tire pressure monitor

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Reference: 2006 Explorer XLT Problem: "Low tire pressure" warning comes on when outside temp drops below 50F degrees. Pressure in all tires check good (32 psi) when the
warning light is on or off. Warning light will stay on for 2-3 days before resetting, even though the outside temp reaches the 60's. A 30+ minute drive at interstate speeds will also reset it. It's damn aggrevating to have to reset the light every time I start the engine, especially all winter long.Two trips to the dealer service rep got nothing more than "mine does that too", or "they all do that". Have any of you Explorer drivers had this problem? Know of any service bulletin fixes?
KC
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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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Ollie wrote:

i drive a ranger for work, we have to run them at 28psi, or slightly less or they ride even worse off road. is this unsafe?
should i say again how much i despise rangers?
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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
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wrote:

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/Goodyear_Tire_Inflation___Load_Charts.pdf . 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)
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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
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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
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KC wrote:

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
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"Mortimer Schnerd, RN" <mschnerdatcarolina.rr.com> wrote in message

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
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C. E. White wrote:

You're taking the use of the word "directly" literally, as if there were a 1:1 relationship. I'm using it as the opposite of "inversely". In an inverse relationship, as one goes up, the other goes down. In a direct relationship, as one goes up, so does the other.
IIRC, a scuba cylinder's internal pressure will raise or fall 4 psi for every degree Farenheit's difference. That's not a 1:1 relationship but it is a direct relationship.
Given that a rubber tire is going to flex more than a steel or aluminum cylinder, I would expect even less of a rise or fall of psi with temperature change.... but it will rise or fall.
--
Mortimer Schnerd, RN
mschnerdatcarolina.rr.com
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"Mortimer Schnerd, RN" wrote:

Not sure the SCUBA cylinder is a good comparison, 3,000+ PSI vs. 30-something PSI...
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Pete C. wrote:

Sure it is. The temperature alone is enough to cause a burst disk to blow out if you leave a tank in the trunk of a car baking in the sun. The burst disk is designed to blow about 1000 psi over the working pressure of the tank. Normally that would preclude such an event but the disk weakens with each cycle of the cylinder... I.E., it flexes as you use and refill the tank. I have personally witnessed a blowout of a burst disk in the trunk of a car sitting in the sun down in Florida. Much noise, much fog (instant condensation of any moisture as the tank pressure drops very rapidly), no damage (except to your drawers).
I don't think the disk would have let go on a cool day. The only other time I've ever seen disks blow was during filling. Rapid flexion, rapid heating of the cylinder as the pressure increases followed by rapid chilling of the valve (it often will ice up) as the tank rapidly empties.
Whether we're talking about a scuba tank or a tire, it's a closed cylinder. Both flex to an extent and both adhere to Charle's Law.
--
Mortimer Schnerd, RN
mschnerdatcarolina.rr.com
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----- Original Message ----- From: "Mortimer Schnerd, RN" <mschnerdatcarolina.rr.com> Newsgroups: alt.autos.ford Sent: Tuesday, November 13, 2007 1:35 PM Subject: Re: tire pressure monitor

Assume you filled the tank to 3000 psi at 32 degree farenheit, and then let it sit in the trunk of your car and drove to Florida, in the summer. Generally you would expect the maximum temperature in a trunk of a car in Florida in the summer to not exceed 140 degrees F. What would the prssure be?
T1 = 32 degrees F = 491.4 degrees R (Rankine, absolute temperature scale); P1 = 3000 psi T2 = 140 degrees F = 599.4 degrees R (Rankine)
P2 = 3000*599.4/491.4 = 3659 psi. (more or less)
Seems like the tanks needs to have fatique life established if the blow out disks are failing at only 66% of the design point.

Tires flex all the time. Every rotation flexes the tire. But tires have a very long fatique life (they'll fail from material deomposition before they fail from flexure, assuming appropriate loads and proper inflation). I am surprised scuba tanks seem so fragile.
This really has little to do with the pressure in the tire, or why the TPMS was going off on cold mornings. As I said before, there is no way a properly inflated tire is going to have the pressure decrease by 25% becasue of a change in the temeprature overnight - at least on earth.

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"C. E. White" wrote:

The annual visual / eddy current inspection of SCUBA tanks is supposed to catch fatigue cracking. Tires and wheels which are subject to far more severe service under highly variable conditions have no such inspections. Both are life safety related too...
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C. E. White wrote:

The tanks themselves aren't fragile. I've never personally witnessed a tank fail in 10 years as a dive shop manager. But those burst disks are designed to fail long before the tank does, much like a fuse should fail before the protected component does. So the combination of numerous flex cycles combined with sitting a tank in the sun can cause a premature failure of the disk.... Charle's Law in action.
I used to slow fill tanks sitting in a water bath inside my air conditioned store.... say at 70 degrees. The idea was to provide an honest fill. Places that just shot the air to the tank might initially read 3000 psi (which made the tank HOT) but by the time you got to the dive site, the temperature inside the tank will have equalized with the environment. You got into 70 degree spring water and read your gauge, you might find you were 300 lbs light. So you either overfill initially or you could fill slowly or you could do both. I did both. Filling a tank to 3150 psi would yield a true 3000 psi at the dive site.... and a damned sight more in the trunk of a hot car.
Better for a $2 burst disk to blow than a $150 tank.
--
Mortimer Schnerd, RN
mschnerdatcarolina.rr.com
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"Mortimer Schnerd, RN" wrote:

Not by a long shot, they are damned beefy. I recently cut two AL80 tanks that had failed inspection apart for my dive shop to use for display in classes. One was cut lengthwise to give a nice overview half and the other just the top section cut off to pass around to show hairline neck fractures. I rough cut them with the plasma cutter which was pretty impressive itself, blowing through up to about 3/4" thick aluminum, and then threw the keeper pieces on the mill and milled the plasma cut surfaces away leaving a nice finish. Given the current metals pricing, a condemned SCUBA tank is still worth a few $ as scrap aluminum.
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"Mortimer Schnerd, RN" <mschnerdatcarolina.rr.com> wrote in message

The amount the pressure changes is a function of the temeprature change and the initial pressure. P1/T1 = P2/T2 for gasses that approximate an ideal gas (air at normal pressure and temepratures is not too bad, as long as it is dry).
Assume intial temperature (T1) of 70 degree Farenheit which is 529 degrees rankine and an inital pressure (P1) of 3000 psi. Assume P2 Pressure of 3004 psi. T2 would have to be:
T2 = T1*P2/P1 = 528*3000/3004 = 527, which is your 1 degree change
However, if the pressure is only 1000 pounds, a 4 psi change would require around a 2 degree temperature change.
T2 = 528*1000/1004 = 526, which is a 2 degree temperature change.

I am not disputing that, but there is no way the pressure in the tire will rise or fall enough to affect the operation of the tire pressure monitoring system over a normal temperature range, if the tire is properly inflated (inflated cold to the vehicle manufacturers recommended pressure).
Tires change volume (but not necessarily shape) very little over the range of normal inflation pressures. The tire carcass is not as rigid as a scuba tank, but it is not a balloon either. The shape of the cross section changes, but the interior volume is relatively constant unless the pressure in the tire is very low, or the tire is drastically overloaded (well beyond any normal load). The change in volume is trivial and can be ignored for a change as little as 25% below the recommended pressure.
Ed
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" I searched the NHTSA Recall, TSB, Defect Investigation, and Complaint databases. There was exactly one entry (in all data bases combined)"
at http://autosmash.com under the TSB section for Exploreres you'll find:
"TIRES:PRESSURE MONITORING AND REGULATING SYSTEMS
SOME OF THE VEHICLES WERE BUILT WITH INSTRUMENT CLUSTERS THAT WERE NOT CONFIGURED PROPERLY FOR USE WITH A REVISED TPMS MODULE. THE TIRE PRESSURE MODULE SYSTEM (TPMS) IS FULLY FUNCTIONAL, BUT THERE IS NO VISUAL INDICATION TO A DRIVER IF THE ..."
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UPDATE: I increased the pressure in all 4 tires from 32 psi to 35 psi. Last night the temp dropped down to 32F degrees. The "low tire pressure" warning did NOT come on.
Based on previous responses, I have no idea why the increase in pressure would solve the problem, but apparently it did. Maybe the sensor is operating with some effing Ford algorithm or something. Anyway, just advised all of the solution in case anyone google searches the problem in the future. Thanks to all who responded.
KC
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