I was driving along and this light come on in the dash, which looks like a horseshoe (open end up) with an exclamation point(!) inside of it. I pull over, look it up in my owners manual and find it's a "low tire pressure indicator". I look at the tires and they all seem OK -- one may be a tad low.

Anyhow, my question is: how does that sensor work? Are there four sensors, one in each tire? Does it detect frame sitting lower than normal?

Until this truck, I've never had a vehicle with low tire pressure sensors.

 

Looking at tires does not tell you air pressure. That can only be done with a pressure gauge. A tire might be down by half and not look low.
As far as how they work, from my experience and what I have read, they go by a percentage of pressure lost. Not sure the exact percentage, but somewhere in the 10-15% as far as I can tell. As I expect you have P series tires on your truck, they will have a recommended pressure of 35. Keep them at that as the P series tires really need to be kept at that pressure to handle the weight of the vehicle. If not kept there, then you could have a problem like a blowout or tread separation. Usually at highway speeds. That is caused by the tire getting hot from too low a pressure.

 

So, there's a sensor in each wheel. Interesting . . . . Thanks.

 

There is a sensor strapped to each wheel. That is with the 07 and 08. 09 up have a sensor attached to the valve stem.

 

I guess a can of fix-a-flat would really screw up the sensors. Or some people put that preventive goop in their tires -- that probably would mess with the sensors too.

 

Most definitely. I wouldn't use a can of that type of product even without sensors. Too many problems with them, including some in the past (maybe still) were pressurized with propane.

 

I carry a can for last resort. If it comes to a choice I have to make to get get out of an emergency situation, you bet I'll give it a try. Now this emergency would have to be two flat tires, on some back road with no cell service, my tire plugger kit doesn't work, or my tire pump doesn't work. Not likely to happen, but if it does, I'm not going to worry about tire pressure sensors.

 

It's actually pretty cool how they work...

http://auto.howstuffworks.com/car-dr...ng-systems.htm

 

Thanks for the link!

 

The way they say it works, is a bit misleading. Ford uses the same sensor no matter what load range tire is used on the F150. By that story, it would wait until about 28 to 35 psi being acceptable. On my vehicle, I will get a fault when the temperature drops. The recommended pressure on mine is 45 (LT LR C tire). It will set a fault at about 40. Ford also puts LR E tires on some of the F150s. These would set a fault at an even higher pressure. That is why I mentioned it uses a percentage. If I were to set my air pressure at 40 and reset the sensors, it would set a fault at around 35.

Here is a link to the NHTSA rule on the tire pressures. There is a chart that shows what pressures a sensor is supposed to be set. That would be balanced though by what tire is on what vehicle. In some cases the proper pressure for a tire would be lower than what the feds allow. Example, on an F150, 45 would be the correct pressure to use with some LT LR E tires with a max of 80. As the limit NHTSA uses for this tire is 46, it would always set the fault if it went specifically by pressure. That is why Ford uses a percentage. That way they do not need a specific sensor for each vehicle. This also helps when aftermarket tires are used with a different load range.

http://www.nhtsa.gov/cars/rules/ruli...ule.html?name=

 

Yes it will mess up the sensors along with Pi$$ing Off the person who will be changing or repairing the tire.

 

Could you explain some more? If the sensor for an '08 is "strapped" to the wheel, is it strapped outside the tire? If so, how can it determine tire pressure?

The warning light came on again today. I aired up all four tires, including the spare. All five tires were pretty close to correct pressure.

Does the spare wheel/tire also have a sensor?

Is this TPMS (tire pressure monitor system) going to be a recurring, but minor, PITA?

 

No it is strapped to the wheel along the centerline. Spare should not have a sensor.

Cold will lower the pressure. Mine is on right now since we had the big cold snap.

Yes, you can expect to get the low pressure light, especially when the weather gets cold. Yes a PITA, but unfortunately thanks to the Feds, Firestone, and Ford (Explorer tire problem) we are stuck with it.

Here are pictures of the strap on type.

https://www.google.com/search?q=%22f...w=1920&bih=889


Here are picture of the valve stem type.

https://www.google.com/search?q=%22f...w=1920&bih=915

 

Greenslime is a much better 'quick fix it' substitute to fix-a-flat. Much easier to clean up and won't harm sensors. Only issue is I believe the entire tire needs deflated to use it, unless over the years they made a spray can type. Either way, at least you have an emergency option. I trust my Load E tires too much.

 

Are you experiencing the low pressure always w/the same tire? You may have a slow leak that may need to be checked out.