On a very cold day, I left my 1996 GMC truck in a parking garage. When I came back, I found that I had lost all my accessories.
No radio, no power windows, no heater blower, no interior lights, etc. Also, the brake warning light was on. The truck did start and the headlights and brakes worked, so I was able to drive home.
The next day, I went to start the truck and everything worked again. The situation has never reoccurred. What do you think happened?
— Bill
Well, this will involve a lot of guesswork, Bill. Just like all my other answers. I’m going to assume that the very cold weather was a factor. We all know about shrinkage, right?
My guess is that some electrical contacts shrank in the cold and ceased to make contact. That’s what interrupted your power. Which contacts were they? Well, I think we can rule out a fuse or a relay, because there are simply too many things that stopped working. A fuse or relay could explain one or two of those things but not all of them.
We can also rule out something universal — something that would affect every electrical connection in the truck. Why? Because the truck started. So, for instance, it could not have been the negative terminal of your battery, because the starter motor needs more power than all of those accessories combined.
So, it’s got to be something general, but not universal. And I’ve got two guesses. The first is your ignition switch. It’s possible that some contacts in the switch were affected by the cold. That could cut off power to your accessories while allowing the truck to crank, and, on many cars, allowing the fuel pump to run.
The second possibility is that it was one of the major wiring harnesses that go between the passenger cabin and the engine compartment. They’re mounted on the firewall. And if one of those got cold enough, or got cold enough combined with some age-related corrosion, some of the metal contacts could have failed to maintain contact.
If you’ve got nothing better to do this weekend, start the truck and turn the radio up so you can hear it from under the hood. Then shake and manipulate all the wiring harnesses on the firewall. You may find that jiggling one of them results in the radio cutting in and out. If so, you’ve found the problem. Then you can either replace that harness, or start a campfire underneath it next time this happens.
