A/C clutch relay does not close
 

I have a 99 jeep cherokee classic. I have a problem where the A/C
clutch does not engage at all. I can manually engage it by manually
engaging the A/C clutch relay in the relay box under the hood. When
the clutch is manually engaged by manually closing the A/C clutch
relay, the A/C blows cold air. I checked the 20A fuse for the A/C
clutch, also located in the relay box, and it is OK. I checked the
relay itself to insure that applying 12V to it would cause it to close,
and it appears to be OK (it does not appear that the relay itself is
bad). I looked at the control unit in the interior (containing the
knobs to control heating and A/C), and it appears that vacuum is used
to control heat and A/C function when the knob is rotated to that A/C
position, but I can't determine yet how the vacuum system ultimately
manages to close the A/C clutch relay. It seems that 12V is not being
applied to the A/C clutch relay for some reason, and I have read that
there is a pressure switch on the system somewhere. Could it be that
the refrigerant is low, even though the system blows cold air when I
manually engage the relay? Also, is there a vacuum switch somewhere
(under the dash, etc) that is actuated by vacuum when the control knob
is rotated into the A/C position, that should be ultimately actuating
the A/C clutch relay? Any help would be appreciated. Help! Thanks.

Re: A/C clutch relay does not close
 

May have a bad low pressure switch:
http://members.cox.net/wilsond/Fixes...s.html#sensors
God Bless America, Bill O|||||||O
mailto:-------------------- http://www.----------.com/

carybriel@hotmail.com wrote:
>
> I have a 99 jeep cherokee classic. I have a problem where the A/C
> clutch does not engage at all. I can manually engage it by manually
> engaging the A/C clutch relay in the relay box under the hood. When
> the clutch is manually engaged by manually closing the A/C clutch
> relay, the A/C blows cold air. I checked the 20A fuse for the A/C
> clutch, also located in the relay box, and it is OK. I checked the
> relay itself to insure that applying 12V to it would cause it to close,
> and it appears to be OK (it does not appear that the relay itself is
> bad). I looked at the control unit in the interior (containing the
> knobs to control heating and A/C), and it appears that vacuum is used
> to control heat and A/C function when the knob is rotated to that A/C
> position, but I can't determine yet how the vacuum system ultimately
> manages to close the A/C clutch relay. It seems that 12V is not being
> applied to the A/C clutch relay for some reason, and I have read that
> there is a pressure switch on the system somewhere. Could it be that
> the refrigerant is low, even though the system blows cold air when I
> manually engage the relay? Also, is there a vacuum switch somewhere
> (under the dash, etc) that is actuated by vacuum when the control knob
> is rotated into the A/C position, that should be ultimately actuating
> the A/C clutch relay? Any help would be appreciated. Help! Thanks.

Re: A/C clutch relay does not close
 

May have a bad low pressure switch:
http://members.cox.net/wilsond/Fixes...s.html#sensors
God Bless America, Bill O|||||||O
mailto:-------------------- http://www.----------.com/

carybriel@hotmail.com wrote:
>
> I have a 99 jeep cherokee classic. I have a problem where the A/C
> clutch does not engage at all. I can manually engage it by manually
> engaging the A/C clutch relay in the relay box under the hood. When
> the clutch is manually engaged by manually closing the A/C clutch
> relay, the A/C blows cold air. I checked the 20A fuse for the A/C
> clutch, also located in the relay box, and it is OK. I checked the
> relay itself to insure that applying 12V to it would cause it to close,
> and it appears to be OK (it does not appear that the relay itself is
> bad). I looked at the control unit in the interior (containing the
> knobs to control heating and A/C), and it appears that vacuum is used
> to control heat and A/C function when the knob is rotated to that A/C
> position, but I can't determine yet how the vacuum system ultimately
> manages to close the A/C clutch relay. It seems that 12V is not being
> applied to the A/C clutch relay for some reason, and I have read that
> there is a pressure switch on the system somewhere. Could it be that
> the refrigerant is low, even though the system blows cold air when I
> manually engage the relay? Also, is there a vacuum switch somewhere
> (under the dash, etc) that is actuated by vacuum when the control knob
> is rotated into the A/C position, that should be ultimately actuating
> the A/C clutch relay? Any help would be appreciated. Help! Thanks.

Re: A/C clutch relay does not close
 

May have a bad low pressure switch:
http://members.cox.net/wilsond/Fixes...s.html#sensors
God Bless America, Bill O|||||||O
mailto:-------------------- http://www.----------.com/

carybriel@hotmail.com wrote:
>
> I have a 99 jeep cherokee classic. I have a problem where the A/C
> clutch does not engage at all. I can manually engage it by manually
> engaging the A/C clutch relay in the relay box under the hood. When
> the clutch is manually engaged by manually closing the A/C clutch
> relay, the A/C blows cold air. I checked the 20A fuse for the A/C
> clutch, also located in the relay box, and it is OK. I checked the
> relay itself to insure that applying 12V to it would cause it to close,
> and it appears to be OK (it does not appear that the relay itself is
> bad). I looked at the control unit in the interior (containing the
> knobs to control heating and A/C), and it appears that vacuum is used
> to control heat and A/C function when the knob is rotated to that A/C
> position, but I can't determine yet how the vacuum system ultimately
> manages to close the A/C clutch relay. It seems that 12V is not being
> applied to the A/C clutch relay for some reason, and I have read that
> there is a pressure switch on the system somewhere. Could it be that
> the refrigerant is low, even though the system blows cold air when I
> manually engage the relay? Also, is there a vacuum switch somewhere
> (under the dash, etc) that is actuated by vacuum when the control knob
> is rotated into the A/C position, that should be ultimately actuating
> the A/C clutch relay? Any help would be appreciated. Help! Thanks.

Re: A/C clutch relay does not close
 

May have a bad low pressure switch:
http://members.cox.net/wilsond/Fixes...s.html#sensors
God Bless America, Bill O|||||||O
mailto:-------------------- http://www.----------.com/

carybriel@hotmail.com wrote:
>
> I have a 99 jeep cherokee classic. I have a problem where the A/C
> clutch does not engage at all. I can manually engage it by manually
> engaging the A/C clutch relay in the relay box under the hood. When
> the clutch is manually engaged by manually closing the A/C clutch
> relay, the A/C blows cold air. I checked the 20A fuse for the A/C
> clutch, also located in the relay box, and it is OK. I checked the
> relay itself to insure that applying 12V to it would cause it to close,
> and it appears to be OK (it does not appear that the relay itself is
> bad). I looked at the control unit in the interior (containing the
> knobs to control heating and A/C), and it appears that vacuum is used
> to control heat and A/C function when the knob is rotated to that A/C
> position, but I can't determine yet how the vacuum system ultimately
> manages to close the A/C clutch relay. It seems that 12V is not being
> applied to the A/C clutch relay for some reason, and I have read that
> there is a pressure switch on the system somewhere. Could it be that
> the refrigerant is low, even though the system blows cold air when I
> manually engage the relay? Also, is there a vacuum switch somewhere
> (under the dash, etc) that is actuated by vacuum when the control knob
> is rotated into the A/C position, that should be ultimately actuating
> the A/C clutch relay? Any help would be appreciated. Help! Thanks.

Re: A/C clutch relay does not close
 

L.W.(Bill) ------ III wrote:
> May have a bad low pressure switch:
> http://members.cox.net/wilsond/Fixes...s.html#sensors

Yep. You can safely jumper that switch for testing (and for
filling) but it's not a fix. The purpose of the low limit is
to prevent the AC compressor from burning out due to lack of oil.
The low limit switch can kick in when the AC is still capable
of delivering cool air.

The vac has nothing to do with the clutch or relay, it's
purely for moving the doors that direct airflow.

The clutch circuit is

battery >-->fuse>-->lowlimit>-->highlimit>-->AC relay-->clutch solinoid-->ground

On the relay side it's a bit more complex and involves the switch inside
and the powertrain control module (to control engine RPM).

The switch should be <1 ohms (actually 0) unless the system pressure is too low.
Thing is system pressure changes based on ambient temperature. That's why
filling is best done by volume and not pressure.

I would hazzard a guess your less than half a can down. But don't just
go throwing more in there, get a pressure gauge kit (not the cheap onthecan one)
but at least a good on on the fill hose. or better yet a high/low side set.
(all depends on how many times you plan on using it)

Most good AC shops will do a refill (and system leak check) for less than 100$
It's been a while since I checked prices.

> carybriel@hotmail.com wrote:
>>
>> I have a 99 jeep cherokee classic. I have a problem where the A/C
>> clutch does not engage at all. I can manually engage it by manually
>> engaging the A/C clutch relay in the relay box under the hood. When
>> the clutch is manually engaged by manually closing the A/C clutch
>> relay, the A/C blows cold air. I checked the 20A fuse for the A/C
>> clutch, also located in the relay box, and it is OK. I checked the
>> relay itself to insure that applying 12V to it would cause it to
>> close, and it appears to be OK (it does not appear that the relay
>> itself is bad). I looked at the control unit in the interior
>> (containing the knobs to control heating and A/C), and it appears
>> that vacuum is used to control heat and A/C function when the knob
>> is rotated to that A/C position, but I can't determine yet how the
>> vacuum system ultimately manages to close the A/C clutch relay. It
>> seems that 12V is not being applied to the A/C clutch relay for some
>> reason, and I have read that there is a pressure switch on the
>> system somewhere. Could it be that the refrigerant is low, even
>> though the system blows cold air when I manually engage the relay?
>> Also, is there a vacuum switch somewhere (under the dash, etc) that
>> is actuated by vacuum when the control knob is rotated into the A/C
>> position, that should be ultimately actuating the A/C clutch relay?
>> Any help would be appreciated. Help! Thanks.

Re: A/C clutch relay does not close
 

L.W.(Bill) ------ III wrote:
> May have a bad low pressure switch:
> http://members.cox.net/wilsond/Fixes...s.html#sensors

Yep. You can safely jumper that switch for testing (and for
filling) but it's not a fix. The purpose of the low limit is
to prevent the AC compressor from burning out due to lack of oil.
The low limit switch can kick in when the AC is still capable
of delivering cool air.

The vac has nothing to do with the clutch or relay, it's
purely for moving the doors that direct airflow.

The clutch circuit is

battery >-->fuse>-->lowlimit>-->highlimit>-->AC relay-->clutch solinoid-->ground

On the relay side it's a bit more complex and involves the switch inside
and the powertrain control module (to control engine RPM).

The switch should be <1 ohms (actually 0) unless the system pressure is too low.
Thing is system pressure changes based on ambient temperature. That's why
filling is best done by volume and not pressure.

I would hazzard a guess your less than half a can down. But don't just
go throwing more in there, get a pressure gauge kit (not the cheap onthecan one)
but at least a good on on the fill hose. or better yet a high/low side set.
(all depends on how many times you plan on using it)

Most good AC shops will do a refill (and system leak check) for less than 100$
It's been a while since I checked prices.

> carybriel@hotmail.com wrote:
>>
>> I have a 99 jeep cherokee classic. I have a problem where the A/C
>> clutch does not engage at all. I can manually engage it by manually
>> engaging the A/C clutch relay in the relay box under the hood. When
>> the clutch is manually engaged by manually closing the A/C clutch
>> relay, the A/C blows cold air. I checked the 20A fuse for the A/C
>> clutch, also located in the relay box, and it is OK. I checked the
>> relay itself to insure that applying 12V to it would cause it to
>> close, and it appears to be OK (it does not appear that the relay
>> itself is bad). I looked at the control unit in the interior
>> (containing the knobs to control heating and A/C), and it appears
>> that vacuum is used to control heat and A/C function when the knob
>> is rotated to that A/C position, but I can't determine yet how the
>> vacuum system ultimately manages to close the A/C clutch relay. It
>> seems that 12V is not being applied to the A/C clutch relay for some
>> reason, and I have read that there is a pressure switch on the
>> system somewhere. Could it be that the refrigerant is low, even
>> though the system blows cold air when I manually engage the relay?
>> Also, is there a vacuum switch somewhere (under the dash, etc) that
>> is actuated by vacuum when the control knob is rotated into the A/C
>> position, that should be ultimately actuating the A/C clutch relay?
>> Any help would be appreciated. Help! Thanks.

Re: A/C clutch relay does not close
 

L.W.(Bill) ------ III wrote:
> May have a bad low pressure switch:
> http://members.cox.net/wilsond/Fixes...s.html#sensors

Yep. You can safely jumper that switch for testing (and for
filling) but it's not a fix. The purpose of the low limit is
to prevent the AC compressor from burning out due to lack of oil.
The low limit switch can kick in when the AC is still capable
of delivering cool air.

The vac has nothing to do with the clutch or relay, it's
purely for moving the doors that direct airflow.

The clutch circuit is

battery >-->fuse>-->lowlimit>-->highlimit>-->AC relay-->clutch solinoid-->ground

On the relay side it's a bit more complex and involves the switch inside
and the powertrain control module (to control engine RPM).

The switch should be <1 ohms (actually 0) unless the system pressure is too low.
Thing is system pressure changes based on ambient temperature. That's why
filling is best done by volume and not pressure.

I would hazzard a guess your less than half a can down. But don't just
go throwing more in there, get a pressure gauge kit (not the cheap onthecan one)
but at least a good on on the fill hose. or better yet a high/low side set.
(all depends on how many times you plan on using it)

Most good AC shops will do a refill (and system leak check) for less than 100$
It's been a while since I checked prices.

> carybriel@hotmail.com wrote:
>>
>> I have a 99 jeep cherokee classic. I have a problem where the A/C
>> clutch does not engage at all. I can manually engage it by manually
>> engaging the A/C clutch relay in the relay box under the hood. When
>> the clutch is manually engaged by manually closing the A/C clutch
>> relay, the A/C blows cold air. I checked the 20A fuse for the A/C
>> clutch, also located in the relay box, and it is OK. I checked the
>> relay itself to insure that applying 12V to it would cause it to
>> close, and it appears to be OK (it does not appear that the relay
>> itself is bad). I looked at the control unit in the interior
>> (containing the knobs to control heating and A/C), and it appears
>> that vacuum is used to control heat and A/C function when the knob
>> is rotated to that A/C position, but I can't determine yet how the
>> vacuum system ultimately manages to close the A/C clutch relay. It
>> seems that 12V is not being applied to the A/C clutch relay for some
>> reason, and I have read that there is a pressure switch on the
>> system somewhere. Could it be that the refrigerant is low, even
>> though the system blows cold air when I manually engage the relay?
>> Also, is there a vacuum switch somewhere (under the dash, etc) that
>> is actuated by vacuum when the control knob is rotated into the A/C
>> position, that should be ultimately actuating the A/C clutch relay?
>> Any help would be appreciated. Help! Thanks.

Re: A/C clutch relay does not close
 

L.W.(Bill) ------ III wrote:
> May have a bad low pressure switch:
> http://members.cox.net/wilsond/Fixes...s.html#sensors

Yep. You can safely jumper that switch for testing (and for
filling) but it's not a fix. The purpose of the low limit is
to prevent the AC compressor from burning out due to lack of oil.
The low limit switch can kick in when the AC is still capable
of delivering cool air.

The vac has nothing to do with the clutch or relay, it's
purely for moving the doors that direct airflow.

The clutch circuit is

battery >-->fuse>-->lowlimit>-->highlimit>-->AC relay-->clutch solinoid-->ground

On the relay side it's a bit more complex and involves the switch inside
and the powertrain control module (to control engine RPM).

The switch should be <1 ohms (actually 0) unless the system pressure is too low.
Thing is system pressure changes based on ambient temperature. That's why
filling is best done by volume and not pressure.

I would hazzard a guess your less than half a can down. But don't just
go throwing more in there, get a pressure gauge kit (not the cheap onthecan one)
but at least a good on on the fill hose. or better yet a high/low side set.
(all depends on how many times you plan on using it)

Most good AC shops will do a refill (and system leak check) for less than 100$
It's been a while since I checked prices.

> carybriel@hotmail.com wrote:
>>
>> I have a 99 jeep cherokee classic. I have a problem where the A/C
>> clutch does not engage at all. I can manually engage it by manually
>> engaging the A/C clutch relay in the relay box under the hood. When
>> the clutch is manually engaged by manually closing the A/C clutch
>> relay, the A/C blows cold air. I checked the 20A fuse for the A/C
>> clutch, also located in the relay box, and it is OK. I checked the
>> relay itself to insure that applying 12V to it would cause it to
>> close, and it appears to be OK (it does not appear that the relay
>> itself is bad). I looked at the control unit in the interior
>> (containing the knobs to control heating and A/C), and it appears
>> that vacuum is used to control heat and A/C function when the knob
>> is rotated to that A/C position, but I can't determine yet how the
>> vacuum system ultimately manages to close the A/C clutch relay. It
>> seems that 12V is not being applied to the A/C clutch relay for some
>> reason, and I have read that there is a pressure switch on the
>> system somewhere. Could it be that the refrigerant is low, even
>> though the system blows cold air when I manually engage the relay?
>> Also, is there a vacuum switch somewhere (under the dash, etc) that
>> is actuated by vacuum when the control knob is rotated into the A/C
>> position, that should be ultimately actuating the A/C clutch relay?
>> Any help would be appreciated. Help! Thanks.

Re: A/C clutch relay does not close
 

I think I understand your reply, and will check the low pressure switch
as you suggest. There is one thing that I don't understand though.
You mention that on the relay side, a switch is involved inside. The
only knob inside the cab that activates the AC is a rotating knob that
allows selection of AC functions, as well as selection of heat related
functions (floor, defrost, etc). Does this rotating knob have an
integrated electrical switch as well as a vacuum switch? I do not
believe that this internal switch is bad, but I'm just wondering what
electically actuates the relay from the inside of the cab. Also, how
does the powertrain control module figure into the logic of the relay
side circuit?
Thanks for the help! I'll check the low pressure switch and let you
know what I find.
Cary Briel


DougW wrote:
> L.W.(Bill) ------ III wrote:
> > May have a bad low pressure switch:
> > http://members.cox.net/wilsond/Fixes...s.html#sensors
>
> Yep. You can safely jumper that switch for testing (and for
> filling) but it's not a fix. The purpose of the low limit is
> to prevent the AC compressor from burning out due to lack of oil.
> The low limit switch can kick in when the AC is still capable
> of delivering cool air.
>
> The vac has nothing to do with the clutch or relay, it's
> purely for moving the doors that direct airflow.
>
> The clutch circuit is
>
> battery >-->fuse>-->lowlimit>-->highlimit>-->AC relay-->clutch solinoid-->ground
>
> On the relay side it's a bit more complex and involves the switch inside
> and the powertrain control module (to control engine RPM).
>
> The switch should be <1 ohms (actually 0) unless the system pressure is too low.
> Thing is system pressure changes based on ambient temperature. That's why
> filling is best done by volume and not pressure.
>
> I would hazzard a guess your less than half a can down. But don't just
> go throwing more in there, get a pressure gauge kit (not the cheap onthecan one)
> but at least a good on on the fill hose. or better yet a high/low side set.
> (all depends on how many times you plan on using it)
>
> Most good AC shops will do a refill (and system leak check) for less than 100$
> It's been a while since I checked prices.
>
> > carybriel@hotmail.com wrote:
> >>
> >> I have a 99 jeep cherokee classic. I have a problem where the A/C
> >> clutch does not engage at all. I can manually engage it by manually
> >> engaging the A/C clutch relay in the relay box under the hood. When
> >> the clutch is manually engaged by manually closing the A/C clutch
> >> relay, the A/C blows cold air. I checked the 20A fuse for the A/C
> >> clutch, also located in the relay box, and it is OK. I checked the
> >> relay itself to insure that applying 12V to it would cause it to
> >> close, and it appears to be OK (it does not appear that the relay
> >> itself is bad). I looked at the control unit in the interior
> >> (containing the knobs to control heating and A/C), and it appears
> >> that vacuum is used to control heat and A/C function when the knob
> >> is rotated to that A/C position, but I can't determine yet how the
> >> vacuum system ultimately manages to close the A/C clutch relay. It
> >> seems that 12V is not being applied to the A/C clutch relay for some
> >> reason, and I have read that there is a pressure switch on the
> >> system somewhere. Could it be that the refrigerant is low, even
> >> though the system blows cold air when I manually engage the relay?
> >> Also, is there a vacuum switch somewhere (under the dash, etc) that
> >> is actuated by vacuum when the control knob is rotated into the A/C
> >> position, that should be ultimately actuating the A/C clutch relay?
> >> Any help would be appreciated. Help! Thanks.