By Paul Weissler, MACS Senior Technical Correspondent
When an HVAC case electric actuator fails and you get a hard code, the fix should be straight- forward: replace the actuator. Ditto if a trouble tree calls for a new actuator. There often is a hesitation to just install the actuator, a fear that maybe the problem is in the harness or the control module. However, if you’ve followed the factory diagnostics without “shortcuts,” and there’s no apparent wiring issue, you have to trust them and get the actuator. And once you’ve bought the actuator, you also have to trust – and use – the factory installation procedure—in the case of General Motors, a choice of two procedures.
First, clear all codes with whatever scan tool you use (or if there’s a factory-listed way with pulling fuses, if you don’t have a suitable scan tool). Turn off the ignition. Replace the part(s) — actuator(s) and/or module – and reconnect the wiring. Then it’s time to initialize (calibrate if you prefer).
GM HVAC control modules and actuators can be initialized with a scan tool if it has the enabling feature, and the long-used Tech 2 has a Body/HVAC Control Module/Special Functions/Recalibrate All Motors listing. It’s the GM-preferred method. Codes cleared? Part(s) installed and wiring reconnected? Plug in the scan tool, start the engine and select the motor recalibration listing. Allow the scan tool to go through its routine (don’t touch the HVAC controls during initialization), and then make sure that no codes have returned (or new ones originated).
If you don’t have a scan tool with the enhanced software that includes a motor recalibration feature, the alternate procedure should work (if not the first time, repeat). If you used a scan tool to clear codes, turn off the ignition, and here again, connect any unplugged components, module and actuator included. Remove the HVAC fuse (typically the HVAC/ECAS or HVAC CTRL,wait 15 seconds or so, then reinstall, start the engine, and allow a minute for the system to initialize. Here again, check for any codes after the calibration routine is over.
It’s important to install and connect the replacement parts and let the module do its thing. Never leave a new actuator out of the HVAC door linkage to “see if it’s working.” The module has to cycle the actuator to find the extremes of its intended travel. If the actuator is just hanging, the module will get fooled, and when you install the actuator it will “hunt.”
GM had a service problem with hunting actuators creating the familiar (intermittent) ticking noises, on 2004-06 full-size SUVs and pickup trucks, affecting the recirc door (code BO229), left temperature door (code BO414), right temperature door (code BO424) and front mode door (code B3770). The BO codes are generic, so they show up with a generic scan tool. The B3770 requires enhanced data, which the OE (such as Tech 2) and premium aftermarket scan tools display.
There was more to the subject than just the occasional ticking. If the recirc door didn’t close, A/C performance would be poor in hot weather. If either of the temperature doors didn’t move properly, the temperature control on that side would be affected, and if the mode door stuck in the wrong position, well that’s obvious.
The cause in each case was a door operating strategy that was looking for a precise position that the door might not be able to settle into, as commanded by the module, because of the mechanicals of the door, shaft and duct. The fix was new software that widened the tolerances, so the door could readily find an acceptable spot, and no need to replace any of the five-wire actuators. But if you don’t have the reprogramming capability, it is possible to shim a recirc door with foam sealer, so it stops a bit earlier.
However, “it is cut-and-try,” so a jury-rig like that isn’t worth the effort for the possible comeback. If necessary, get a shop with a GM Service Programming System to do the job for you (and keep a list of all the “re-flashes” you send out, so you can determine when/if it’s worth buying the capability).
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