Question about 1998 Lexus LS 400

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Why HVAC Controls Dead? - 1998 Lexus LS 400

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Posted on Jun 11, 2014


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SOURCE: I have freestanding Series 8 dishwasher. Lately during the filling cycle water hammer is occurring. How can this be resolved

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1 Answer

What caused heat to stop working?

same answer on all cars. nice photo, so rare here, shocking that.
in order,......
1: sure the blower must blow.
2: the engine must get to min. 180F , if not, thermostat bad in engine.
3: ok, engine is hot, so the heater core must get hot next. it has 2 hoses, we feel them with hands, are both hot?, yes, then the core is most-likely hot. (inside) (if not core is packed in gunk inside)
ever do 2year cooling flushes< (oops. zero service lands you here)
4: the controls must open the dampers, if not? bad controls
5: if the controls work and dampers fail, then the dampers are bad.

all answers assumes , cooling system full to top of RAD.
your car uses 3 damper motors. (electric)
that operate them by controls
the motor for temp is called temperature actuator motor.
the controller is called HVAC controller
did you scan the PLC to see if HVAC controller has errors.
the controller needs fuse 15 good. Cab fuse box ."hvac-rfa"
the controller use a network, data comm .
if you scan the PCM and it tells you the HVac comms are dead
what do you know? (means you cant fix it)
its not just a simple switch like in 1990 (gone)
its all computer controlled now.

check fuse first
scan 2nd.
like we do on all modern cars, or , its all just a guessing game.

Feb 14, 2016 | 2008 Suzuki XL7

1 Answer

Heat and ac controls quit working. Changed the control unit and still nothing. Checked fuses and all good.

If your saying the blower motor doesn't work, turn on the hvac and check voltage and ground at the blower motor connector. Go from there.
Can you check for applicable hvac trouble codes? Some of the national brand auto stores will check codes for free, if there are any.
At the applicable fuses, you need to check fuse circuit for voltage. Even if fuse is good, if no voltage on the fuse circuit, the circuit is still dead. Use a testlight to check voltage.

Dec 05, 2015 | 2003 Chevrolet Silverado 1500

2 Answers

Side windows on passenger side fog up. Not on driver's side, only on passenger.

so find out which vent is dead,?
before 5 gallon LYSOL flush an a week long GAG !
make sure first never to use RECRIC, when wanting to defog.
and is the A/C running in dash defrost mode (compressor main shaft spins)

if a vent is dead, its a bad damper valve, (not saying a part, a function)
the book is free, you can read it, HVAC. see venting and dampers.
its very simple in concept, a fan, a damper and dry air. hot.
the heater core makes heat the non recirc, damage gives you fresh air, the A/C makes that air dry. and the dampers the supply all vents work work.
so make sure the controls are set right,
then see if the upper vents all blow, with hand waved on them.
see? that is easy, and not reporting a dead vent ,makes help harder.
the FSM is on line, ill look for you.
but first HVAC 101
  1. does blower work on high?
  2. does the recirc turn to fresh are,and you get fresh air with blower off moving, at all top vents. y/n
  3. do you have full heat? 180F + engine heat (scan tool shows this as ECT)
  4. do all vents top work , blower on?
  5. is the A/C compressor working in A/c mode (cold) and Defrost?
  6. more... below.
Rule one, dont work in dash until air bags are DEFEATED (injury) (pull battery neg lug, and wait 1hour)

got fsm page open, ill state some facts needed to fix this.

first ill prove the dash system does not need total dismantle
for all jobs. Does NOT.
Air /mix blend door
Single Zone/Dual Zone Driver's Side(FSM)
  1. Disconnect and isolate the negative battery cable.
  2. Remove the instrument panel silencer from the driver side of the instrument panel.
  3. Remove the three screws that secure the blend door actuator to the driver side of the HVAC air distribution housing .
  4. Remove the blend door actuator from the air distribution housing and disconnect the HVAC wire harness connector from the actuator.
  5. Remove the blend door actuator from the vehicle.

next page, is blend door 2, type.
has vacuum servos.
we move the controls and make sure all servo vacuum motors

this car has driver side and passenger side seperate blend doors.
so,,, we are getting close here>
if the defroster fails,(seems not) then that is electric motor drive
and very hard to test . (if not door jam)

in any case, we need to know what part is failing,
if the passenger blend is dead.
seen below,
we then must find out why
1: just stuck.
2: servo actuator failure
3: controls to #2 above bad, or connections to each.

login to see HVAC chapter, read how it works and the tests.

Feb 11, 2015 | 2001 Jeep Grand Cherokee

2 Answers

I have a 1995 jeep grand cherokee that blows out defrost and floor but wont blow out dash vents

Replace the vent motor. Most likely vacuum. make sure the vacuum tube isn't broken.

Jan 09, 2015 | 1995 Jeep Grand Cherokee

1 Answer

How do I reset the a/c modes?

its not programmable.
the modes are controls by dampers, those are controlled by the dash controller, HVAC.
the FSM cover these tests. so does
its lots of pages, i cant post them.
check all fuses
does blower work on high ,
is only one vent bad? or all vents dead.??
are all modes dead.? (direction, and recirc?)

Jun 30, 2014 | 2002 Cadillac Seville SLS

1 Answer

Changed battery in my 2004 GMC Envoy and now my a/c only blows from the dash and rear vents. What could be wrong and where is the part located on the vehicle?

There are a couple of possibilities, one is that the HVAC system lost its calibration during the battery change or the actuator that controls the vent door has gone bad.
To recalibrate the HVAC system, set all HVAC controls and switches to OFF or fully CCW positions. Disconnect the battery for about 3 - 4 minutes, then reconnect the battery. Do not touch any of the HVAC controls for a few minutes, then try the controls to see if the air flows correctly.

Aug 01, 2012 | 2004 GMC Envoy

1 Answer

I do not have any air coming out of the dash. The battery went dead while on vacation. I changed the climate control acuator and still have no air coming out of the dash. Only the floor and defroster.

When the battery went dead, the HVAC system lost its calibration. Try setting all switches and controls to their full OFF or CCW position, then disconnect the battery for about 3 minutes. Reconnect battery and don't touch any HVAC controls for a few minutes. If this does not solve the problem, you may have changed the wrong actuator. Below is a diagram of the HVAC system and actuator locations.


Jun 04, 2012 | 2004 GMC Envoy

1 Answer

2004 GMC Yukon XL: The A/C System will only blow air from defroster ducts. I had to disconnect the battery to replace an SRS Sensor. This problem happened after a dead battery but corrected itself. How can...

It's possible the HVAC needs to recalibrate itself, try the following;
With key on, set all HVAC controls and switches to their OFF or CCW positions. Turn key off, disconnect battery for a few minutes, then reconnect battery. Turn key on, but do not touch any HVAC controls for about 2 - 3 minutes. After that, see if the vents and all other functions work. If this does not solve the problem, either the mode control actuator(s) or mode door(s) are stuck or defective.

Sep 08, 2011 | 2005 GMC Yukon Xl Denali

1 Answer

Had dead battery put new battery in and now the ac works but no air comes out of 4 dash vents. Checked fuse and relay. What next can I do?

When the battery was reconnected, the HVAC went through a recalibration, sometimes this caused the door actuators or the door gears to break or get stuck. Try the following; with the key ON, set all HVAC controls and switches to their OFF or CCW position, key OFF, disconnect battery and reconnect. Set key to ON (don't have to start vehicle) and don't touch any HVAC controls for about 2 minutes. Then try HVAC operation, if vents still don't work, you will need to have a mechanic look at it. Attached is a diagram of the actuator locations.


Aug 30, 2011 | 2004 GMC Envoy

1 Answer

AIR FLOW CONTROL HEAT an aircon. Will not go to floor or defrost.

i had that problem but in reverse i had no vents, its a hvac mode acuator, it controls where the air goes sounds like yours is dead

Mar 18, 2010 | 2001 GMC Jimmy

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