Installation of SmarTire

Active Tire Pressure Monitoring System

in a 2001 Porsche 996 Turbo

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SmarTire web site

January 25, 2002
By Greg Heumann

It can be very difficult to feel a low tire from the driver's seat, or even see it upon visual inspection. Low profile tires with stiff sidewalls compound this problem. Yet driving on a low tire produces excess heat which temporarily raises the pressure in the tire, potentially masking a serious problem. Driving on a very low tire can quickly ruin it. Yet in many cases, a slow leak due to a nail or screw is easily repaired. An early warning to low tire pressure certainly increases your margin of safety.

The SmarTire system is a pressure monitoring system that can be installed in virtually any car. Pressure senders in each wheel are activated by centrifugal force as soon as you start to drive, and send information to a central receiver. The system alerts the driver to low pressure in any tire. If it saves one tire, it practically pays for itself.

The "basic" system includes the senders and receiver, which has LED lights and a horn to alert you to low pressure. I opted for the additional "full function remote" display, which displays much more information. (See Driving with SmarTire)

Step 1: Install Sending Units

The sending units mount on the wheel, so you must have your tires dismounted and then remounted and balanced. The senders are color coded, and if you install the right color on the right wheel, you'll save yourself some steps later. If you were to rotate your tires, you would need to tell the system which sender is where, so there is a "learn" facility to handle this situation. However, the system ships with the system expecting a particular color at each corner, so this step is not necessary for the initial installation. The "hose clamp" is included with the system. Although it is difficult to see in this photo, the clamp and sender are riding in the wheel's "drop center", which is lower than the rest of the wheel. The Porsche wheels are tapered, so the sender could not me mounted in the center.

The installer must be careful that the tire installation equipment will not damage the sender during installation. Further, when the tire is removed in the future, you must ensure the technician knows where the sender is located, so that he breaks the bead of the tire from the rim at a different location. For this reason, the sender is mounted near the valve stem on each tire.

The sender weighs about 2 oz; my wheels each required 2 oz. of balance weights at 180 degrees to the sender to achieve proper balance.

Step 2: Test the System

The "basic" SmarTire system needs only to be plugged into your cigarette lighter to operate. The Full Function Remote display plugs into the receiver. I left these devices unmounted to ensure that my choice of receiver location would be free of any interference, and to see how various mounting locations for the display would work.

The photo to the right shows my "test setup", and below, another location I considered for the display.

After driving with the system both during the day and at night, I finally settled on yet another location. Although once the novelty wears off I know I don't need to see this all that easily, this location is nicely visible. It is mounted with velcro, and can easily be repositioned. I don't carry passengers too often in this car, and only the very tallest off them might have a problem with their knees hitting the display. If this is a problem I will simply add another path of velcro a little more out of the way, allowing me to temporarily reposition the display there.

Once you have identified mounting locations you are satisfied with, it is time to hardwire the receiver to 12V, and hide the wires.

Step 3: Remove the leather cover

This cover is hooked over the edge to the rear of the car, and snaps in toward the front. Pull from the edge closest to the front of the car and swing it out slightly to remove.

Step 4: Remove the carpet panel

The carpet panel clips are also at the front of the car; the rearward edge of the carpet simply tucks under the plastic. Pull out at the front, then slide forward.

Step 5: Expose the power source

Back in the good old days, I'd have gone hunting through a bunch of wires looking for a source of switched 12V. Thanks to the internet, this time I just asked "where can I find a good source" on the Funcarsonline Porsche Board, and in 30 minutes I had the answer. This harness is evidently there to power the optional factory phone. The left-most wire is green with red, and has 12V present all the time. I chose the green-with-black wire which has 12V available when the key is on only.

Step 6: Splice in the SmarTire receiver power

I didn't order the optional power cable, so I simply checked the polarity and output voltage of the cigarette lighter cable that came with the basic unit, and then cut it. The photo doesn't show that there is a black wire in addition to the white one; and it doesn't show that other side of the butt connector has both the white wire to the receiver, and the green-black wire from the harness going into it - but that's what's there. I opted to cut the green-black wire, and twist the white and one side of the green-black together, then crimp them in this connector, because the white wire is a bit too small to work reliably in the popular "in-line tap" style connectors.

Crimp a horseshoe lug onto the black wire. Loosen the bolt there that is handy (10mm), insert the lug and tighten again. (The 11mm nut on the back side is a fixed part of the steel riser - you don't need to get a wrench on it.)

Step 7: Mount the receiver, clean up

The receiver is mounted on top of the existing velcro that holds the carpet sides together, using more velcro tape. I like using Velcro tape when possible - the unit won't go anywhere, but is easily removed later, and you don't risk creating any rust spots, leaks, or accidentally puncturing the brake line on the other side of the bit you just drilled through for a screwed-in mount.

The cable between the receiver and the remote display is multi-conductor, so shortening it is not worth the effort. This also leaves me free to move the display to another location if I decide I don't like it where it is. Cable ties, please. Lots of cable ties.

Step 8: Reinstall cover plates

Of course you don't remember where that little tab on the front of the carpet panel goes, and it isn't obvious when you go to put it back together. Slide the carpet panel into place with its tab as shown. Feel around behind the forward section to ensure the clips are lined up with their sockets and pop them back in. There is a convenient slot for the cable to make its way out here.

Hook the leather plate over the rearward vertical edge, then snap it into place. There is enough room between this panel and the lower dash for the cable to emerge over the top of the panel.

Step 9: That's it. Time for a test drive!

The system comes pre-programmed with a default cold pressure, low pressure alert level and pressure differential warning point. These are all easily changed, and are well documented in the system documentation so I won't cover it here. However if you're interested to see what the system looks like in operation, read on.

Driving with SmarTire: Proper display

When power is applied, the system will beep once and exercise its display. Very shortly after, the display will blank, showing only the vehicle outline. However the centrifugal switches in the sensors will trigger as soon as you go a half a block. If the system is receiving properly, the four tire locations will turn black, indicating a signal is received from each.

Pressure Reading

Press the top "tire" button to select any tire. Here, the right rear tire is showing 44PSI. (You can also set the system to display pressure in Bar.)

Temperature Reading

Press the center "information" button to choose between pressure, temperature and pressure differential. Here, the front left tire is showing 75 degrees Fahrenheit. (You can also set the system to display temperature in degrees Celsius.)

Pressure Differential Reading

Now this is the least obvious, and perhaps most useful feature. The display says the front right tire is down a pound. But this reading is temperature-compensated.

Suppose I want my front tires at 40PSI, cold. Suppose that, after driving, the pressure display for this tire reads 44PSI, and the temperature reads 102 degrees. Quick - is that right? Is that right?

This display is telling me what I really want to know - that I should add air. The pressure should read 45PSI at this temperature in order to be 40 PSI when cold.

This will make consistently setting my pressures at the track much easier!

Feel free to share this information. It is free! Your mileage may vary.

More pictures of Greg's 996TT

Questions? Email Greg.