A/C fan slows, A/C gets warmer when stepping on brake

[pet575]

This is completely crazy to me, but this summer for the first time (I bought my 2004 in 2005 when it was a year old) my A/C loses its coolness when I step on the brake pedal for more than a few seconds. If you have the fan up on one of the higher speeds, you can actually hear the fan slow down as well.

I've noticed over the past few years that, at night time, the headlights will actually dim a bit when I step on the brake and then brighten back to normal when I let off the brake pedal and speed back up. Have also noticed the power windows and moonroof go up MUCH more slowly lately-especially with the engine not running but with the key turned on. Battery and alternator were my suspicions, but they have have been checked and verified as good.

Possible bad battery connection or battery cable? Negative cable and terminal have all been replaced recently, making the problem no different. Wondering if the positive battery cable and/or connector are the problem?

 

[pet575]

Also, this does not appear to be a slipping belt problem or anything like that. Belt was replaced back in February without issue. A/C will stay nice and cold as it always have if you put the vehicle in park and just sit and idle, so I do not believe this issue is related to increasing/decreasing RPMs.

 

[charli]

Check your alternator output.

 

[pet575]

Battery and alternator were my suspicions, but they have have been checked and verified as good.

 

[general x]

Does the A/C go to ambient air temp if you hold the brake pedal or is it mearly a fan speed issue? If so maybe try to get someone to hold the brake pedal with the engine running and see if the A/C compressor stops cycling. Don't know why it would quit but for the air to warm with the A/C on, the compressor would have to stop working

All the other issues (and maybe even the A/C) sound like a bad connection somewhere, and since it's effecting plenty of the electrical it must be somewhere important like a major connection. How well does the starter work, and what happens when you turn everything on? I would check the positive cable circuit for corrosion. Wherever the problem is, it will be fairly obvious once you find it. The connection will have a fluffy greenish coating on it or will be just plain rusty. Once you find it, clean the corrosion off with emery cloth, wire wheel or wire brush and make sure to coat the cleaned connections with dielectric grease or something similar to prevent further corrosion down the road. It doesn't take much to muck up an electrical system so hopefully it's something simple like a bad connection!

 

[pet575]

Does the A/C go to ambient air temp if you hold the brake pedal or is it mearly a fan speed issue?

It doesn't go to "warm" it just becomes "less cool" and it is not for very long. The fan also slows a bit for just a second, but then both the fan and the coolness "recover" if you hold the brake down for awhile or if you let off the brake and accelerate.

Odd that there is no effect on the A/C or fan speed if you were to put all the windows down and then put theme back up while the headlights and radio are on. It is ONLY when you step on the brake. Pretty confusing deal. I'll start with the positive cable when I get a chance this weekend.

 

[general x]

Yeah, I don't know what to say for sure. Maybe the initial momentary draw from the brake light circuit is higher than the combined draw from the windows and headlights causing the A/C clutch to momentarily disengage and the fan to slow down. What would cause that I do not know, perhaps part of the ABS or some other system that is momentarily cycled by the pressing of the brake pedal? Whatever is causing the entire electrical system to perform poorly should be addressed first, as fixing it will likely cause any other problems to go away.

 

[pet575]

Update: since I posted this I've kept a closer eye on the cause of the symptoms. I'm still confused but I DO know this now-it is not JUST the stepping on the brakes that this happens. Sitting with the engine idling in Park, with the windows up and the A/C going, I can duplicate this symptom by pushing the power window switches UP (even though the windows are already up).

The fan speed slows considerably and then recovers on its own even if I continue to hold down the buttons on the window switches. So, this is obviously something electrical rather than brake-related. Thinking I'll get a second opinion on the alternator output and also check the cables very closely. For a few years now, I've noticed the headlights dimming just a tiny bit when in the garage or something if I step on the brake, push the windows buttons with the windows already up, or something similar.

Anyone have anything to add now that I know this?

 

[matt0248]

This is a normal condition. As electric motors get old, window motors, fans, ect. they draw more current to operate. So, when your ex is idling, the alternator can only output so much current, when you change the load on the alternator by asking the wondow motor to stall (asking the motor to raise the glass up when it already is) the increased current lowers the voltage output of the alternator enough to slow the blower fan, dim the headlights, and so on. It is a cumulative effect of several things and probably not a problem. The A/C compressor also adds mechanical load to the engine, you should hear the idle speed of the engine increase slightly when the compressor comes on. The air coming out of the vents feels warmer for two reasons, the compressor is truning slowly and there is lower air flow arcoss the condensor.

IF this is really bugging you there are aftermarket alternators with higher idle output, problem solved.

 

[pet575]

Thanks matt.

If I'm understanding you correctly, this normal condition is an indicator only of age and wear and not failure. Is that correct or should I change out the alternator in the interests of being proactive and not having it fail on me where it is not convenient?

 

[matt0248]

A good test, if you have access to a volt meter, is to connect the meter across the battery and let the truck idle with the AC on high speed and the headlights on. See if the alternator can keep the voltage level constant. take note of the charging voltage every couple minutes. The alternator should be able to keep a stable voltage at idle indefinately. If the voltage seems to slowly drop over time it could be that the alternator needs to be replaced.

 

[pet575]

I've been MIA for awhile, but I wanted to update this thread to let you guys know that I discovered some time ago that my battery had a bad cell in it. I was more than likely living purely on the alternator current and what little charge the battery was holding. Replaced the battery in November but won't know until it gets hot whether this will fix the A/C issue. I'll get to test out the fan speed issue once we get some snow, I'm sure.