Heating and Automatic Air Conditioning System in Mk2 Supras

 

1. Blower Motor

1.1 Blower motor works on some speeds, but not all.

Check for faulty blower motor resistor. Located behind glove box, red painted screws on a rectangular plate. Carefully remove it from the air duct. It should be a white ceramic block, but it may be disintegrated if faulty.

Most found in junkyards will be in bad shape, best to buy new from Toyota dealer. Resistor Assembly, part number 88635-14080.

1.2 Blower motor doesn't work on any speed.

Check fuses: 10 A GAUGE fuse, 10 A A/C fuse,

Check A/C circuit breaker on passenger side kick panel, to right of front seat passengers feet.

Check heater relay on passenger side kick panel.

Test blower motor by applying 12 V directly to the electrical connector leading to motor. Replace if faulty. Usually, you can just replace the brushes, and it will be as good as new. Note that the motor can be removed from the housing without removing the entire dash from the car! It is a bit tricky, but remove the three bolts holding it in, then turn it about half a turn and it should lower out of the housing.

You may find that an inch long threaded stud sticking out of the firewall gets in the way. Some people cut this shorter to make removal easier. It should not be necessary. Remove the passenger side kick cover. Under the long stud is a little J jig that's held on with a gold Phillips screw. Remove that jig. Now rotate the fan motor while withdrawing. It will try to get stuck, but just keep rotating and pulling down. It has a certain sweet spot where it will drop right out. After you've done it a couple times, it takes only a few minutes to remove it. While you're in there, clean the blower housing of all leaves and pine needles.

 

2. Heating and Air Conditioning system

2.1 No Heat from Heater

1. Coolant system not totally full. This may sound simple, but it really isnít. An air bubble can form in the upper areas near the heater core, and MK II engine will stay in temperature specifications even when slightly drained, but water wonít go through the heater core because of the air bubble at the high point. Make sure the level in the plastic coolant reservoir is OK - I sometimes have to take the cap off (the plastic reservoir, not the radiator) and use a flashlight to see (also make sure you have a good gasket under the cap). A related problem could be a tear in one of the hoses going to the firewall (and heater core), particularly the one going out of the back of the engine block. Look for yellow/green stains from leaked coolant. I once had a tear just where it left the block.

  1. Second, If your thermostat is stuck open, it will take a long time for the car to warm up, so it will take a long time for the heater to give you hot air. You can of course check this by watching your temp gauge and seeing if it takes a long time to rise. You should start to get some warm air from the heater as soon as the temp gauge rises from its resting place. Air should be pretty hot when it gets up about a quarter of the way up the gauge. If not, start to check the heating system.

3. Heater valve problem (located on right firewall next to the wiper motor). A likely problem.

4. Vacuum switching valve (VSV) problem (located on right fenderwell, under the washer fluid reservoir). A likely problem.

5. Blocked heater core (UK: matrix). Good luck if this is the problem as you have a days worth of work disassembling the passenger side dash. Plus cost of a new core. To test take off the lines going through the firewall (engine side) and try to put water through under pressure. If water flows, youíre OK.

6. Something electronic in the Automatic climate control system. See the following sections to diagnose problems.

 

It IS possible to have toasty warm heater hoses and still not get heat in the cabin. I had heater/climate control system issues to solve in BOTH of my Supras, and I ended up learning a LOT about how the system works. So here comes Climate Control 101 (note that almost all of this applies to both the MKII and MKIII):

So if the temperature sensor circuit has a short in it somewhere, you can play with the dash controls and all the stuff under the hood forever and you wonít get a bit of heat out of it. The quickest way I can think of to test it would be to remove the center console (the rear part, from just behind the gearshift back) and unplug the wires to the sensor. If you get heat, and lots of it, you have a shorted temp sensor. If you still donít get heat, measure the resistance on the "car" side of the sensorís plug to make sure you donít have a short further up the internal wiring. Youíll need to check your manual for which two wires to measure between. If you definitely have an open circuit and STILL canít get heat, you probably have a failure somewhere else in the electric part of the climate control. This could include broken wires going to the temperature flap servo, broken wires going to the CCC, or even a bad CCC.

 

3. Clicking From Heater System Behind Dash

The solution to the >>"mystery" click problem, contributed by Vern. >>

This is a common problem with Mk2's. The noise is from the relays that control the heater valve actuator (the gold thing behind the radio). The slide resistors in the heater controls can wear out, and have bad internal connections with the sliding arms (levers). Basically, one second they would send a voltage signal to the actuator circuits, turning on a relay, and the next, the internal connection would break, the voltage signal would go away, and the relay would click back off. The signal can change very rapidly due to road vibration, or whatever, and that change between on and off can wreak havoc on the relay systems back there. The noise can be really annoying, especially if you are cold and the heater just wants to make noise, and not heat!

As soon as I replaced all those switches the noises pretty much stopped, and the heater works just fine. Major PITA job, but worth it.

All this crap could have been avoided if Toyota had just given our cars cable controlled manual heater valves like every other car, but then we couldnít have had "Automatic Air Conditioning". But luckily, the Supra is *not* like every other car, and I love it!

Vern

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The fan in the console draws in air to read the cabin temperature.

There is a thermistor just in front of it.

Along with it, the outside temperature sensor, the solar sensor and a few other water temp sensors and relays provides the data. Note the test connector bypasses all of this.

With this data, the temp lever selection, the AC amplifier 'tries' to keep the cabin temperature at a consistent level.

If fact, having this thermistor at the back of the console is a very good idea, as most are located in the dash, which doesn't give that accurate of a read.

The mystery is why the AC amplifier continues to send a small voltage to DVV, located left and behind the radio. It's a vacuum valve driven by magnetic relays. The relays are generating the clicking.

I tried a new factory amplifier and it still clicked.

Based on the circuit, a *very* small vacuum leak would cause the servo to slid right, sending a new resistance value to the amplifier. So, if either the servo or DVV was leaking, this could be a cause.

However, if it were a consistent vacuum leak, it would do this *all* the time.

One test I would like to see is with a MK2 that clicks, and try pulling vacuum against the servo. Increased by small amounts, because it could be possible the rubber diaphragm is leaking at a certain point. Same could be with the DVV.

I do know they switched to electric servo motors in later Toyotas.

Jim King