When Fan on HI spd, AC Compressor Clutch releases...?

When Fan on HI spd, AC Compressor Clutch releases...?

90 Nissan 240SX w/ factory AC - the HVAC fan has 4 speeds + off. Using
the fan at up to speed 3: AC does a pretty good job of keeping the car
cool almost up to triple digit temps outside. Once outside hits 100+ it
can take a while to cool car down but it does 'OK' (again max fan speed
of 3 not 4).

However when I turn the fan speed to 4 (HI) the AC compressor (clutch)
kicks off after a SHORT WHILE - maybe a few minutes. And then it only
intermittently cycles with long off duration (say on for 2 min and off
for 10 min...?) and cooling is of course dramatically reduced.

It has a thermostatic temp probe in the evap coil and cycles the
compressor appropriately at speeds 1-3 keeping the car cool. It's just
at speed 4 that the compressor stops cycling MOST of the time.

I'm at a loss as to what would cause the compressor to cycle only
intermittently or sporadically only when the fan speed is 4 (HI). Could
there be enough electric draw of the fan to affect electric current to
the AC compressor's clutch? I have to admit I'm kinda grasping on this
one. Thanks for any suggestions or ideas.

"loadhawg" > wrote in news:1151345972.971100.138200
@u72g2000cwu.googlegroups.com:

> When Fan on HI spd, AC Compressor Clutch releases...?
>
> 90 Nissan 240SX w/ factory AC - the HVAC fan has 4 speeds + off. Using
> the fan at up to speed 3: AC does a pretty good job of keeping the car
> cool almost up to triple digit temps outside. Once outside hits 100+ it
> can take a while to cool car down but it does 'OK' (again max fan speed
> of 3 not 4).
>
> However when I turn the fan speed to 4 (HI) the AC compressor (clutch)
> kicks off after a SHORT WHILE - maybe a few minutes. And then it only
> intermittently cycles with long off duration (say on for 2 min and off
> for 10 min...?) and cooling is of course dramatically reduced.
>
> It has a thermostatic temp probe in the evap coil and cycles the
> compressor appropriately at speeds 1-3 keeping the car cool. It's just
> at speed 4 that the compressor stops cycling MOST of the time.
>
> I'm at a loss as to what would cause the compressor to cycle only
> intermittently or sporadically only when the fan speed is 4 (HI). Could
> there be enough electric draw of the fan to affect electric current to
> the AC compressor's clutch? I have to admit I'm kinda grasping on this
> one. Thanks for any suggestions or ideas.
>
>

I wouuld first suggest you hook up manifold gauges to see if high side
pres. is bumping up high enuff to kick comp. off since you say it is blower
high speed only..blower high speed is removing the most heat possible at
the evap. and putting it in the condensor which is part of the high side..
if you had excessive comp. cycling at low blower speed i would think a low
freon charge was the culprit. you do need an accurate idea of your pres.
readings tho...................kjun

"KjunRaven" > wrote in message
...
> "loadhawg" > wrote in news:1151345972.971100.138200
> @u72g2000cwu.googlegroups.com:
>
> > When Fan on HI spd, AC Compressor Clutch releases...?
> >
> > 90 Nissan 240SX w/ factory AC - the HVAC fan has 4 speeds + off. Using
> > the fan at up to speed 3: AC does a pretty good job of keeping the car
> > cool almost up to triple digit temps outside. Once outside hits 100+ it
> > can take a while to cool car down but it does 'OK' (again max fan speed
> > of 3 not 4).
> >
> > However when I turn the fan speed to 4 (HI) the AC compressor (clutch)
> > kicks off after a SHORT WHILE - maybe a few minutes. And then it only
> > intermittently cycles with long off duration (say on for 2 min and off
> > for 10 min...?) and cooling is of course dramatically reduced.
> >
> > It has a thermostatic temp probe in the evap coil and cycles the
> > compressor appropriately at speeds 1-3 keeping the car cool. It's just
> > at speed 4 that the compressor stops cycling MOST of the time.
> >
> > I'm at a loss as to what would cause the compressor to cycle only
> > intermittently or sporadically only when the fan speed is 4 (HI). Could
> > there be enough electric draw of the fan to affect electric current to
> > the AC compressor's clutch? I have to admit I'm kinda grasping on this
> > one. Thanks for any suggestions or ideas.
> >
> >
>
> I wouuld first suggest you hook up manifold gauges to see if high side
> pres. is bumping up high enuff to kick comp. off since you say it is
blower
> high speed only..blower high speed is removing the most heat possible at
> the evap. and putting it in the condensor which is part of the high side..
> if you had excessive comp. cycling at low blower speed i would think a low
> freon charge was the culprit. you do need an accurate idea of your pres.
> readings tho...................kjun

kjun, Hmmmm. I though the slower the air moved across the evaporator coils
the more heat would be removed from it. As a matter of fact, you can prove
it by checking outlet vent air temperatures at different blower speeds. You
will find that the lower the blower speed the colder the outlet air
temperature will be... Kinda contradicts your theory.

To the OP. Sorry I am not familiar enough with your make and model to offer
a rational theory as to the cause of your symptoms. Sounds pretty weird to
me. Perhaps the high blower speed and the compressor cycling problem is just
a coincidence. I can't think of anything that one would have to do with the
other. Maybe some sort of controller problem?
..--
Kevin Mouton
Automotive Technology Instructor
"If women don't find you handsome, they should at least find you handy"
Red Green

On Tue, 27 Jun 2006 22:41:50 GMT, KjunRaven > wrote:
>I wouuld first suggest you hook up manifold gauges to see if high side
>pres. is bumping up high enuff to kick comp. off since you say it is blower

I haven't had them on recently but had them on at least twice since the problem started a couple years ago - pressure is
fine on both sides. It also does it with the pressure cutout switch bypassed.

>high speed only..blower high speed is removing the most heat possible at
>the evap. and putting it in the condensor which is part of the high side..

Why would the compressor stay OFF for 20 or 30 minutes while the fan is at the HI(4) speed? And only occasoinally kick
on for 1-2 minutes and be off for another 15, 20 or 30 minutes...?

>if you had excessive comp. cycling at low blower speed i would think a low
>freon charge was the culprit. you do need an accurate idea of your pres.
>readings tho...................kjun

Pressures have been within spec each time I've checked in the past. Maybe a tad on the high side but within the spec
range. System works excellent and cycles just right at fan speeds 1-3 - just not 4.

On Tue, 27 Jun 2006 21:33:36 -0500, "Kevin" > wrote:

>To the OP. Sorry I am not familiar enough with your make and model to offer
>a rational theory as to the cause of your symptoms. Sounds pretty weird to
>me. Perhaps the high blower speed and the compressor cycling problem is just
>a coincidence. I can't think of anything that one would have to do with the
>other. Maybe some sort of controller problem?

Maybe. I was kinda sorta wondering if there was some kind of issue with the amps draw for the fan at high speed that is
goofing with the controls ... ? I'm kinda graspin' at straws on this one...?

Not likely coincidental - it's been behaving like this for a couple years now.

Thus spake loadhawg:

> When Fan on HI spd, AC Compressor Clutch releases...?

This should be easy to diagnose using a wiring diagram. You need to see where
the voltage cuts off when it cycles. There is a path from the fuse supplying
the compressor clutch to the clutch itself. Somewhere that is cutting out --
you need to trace that and find out exactly where.

Get a wiring diagram and a meter, then get back to us.

Good luck,
--
DaveC

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