SL/R129: R129 300 heater
Banned
Joined: Feb 2014
Posts: 276
Likes: 33
SL500, Miata
I have the notion that temperature control works like this.
If it is really hot inside the car, the temperature is set to 72, and the mode is automatic, then there will be no coolant to the heater core, and the blower speed will be "4". As the interior cools the blower speed slows. On the lowest blower speed the interior may be too cool, so rather than allow the evaporator temperature to rise, the control introduces hot coolant. If you followed that and it is correct, then you can see that the interior temperature sensor could be a problem, though there shouldn't be hot coolant and a blower speed greater than "1". Another and more likely possibility is the valve which regulates the flow of coolant to the heater core.
Given the above, a logical starting point is to run diagnostics and view the input of various signals. Read their values and consider their plausibility. Note: on the very early control units diagnostics are not available.
If it is really hot inside the car, the temperature is set to 72, and the mode is automatic, then there will be no coolant to the heater core, and the blower speed will be "4". As the interior cools the blower speed slows. On the lowest blower speed the interior may be too cool, so rather than allow the evaporator temperature to rise, the control introduces hot coolant. If you followed that and it is correct, then you can see that the interior temperature sensor could be a problem, though there shouldn't be hot coolant and a blower speed greater than "1". Another and more likely possibility is the valve which regulates the flow of coolant to the heater core.
Given the above, a logical starting point is to run diagnostics and view the input of various signals. Read their values and consider their plausibility. Note: on the very early control units diagnostics are not available.
