and brake read easier? I imagine having the obd2 reader honda. It is happening to me recently. Ac is cold and when I am off accell then clutch in it will die. As mentioned slowing to a stop or a light. I was going to try a reset on the computer since my battery went dead and it reset badly.
2000 nsx
Exhaust is only mod. Came with car and had no issues before

 

[Honcho]

This thread is getting technical. Is there any real way to check if the inputs from the clutch and brake read easier? I imagine having the obd2 reader honda. It is happening to me recently. Ac is cold and when I am off accell then clutch in it will die. As mentioned slowing to a stop or a light. I was going to try a reset on the computer since my battery went dead and it reset badly.
2000 nsx
Exhaust is only mod. Came with car and had no issues before

So are you saying the engine dies when you push in the clutch pedal and the A/C is on?

 

[Old Guy]

There are a whole pile of things that can cause the engine to die / fail to idle. Unless your problem specifically matches up with the problem described in the previous posts I would not focus on just the brake switch.

An OBDII scanner will not help you. The OBDII diagnostics on the NSX are all primarily focused on compliance with emission requirements with a smattering of automatic transmission diagnostics thrown in to the mix along with a couple of DTCs for the drive by wire servo system. It does have a single error code dedicated to the idle control system P0505; however, according to the service manual that is not related to the operation of the air conditioning system. The NSX does not have a CAN bus system with the large scale diagnostics that modern cars have. So if "getting technical" means having to unplug stuff and use a multimeter to test then the only way to avoid that is a trip to your dealership service department and let them do the grunt for you. Otherwise, get the service manual and multimeter out and start measuring.

In the section of the service manual (starting at 11-117 in the 1997 version) dedicated to problems with the DBW idle control, the list of things to check are
- air conditioning signal
- alternator signal
- neutral switch signal
- clutch switch signal
- brake switch signal
The service manual provides no guidance on the function of each of these signals in terms of idle control, and no guidance as to what problem = what cause so you have to be guided by your symptoms. However, the brake switch signal is listed as the last thing to check in the probable cause list for engine stalling so that is a good place to start absent other symptoms.

All of this presumes that your base idle speed is correctly set. No point in trying to diagnose an idle up problem if the base idle speed is incorrect.

As a footnote to my 5 year old post, the brake and clutch signals are definitely required in the DBW cars because the ECM does cruise control via the DBW system and brake and clutch are required cruise control inputs. They may also be required for idle control as implied; but, not explicitly stated by the service manual