I have a 2017 XLT with the TPMS set to 35 psi. The spare doesn't have a sender, so I am going to put a sender on the valve stem for my TPMS on my travel trailer. (At least when towing the trailer I will know the truck spare is inflated, or I can turn on the trailer TPMS occasionally just to check the truck spare.) What is the upper and lower pressures at which it will alarm? I want to match those numbers on the spare.

 

There is no "alarm" limits. The new TPMS sensors send an actual PSI reading to the truck, unlike the old sensors which just had "too low" and "too high" signals.

 

The sensors always provided pressure, the system was set to show a too low or too high warning at certain pressures.

 

Can't speak for Ford, but not all sensors worked like this. 1999 Grand AM definitely did not. It just routinely sent "ok" or "not ok" signals. I'm sure all modern sensors and aftermarket TPMS systems send full pressure reading now, but not all older ones did. There were even indirect TPMS systems that didn't read pressure at all, but instead would notice if one tire was spinning slightly faster via the ABS sensors, which would indicate a low tire


I'm looking at aftermarket systems for my travel trailer so I can stick a display on my dash as we have some long trips planned. Since it's a tandem axle, I may not notice right away if one of the tires starts getting low. Seems all the ones being sold now show PSI readings. Not sure what kind of system OP has though. Transmission frequency will also need to be checked.

 

Much simpler solution. Drop the spare, turn it over, rehang with valve stem down, now it is simple matter to check the pressure with a gauge when you check the other tires. You do check the other tires don't you?
Side note I run my spare at 10 psi higher than the tires on the ground so if it loses a little air no big deal, if I ever have to install it I will bleed it down to correct pressure.
Side note 2 If you have a Discount Tire store near you they have a drive thru lane to check and adjust pressure for free I use it once a month.

 

I personally never check the spare other than giving it a kick now and then to make sure it's not so low that it'll come off the bead. I always have my electric air pump in the truck at all times.

 

OP here. So, unless I actually have the tire pressure screen showing on the dash, the truck has no way to alert me that a tire is low? That doesn't seem right. It seems like it should have something to get your attention.

The TPMS system I have on the travel trailer is a TST 507RV system. I have used it about 6 years. It works great. It saved me from a possible RV fire once when the Class C I had back then had a brake caliper dragging. The wheel was so hot when the TPMS alarmed and I stopped that it was smoking.

 

No what I meant was some older sensors worked different that new ones. Our trucks show the actual PSI. Older vehicles just had a light that came on when the pressure was too low or too high. Your system is new and shows the PSI. However, it might not run on the same frequency as the OEM Ford sensors, or might just be incompatible altogether. That's something you'll have to look into.

 

Actually, my question is with the pressure set at 35 psi, what pressure must the tires get down to before the truck says "Ahem, you have a problem."

 

Good question, I've thought about that also. Especially knowing that the set PSI can be changed in FORScan. I looked in the service manual and although there is a lot of info about the TPMS system, I didn't find anything specific about a PSI trigger point. It only states that in the event of a tire being significantly under inflated.

I have attached a part from the manual about this. Not that I'm one of those RTFM guys, because I don't lol. Unless I am looking for something specific. However, this thread has prompted me to look into the TPMS system, and there is some interesting info I didn't know. For example, if the low tire light stays illuminated, of course you have a low tire. But if it flashes at startup for one minute then stays lit, this indicates that you have a faulty sensor. Battery could be dead. The sensors do not turn on until you reach 20 MPH for two minutes.

Please, no lectures from any Manual ***** about RTFM. lol