Heater is the last thing I need in these hot summer days. But my 96 GCaravan has its own brain and set the heater to full strength on the driver side. (It's a dual zone system). Passanger side is perfect OK, even A/C works fine. Sometimes it return to normal 5 minutes after I hit the road, but it would repeat this 5 min delay every time I shut down and restart the engine. In the past few days it seem to stuck in the MAX state for good. I also noticed if I slide the temp setting higher the hot air even gets hotter. Do I have a valve stuck open or a faulty temperature sensor gives computer a wrong signal? If I want to manually shut off the valve is it in the cab or under the hood? How complicate is it try to reach there? help! help! help! Sweating in S. California (It says 105 outside, inside got to be higher)
gizmonix
07-22-2006, 04:48 PM
OK, guys, I learned a lot more on this forum to look deeper into the problem. The HVAC Diagnostics indicates "Driver Actuator Did Not Reach Cold Stop". I'm not sure if 1. the actuator is mechanically stuck, 2. Actuator motor failed, or 3. the bridge output driver in the HVAC controller that drives the actuator failed.
The document posted by one of the gentlemen says Driving Voltage on pin 15 should be about 11.5V and pin 13 at 1.5V to drive actuator motor toward Cold position. Are these the pin # on the connector that plugs into the HVAC controller? I haven't take it off, so do not know if the pin #s are clearly marked. In case I need to access the actuator, where is it? Can somebody give me a hint?
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The problem is in the Control Unit !!!!!!!!!!! Made a best guess on the pin #s and sure enough, the voltage moves on two other actuators (mode actuator should be on Pin 18 and 19, not Pin 18 and 12 as posted before by somebody) when I turn the mode knob or temp slide. Pin 13 and 15, for the driver side actuator, stubbornly stuck at 0.5V no matter how I slid the driver side temp. slide. Took the back cover off and found 2 driver chips U2,U3, both L293D made by STmicroelectronics. U3 is wired to drive the driver side and passenger side actuators, U2 is wired to drive the Mode actuator and to pin 1 and pin 3 of the connector (destination unknown). I re-installed the control unit with the circuit board exposed and re-measure the voltages directly on the driver chips. This is what I found, 1. the chips are awfully hot, burned the skin of my finger tip. 2. Input to the chip seems to working OK, when I slid the driver side temp. slide I can see 5V logic control voltage appears on the input pins of U3. Hopefully U3 is the only component need to be replaced (cost <$5 at DigiKey).
gizmonix
07-25-2006, 06:59 PM
Root of the problem is NOT in the Control Unit !!!!!!!!!!!!!!!!!
While waiting for the arrival of the replacement part, I decided to do a little experiment. By connecting pin 15 to the positive and pin 13 to the negative ends of a 12V battery I was able to force the actuator to the COLD position.
Since I'm going to replace U3 in the control unit I cut off the leads of the IC that connect to those 2 pins. Now nothing is driving the actuator motor, it should stay in the COLD position, right? I WAS WRONG !!!! the actuator keeps swing back to the HOT position every time I start the engine.
This really puzzled me for a while. Apparently something else was driving the motor, and one side of the motor must be shorted to ground. After ohmed all 3 actuators I found those 2 functioning actuators are floating from ground(chassis) but pin 15 of the driver side actuator has a 10 ohm path to chassis. Also found pin 13 is connected to pin 16 on the circuit board and measures 13V when installed in the car (with IC leads cut off).
I suspect there is a short circuit in the motor winding so pin 15 could never be driven high and over current causes overheat, eventually kills the driver transistor in U3. Pin 16 might be the actuator position sensor feedback voltage happened to have enough current that drives pin 13 positive then pushed the actuator to the HOT position every time the system is powered up.
My conclusion is that even after I replace the motor driver IC the problem's going to come back shortly. So here is my solution: Force the actuator to COLD (use battery power) then remove pin 13 from the connector. Now, I finally get AC on the driver side. If the actuator is too hard to reach I'll just leave it like that. GC provides plenty of heat, just passenger side alone is more than enough. The only draw back is the 2 bothersome blinking lites. BWT, AC and recirculating LEDs no longer works even though the entire system is functioning.
