Climate control - cold/hot air mixing problem

[Zapper35 Lurker Find all posts View Profile Close Zapper35]

My faithful 1999 KD-KZJ95W-GKPST-R Prado is getting a bit hot under the collar

A couple of days ago whilst driving and adjusting the temperature lower in my 90 series I heard a repeated mechanical ‘clicking’ noise down and forward of the passengers seat close to the centre line of the cabin. Pic shows roughly where it sounded to be coming from. The clicking occurred when selecting temperature about 22 degrees and lower. It was as if a small motor was trying to drive something that would not move. The clicking stopped after a short time. The temperature of the air did decrease a bit but not down to anywhere near the 18 to 20 degrees expected. It has been working fine for years with no issues in either manual or auto and with aircon selected on or off. I therefore think the failure is a stuck flapper door that controls the warm air being supplied to the mixing chamber/plenum that is not closing off enough and so letting too much warm air into the mix.

I have a basic understanding of climate control as follows:

1. Climate control is achieved by mixing warm and cold air and then directing to selected areas/zones of the cabin.
2. Water from engine cooling system circulates through a heat exchanger/radiator in the left forward cabin on the rear of the firewall and is the supply of warm air to climate control.
3. The aircon compressor provides cold air to the climate control. The operation of the compressor is governed by the A/C switch - you can stop the compressor supplying cold air to the system if desired.
4. A mixing system of flapper doors combines the two airstreams to supply cabin air at the selected temperature on the rotary selector between 18 and 30 odd degrees. I THINK THIS IS WHERE MY PROBLEM IS.
5. This mixed air is then sent to the user selected zone using the MODE push button (= AUTO is deselected) or is directed to cabin zone based on mixed air temperature relative to ambient temperature ( = AUTO sends warm air to footwell or cool air goes to face vents.

I think the issue is a stuck flapper door or actuator allowing warm air in when it shouldn’t be.

We are supposed to head away later today for a trip to the North and do not need warm air (lovely summer here in NZ) at all so perhaps I could disconnect the warm air supply to the mixin chamber as a stop gap measure until I have time to pull it apart. Could someone please give me a rough idea what to look for or where to look please. A guide using the supplied pic would be of great assistance. Many thanks, Zap.

[Zapper35 Lurker Find all posts View Profile Close Zapper35]

Just inboard of the passenger footwell is a stepper motor that controls (via a cable) a valve (on the forward side of the firewall) in the hot water feed to the system. That stepper motor is malfunctioning. As a trial I disconnected the cable at the motor and manually positioned the valve to closed but I am still not getting cold air from system. I have checked the refrigerant pipe out of the a/c compressor and it is quite cold so that part of system is functioning. Could anyone explain how the system works please as my initial assumptions are a bit wide of the mark.

[Zapper35 Lurker Find all posts View Profile Close Zapper35]

Now solved. The nylon anti-lash gear in the servo-motor that controls the hot water valve had stripped teeth. It was a right bastard to get that servo-motor out as I couldn’t see how to remove the footwell duct that directs air to passenger footwell. As a result it was very very difficult to get the last of the three mounting screws out. I ended up using a small 1/4” drive and welded a cutoff phillps-head screwdriver into a spare 1/4” socket. It was still a tight fit as it is a very tight squeeze. Again it would be easier if I could have removed that footwell duct.

Anyway, I am guessing that when folks lose temperature control then this serve motor is a prime suspect. It is easy to diagnose as all you do is look in towards the drive tunnel just inboard of passengers feet and you should see the servo working and the cable being operated as you turn the temperature selector.

[Zapper35 Lurker Find all posts View Profile Close Zapper35]

Oh, and the full answer was to disassemble the servo-motor and pull nylon gear out of mesh with motor gear and rotate nylon gear 180 degrees and put it back in as it only rotates 90 degrees for full travel. Just make sure you get the lever arm back on the correct spline as if it is out one tooth you will see the motor trying to continue to run as it isn’t getting the travel limiter signal for full travel so strains against the Bowden cable to hot water valve as it has already reached full travel and is fighting the servo-motor.