When Defrost on, engine RPMS jumps

Hello All,

Thought I finally had the major things fixed, but then its the Old Murphy's Law.
Anyway, today I noticed that while driving,at idle or park, the rpms will jump by about 200-250RPMs. It will only happen when the heat is turned on or the AC. I hear what seem to be a clicking from the glove box area, then the RPMS shoot up, then quickly back down. It will keep repeating this as long as the heater is turned on.I think it may be low on freon( hopefully).
What could the problem be, and where should I start to look?

Thanks Again
Dion

I think you're pretty close. The engine is revved up by the PCM ... which expects the ac to be loading up. If the ac is working normally, it's supposed to be a hardly noticeable thing ... but it always is somewhat noticeable.

I think you're pretty close. The engine is revved up by the PCM ... which expects the ac to be loading up. If the ac is working normally, it's supposed to be a hardly noticeable thing ... but it always is somewhat noticeable.


I read that both the AC and heater operate thru the AC unit. if this is true, is there a way I can disconnect only the AC unit to verify if it is indeed low on freon? Or is there another test I could do to narrow down my problem?

Usually an A/C compressor cycling on and off quickly is a good indication that it is low on refrigerant. The prob is that the system is unable to build pressure. It shouldn't matter if the heat is on unless you mean when the front defrost is on. The defrost setting will cycle a/c to dry the air out. Try unpluging the harness that leads to the compressor to see if it quits clicking. If so, you should be pretty sure that it is low on refrigerant...if not you may have something else going on.

I read that both the AC and heater operate thru the AC unit. if this is true, is there a way I can disconnect only the AC unit to verify if it is indeed low on freon? Or is there another test I could do to narrow down my problem?

Your heat should operate normally whether or not the A/c compressor is operable. From that standpoint, the two are independent although they can work together when you select a warmer temp. while A/C is active.

I agree that rapid A/C cycling is often a harbinger of low freon, but I wanted to point out, that it will also cycle faster when outside temp is lower. For instance at 60 degrees it may cycle several times per minute, whereas at 90 degrees it may not cycle at all. The best way to ultimately check your freon level is with a guage as rapid cycling can be caused by other system problems as well.

Is the connector on the front or back of the compressor? Also, I was told that there is another type of coolant that has bigger particles than the A134 refridgerant and if there is a leak in the lines , it won't be as noticable. Any idea what the name might be?Suppose to help eliminate replacing any o-rings unless the leak is really bad.

Thanks
Dion

Is the connector on the front or back of the compressor? Also, I was told that there is another type of coolant that has bigger particles than the A134 refridgerant and if there is a leak in the lines , it won't be as noticable. Any idea what the name might be?Suppose to help eliminate replacing any o-rings unless the leak is really bad.

Thanks
Dion

If you mean the electrical connector for the A/c compressor clutch, it is on what I would call the front, just above the pulley on the "belt" end. It is a latching plug, so you will need to pry the locking "foot" away from the plug as you pull it up to disconnect it.

Hope that helps

Will try this tonight when my Wife gets back from shopping. If I disconnect this, I should see no fluctuation in the rpms when the heat on right? This would indicate it is the ac clutch and possibility of low freon?

Will try this tonight when my Wife gets back from shopping. If I disconnect this, I should see no fluctuation in the rpms when the heat on right? This would indicate it is the ac clutch and possibility of low freon?

Hi
If your vehicle does not command the A/C to run in the floor heat position (I have read on this forum where some say theirs does) I do not think you will see the "jump" in RPM even if you leave the A/C clutch power connected. You might try that first - i.e. leave it connected, all heat/Ac off. Start the van, select heat only and see what you get. My van does not command A/C with heat, but does with defrost. Then you can select defrost and see if the fluctuation returns. It is most likely caused by the cycling of your A/c compressor which you can observe visually while the engine is running to confirm. I would then either purchase a guage to measure the system pressure or have it checked prior to simply dumping in a couple of cans of 134a.

I removed the connector on the AC. There are 2, one on the AC unit itself and the other on one of the lines. Anyway, disconnected the one on the unit itself and it appears to have stopped the fluctuation of RPMS. Also noted that when I turned the AC on, it blew out hot air only. I suspect the it is indeed the freon. Will post when I know for sure.
By the way, what is the second connector?(The one on the line itself)

The one on the line is probable a pressure switch. They are usually on the accumulator, but it appears it's on the line on the Windstar. Check the pressure w/ a gauge as the previous poster suggested...it may be just a bad switch. If you have good pressure it may be the switch, if not then it's probably the refrigerant, but it could ba a combination of both. Let us know what you find. MM.

If you have low / no coolant, then you most likely have a leak....so you will want to have that checked out.

You mention that you have read about a different kind of from the R-134.
I do not know much about the coolant, but I would NOT put anything in other than what is called for. Your system is designed for a specific product, and putting something else in might cause MAJOR problems.

You have 2 "pressure" switches, 1 is near the compressor and is the HIGH pressure switch. It causes the compressor to turn off if too high of a pressure is detected. The other switch is on the "acumulator" which is up near the passenger side firewall. That switch keeps the compressor from running with too low of pressure, like in super cold weather.
At least that is what the manual states for my '96.

The heat function works as follows.
The antifreeze flows through the heater core ALL the time when the engine is on.
When you adjust the temperature from cold to hot, you cause a "blend door" to move to cause no (cold setting) to most (hot setting) to flow over the hot heater core.
From my understanding, the air always flows over the "evaporator" coils for the A/C. In the winter, in the Defrost setting, the air flows over the evaporator coils to remove moisture, and prevent the windows from fogging up.
My '96 A/C is working in all settings except OFF and VENT.

I found a leak near a connector n the compressor.I had a garage remove the little freon that was left. They then filled it up with a coolant called Dxtron( something like that). Apparently you cannot re-fill with the 134A when there is a leak.This new coolant has bigger molecules than the 134A and dif the leak is small enough would have a harder time escaping. we put this in to verify the RPMS with the low freon.If it works, then I will get the leak fixed.

The name of the coolant that was used was called Duracool.So far all seems well. Will post back if anything comes up

Hopefully the shop knows what they are doing, which is why you took it to them.
To work with A/C systems, they are supposed to be trained and certified.
They should know what products can be used safely and what cannot be used.
My caution was to any DIY'er thinking of putting something different into their A/C system.

WisWind,
Thanks for the headsup and concern. It is people like you who make this type of site so valuable.
Will let you know how it goes.

Thanks Dion