Higer RPM when AC is engaged

[B26Marauder]

I had to have the compressor replaced (with a new one, not rebuilt) 2 months ago. The AC has, and continues, to work fine. About 2 weeks ago I was stopped in traffic when the engine suddenly surged about another 300-500 rpm. The car just wanted to pull away from me. I had to press the brake harder to keep it from moving. Ever since that time, whenever the AC compressor engages, the rpm will shoot up the addtional 400 rpm or so. This is making it hard on the engine, brakes and transmission. Whenever the transmission is in the process of switching gears, the engine surges during that half second instead of dropping down in rpm. At speeds below 40, I have to brake to slow down, I just can't take my foot off the gas. This additional rpm acts like a cruise control. I can just take my foot off the gas and the car will continue at the same speed. If I'm doing 15-20 and take my foot off the gas, the car will actually slowly increase in speed and actually change gears and continue to increase if left unchecked. Is there a speed sensor of some kind that only comes into play when the additional load of the AC is placed on the engine? While the car is exhibiting this behavior, if I just reach over and turn off the AC, the engine immediately drops back that 400 rpm, and the rpm and transmission shifting return to normal.

 

[budwich]

Although your problem seems to be related to your A/C, a surge condition may point at your MAF sensor. Have you cleaned or replaced or is it original? The added A/C load is compensated for by the PCM based on various inputs from the sensors. Perhaps the scenario is like this... the computer "detects" an added load or demand for power... means increase fuel (injectors times) and maybe increase timing. Much like if you had pressed on your gas manually. The "detection" could be a "change" in the throttle position, "decreasing" idle, or "increase" in air flow coming into the engine.... etc. Having said that, is your TPS original? A bad TPS can also cause similar problems.

I assume that your "reaching over and turning off A/C and drops back x.. rpm" is at idle. Is this at idle (park) and idle (drive) with foot on brake???

 

[B26Marauder]

Thanks for the reply Budwig. The MAF has been replaced, but that was about 100K ago. I'll look at it this weekend and see if I can clean it up at all.

TPS? Does that mean Throttle Position Switch? If so, it's original but would it behave differently depending on the AC or not?

With the car in Park, the rpm does not increase very much when the AC is turned on. With the car in drive, foot on the brake and at idle, the engine does increase by 300-500 rpm when the AC is turned on. Decreases if the AC is turned off. With the car in drive, at any moving speed, an increase in rpm does occur when the AC is turned on and does decrease when the AC is turned off.

 

[budwich]

thanks for the info. Yes you got the tps right. Would it be different under AC or not? good question. probably not but maybe the "compensation algorithem" it uses the input a bit differently.... don't know. I have a similar surging problem sometimes.... haven't related it to my AC but sometimes when coming to a stop, system will want to rev towards 2000 or so. It improved a bit by replacing the TPS but not entirely. There have been some threads on surging with people having different successes and failures attacking the area. My ford truck cd indicates that a surge problem may be MAF related. I haven't changed mine cause it is expensive and I don't have a lot of spare cash. I have taken it apart and cleaned it (again some threads) and have even played with it. On my 96, the MAF has a metal plate that forms some sort of flow "barrier" for the air moving by the resistive elements. I took the plate off for a test drive..... wow was the thing ugly then, it would go from almost dying to high idle and then normal. When going down the highway, it was stable one minute and then having surge problems, lost power, even maybe shift problems. Again wow, is this thing sensitive.... who would have figured... coming from a carburetor type guy...

I guess my question to you would be...why did you replace the MAF 100K ago...ie. what was the problem / symptoms???

 

[shamaal]

On the 91-99s, the output of the A/C pressure switch goes to the computer and the Wide Open Throttle (WOT) cutout switch and then on to the compressor clutch. I have no reason to believe it is different for a 2000.

It almost sounds as if the compressor clutch is not really engaging, but the computer thinks it is and increases the RPM. However if it's really cooling that theory's no good.

Out of curiosity, does the problem change if you unplug the compressor? The computer would still thinks it's engaged and increase the RPM.

Just a thought

 

[B26Marauder]

I had the Cold Start Problem and when I took it to the dealer to have the O-rings done, they discovered that the MAF was also bad. They were almost insistent that it needed to be replaced. Ouch, I remember it being like $500 or something like that. I think I had around 90K on it when that occured. I should mention that I drive a long way to work daily and at this point I have 226K so like I said, the MAF was replaced over 100K/3yrs ago. I'll look at it this weekend and see if it's dirty.

Shamaal, yes it cools fine (Thank God, it's Aug in Texas). When you say unplug the compressor, are you saying to pull the plug from the back of the compressor while the car is running and the AC is engaged?

 

[shamaal]

I'd do it with the engine off. Then start it up and let it run and see if the idle shoots up the same as before. There's no harm to the engine, just no A/C.

The switch and signal to the computer will work the same, the only thing different is no bogging from the compressor.