Jonttt's 993 C4S Journal

jonttt said:

TPMS

Stage 1 - Sourcing and Tyre valve stem preparation


Tyre Pressure Monitoring System (or TPMS) is a technology that is becoming more and more common place.

Back in the late late 80's early 90's it was a new technology. In fact the Porsche 959 was the first passenger car fitted with a TPMS system.

Early basic systems saw some luxury cars fitted with a system that measured the rotational speed of a wheel and could detect a tyre that was losing pressure from a reduction in overall diameter and subsequent affect on rotational speed (ie a smaller diameter wheel has to rotate faster to to achieve the same linear speed). This basic system is known as iTPMS (i for indirect as it uses the ABS wheel speed sensor fitted outside of the wheel itself so cannot provide actual pressure readings) For example my 2001 BMW Z3m has such a system fitted but it was only during the last year or so of production of Z3's that the system was fitted.

The technology has moved a long way and now many cars are fitted with systems that can accurately measure and display both tyre pressure and temperature whilst providing warnings to drivers beyond predefined boundaries. So far as I know TPMS will be mandatory on all new passenger cars in the near future.

I learned a long time ago the benefits of correctly set tyre pressure especially on performance cars and not just for the obvious reasons but it can have a massive effect on tyre wear. I once went through 4 sets of front tyres on a Mini Cooper S Works in 20,000 miles only to discover the tyre pressure had been completely wrong all the time !

The end result is that I am now paranoid about correctly set tyre pressures. Given some of my cars are nowhere near daily drivers and I feel as though I am checking tyre pressures manually every time I drive them.

So when I purchased my 993 one of the first projects I set myself was to identify and install a TPMS system.

The industry standard way of fitting a TPMS system (including OEM) is for small lightweight sensor / transmitters to be fitted within the tyre attached to the bottom of the valve stem. There is an alternative method using metal straps which are secured around the alloy (within the tyre). I did not really explore the metal strap option as I wanted to try an OEM style install.

Initial research showed that the first stumbling block I would encounter would be due to the fact that I have Hollow Spoke alloy wheels c:
These use a none standard tyre valve stem which is both longer and thinner than "normal".

As most TPMS kits come with valve stems adapted to secure the TPMS sensors onto then I would need to find either a suitable valve to fit or adapt the Hollow Spoke Stems.

I narrowed the choice of TPMS to the TyreSure (a rebranded Orange) system. This is a few years old now but well proven, they manufacture OEM sensors (not Porsche) and their retrofit kit is offered for sale by Porsche resellers.

The UK retail price for the system is c£170 but I managed to secure an Ebay purchase (via Malta) for <£100 delivered of a new sealed kit.

It is worth noting that the x4 tyre sensors / transmitters have built in batteries which are none replaceable ie you have to replace the whole sensor when the battery goes. They are marketed as having a 7 year operational life but I am always sceptical of such claims as they tend to be maximum in ideal conditions etc... Plus they have no "on/off" switch and I do not know how they "sleep" etc... (edit: I believe they "sense" rotation) so I have taken a chance purchasing what could be old stock with a shorter shelf life. But for my initial trails I was happy with the significant discount.

TPMS system received....




The kit is made up of x1 display unit for inside the car with a USB cigarette socket power lead, x4 sensors / transmitters for attaching to the tyre valve stems, x4 adapted standard tyre valve stems, x4 securing screws.




My car has had the standard 993 4S "solids" upgraded at some point in its life to "Hollow Spokes" but there is no history to show when this happened. I therefore have no ideal how old the current valve stems are. As these special valve stems use rubber seals I decided it would be easier / quicker / safer to buy x4 new valve stems from Porsche...... but they are not cheap so factor in c£40 for a set of 4.




So I set about closer examination of how the kit is supposed to work and how I could get it to fit to the Hollow Spokes.

This is how a kit valve stem fits together....








Essentially it is a standard valve stem with a ball socket at the end. The ball socket has a hole drilled through it (to maintain air flow through the valve) which is tapped to accept a screw. A hollow (to maintain the air way again) screw is then used to secure the sensor / transmitter to the valve stem.


If you compare the kit valve stem to the OEM Hollow Spoke stem you can see that the stem is both too wide to go through the valve hole in the Hollow Spoke alloy and it is not deep enough to sit on the outer and inner edges of the alloy.






I had read on the internet that some people had got around this problem by fitting the ball joint from the kit valve to the OEM valve but this would mean a much longer securing screw would be needed.

I did not see the need for this. Although the OEM valve does not have a ball joint it was basically the correct diameter to fit into the socket on the sensor and allow rotational movement for adjustment when fitting and be secured by the screw. So I thought I would give a direct connection a go by simply drilling and tapping the OEM valve......




There was a big unknowns ie would the OEM valve accept a 4mm wider hole and retain integrity ? Only one way to find out......

Ideally this would be carried out with a bench drill but I don't have one so a vice would have to do......

I started off by drilling a "half way house / pilot hole" using a dremel and 3mm drill (the biggest I had for a dremel).




This went through easily and you realise there is an internal "step" in the hole diameter so the 3mm drill only needs to pass through a few mm.

I then used a standard drill with 4mm bit. Again internet research implied a 10mm hole but I was worried that this would affect the strength of the valve stem so I went with what I thought was enough to allow secure attachment of the screw.






The drilling process went well and I am as happy as I can be that there was no detrimental effect on the strength of the valve stem from the increased diameter hole.

So it was then a striaght forward process of tapping the hole with an M5 x 0.8 thread.....






Once tapped the securing screw (using a T20 torx driver) could be tested to ensure enough thread to secure (and to help clear excess metal swarf)....






It was then a matter of trial fitting and to see if the unit could be put together and secured without the need for a ball socket....... I found that the securing screw actually ended up being too long. Rather than drill the hole deeper and risk weakening the valve I decided to simply pad it with a nut.

Once tightened this worked really well. The unit can be tightened as securely as the original kit valve.....




So it was simply a matter of repeating for the other 3 units....




So that leaves 2 stages to go:

1) fitting of the tyre valves (requires tyre fitter to remove tyre, fit new valves with sensors attached, refit wheels and balance

2) fitting of the display unit inside the car (I will trial via cigarette lighter socket and then hard wire in)

I'm off on holiday for a while though and my car is still having some jobs done on it so those stages will need to wait a few weeks............

ps I have no idea how easy it is to fit the kit valves to none Hollow Spoke 993 Alloys (eg Cup II's or solid spoke turbo twists). If they fit then obviously none of the above adapation of OEM valve stems is needed ;-)

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