Changing a switch not as simple as it seems any more

Dear Ask a Mechanic,

I consider myself to be handy, so when the power window switch in my 2013 GMC Sierra broke, I was confident that I could replace it. I bought a new one online, and it was easy to install, but now the only window I can control from that switch is the driver’s window. In fact, nothing else works from that door anymore. Even the heated seats, which aren’t part of that switch, don’t work.

When I reinstalled the old switch, everything worked except the driver’s window (and only because that part of the switch is broken). I’ve double-checked that the switch I bought is correct. Why is this happening? – Pickup Puzzled

The simple answer is that the truck doesn’t recognize the new switch, but to understand why requires a bit of background.

Consider the way that power windows were traditionally wired. Look inside the driver’s side door and you would find power and ground wires for that window switch. You would also find wires running from the switch to the vehicle’s three other door windows. That is eight wires.

Now add wires for the speakers, power locks and mirror directions. You get the idea. Pretty soon the door harness looks like a trans-Atlantic phone cable. All those wires add to the weight, complexity and cost of a vehicle – and take up a lot of space.

The solution is that many modern vehicles use door modules and communication networks instead. The driver’s side switch is a micro-computer, sending and responding to signals over a data network to the modules in the other doors or parts of the vehicle. This means fewer wires and allows for some neat functions, like windows that can be rolled down using your key fob.

In higher-trim versions of GMC trucks like yours, the power locks, heated seats and mirror controls all pass through that switch. The automaker has made those switches VIN and configuration specific, which means that to work properly they must be programmed so the vehicle recognizes them.

Unfortunately for handy people like yourself, this means General Motor’s Service Programming System needs to be used, so it can only be done by a dealership or an independent shop. This also means a previously installed and used switch also probably won’t work, since it could confuse other modules in the Sierra when it shares its incorrect ID and data over the vehicle’s network.

Whether this is a clever theft-deterrent or a money-grab depends on your perspective.

Ask a Mechanic is written by Brian Early, a Red Seal-certified automotive technician. You can send your questions to wheels@thestar.ca. These answers are for informational purposes only. Please consult a certified mechanic before having any work done to your vehicle.