tsty
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response 32 of 40:
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Apr 6 02:13 UTC 2003 |
re giznoirt above :
When taking it for a test drive (always test drive it, no matter who your
buying it from) drive it for at least 20 minutes, let it idle for at least
2 mins, hard brake it once (Panic stop from minimum of 40 mph to zero, see
how long it takes for the car to stop.) Get a mechanic to look it over.
i didn;'t add this but i should have ... if if has gages (or not, i guess)
at idle turn on ALL the electrical stuff! inclluding cigartette lighter
and see what happens. it should increase the idle (a/c shold do that
all by itself) and (as i found out with fuel injectin) not stall.
nursing a bad-battery/charging system car home one night recently
i found out that teh non-carbureted cars stall on low-volts. sure
was a surprise to figger that one out on the fly.
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mdw
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response 33 of 40:
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Apr 6 10:55 UTC 2003 |
Older cars with open loop control systems & carburetors had to do weird
things with A/C & automatic transmission for the idle speed. So, yes,
on those cars, engaging either would make a difference (of course,
engaging an automatic is going to affect engine speed anyways). I have
never heard of a car where engaging electrical accessories will increase
idle. Most newer cars have closed loop control systems and "constant
idle"; there's no reason why idle would increase for any accessory on
such a car. On my car (1988 volvo, efi) when the a/c cycles on or off,
the engine idle speed glitches slightly.
On *very* old cars, at idle, the battery would not charge. On those
cars, the headlights would get noticeably brighter when engine RPMs were
increased. This is still true on many modern motorcycles.
Nearly all modern spark ignition engines depend on a functional
battery/electrical system. The only exception I can think of is some
lawnmowers which use a magneto instead. EFI certainly depends
electricity; E = "electronic". If the battery fails "in operation", the
car may continue to operate until the electrical load exceeds the
charging sytem's supply -- ie, high load, low supply - headlights & all
accessories on and engine idling "in gear" stopped with an automatic. I
was once in a car where the battery post had rotted through (battery
acid) and snapped. Car worked fine until a stop light, then it was a
total systems failure.
Older diesel engines did not need electricity - so on such vehicles
(probably mostly trucks) a battery failure won't kill the engine.
Starting it might be a problem (although there are diesels that can be
started using compressed air or other means). I believe modern diesels
are switching to the use of electronic closed loop systems (pollution,
economy, efficiency), so a functional electrical system will be required
there.
The april issue of CR has a much more complete list of things to look
for in a used car. I don't think they mention anything about idle speed
(which on most cars is hard to judge anyways) but they do mention hard
braking, also having a friend watch to see if the wheels line up with
the car's motion (if not, could be frame or wheel alignment issues),
also looking for signs of car repair work, and other stuff. A good
mechanic should be able to tell you if there's anything "off" about the
car's idling or other characteristics, and can look for a lot of other
things (wear in suspension components, brake pad life, battery
condition) that you could not necessarily tell just by driving.
You should still budget money for things the mechanic might not catch.
The older the car, the more likely it is that really strange stuff could
fail. Not all of these are practical to find, so any older car *is* a
gamble. This is why they're worth less money.
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gull
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response 34 of 40:
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Apr 7 17:42 UTC 2003 |
Re #33: My Honda will increase the idle if the electrical load is high
enough. Actually, it's kind of interesting. Honda lets the engine
computer control the alternator field voltage. It's not uncommon, on a
summer day with the blower fan and most other accessories off, for me to
look down when the car's idling and see only 12V on my voltmeter. If I
switch on some high-current accessories, or idle long enough for the
battery voltage to get down farther, the ECU will switch the alternator
back on, the voltage will jump up to 13.75 or so, and the idle will kick
up a hundred RPM or so. I suspect this is a way to improve fuel economy
and idle emissions. I've also noticed that in this mode, the ECU will
sometimes switch the alternator off when I accellerate and back on when
I coast in gear.
I've noted three distinct charging "modes" on this car. One is no
charging -- the alternator field is essentially switched off, and the
car is running on the battery, with an electrical system voltage around
12V. Another is a sort of float charge, where the battery is being kept
charged but the voltage is around 13.5-13.75V. (I don't have a digital
voltmeter, just an expanded-scale analog one, so this is approximate.)
Then there's the "the battery is really low and we need to charge it
fast" mode, like when I start the car and turn the heater blower on high
on a very cold winter day. Under those circumstances I've seen charging
voltages as high as 14V.
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