carsnob

Kind of a minor thing but interesting...
You know those nice wheel/valve stem covers that the dealer parts dept. sell? They're made of metal and with the MB logo give a nice little detail look to your rims. They're a bit bigger than the usual valve stem cover.
Turns out that if you have the tire pressure monitoring system (standard on the '07 600's and perhaps standard on the '08 550's), then you can't use them! They require plastic valve stem caps!
Apparently, there was some memo from MB head office regarding this.
Something about the "antennae" of the tpm getting thrown off or mislead by the metal of the stem cap. Supposedly the tpm's have some sort of battery in them.
My question is if the whole rim is metal, (alloy) then how can a little metal stem cap throw off the tpm system?? Wierd....

whoover

The real reason they don't recommend them is bimetallic corrosion. Unlike old brass valve stems, TPMS stems are aluminum and steel caps can corrode the aluminum, causing the caps to weld on. This is more a theoretical than real problem in California (salt would accelerate the corrosion, but we never see any) so I ignore it. I've got metal caps with more than two years on them, with no problems. It has nothing to do with the transmitters.

WSH

Interesting....

Perhaps proximity-related phenomena...

After all, DistPlus/Parktronic can occasionally (perhaps once every 6mos in my expce) be set-off into temporary (<5mins) malfunc mode b/c of various extraneous electronic signals....

Would like MB/AMG to offer on-board tire temps, in add to pressures (like 599 has)...not very relevant for me, as I have some 10mins of slow urban driving before getting on a fwy to warm up tires (esp on a bitter cold 45F winter CA AM)....but perhaps more relevant to guys who live in suburban locales very close to mtn twisties, where early AM temps are borderline vs perf tires and their trac vs temp curves....

WSH

Ahhh, thx for explanation, whoover....

Nonsense we don't deal w/in CA, along w/those girlie winter tires/wheels.....

carsnob

Whoover, you're right about the corrosion! I live in the great white north and in Toronto there's lots of salt. When I changed my snows and removed the caps, I had to use a pliers. Couldn't figure out how they would be corroded on.
Turns out your explanation makes perfect sense!
Come to think of it, the tpms worked fine all winter hence that bi-metal thing must be the issue. I wonder if a little wd 40 would offset the bi metal reaction...
Anyway those little stem caps do offer a nicer detail...

EinMD

I had the same issue where the valve stem covers fused onto valve stems. The salesman gave them to me when I purchased the car. Then when I went to put air in the tires I could not take the covers off. I went to my service advisor who told me about the issue. They wanted to charge me for labor to R&R the tires and replace the valve stem. I told them they MUST be kidding me. They ended up replacing them under warranty.

Z356

I also have a MY07 BMW 650 with a version of the Tire Pressure Monitoring system like one on my new '07 CL600. It requires special plastic caps too. Regardless of what theories you believe (corrosion, antenna interference), I would recommend to you all with TPM in your wheels to follow factory recommendation and keep the plastic caps ON. Unlike the MB and Porsche system where the TPM displays actual tire pressures, the BMW system displays 'green' tires on a virtual car sketch, which shows you that tires are within the pressure range you inflated them. They will turn color (guess to red/orange) if they are losing pressure from their original 'setting'. Believe the US DOT has set a requirement for all manufacturers that want to sell cars in US that my certain date all vehicles sold here most have a TPM system to alert American drivers about low tire pressures. Besides the obvious safety ramifications (puncture alerts), its purpose is mainly to have owners keep tires better inflated to increase rolling efficiencies, decrease gasoline consumption and help us import less foreign oil. One of the few regulations from these folks that I think it's really smart. Z356

whoover

The "theory" matters because, if you live in a salt-free environment, you can fit any cap you want to and not be stuck with the ugly plastic ones. I've had mine for two years, and they've never shown any signs of problems. The multi-part S65 wheels look way better with a stainless cap that blends in with the rim bolts. Moreover, I'm a stickler about tire pressure and would know very quickly if the TPMS was acting up. I often monitor it to watch pressure variations as the tires heat up during cornering. As any sticker will tell you, unless you're topping up your tires once a month, you're not a stickler. Which is another reason I can be sure that galvanic corrosion is not a problem in electrolyte-free California.

It's always good to know the facts.

AustinGuy

Okay, so what's the verdict? Can you use the brass/metal stem caps or not?

I'm on the verge of getting some of the same brass/metal valve stem caps and want to know for sure before I purchase...

Oh, BTW, has anyone bought stuff from silverstaroutfitters.com? Any experiences? Their prices are A LOT lower than MB

carsnob

Good comments guys, much appreciated. Seems like the nice steel decorated caps would be no problem based on a few of our experiences.
As for the bi-metal/corrosion thing, then maybe to be sure when detailing your wheels, it probably wouldn't be a bad idea to unscrew the caps and put them back on every once in a while. Or better yet as Whoover does, check your air pressures anyway which seems like a good idea to see how they're doing plus that keeps the tpms honest.
...so Austin Guy, since you're on the "verge" of this big committment to get those stem caps, I would say the verdict is -- take the plunge and go for it!
wtf...

EinMD

For the $15 it costs for the valve stem caps, it can cost you a lot more in time and aggravation. I had them and they fused on within a month. I live in the DC area. Not worth it.

whoover

Galvanic corrosion requires an electrolyte, normally salt. If you live in an are where it snows, metal caps are a bad idea on TPMS stems. If you don't, they are not likely to be a problem. It's not rocket science, just simple chemistry. See "lasagna cell" in http://en.wikipedia.org/wiki/Galvanic_corrosion .

The steel pan and aluminum foil are the cap and stem. The salt on the road is the tomato sauce.

AustinGuy

WTF?