Tire Pressure Loss warning light
> I've noticed in one of the S4's I've test driven that it has a
> tyre pressure warning message that comes up on the dash display. Does
> anyone know how exactly this works, are there some sort of pressure
> sensors in the wheels?
I recently purchased a 928 with the RDK system. One of my wheels, which had been after-market chromed by the prev. owner, had a non-functioning RDK sensor, meaning that no matter how hight the pressure, I'd always have a "TIREPRESSURE LOSS" warning and the accompanying idiot light. There was no reasonable fix for it. Since, I bought after-market wheels sans RDK sensors. To kill the warning, I disabled the RDK computer using a little jumper kit from Devek (http://www.devek.net). It installs easily and bypassed the whole RDK system. It's really too sensitive to be all that useful, and I don't miss it a bit. $15 fix.
Note: A do it youself fix is to unplug the connector (leaving the RDK computer disconnected) and put a jumper between pins 2 and 13 on the harness connector.
Here's my observations on the RDK system (German for Rad Druck Kontrol or tyre pressure control).
The pressure that you measure is in the tyre is a function of temperature and the external barometric pressure; you're actually measuring the difference between the internal pressure and external pressures. So, if there is no BIG leak [all tyres leak, some more than others], pressure will decrease when the ambient temperature drops such as overnight. Also, if there is a barometric increase (high) pressure zone, it could be enough to tweak the sensor.
My GTS RDK sometimes alarms when the temps drop say 7F/2C below the normal temp trends. Nothing wrong, the system is just being "too literal" in reporting a pressure drop [nothing like that big RED alarm]. If I drive a bit, turn the key off then restart, the tyres warm enough to off-set the overnight pressure drop.
The fix? Boost your pressure about 2 psi/14 kpa (.14 bar) above the sensor trip point. This is enough to compensate for the pressure drop variables. In my GTS, recommended pressure is 36 psi, I inflate to 38 psi/262 kpa and don't have many RDK alarms.
IMHO these small problems don't outweigh knowing when the pressure is off. It keeps me on my toes so the pressure never drops too low. I guess too many owners complained about RDK, it was dropped in MY 94 from the 928.
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