Dandy. That doesn't have anything to do with the Engine Computer.
In the past, radios had two power wires. One was hot all the time for the station presets and clock memory. That was always tied in with something else that was always live. That could be the horn, or on some models, the cigarette lighter, but on most Chrysler products that was the interior lights. That's why you wouldn't find a separate fuse labeled for the radio's memory. The second power circuit came from the ignition switch. That's the one that turned the radio on.
For information only, when that memory feed was lost, different radio models had different symptoms. Some played normally except the station presets went back to factory defaults every time you turned it on, and the clock reset to 12:00. Some models would be totally dead. Some others appeared to tune stations, but had no sound. Any time you run into anything like those symptoms, always look for at least two fuses, and a third one if the car has a factory-installed external amplifier.
By around 2002 to 2004, the power system changed. I added two diagrams for your car to show what I mean. In the first diagram, at the top left, is the 12-volt feed from the Totally Integrated Power Module, (TIPM). That's the fuse box under the hood, but it includes a lot of computer circuitry inside it. If you follow the red wire down to the radio, you'll see it listed as "Fused B+ (I.O.D.). That stands for "Ignition-off Draw". Since the late '80s, that is the circuit that stays on all the time to keep the memories alive in all of the computers. With the ignition switch off, the maximum current allowed through that circuit is 35 milliamps, (0.035 amps). That is the industry standard now unless the manufacturer states otherwise. Cadillac, for one, allows up to 50 milliamps. Chrysler says with a drain on the battery of 35 milliamps, a good, fully-charged battery will have enough charge left to crank the engine fast enough to start after sitting for three weeks. That's the wire you should use for your new radio if it uses the separate memory wire.
Where things really get different is there is no 12-volt wire coming from the ignition switch to turn on the radio like they did in the past. That same red wire is the power supply, but the radio turns on internally when it gets a digital signal from the TIPM over the data buss. That's shown in the second diagram. In fact, almost everything on the car is turned on by the TIPM or by the Body Computer when they see the ignition switch has been turned on. All of the dozens of computer modules communicate back and forth over one of three data busses. Each data buss is a pair of wires that send the digital signals back and forth. Those signals are only about 0.4 volts. Being so weak makes them susceptible to electromagnetic interference that can confuse those computers. To prevent that, they twist the two wires around each other. That way the same stray voltages are "induced" into both wires equally causing them to cancel each other out. Those data buss wires are the ones shown as "twisted pair".
I'm sure there are some aftermarket radios that are designed to just plug into these systems and operate through the data buss, but unless you tell me different, I'm going to start by assuming your replacement radio is of the common sense design that uses the two 12-volt power wires. My suspicion is you have the "12-volt switched" wire connected to something that stays live all the time. If you do, the radio should not drain the battery if you remember to turn it off every time you leave the car. The better solution is to connect that 12-volt switched wire to something that does turn off without you having to do it yourself each time. If this is what is happening, a radio by itself can draw as much as an amp, even with the volume turned way down. That's about the same as two glove box lights staying on all the time. That can drain a battery overnight.
The next question has to do with the remote amplifier. If your car has one from the factory, that had to be bypassed for the new radio to work. Did you have to do that? If so, how was that done? This shouldn't be related to the battery drain. It's more for my information. Factory remote amps used to be turned on by the radio when it was switched on, but on newer cars like yours, it is also switched on separately through a digital signal over the data buss. That means the amp will turn off automatically when you turn the ignition switch off.
If this sounds like what you're running into, let me know if you need help selecting a wire or a circuit to connect the new radio to. I have the wiring diagram for your car online and I should be able to find a suitable wire. There might even be an open tap in the TIPM, but that would require running a new wire through the firewall.
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Saturday, April 24th, 2021 AT 6:42 PM
