Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Issues with a modern car.

  1. Aug 16, 2009 #1


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I drive a '99 Chrysler 300m. Recently on an hour long freeway trip the interior lights were randomly turning on, then after a random time from a few secs to minutes they would turn off. At first I thought a door ajar sensor was flaky, but then realized that the dash display did not show a door open.

    I was not real concerned with this behavior until one morning I found a dead battery. Could be a lot of things, so I jumped started the car and went about my business. Next morning same thing, now I had problem. I reasoned that those blinking lights might be the source of my problem. Chilton's pointed me at the fuse for the interior lights, each night I pulled that fuse and sure enough no more battery problems. I also learned that all of these functions were controlled by the BCM (Body Control Module). Apparently it was getting a bad signal or was spontaneously producing one.

    I checked some Chrysler forums and found others discussing similar problems. The most common fix was usually something to do with a bad ground. In the forums I also discovered that the trunk light was also controlled by the BCM. When we first bought the car (used) the trunk light did not work. I found a severed wire in the bundle at the flex point. I repaired that and the trunk light has worked fine since. Figuring that if it was a trouble once it might be again I reopened the bundle and inspected the wires for anything that might cause troubles. I found nothing of interest except that at one of my solder joints the end a single strand of copper was poking through the shrink wrap by something less then a mm. I did not see how that could cause a problem as there was nothing for it to touch. In any case I carefully wrapped it in several layers of black tape then re-taped the bundle and attempted to ease a bend near the trunk hinge bracket.

    Well even though I really do not feel I fixed anything the lights have quit blinking. I have taken several signigant drives and it has not recurred. I have not been pulling the fuse and have had no further battery problems.

    Is there anyone here that knows something about the operation of the BCM in this sort of car. Could it be that this is really fixed or is it just intermittent and waiting spontaneously fail again?
  2. jcsd
  3. Sep 13, 2009 #2

    and most of them are not in proper ducts
    they just drape and snake everywhere
    go on line and hunt up a full body wire schematic
    hint try [carefully ] russian sites

    first check clean tighten ect the GROUNDS
    hint there are many of them

    and then look under mats carpeting and trim panels
    for wires snaking around sharp metal edges
    or other wire problems

    BTW the cheap but time and effort way to fix bad a harness [wire loom run]
    is go to pick and pull yards and find your cars EXACT year model ect
    and remove what you need and replace the whole clip to socket run of the harness
    [good practice for installing on your car]

    VOM with extra long wire or extenders is helpful
  4. Sep 14, 2009 #3


    User Avatar
    Science Advisor

    I would check the alternator and the voltage regulator. If you're not familiar with the workings of cars, after your car starts, your alternator acts as an electrical generator, supplying all of the juice to your car (it also recharges your battery). You have some sort of voltage regulator inline with it to ensure that everything gets its proper 12v with no spikes or valleys.

    If the alternator starts to go bad, then your car will use your battery as souce of energy. With nothing to recharge your battery, it will die, and keep dying. So, if you have problems in these areas, you can get voltage spikes and things like this that can cause really weird problems with the electrical system.

    So, as mentioned, make sure your battery and major components are grounded well, then take it in to make sure your alternator doesn't need replaced.
  5. Sep 14, 2009 #4


    User Avatar
    Gold Member

    I have a 300M too. Had the same problem (except the dead battery). I joined the 300M Enthusiast's Forum, (which I highly recommmend). They suggested I play with the interior light dimmer slider. It seems to have done the trick for me.
  6. Sep 16, 2009 #5
    anything over 6 years old these days is just about ready to have a lot of bits replaced.
    i just replaced a relay switch on my, and its only not even 6 years old yet, not too mention all lot of other bits that have failed. Fair enough some things need to be replaced quite frequently, but these days tooo many vehicles are just built to last the warranty period.
  7. Oct 7, 2009 #6
    You're kidding right? I remember, when growing up (1980s), that cars were delivered broken to start with and just got worse! reliability has increased massively over the last 20 years. I think the U.K suffered badly with British Leyland but I don't think Detroit was producing exactly, high quality iron back then either. I'd imagine the situation only improved when the Japanese demonstrated that relaibility wasn't impossible.
  8. Dec 15, 2009 #7
    Good for you!

    Sounds to me like it was probably a door ajar signal. Alternatively, you might look at your manual overhead light switch (usually mounted on the dash).
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook