A Little Night Music

Dear Car Talk

Dear Car Talk | Sep 25, 2014

Dear Tom and Ray:

I have a 1999 Ford F-150 with 94,000 miles. The truck runs great, but I have a problem with the radio. Starting last Thanksgiving, the radio comes on all by itself, even without the key in the ignition. It has happened at least half a dozen times since then. I've gotten to the point that I make sure the radio is physically turned off before I shut off the engine, so that if it does come on, it won't run down the battery. My son suggested taking out the radio and cleaning the plug-in connectors, thinking they might be corroded and causing a short. But a local mechanic advised against this because of the truck's anti-theft feature; he said if we disconnect the radio, we might possibly have to take it to a dealer to get it reprogrammed to work again. One thing I did notice is that when it happens, it usually is damp outside, which is why we suspect corrosion on the connecting terminals. Any help would be greatly appreciated.

-- Dennis

TOM: You've heard of sleepwalking, right, Dennis? Well, maybe you're walking out to the truck in the middle of the night and listening to the ballgame?

RAY: I don't think the radio connectors have anything to do with it. If they were corroded, your problem would more likely be that the radio would be turning off unexpectedly, rather than turning on.

TOM: So I think what's happening is that your truck is going into the ignition's "accessory" mode sometimes, probably because of a faulty ignition relay.

RAY: When you put the key in the ignition, there are several positions. There's "off," "accessory," "run" and "start." In accessory mode, only certain low-power devices, like the radio, get power. That allows you to listen to the radio without killing your battery while you're waiting for your wife to finish shopping at the bait-and-tackle shack.

TOM: And for whatever reason (it could be corrosion), one of your ignition relays is closing on its own and putting the truck into accessory mode while you count sheep.

RAY: You can figure out which relay it is by trial and error. There are two power-distribution centers in this truck, and each houses a bunch of relays. One is under the dash, and the other one's under the hood.

TOM: Next time you find that the radio is getting power with the ignition off, pull out these relays one at a time. And when the radio goes off, eureka! Then all you need to do is take that relay to an auto parts store and get a replacement.


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