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lj_cox
06-03-2020, 12:36 PM
So yesterday we took the fifthwheel in to have new tires put on at a tire shop we’ve used before for the truck (Sailuns from SimpleTire, for the curious.) The guy who checked us in was fine, we discussed not jacking on the axles and they worked out a safe way to change the tires, he reminded me to pull the tire monitor valve caps, great. After he stepped away I remembered I hadn’t mentioned inflation pressure so I told the mechanic the tires needed to be inflated to max pressure: 110 psi. He said yeah, with these heavy units they need to be at max. Plus, I say back, they’re ST tires – they need full inflation no matter what load. All good, tire change goes well, we get the receipt, we get in the truck, we go to pull out… and the tire monitor is going nuts! BEEP BEEP BEEP! All tires are LOW! 85-87 PSI! Flashing lights! BEEP BEEP BEEP!

I say well this is bulls**t, I told the guy 110, obviously the tire monitor is wrong, and restarted it. Nope; BEEP BEEP BEEP! Ok, well, we have the 30 second rule. If something is concerning about the rig, take 30 seconds and check it out. It’s likely nothing, but it’s only 30 seconds for peace of mind. So I shut off the truck and get out and get the air compressor inflator out and check a tire. 90 psi. Now we have a problem. Back into the store, wait for someone to free up. “Hey I told you guys 110 and my tires are at 90, WTH?” “We don’t inflate anything to 110. Our max inflation is 90.”

I wouldn’t have been upset with that, except for the fact that they DIDN’T TELL ME THAT UP FRONT. Said something to the mechanic and he’s like, oh yeah, 90 is the most we do. Do you want me to get someone? I’m like no, we’ll do it ourselves, hooked up the air compressor and aired all the tires to 110 (took about 15 minutes, nbd) while DH went in to ask them WTH?!?

Turns out they CAN inflate tires over 90 psi. They just have to use a manual system instead of the automatic system, and apparently the gauge is broken? Only one guy seemed to know about this super sekrit method. He offered to fill the tires the rest of the way but by then I was almost done. We finished up and left in a pale cream colored huff.

Lessons learned: 1) I am SO GLAD we had the tire monitor. 2) The 30 second rule, well, rules! And 3) our TPMS runs about 3-5 psi low (within tolerance), but the air compressor gauge is spot on (good to know!)

DmaxDually
06-11-2020, 08:25 AM
Just curious, how do you know your air compressor gauge is spot on? Do you have a calibrated test gauge to compare it too?

I ask because this is what happened to me and it confirmed what I already knew. That is, tire pressure gauges are not accurate. They can be off by only a couple of PSI or up to 10 or more psi.

I purchased my camper used. It was a year and a half old and in like new condition. It had the factory Marathon tires on it. I knew about the problems with the tires. So I kept a close eye on my tires and pressures to make sure they were always at or close to 110 psi.
After about a year or so and a couple of local camping trips I had one of my tires fail. Luckily I was on a local road with stop lights and not the highway. A guy pulled up along side me at the light and told me I had a flat. There was no damage at all to the camper.

Before my next trip I replaced all my tires, including the spare, with Good Year G614's. I also installed metal valve stems to accommodate the TST TPMS with the flow through sensors which I also added to all my tires including my dually TV.

I purchased the tires online (Simple Tires) and I did all the work myself except for actually mounting the tires on the rims. I took the rims to a local tire shop to have them mounted and balanced.
After all was said and done, everything aired up, 110 psi, TPMS temp and press warning levels were set. It was time to check the system. I turned on the TPMS and let the tire temps and pressure stabilize for a few hours.
I came back to check the readings and found that the reading on the TPMS were reading anywhere from 100 to 102 psi.

My first thought was the TPMS was junk. So I took out my gauge and rechecked the tires. The pressure were reading 108 to 110 psi on the gauge. I'm think I just wasted almost $400 on a TPMS..
I have a few different tire gauge and decided to check the tire with each one. None of them matched. They were all off. Some read higher and some read lower.

To make a long story longer... After some research I purchased a calibrated tire gauge check station. I checked my tire gauges and no surprise they were all off anywhere from 8 to 12 psi..I got rid of all those gauges and purchased a couple of good quality adjustable inflator gauges. I checked and adjusted them with the check station. Inflated all my tires to 110psi using the new gauge and now my TPMS is reading 108-110psi.. within the designed 2 to 3 % error limit..:thumbsup:

lj_cox
06-11-2020, 08:34 AM
It's kind of recursive: I know the tire gauge is spot on because they told me they inflated the tires to 90 psi on the automated system and that's what it read on all 4 tires.

(Also I've taken it in and had it checked in the past.)

On my TPMS, reading 3-5 psi low is within tolerance. So I'm ok not fixing that, just allowing for it when I check the tire pressures using that system.

Montana Man
06-11-2020, 08:39 AM
The same can be said of torque wrenches. We torque wheels and such with tools that usually are not calibrated.

PSFORD99
06-11-2020, 08:48 AM
The same can be said of torque wrenches. We torque wheels and such with tools that usually are not calibrated.

Very true, I had to have our torque wrenches recalibrated every six months, we used them to torque the bolts on breakaway sign post on our roads ,and freeways, they had to be right on, to function properly.

PSFORD99
06-11-2020, 08:56 AM
It's kind of recursive: I know the tire gauge is spot on because they told me they inflated the tires to 90 psi on the automated system and that's what it read on all 4 tires.

(Also I've taken it in and had it checked in the past.)

On my TPMS, reading 3-5 psi low is within tolerance. So I'm ok not fixing that, just allowing for it when I check the tire pressures using that system.

I compare my old stick tire gauge to my truck TPMS ,and its right on almost to the psi. My TST is usually reading about 3 psi different then my tire gauge . I couldn't tell you how old that gauge is, its the one that just pushes up the stem with the readout. I have used it for years, and will continue too.

I check mine by comparing to my truck readout, still continues to be accurate enough. IMO the TST 507 is off on the readout, but 3 psi is no concern to me, even as much as 5 psi is not a problem.