How do I stop myself getting hot feet in my '99-on...?

by Gavin McKeown -

A common problem with Miatas is that the heating system puts out warm air even on the cold setting. Whilst it is true that the heater core always has hot water running through it, this is not directly the cause of the problem. Almost all modern cars have the core contiunuosly in the circuit and they don't all have this problem. There is a door in the heater core which opens and closes to allow the warm air out. When the door is closed, no warm air gets out and you can get cold air to the vents. The cause of the problem in Miatas is actually that the range of travel on the control knob and it's connecting wire is less than the range of travel on the door. This means that the door doesn't reach fully open or fully closed, so some warm air always gets out.

Luckily, it is easy to move the cable so that the range of travel allows the door to close.

(Note: this fix works on earlier generations as well)

Step One

Remove the glove box by uncliping the hinge on the door side and then sliding out the rod from the hole in the center console. This allows you to see the edge of the heater matrix.

rod_hinge.jpg (64973 bytes) cold.jpg (54420 bytes)


Step Two

hot.jpg (55282 bytes)You will see a cable coming from the control knob with a loop in the end. This loop goes around a lever. At the other end of the lever is a rod which is connected to another lever that actually controls the door that lets the air out from the heater matrix. At first I thought I needed to adjust the threaded end of this rod, but it's even simpler than than. The cable is held to the plastic of the heater by a metal clip. If you lift this clip and pull the cable, you can get the cable free. Make sure the loop is still over the lever and get used to which way the cable and lever move for hot/cold. The cable extends to make the air hotter. Now, move the lever by hand to the fully closed (cold) position. You should feel a slight 'click' as the door closes fully. Turn the control knob so that it is one click off of fully cold and push the cable back into the clip.

clip_hinge.jpg (51917 bytes)Step Three

Check the travel towards the hot end. The range of adjustment may now not give you the hottest air possible, but you will be able to get fully cold. Then, just replace the glove box and you're good to go. A 5 minute job and completely reversable. In the winter, if you really need the hottest air possible, you can repeat the procedure and set it to fully open at the hot position.

Another method from Dave Hanna...

My air dash vents blew warm air, even when the outside temp was quite cool and the temp knob was turned to max cool. It did the same thing even with the AC turned on. The problem is that the heat/cool flapper door does not close completely on the cool setting. This door or flapper is connected to the hot/cold control on the dash by a spring steel wire cable. There is an easy fix.

Go to your nearest hardware store and purchase a small spring. Most good hardware stores keep an orange metal box with lots of drawers filled with hundreds of different springs. You want a number 106 (I think I remember that correctly). This is a very weak spring, about 3cm. long, 1.5cm diameter, with hooks on both ends. Tension strength should be about a pound. The wire is approx. .03-.04 mm. (The Miata is foreign so we work with metric measurements here.)

How to install: Sorry I can't provide photos, but the install is not too hard to visualize from written instructions. First remove the glove box: open the box and pull firmly on the bottom right side: it pops out (and back in easily later on). After you pull the glove box all the way out and set it way out of the way, look just behind the center column on the dash, about 20-30 cm in front of the AC controls (i.e., under the dash center). You will see a black cable, like a bicycle brake cable with a hard steel wire coming out of it. A loop of this spring steel wire is attached or pushed over a lever arm. On the other end of the lever arm is the hot/cold selection door. To verify you are looking at the right thing, turn the hot/cold selector knob. The spring steel wire should move in and out (fore and aft in relation to the car).

My solution: I hooked one end of the #106 spring on the spring steel cable loop and the other on a hole in the sheet metal supporting the dash.

Just to the left of the opening you are looking at under the dash -- and just behind the edge of the console -- are two 1cm X 1cm holes. I attached a straightened out paper clip (then hooks formed on each end) from the top hole to the other end of the #`106 spring. See diagram.

This change should cause the flapper door to close fully and stay closed as you run the AC, or just cold vent. It makes the hot/cold selector knob a tiny bit harder to turn to the right (towards hot). If the knob is real hard to turn after you installed the spring, get a weaker (thinner wire, larger coils) spring.

This won't solve the problem completely because we have the new AC refrigerant which is not as good as the old stuff.

Here's another item that could help reduce the heat of the air coming through your vents. We're not sure this works in all cases, but you might try it if the items above don't help. 

Back to the Garage

23 August, 2001