Does the computer just turn on the light after the engine has been run
so many hundred hours of operation? Or does it compensate for driving
conditions, such as decreasing the change interval if a lot of
short-trip driving or hard acceleration is done?
If there's an optical sensor, how does it compensate for some oils
being a lot darker than others even when new? Or do they measure the
capacitance between a pair of plates immersed in the oil?
You did not identify a specific vehicle. Different models do it differently.
There are some that actually measure crap in the oil. On my newer GM model, the
computer simply counts up miles. If you travel less than three miles between
startups, it counts the miles against a 3000 mile oil change period. If you
travel more than three miles between startups, it counts the miles against a
7500 mile oil change period. This is displayed on the dashboard's Driver
Information Center and it shows from 100% down to 0%.
IIRC the GM algorithm for most gasoline engines is 300,000,000 engine
revolutions, minus "penalty factors" for time spent below or above
normal operating coolant temperature. Simple as that.
Last May I put 12,000 miles on my bike, driving about 10 hours a day
averaging 3200 rpm. That would have be about
3200 rpm * 60 min/hr * 10 Hr/day * 31 daysY,500,000 revolutions.
I waited till I got home to change oil, but all the Harleys along on
the trip got one or two oil changes. If you are right, there is a lot
of good oil being dumped out.
I think the penalty factors likely drag that number down significantly
though. You wouldn't see near that amount of time expire between oil changes
unless you only drove it on very long trips.
I believe they also say not to leave it over 6 months regardless of what the
Robert Hancock Saskatoon, SK, Canada
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it goes by revs times a factor. it counts revs until it reaches a certain
set limit, but the "factor" keeps changing depending on engine condition.
when the engine is at normal operating condition, this factor would be 1.0,
but when it's cold or overheating, this factor would be greater. someone
constantly running at high revs or constantly driving a cold engine (short
trips) will therefore get the light on sooner than others. there is
absolutely no optical sensor to speak of, and this is pretty consistent
across the entire gm line.
the only difference i know of is that the full-size trucks get an engine
operating time (in addition to odometer) on their dash, but i don't think it
affects the oil change interval system.
what it doesn't do is actually look at the oil, or even know when the oil
was changed unless you reset it. if you use regular oil and always remember
to reset the light, i guess you can go by the light. if you use synthetic,
you could probably get away with letting that light go on 3 times or so
between changes, always making sure you reset it as soon as it goes on, of
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If I had to guess (And this is a guess) I would say that they use a ....
crap what is the word... spectograph (you know the dealy you use in
chemestry to shine a light through an opject and it reads how many
contaminent entities are flying around...) specrum something... anyways, I
would assume that when it gets to a certain percentage it would turn the
light on. So much less programming then reading short trips and all, and
then it couldn't compensate for outside factors.
On Fri, 08 Aug 2003 12:53:15 GMT, the renowned "Girgath"
I doubt they use a turbidity sensor, probably just operating hours or
maybe f(operating hours, speed, total elapsed time)- kind of an
electronic version of the little plastic sticky the oil change places
Of course if someone can point out an oil turbidity sensor as a spare
part on a car that will disprove this guess, at least in that case.
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I agree. But the next Q is, how do you reset the light after you do
your own oil change? Do you have to have a fancy terminal to
interface wih the OBD stuff on the newer cars? Or can you just do it
easily with a jumper or switch?
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Different vehicles do it differently. On mine, I push buttons to the DIC to
show the oil change life, and then I push RESET and hold it for five seconds.
Some vehicles do not have the DIC.
The ones fitted to vehicles which have long variable service intervals
do indeed measure the various parameters mentioned to compute a
suitable service interval. They also have oil sensors of some kind
that measure the condition of the oil, though this is unlikely to be
by viscosity. I know this because I once changed the oil in a Mercedes
some considerable time before the computer said it was needed without
resetting the service computer. Over the course of a couple of hundred
miles the computer increased the service interval it thought was safe,
not by a mile or two but by nearly 2500 miles. For this time the
computer was actually counting up the milage to the next service, not
The BMW 3 is similar but the 5 uses lights that progressively
extinguish to the next service.
But the next Q is, how do you reset the light after you do
Depends on the vehicle. Mercedes tell you in the operators handbook
how to reset with a sequence of button pressing. BMW can be done with
a tool or by grounding a pin in the diagnosic socket.
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On Fri, 8 Aug 2003 12:03:17 -0700, Watson A.Name - 'Watt Sun'
Again, some yes, some no. It depends on the vehicle.
BMW varies the Oil Service and Inspection Indicator lamp system that
uses Mileage, Engine Temperature, and Engine Speed to calculate
'Estimated Mileage' that triggers the lights depending on these factors.
Older fords and Cadillacs used something similar.
I can't remember the last time I saw one that was triggered just with
mileage (but I'm sure it was a model before 87).
GM has a web page describing their system, based on engine run time (total
and per start), coolant temp, rpms, and maybe ambient temp as well. I think
I remember being told that to clear the light you turn the key on but don't
crank the engine, then floor and release the gas pedal three times.
Carl Ijames firstname.lastname@example.org
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