From: jeh@cmkrnl.com
Newsgroups: rec.audio.car
Subject: Re: Car wont start when amp connected
Date: 21 Jan 93 15:35:14 PST
Organization: Kernel Mode Systems, San Diego, CA

In article <1993Jan21.085847.19908@news.unomaha.edu>, mgolden@cwis.unomaha.edu (Brian Golden) writes:
> ychoi@unixg.ubc.ca  writes:
>> I have this unusual problem.
>> 
>> I have partially installed an amp in my trunk, and the only wires that are
>> connected are the power, and ground.
>> 
>> BUT when I try to start my car, it wont start.  at first I thought it was
>> because my battery was dead, so I got a jump. The car started fine, but
>> the next day, it would not start.  So I disconnected the power cable from
>> the amp and the car started just fine. 

What do you mean by "wouldn't start"?  

	- Did the starter relay "click" or not?

	- If it did, was the starter able to turn the engine or not?

	- If yes, then you must mean that the starter could turn the engine
	but it wouldn't start running, which suggests a problem in fuel or
	ignition systems.  (An experienced mechanic friend says that almost
	all "car won't start" problems are electrical, not fuel problems.)

>> 
>> Could it be that my battery was not charged enough to start the engine (
>> I thought that the amp would not drain power, the remote wire was not
>> connected)
> 
> Check and re-check all of you connections.  It sounds like the positive wire
> is grounding to the frame of the car, and it is draining the battery.  This
> is also pretty dangerous 'cuz the short will make heat which could lead to
> a fire.  Be sure to disconnect the amp until you get it figured out.

I doubt this mightily unless it's a rather high-resistance "ground".  If the
positive wire is really shorted to ground it will not just drain the battery.
It will burn the insulation (unless it's Teflon) right off the wire in about a
second and a half; once the insulation is burned off, there will be lots more
places where the wire shorts to ground, drawing even more current; the
insulation will produce some of the nastiest fumes that you'd ever go a long
way to avoid (this is why fire codes require Teflon-insulated within ceilings,
etc., of office buildings); the wire will be heated well past red-hot; and it
will have a good chance of setting on fire anything flammable that's touching
it (or almost touching it).  the BEST thing you can hope for is that it melts
through the wire, removing the short, as soon as possible. 

No, it doesn't matter if it's "really heavy wire".  I've seen this happen to
jumper cables made with #4 wire.  

(Moral:  If you are going straight to the positive battery terminal, you must
always use a fuse as close to the battery as possible.  Or you can go to the
load side of the car's fusible link, which serves the same purpose.  The
fusible link is a sort of fuse of last resort.  But there is just one, or at
most two or three, fusible links for the whole car.  So connecting to the
fusible link is NOT a substitute for having a fuse close to the amplifier --
not unless you want a short in your amp or its wiring to disable just about
everything, or most everything, electrical in the car, ignition system
included.) 

In short, if there is a notable absence of pyrotechnic effects in and around
the car when the amp is connected, there is probably NOT a short circuit. 

I'd even rule out a relatively high-resistance leak to ground.  A typical car
battery should have enough oomph to start the car even with several
high-amperage things (high beam headlights, for instance, which at 65 watts
apiece, take a total of around 10A) running.  

Besides, you said that it was fine as soon as you disconnected the amp.  This
says that leaving it connected overnight probably did not drain the battery. 

If I were you I would get (or borrow) a digital (or even analog) multimeter
with a 10A current scale:  

	- Check the battery voltage with and without the amp power connection.  

	- Check the current drawn by the amp when it isn't powered on.  You
	do this by connecting the meter, set for DC current measurements, in
	series with the amp.  It should be extremely low.  

My suspicion is that there is something in your mechanical connection to the
battery that is disturbing the current path to the starter.  You really have
no idea just how low the resistance of this path has to be:  A starter can
easily need several hundred amps to start to turn a cold engine; a resistance 
of just a tenth of an ohm (which ain't much!) is much too much to have in the
way.  

	--- Jamie Hanrahan, Kernel Mode Systems, San Diego CA
Internet:  jeh@cmkrnl.com, or hanrahan@eisner.decus.org  Uucp: uunet!cmkrnl!jeh