Heater core problem?

[89_3SFE]

I need some help in figuring out this problem my 89 celica has. It's a 2.0 L 3SFE and my heater is wiggin out on me. When I'm just driving around town I can hear the temperature controller move back and forth, opening and closing the valve that lets the water in. It usually only does it when I'm running high rpms, but sometimes it doesn't go back to heat. I'll turn it on and drive around and all of the sudden the heat turns cold and I can't get it back to warm unless I turn the car off and let it sit for a little while. Anyone know what my problem is?? Thanks for your help.

[DanoT]

My 88 has the same problem. There is a wire attached to a control unit mounted on the firewall with a 7/8" OD heater hose leading in and out of it. When I remove the wire from the control unit and manually move the post/lever to the heat postion I get heat but then the wire moves on its own to the cool position and shuts down the heat while the post/lever is still in the heat position. If I shut the engine off for a while heat returns. I want to bypass the heat control unit with an on/off valve plumbed in under the hood but I am affraid the stupid wire will still be able to shut the heat off.

How do I deactivate the litte motor that operates that control wire?

[89_3SFE]

I tried the same thing and it didn't work. I haven't found where the motor is but I did temporarily keep that from happening during the winter. Just place a block of wood behind the radio where the lever moves back and forth. Set it to heat and then place the piece of wood in there so it's wedged against something. That will temporarily get rid of the problem, but does anybody know how to permanently get rid of it?