chilledbenz

I've seen some questions on the forum about changing rims and tires and what to do with the tire pressure sensors. When I had new tires installed on my 06 ML I told the guy replacing them to be careful of the sensors, but when he removed the tire there wasn't a sensor inside the tire or on the rim itself. So I did some research and heres what I came up with. This fits my system exactly because the one in the ML doesnt tell me the exact pressure in each wheel, rather it just says if one goes down. If the 08 C system has changed and now tells you the exact pressure in each tire ignore this thread. Hope this helps those thinking of getting new rims or tires.

http://www.tirerack.com/tires/tirete....jsp?techid=44

Indirect Monitoring Systems

In the interest of providing a lower cost Original Equipment system, indirect tire pressure monitoring systems were developed by vehicle manufacturers wishing to comply with the law while minimizing development time and cost. Indirect systems use the vehicle's antilock braking system's wheel speed sensors to compare the rotational speed of one tire vs. the other three positions on the vehicle. If one tire is low on pressure, its circumference changes enough to roll at a slightly different number of revolutions per mile than the other three tires. Reading the same signal used to modulate ABS systems, the vehicle manufacturers have programmed another function into the vehicle's onboard computer to warn the driver when a single tire is running at a reduced inflation pressure compared to the other three.

Unfortunately, indirect tire pressure monitoring systems have several shortcomings. Indirect systems won't tell the driver which tire is low on pressure, and won't warn the driver if all four tires are losing pressure at the same rate (as occurs during the fall and winter months when temperatures turn colder). Additionally, our current experience with indirect systems indicates that they generate frequent false warnings. We have found that the false warnings occur when the vehicle is driven around a long curve that causes the outside tires to rotate faster than the inside tires, or when the tires spin on ice and snow-covered roads. In both of these cases, the false alarms would train the driver to disregard the tire pressure monitoring system's warnings, negating its purpose completely.

zakklp

Just checked mine. Amg 18's Front 31 rear 36. They were close, but I set them to factory specs. Anyone know about how much it cost to have them filled with nitrogen?

Chaz00Blue

I don't know, but that is a great idea. You will have much less fluctuation of pressures based on weather and if you buy it dry it also eliminates any issues with moisture.

Kev204

Do you guys know if we can change the wheels and tires without putting the tire pressure sensors?

RLE

<<Do you guys know if we can change the wheels and tires without putting the tire pressure sensors?>>

Yes, you can. If the receiver in the trunk doesn't receive any signals, it's happy.
The factory wheels have a little support boss inside that each sensor sits on. The valve stem and sensor are a unit. The big AMG logo opposite the valve stem on the AMG wheels compensates for the weight of the sensor assembly opposite.

chilledbenz

So are you saying that the w204 C has the system where it tells you the tire pressure in each individual wheel? If so thats pretty cool.

jrct9454

The W204 uses active sensors, which means one attached to each valve inside the wheel, and yes, they measure each separately. And they are ridiculously expensive to replace, so take care when opening up a tire that uses this system.

chilledbenz

Big thanks, hope I don't have to replace any tires on it but if I have to I now have the extra info.

RLE

<<The W204 uses active sensors, which means one attached to each valve inside the wheel, and yes, they measure each separately. And they are ridiculously expensive to replace, so take care when opening up a tire that uses this system.>>

Since I've swapped wheels and held a sensor in my hand, they are all identical and not assigned to any particular wheel. They do not indicate which tire is under inflated.

The sensor and valve stem are one unit ans sit on a boss inside the wheel. They are held in place by a nut on the outside of the threaded valve stem and are sealed by an o-ring.

What has occurred to me that if sensors are not installed, the wheel will require considerable extra weight opposite the valve stem for balance.

So, when I use my C300's OE wheels for winter tires, I will find out how much of a problem this will be.

peabers

I'm really disappointed that the W204 does not give actual measurements at each of the wheels. My MIL's $46K RDX has it, and helps to tell me which tyre I should be pumping. She'll even watch the readings from her dash and tell me when to stop -- how helpful.

So it's about as good as a "Check Engine" light... umm, yep, the engine is still there.

jrct9454

And I find the whole tire pressure monitoring business a colossal waste of money and a needless complication....all because some people managed to kill themselves by refusing to check tire pressures regularly.

I remember during the Firestone / Explorer fiasco, which led directly to the TPMS requirement in the US, watching one after another of these 5000 lb missiles going by with tires that were CLEARLY underinflated...so you could see it from 50 yards away....and this was when the story was on the TV news nonstop for a month.

I swapped the tires/wheels on our '06 Prius, storing the originals until resale, and simply cut a small piece of black tape to cover the continuously lit warning light....the tire gauge works just fine....

peabers

I believe the introduction TPMS was necessitated by RFTs ....

IMHO, checking tyre pressure should be standard practice, along with topping up windshielf fluids. How hard is that?

People how don't take care of their cars deserve to be sitting in the ditch on the side of the highway. They should not be driving... as a matter of fact.

Bokx_350

I agree that checking tire pressure should be common knowledge for any driver. You can buy a digital tire pressure gauge at Wal-Mart for $10.

I was getting an “adjust tire pressure” msg. I checked tire pressure (with my $10 gauge) and I had 27 psi on both front tires and 30 psi on both rear tires, so I decided to run some test to understand the tpms better. I deflated the front tire to 10 psi, started the car and immediately the screen by the tach went red with a warning “low tire pressure”. This warning was different than the one before, which was ADJUST tire pressure (and this first one didn’t turn the screen red). I inflated the tire back to 27 psi with a 12v pump and then adjusted the system (through the menu on the cluster) to take the current psi values as the standard. Everything work fine, no more msgs, and I feel safe having tested the system and knowing that it works flawlessly.

chilledbenz

great info! So there is a way to set your own psi as the default setting if I understand you. This is good, because I never end up running my tires at the o.e.m. settings, they always ride way too rough for that.

Bokx_350

Yes, through the menu on the steering wheel.

There are only 2 minor limitations on the system:
1. It won’t tell you which tire has lost pressure, so you must check them all (I would have done this anyways).
2. It won’t display the current psi, its either OK or “low tire pressure” warning.

This really doesn’t affect its functionality at all.

SilverMPower

I set my cold tire pressure to 33/38 per spec. As the tires heat up, they read around 36/40. Does this sound right? I am getting TPMS alerts and am wondering if maybe it's telling me I have too much air???

stealtheo

Took delivery of my C350 in July. Had same issues as some of the post above. Had the car in for the first 1k-3k service and when I picked it up the paperwork stated that they replaved my TPS control unit. I asked the dealer about it and told me they had some issues with them, and even though when I took it in I was not having the problem they replaced it anyway.

JimPap

Even finding nitrogen can be a pain. In my market, only Costco and one tire store even has nitrogen and Costco will only let you do refills if you purchase your tires from them. I wound up leasing a nitrogen cylinder from a welding supply house and buying the regulator and hose.

My experience has been that tire pressure varies from cold to hot by 3 psi using the original factory tires and nitrogen. By the way, the air in my tires was originally replaced with nitrogen by my MB dealer. Its part of their dealer prep.

In the C300 owner's manual, it states that tires which change by 4 psi when going from cold to hot, meaning same ambient temperature but from having sat for more than 3 hours to having driven them a few miles. I'm sure they're referring to tires inflated with air. Also, with air, inflation pressure varies 1psi for each 18 degrees in outside temperature which brings up another question about inflating your tires in the morning and what to do later in the day when its 30 degrees warmer. That's our normal temperature swings here.

Now here's the rub. With nitrogen it's a 3 psi difference, with air its a 4 psi difference. It looks to me that you still get most of the shift in tire pressure for a lot more hassel trying to get nitrogen.

The other selling point of nitrogen is that it doesn't seep through your tire as fast (larger molecules). I've heard that you loose air at something like 1 psi per month. Nitrogen is suppose to take longer. Now here is where I see the only advantage with nitrogen. With my original equipment tires, I can tell a difference when over inflating (harsh ride) and underinflating (not to mention you probably wear out the tires early) by as little as 1 psi in either direction. Provided you really do loose inflation slower with nitrogen, its not like you're checking your tire pressure every couple of weeks but can be a bit less compulsive obsessive about it.

MBTex

After the cold front that came through this past week (Temps down to 50 down here...brrrr), my TPS system has started to act up giving me warnings. I've checked all tires (including the spare) and have determined that it is simply due to the change in tempurature. But the warning keeps going off everytime I start the car now....

peabers

It's around 0 here (that's 32 to you people south of the border), and I've not had any problems with my TPMS.

V8 AMG

Hey guys, i heard somewhere that the TPMS system on the W204s dont use any RF sensor inside the wheel. Instead, it supposedly uses the cars ESP system to detect if any wheel is rolling with a smaller circumferential diameter; hence indicative of pressure loss. This then triggers the Check tyre pressure warning.

If I am wrong and it does use a sensor, can someone take a post a picture of it as i'm curious to see what it looks like. Also, the guys at the tyre shop tells me that any car that uses the RF sensor system will generally have a metal valve stem protruding, so can someone also take a close up picture of their tyre's valve stem too.

Thanks.

JimPap

Sounds like you crossed the threshold for when the signal goes off. During the day when it has warmed up, does it still go off?

MBTex

Nope, just when it is cold. It has warmed up to the 70's in the afternoon here and I have no problems then. But, sure enough I get in it this morning (temp 46) and it goes off after about a mile of driving.

I'm thinking I may need to adjust the settings on it between summer and winter. The problem is that where I live it can be 35 degrees one day and 75 the next.

peabers

V8 AMG -- this idea has been posted on this forum before... but so far all the people who have replaced their tyres/rims have had to deal with an actual piece of sensor hardware... so I am not sure if the idea of ESP detection is true.

JimPap

Sounds like the system was set when the tires were hot and on a warm day.

Time for a reset. Isn't it under menu accessed by the "system" button?

Also be sure to check your inflation pressure before resetting it.