A/C cools great initially then tapers off a bit, why??

[rothsean]

I just replaced all of my a/c components, except the compressor (it was new from DMCH a year ago).


So, that being said, the a/c cools great, but today when I drove on the highway the cooling seemed to taper off a bit (it was warmer). It still had a chill in it, but not as much as I would like.

After the car set for 2 hours, it cooled great again initially, then on the highway the cooling was less.


Any ideas why?

[djdogbone]

Are both fans running? Usually, when the AC gets warmerciscbecuase there is not enough heat exchanging. One of your fans may be slow or non-functional

[rothsean]

Are both fans running? Usually, when the AC gets warmerciscbecuase there is not enough heat exchanging. One of your fans may be slow or non-functional

That is true, it seems like the fan is running when parked. I can't hear it otherwise when driving. Are both fans supposed to be on?

[rothsean]

Are both fans running? Usually, when the AC gets warmerciscbecuase there is not enough heat exchanging. One of your fans may be slow or non-functional



Also, when I am going 60 mph down the highway I would think that there is enough airflow? Or maybe not?

[Bitsyncmaster]

My guess is your drain is not open enough. After running a long time the water is blocking a lot of the evaporator.

[SS Spoiler]

Possible the evaporator is freezing up, blocking air flow?

[LordFly]

Mine does something very similar to this. Sometimes turning the fans to 0 for a few seconds, then back on fixes it (but rarely). Sometimes it'll just kick back in while driving. A block in the drain might explain it.... I'll have to go look :)

[Ron]

+1 on all suggestions so far.

Yes, both fans should be on and 60 MPH should give all of the air flow you need.
Your charge could be off -- How much did you put in exactly? What kind?

(It would probably be better if you kept all your AC stuff in the same thread...easier to put things together and not miss something.)

[rothsean]

+1 on all suggestions so far.

Yes, both fans should be on and 60 MPH should give all of the air flow you need.
Your charge could be off -- How much did you put in exactly? What kind?

(It would probably be better if you kept all your AC stuff in the same thread...easier to put things together and not miss something.)


I used r12. It may be a little low, I will check pressures tomorrow. When I added the freon this past Thursday I added until the pressures were 41/175 (low/high). 90 degrees F outside.

[rothsean]

My guess is your drain is not open enough. After running a long time the water is blocking a lot of the evaporator.

I just tested the pressures.

80 degrees outside...

Low pressure 40psi and high pressure 175psi, this was at 750rpm.


I revved the motor to 2500-3000rpm and the pressures went to 20/210psi (low/high). Is that normal?

Does the 40psi on the low side at idle mean that I am overcharged?


The fans are working.

[Ron]

You should take the readings right when the compressor kicks off.

Norms:
80F= 25-32 psi LOW -- 145-190 psi HIGH
90F= 25-32 psi LOW -- 170-200 psi HIGH

It should have kicked out at 20/210 (suspect low pressure/cycle switch).

[rothsean]

You should take the readings right when the compressor kicks off.

Norms:
80F= 25-32 psi LOW -- 145-190 psi HIGH
90F= 25-32 psi LOW -- 170-200 psi HIGH

It should have kicked out at 20/210 (suspect low pressure/cycle switch).




My low pressure switch is brand new from SpecialT.

My compressor was on the entire time I tested pressures. I ran the car for 10 minutes and did many variable RPM settings. Should it have cycled off?

Does it matter how the switch wires are connected to the switch? or is it just a relay that flows electricity either direction?


Does the 40psi reading on the low end indicate too much Freon in the system?

[Ron]

Test it by disconnecting the wires, unscrew it (there should be a valve there to prevent leakage), turn the AC on and hook the wires back up (it don't matter which way you connect them). If the compressor kicks in, the switch is bad.
If not, you may have over tightened it (easy to do with some). Look for mis-aligned/old pieces of o-ring in it.
You can also test it with an ohmmeter -- It should not conduct when off the car.
Yes, it should have cycled out before it got to 20 psi.

The chart calls for readings when the clutch cycles out, so the 40 is kind of meaningless.

[rothsean]

Test it by disconnecting the wires, unscrew it (there should be a valve there to prevent leakage), turn the AC on and hook the wires back up (it don't matter which way you connect them). If the compressor kicks in, the switch is bad.
If not, you may have over tightened it (easy to do with some). Look for mis-aligned/old pieces of o-ring in it.
You can also test it with an ohmmeter -- It should not conduct when off the car.
Yes, it should have cycled out before it got to 20 psi.

The chart calls for readings when the clutch cycles out, so the 40 is kind of meaningless.

Sounds good. How do you check for readings when it cycles off? Seems like that only gives you a few seconds to check. I imagine that the pressures change rapidly once the compressor kicks off.

I will check that switch, thanks for the info.

[Ron]

Sounds good. How do you check for readings when it cycles off? Seems like that only gives you a few seconds to check. I imagine that the pressures change rapidly once the compressor kicks off.

I will check that switch, thanks for the info.Yep, I just let it cycle a few times and catch them one at a time...

[Bitsyncmaster]

You can not determine the charge using pressure readings. Pressure reading can help to prevent over charge but the way I top off is use a thermometer in the center vent, doors open, AC on normal, fan on max. Then I keep adding freon till the thermometer starts to rise (it should have fallen as you were adding freon). Keep watching the high pressure just to be sure you don't overcharge.

[rothsean]

Test it by disconnecting the wires, unscrew it (there should be a valve there to prevent leakage), turn the AC on and hook the wires back up (it don't matter which way you connect them). If the compressor kicks in, the switch is bad.
If not, you may have over tightened it (easy to do with some). Look for mis-aligned/old pieces of o-ring in it.
You can also test it with an ohmmeter -- It should not conduct when off the car.
Yes, it should have cycled out before it got to 20 psi.

The chart calls for readings when the clutch cycles out, so the 40 is kind of meaningless.

I tested the switch, it is fine.

My compressor is still not cycling on and off.

[rothsean]

You can not determine the charge using pressure readings. Pressure reading can help to prevent over charge but the way I top off is use a thermometer in the center vent, doors open, AC on normal, fan on max. Then I keep adding freon till the thermometer starts to rise (it should have fallen as you were adding freon). Keep watching the high pressure just to be sure you don't overcharge.

So, when you say to watch the high pressure I assume that I set it to the spec while at idle?


Also, my compressor is not cycling on and off, it just stays on. So I guess that means that my pressures can,t be read. Should I manually turn the compressor off by switching to vent position, then read pressures?



2 weeks ago when I had a freon leak my compressor would cycle on and off every 2 to 3 seconds. Now with everything fixed it just stays on. Makes no sense to me unless it is overcharged which is not letting pressure drop enough, although last night when I revved the motor the low pressure drops to about 20psi at 2500+ rpm.

[rothsean]

Test it by disconnecting the wires, unscrew it (there should be a valve there to prevent leakage), turn the AC on and hook the wires back up (it don't matter which way you connect them). If the compressor kicks in, the switch is bad.
If not, you may have over tightened it (easy to do with some). Look for mis-aligned/old pieces of o-ring in it.
You can also test it with an ohmmeter -- It should not conduct when off the car.
Yes, it should have cycled out before it got to 20 psi.

The chart calls for readings when the clutch cycles out, so the 40 is kind of meaningless.

Did you mean over tightened the pressure switch?

[rothsean]

Not sure if it matters, but all of my testing above was done with the a/c in the max position.

[rothsean]

Also, I am in Houston so it is hot.

[Ron]

You can not determine the charge using pressure readings.
...You don't need to determine the charge, as such. IF the system is stock and he has 90F, 25-32 psi LOW -- 170-200 psi HIGH, and the output is 35-39F, when the clutch cycles out, the charge is correct.
Additionally fwiw, N:06:01, referring to the chart says, "12. If pressures correct and clutch operating the system is all correct."
Note other vehicles will have different field data and/or procedures to follow.

[Ron]

Did you mean over tightened the pressure switch?Yes. Foreign matter or over tightening the switch can make the contacts connect internally. (It depends on the shape and size of the port where the o-ring seats as to how easy it is to do...mine is real easy but some you would crack the unit first. Hand tighten.


Not sure if it matters, but all of my testing above was done with the a/c in the max position.
As it and the fan should be.

[rothsean]

Yes. Foreign matter or over tightening the switch can make the contacts connect internally. (It depends on the shape and size of the port where the o-ring seats as to how easy it is to do...mine is real easy but some you would crack the unit first. Hand tighten.


As it and the fan should be.

Ok, so any ideas as to why the compressor is not cycling on and off?

[Bitsyncmaster]

You don't need to determine the charge, as such. IF the system is stock and he has 90F, 25-32 psi LOW -- 170-200 psi HIGH, and the output is 35-39F, when the clutch cycles out, the charge is correct.
Additionally fwiw, N:06:01, referring to the chart says, "12. If pressures correct and clutch operating the system is all correct."
Note other vehicles will have different field data and/or procedures to follow.

Yes you need to measure temps from the vents. Bill insists he can determin charge with pressure guages.

Low pressure is dependant on the low pressure switch not related to ambient air temp. High pressure is dependant on air flow over the condenser, engine RPM, ambient air temp and amount of charge. To many variables to use just to determine charge level.

[Ron]

Ok, so any ideas as to why the compressor is not cycling on and off?The switch should turn off before it gets down to 20psi, new or not, it is bad otherwise. I would see if the clutch drops off when unplugging the cycle switch. If not power is probably coming from the diode(s) between it and the otterstat.

[rothsean]

The switch should turn off before it gets down to 20psi, new or not, it is bad otherwise. I would see if the clutch drops off when unplugging the cycle switch. If not power is probably coming from the diode(s) between it and the otterstat.

The clutch does drop off when I unplug the low pressure switch. I also took the switch off and connected the wires, clutch did not re-engage. Once I connected the switch back on the accumulator and connected wires the clutched kicked back in.

The switch seems to be working correctly. I will test the vent temps.

[Ron]

Yes you need to measure temps from the vents.
Output temp, as mentioned , is in the chart and accounted for???


Bill insists he can determin charge with pressure guages.
um..never mind LOL

I agree with you, you can't. BUT, you can use the field data chart to deduce it is correct. That is THE purpose of the chart!
See?


Low pressure is dependant on the low pressure switch not related to ambient air temp. High pressure is dependant on air flow over the condenser, engine RPM, ambient air temp and amount of charge. To many variables to use just to determine charge level.
Here (only) the low pressure doesn't matter...it is just to prevent freeze over and cutting in when the charge is too low (I'm sure you know that). Yes High pressure is dependent on the ambient airflow over the condenser (and resulting heat exchange), but the procedure takes the ambient temp into account (even a box fan if required). All of the variables are. They just took a "perfect" system, exposed it to various conditions and recorded the data...so one can deduce whether it is correct or not. If it is not within the parameters shown, something is wrong...you are on your own then to figure out what. Of course you can usually guess. In the example above, if the High pressure was low and the output temp was high while everything else was on spec, you would increase the charge.

Kind of like a Go-No Go gauge -- you don't get to know what it is if it is off spec -- you just know if it is correct.

[rothsean]

Output temp, as mentioned , is in the chart and accounted for???


um..never mind LOL

I agree with you, you can't. BUT, you can use the field data chart to deduce it is correct. That is THE purpose of the chart!
See?


Here (only) the low pressure doesn't matter...it is just to prevent freeze over and cutting in when the charge is too low (I'm sure you know that). Yes High pressure is dependent on the ambient airflow over the condenser (and resulting heat exchange), but the procedure takes the ambient temp into account (even a box fan if required). All of the variables are. They just took a "perfect" system, exposed it to various conditions and recorded the data...so one can deduce whether it is correct or not. If it is not within the parameters shown, something is wrong...you are on your own then to figure out what. Of course you can usually guess. In the example above, if the High pressure was low and the out put temp was high while everything else was on spec, you would increase the charge.

Kind of like a Go-No Go gauge -- you don't get to know what it is if it is off spec -- you just know if it is correct.



Why would my clutch not be cycling?

[Ron]

The clutch does drop off when I unplug the low pressure switch. I also took the switch off and connected the wires, clutch did not re-engage. Once I connected the switch back on the accumulator and connected wires the clutched kicked back in.

The switch seems to be working correctly. I will test the vent temps.
That much is ok, but you didn't mention how low the pressure went...if still 20 psi and no cut out, something is wrong.

Like Dave said, the low pressure switch only looks at the low pressure, independent of anything else (no need to check the output temp until you get it cutting out correctly). Since it does as above, it sounds to me like the diodes, wiring, etc are correct -- Seems it is what is passing power to the clutch, as it should, but not cutting off when it should. It must cycle. Sometimes it takes a long time if conditions are just right, but at 90F i wouldn't think that would be the case. Can you get you hands on another one? Just about any one that screws on there is OK for a test. What's with the old one?

You did pull a good vacuum?

[rothsean]

That much is ok, but you didn't mention how low the pressure went...if still 20 psi and no cut out, something is wrong.

Like Dave said, the low pressure switch only looks at the low pressure, independent of anything else (no need to check the output temp until you get it cutting out correctly). Since it does as above, it sounds to me like the diodes, wiring, etc are correct -- Seems it is what is passing power to the clutch, as it should, but not cutting off when it should. It must cycle. Sometimes it takes a long time if conditions are just right, but at 90F i wouldn't think that would be the case. Can you get you hands on another one? Just about any one that screws on there is OK for a test. What's with the old one?

You did pull a good vacuum?

I did vacuum for 3 hours.

Only mistake that I can think of is that on my second can of freon I forgot to purge the line when I open even the valve to add freon, but that would be such a small amount of air that I can't see that being an issue. I did purge on the first and third can.

I will try my old switch again.

[Ron]

Yeah, I doubt the purge would matter.
Unless I misunderstood something or we have our wires crossed, I bet the old one will make it cycle.

Good Luck!

[dmc6960]

What size cans were these? How many did you put in? Did you empty all of them?

[Ron]

fwiw- I have found that, accounting for purges and the revised spec, three 12 oz cans is just right for R12.

EDIT: Found it.

Service Bulletin ST-29-11/81: "The freon charge has changed to 2.2 lbs. from the previous 2.75 lbs.."


1 lbs = 16 oz
12 oz x 3 = 36 oz
36 oz/16 = 2.25 lbs

[rothsean]

What size cans were these? How many did you put in? Did you empty all of them?

14 oz cans, I used most of the third one.

[rothsean]

Yeah, I doubt the purge would matter.
Unless I misunderstood something or we have our wires crossed, I bet the old one will make it cycle.

Good Luck!

I put my old switch back on, no changes. The compressor does not cycle.

[Ron]

Dang??
How low did the pressure go?

Are there any marking like S 34, R 10 on either of them? (That would mean sets at 34 psi, resets at 10 psi) a lot of them are that low. In that case, you might have a little too much in it, which could fit, not knowing exactly how much and using larger cans...

[David T]

How low did the pressure go?

Are there any marking like S 34, R 10 on either of them? (That would mean sets at 34 psi, resets at 10 psi) a lot of them are that low. In that case, you might have a little too much in it, which could fit not knowing exactly how much and using larger cans...

If the compressor does not cycle you WILL freeze up the evap coil and you will notice reduced cooling. Causes for non-cycling and freezing include:
Not enough or too much refrigerant
Misadjusted or defective low pressure switch
Air in the system
Blocked condensor coil
Wiring modifications
Heater fan running the wrong direction because it is not the correct replacement

Typically it will be more than one thing so you have to check them all. One scenario is that the compressor won't run so someone either adjusts the low pressure switch or maybe just jumps it out! Another is, again, the compressor isn't running so someone shoots some refrigerant in, too much, and now it doesn't cycle. Most problems start with a leak and low refrigerant. The "fixes" vary.
David Teitelbaum

[rothsean]

If the compressor does not cycle you WILL freeze up the evap coil and you will notice reduced cooling. Causes for non-cycling and freezing include:
Not enough or too much refrigerant
Misadjusted or defective low pressure switch
Air in the system
Blocked condensor coil
Wiring modifications
Heater fan running the wrong direction because it is not the correct replacement

Typically it will be more than one thing so you have to check them all. One scenario is that the compressor won't run so someone either adjusts the low pressure switch or maybe just jumps it out! Another is, again, the compressor isn't running so someone shoots some refrigerant in, too much, and now it doesn't cycle. Most problems start with a leak and low refrigerant. The "fixes" vary.
David Teitelbaum



It worked fine today. I did release some Freon pressure last night so maybe that did the trick, if it was over charged.

I will see how it does on future rides. Thanks for all of the replies.

[rothsean]

BTW, I did install the new VOV (orifice) from SpecialTAuto last week when I replaced everything.