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Old 09-17-2018, 11:09 AM   #1
Scooby Newbie
Member#: 431461
Join Date: Oct 2015
Chapter/Region: MWSOC
Location: Grand Rapids, MI
2009 STI

Default The TPMS Simulator - how I turned the dash light off with 28201FG000

I haven't found anything online recapping someone's successful effort to do this, so here goes. The TPMS in my 09 STi started misbehaving this summer (dash light) - I assume the sensor batteries finally died.

After reviewing a couple posts and the wiring and TPMS chapters in the service manual I decided to make a TPMS simulator board to replace the board in the TPMS module in the trunk. It takes ignition switched battery voltage and ground and generates a 2.5Hz 50% duty cycle square wave on pin 2. That signal on pin 2 indicates to the dash that all is well in the tires.
  1. Remove the module from under some evap canister thing in the hatch under the floor
  2. Remove the module cover and board.
  3. Carefully chop the connector off the board and depop the unused pins
  4. Assemble the board (bread board bench test, bread board in-car test, real board bench test, then real board in car test)
  5. Reassemble the module (in car test), and reassemble the trunk (in car test).

It's just a CD4541B timer, a 1Mohm sense resistor, a 470pF ceramic capacitor, and a 100kohm trim pot. Ground chip pins 5, 6, 7, 9, 12, and 13; power chip pins 10 and 14. Set the pot (~30kohm) so that chip pin 1 oscillates at 20.5kHz (2.5*8192), and chip pin 8 is the signal you feed to module connector pin 2. Beware measurement loading pin 1 changing the frequency measurement. My DMM presented some capacitance to the item being measured, and I was able to measure several different frequencies at various nodes in the oscillator part of the circuit depending on how I measured it. I had the best luck measuring chip pin 1 with one probe floating (not connected to anything).

Note that the original board is a 3"x3" square (it is shown post-hack) with small corner chamfers.

bonus points for being better at soldering than this.

No more dash light for about 10 dollars of parts. It's more if you buy a spare module off of eBay like I did to ensure reversibility. If you light your car on fire trying to do this it's not my fault. Attempt to replicate at your own risk.

Service Manual Wiring Page WI-84
Service Manual TPMS Diagnostics Page 25
TI CD4541B datasheet

My first iteration used a 555 timer, but it didn't didn't work for me. My cheapo DMM only resolves 0.1Hz, and that's approximately the same as the tolerance that the dash allows. I also found unacceptable frequency and duty cycle variations due to the output high and low being significantly different than Vbatt/ground, asymmetrically so, and dependent on input voltage. The CD4541 - being CMOS - avoids this and is apparently well suited for this task.
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Last edited by Tommywantwingy; 09-17-2018 at 04:47 PM. Reason: misspelled TPMS
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