940 After market tyre pressure sensors dissapointing.

Just installed an after market kit which, being obsessive about my tyre
pressures, thought this would be ideal. While it does work (on 3 wheels
anyway) it is rather dissapointing.

System: Tyre Alert from Richard Grant Mouldings in the UK (no manufacturer
declared).

1. Valves were difficult to fit and wern't long enough for the wheels to
properly tighten the retaining bolt, I think this was the reason the fitter
broke one! (alloys: 15" multispoke 1995)

2. 2/5 valves wouldn't seal completely, even with the retaining bolt on the
outside well tightened. They've sent me different valves that don't have
nuts and seem to pop in from the inside. They are prettier with chrome
sleeves, so seems protected as long as its real chome and sealed.

3. The original valves were bare aluminium, this is bound to corrode.

4. The accuracy seems abismal. I was slightly concerned when I read the
brochure with 1.5psi tolerance (5C tolerance for temperature), but thought
it that would be a maximum, however my digital pressure gauge which I
thought reads a little high, reads 28.5 when the remote system is reading
30. So its at the limit or more. They all read the same offset.

5. The update is rather slow (from No. 5), and it read 3C for 10 mins when
the others read 10-15C after a bit of driving and braking. Measurement
update is by pressure change apparently.

Perhaps I'm expecting too much and I'm sure these will get better, but seem
to need a bit of development.

 

Just explain this kit a bit more:

These replace the valves in the wheel rims, and you can read off the
pressure and temperature from them? Where do you read it from - the
valves themselves (I've seen something that replaced the valve cap and
showed red if the pressure dropped low)? Does it affect inflating the
tyre? Batteries? Cost? Can they get nicked? I wonder how prolonged
high speed use will affect them. Or rain and salt.
--

TSH


For email, replace 'SpamOnlyToHere' with my initials

 

Following up my own post - I just found a website:

http://www.needforspeed.co.uk/tyrealert/

Feck me, that's expensive for a tyre gauge.

Aren't cars with ABS able to warn of low tyre pressure? Something to
do with the wheels having different average angular velocity across an
axle.
--

TSH


For email, replace 'SpamOnlyToHere' with my initials

 

Technically it'd be possible to measure relative difference so you could
tell if one tire was lower than the other, I'm not aware of any cars
actually doing this though.

 

From http://www.asashop.org/autoinc/feb2004/mech.cfm

"General Motors Corp. uses a system that will compare the speed of
wheels on the same axle and determine if a tire has insufficient air
pressure. This is not the same system used earlier to actually measure
tire pressure using a monitor ring mounted in the tire. This system
would operate by observing a faster wheel speed, which would indicate
low tire pressure. This system would also look at other inputs, such as
a steering wheel angle sensor, before determining a low tire pressure
situation. "

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

You probably could do this but it would be very insensitive, and would
really just detect severe punctures.

Sorry for the lack of info, thought these things were all the rage,
especially when there is talk about fitting them to new cars. The tyre
fitter tells me the Renault Lagunas with it have continual problems with the
alarm going off too often, apparently problems with sensor accuracy and very
tight and unadjustable alarm limits. You can program your own limits with
these aftermarket units.

Anyway technology:
The new valve is just something to bolt the transmitter to, some systems use
a big clamp that goes round the entire rim.. probably more desirable than
messing about wth unstandard valves that won't seal.
They work by transmitting a RF signal with data on sensor number, battery
voltage, pressure and temperature. Apparently this system triggers a
transmission when pressure varies by 0.2psi so it doesn't transmit all the
time.
The RF signals are received and logged by a unit on the dash/stuck on
window. This is powered by a battery line and ignition on/off. It always
receives, so if your tyre goes flat when the engine is off it warns you when
you switch on otherwise it cycles round each tyre displaying the last
reading logged.

The wheel electronics are sealed and not subject to the elements anyway,
other than temperature and whatever is inside the wheel. Batteries are
reported to last 5 to 7 years, I've no idea what the plan is for
replacement, but the back of the modules are silicone filled so you could
probably remove that.