A Tire Pressure Monitoring Saga

[From Craig] Before last April's Alfa Owners National rally, my wife and I agreed that a tire pressure monitoring system (TPMS) would be a worthwhile thing to buy. I asked people at the rally and elsewhere on the internet, which manufacturer to buy, and got a bunch of good responses but no clear favorite. 

Finally I decided on the Truck System Technologies (TST) 507 system because:

1. Their specs are the most accurate (in the RV tire pressure range, see below).
2. Their sensors monitor both pressure and temperature.
3. They offer both flow-through and cap models.
4. Their sensors are lighter or comparable in weight vs. competing sensors.
5. Owners can replace sensor batteries, at a recommended interval of 1 year.
6. Their customers said TST has excellent customer service (I agree).

I ordered the system directly from TST, flow-through sensors for the RV and caps for our Jeep, plus the indicator screen and a repeater. Ordered 6 of each kind of sensor because I couldn't decide whether our outside-dual tires could be filled using a flow-through sensor. It turned out they can, and I returned two cap sensors for a full refund.

Placed the screen in an unused space on the dashboard and hard-wired its cord to the back of a nearby 12V power outlet. Installed the repeater in a cabinet above the bed, and wired its power cord to a light in the bottom of the cabinet. Paired each sensor to its position on the two vehicles, and installed them one at a time.

On installing the sensors on our Jeep, it turned out that they register quite a bit lower than the Jeep's built-in TPMS, like 28 PSI vs. 36. Our several stand-alone gauges agreed with the Jeep sensors, so I think that the TST sensors' excellent spec of +-1.5 PSI applies only in the range of truck tire pressures, like 80-140, and rolls off to lower reading in the range of car tire pressures. This doesn't seem like a deal-breaking flaw to me, since the purpose of a TPMS is to give early warning of problems while driving.

Also, the Jeep came with rubber valve stems rather than metal, which means the sensors' temperature sensing function relates more to the ambient air than the air inside the tire. Readings while driving confirm this difference. I could have had a tire dealer replace the OEM valves with metal-stemmed ones, but then I would have lost the Jeep's TPMS sensors, which would be hard to explain if it ever comes time to sell it or trade it in.

The final problem related to the inside-dual tires. Our Alfa came with 6 inch brass extenders on the inside duals. With the original tires, the extenders touched the holes in the wheel covers, and the first 24K miles of vibration had worn deep gouges in the extenders. This contact was fixed when new tires were installed, but the flow-through sensors again touched the wheel covers. I was afraid of either vibrating the sensors into premature failure, or breaking the gouged extenders with the added weight of the sensors. First I ordered 4" valve extenders, but the tip of the flow-though sensor still touched the wheel covers, so I ordered 3 inch extenders. These were mechanically right, but resulted in air leakage when used with the flow-through sensors. 

After some experimenting with the help of TST's VP and chief troubleshooter Mike Benson, we found that attaching the sensor to the tire valve, and the extender to the top of the sensor, eliminated the leaks. This suggests that the female side of the flow-through sensor does not allow full seating when used with an extender that had a slightly larger outside diameter on its main stem than on its threads.  This arrangement of the sensors and extenders also has an anti-theft aspect, because it's vary hard to see the sensors way back inside the width of the tire.

So far I'm pleased with the operation of the TST TPMS. One startling thing is how high the pressure in the RV tires rises while driving, although they do stay below the +20% warning threshhold suggested by TST. Tire pressure increases much less on the towed Jeep.  Final judgement will come after several months of use.