Tire pressure monitor

My wife has a '05 and those tires hold 35 psi, and yes the lite comes on
when they drop below 32psi on cold mornings

Better double check the tire pressure sticker

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OR, double check your gauge?

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According to the owners manual, the light should come on if one of the
pressure in one of the road wheels drops 25% below the recommended pressure.
So, assuming the recommended pressure is 35 psi, the light should come on if
the pressure in one or more tires drops below 26 psi. I would be suspicious
of your pressure gauge if you are constantly having this problem.

Are you still using the OEM tires? Tires with steel belts in the sidewalls
are a no-no.

As for the "they all do that line." Pure BS. Find a good gauge and set the
pressure properly (cold - The tire pressure when the vehicle has been
stationary and out of direct sunlight for an hour or more and prior to the
vehicle being driven for 1 mile). If the light is on in the morning when you
get ready to go, check the pressure. If it is less than 25% below the
recommended pressure, take it to the dealer and tell them to fix it. Don't
let them BS you. If they start that, mention "safety problem." I searched
the NHTSA Recall, TSB, Defect Investigation, and Complaint databases. There
was exactly one entry (in all data bases combined) related to the 2006
Explorer tire pressure monitoring system and it was a weird one. Here it
is -

Make: FORD
Model: EXPLORER

Year: 2006
Complaint Number: 10169074
Summary:
2006 FORD EXPLORER LIMITED DRIVER SEAT WOULD NOT RETAIN MEMORY ON #2
SETTING. WHEN EXITING THE VEHICLE THE SEAT WONT GLIDE BACK UNTIL THE #2
BUTTON IS PUSHED. TIRE PRESSURE LIGHT ILLUMINATED. *KB THE DEALER HAD TO
REPROGRAM THE SEAT MEMORY MODULE AND REPROGRAM BOTH REMOTES. THE DEALER
RESET THE TIRE PRESSURE SENSORS. *NM

If it was true that they "all did that," there would be more than one weird
complaint in the complaint database. You can always add one if you like -
see <http://www-odi.nhtsa.dot.gov/ivoq/index.cfm
Ed

You're right, the sticker does say 35 psi. Damn, after a lifetime of
32 psi, I never thought to look. But on the other hand, 32 is within
10% of 35 and "shouldn't" cause a problem. But I'll check at 35 psi
and see if it makes a difference.
Is the higher pressures due to larger tires (16" vs 15")?

KC

As I said, all 4 tires read the same pressure with very minor change
in psi between the warning light on or off. And I have used several
pressure guages with only minor variations between them. As a side
note, during the summer I picked up a nail and had a slow leak. The
warning light did come on when that tire got down to the mid 20's
pressure. Repaired the tire and the light went out. So that part of
the system works. This problem seems to be temp related rather than
pressure related.

Still using the OEM tires.

That's what I thought also, but wanted some input.

Thanks for the feedback and link.

KC

Except that temperature and pressure are directly related (Charles's Law). If
the pressure increases, so does the temperature. If the temperature increases,
so does the pressure... at least in a closed vessel.

--
Mortimer Schnerd, RN
mschnerdatcarolina.rr.com

True, but not by 25%.

In order for the pressure to drop by 25%, the temperature would have
to drop by 25% - and you have to use an absolute temperature scale.
For instance:

70 degrees F = 529 degrees Rankine (an absolute temperature scale
using the same "size" degrees as Fahrenheit), so for the pressure in
the tire to drop 25%, the absolute temperature of the air in the tire
would have to drop by 25%, or 132 degrees F. So if the temperature
dropped from 70 degrees F to -62 degrees F overnight, you would expect
the tire pressure warning light to come on. I don' t think there is
any place on Earth with an overnight temperature swing of 132 degrees
F. Maybe the moon

Ed

...

Is 35 the pressure on the vehicle sticker (probably located on the
driver's door jam). The pressure chosen is related to a lot of
factors, vehicle weight, tire type, expected usage, fuel economy
concerns, safety, ride, handling, etc. There are industry standard
load vs. inflation pressure tables for all tires of a given size and
type.

I would expect the tires on your truck have a maximum pressure of 44
psi. However, the maximum load they can carry is probably the same
from 35 psi to 44 psi (assuming they are 104S tires). The "extra" 9
psi allows for higher pressure recommendations for high speed driving,
or better performance. When P series tires are used on Light Trucks
and SUVs, the maximum load is derated (usually by 10%). So, when you
mount p series tires on a light truck or SUV, the maximum load rating
shown on the tires sidewall is higher than recommended by the tire
manufacturer. The tires on your truck are probably rated to carry 1804
lbs when used on an SUV (at any pressure from 35 psi up to the maximum
shown on the sidewall). The sidewall probably says the maximum load is
1985 lbs (if you reduce 1985 by 10%, you get 1804 lb)

There are load inflation tables for some tires available at
http://www.homestead.com/hmcclub/Goodyea...
This is a Goodyear chart, but all tires of a given size and type are
the same. The load vs. inflation table for a P235/70R16 mounted on an
SUV is as follows:

PSI Load (lb.)
26 1544
29 1644
32 1714
35 1804 (maximum load for load range 104S tires - which is what you
have)
38 1864 (applies to load range X tires only)
41 1954 (applies to load range X tires only)