Well.... the challenges continue. Before we get into all the latest trials and tribulations just to get the "Check Engine" light working, I thought it would be good to review that the tachometer is now working and how! Turns out both Porsche and the GM LS3 use 58 tooth reluctor wheels! We just needed to find and scope the 5V square wave from the GM reluctor wheel and then route it to the Porsche's DME harness so it could pass the pulse count onto the microprocessor contained on the instrument cluster's circuit card. Sadly that was easy compared to how the check engine light and other idiot lights operate.... SO HERE GOES!! (more photos at the end of the text below)
Idiot light operational methods in Porsche 987 and similar models:
The DME (engine computer) along with running the engine also monitors various sensors for proper compliance and also for protection.... whenever a fault is found, the DME does its thing to ascertain if an idiot light or message needs to be brought to the driver's attention. Now comes the tricky part - instead of just lighting up a idiot light like in previous technologies - this guy sends out a message on the CANBUS (car tallk for local area network) setting a flag addressed to the microprocessor that is on the instrument cluster logic card to receive for handling. The message is interpreted to ascertain what light it should turn on and there you have it... Oh boy!
This method saves the running of multiple unique wires in the harness for every individual idiot light you have. Instead there is just one local area network (CANBUS) that a message can be sent on and all that wiring disappears as its all contained within a logic card on the instrument cluster that is under its own microprocessor control.
NOW THAT WE KNOW THIS... how do we get the the GM computer to turn the "check engine" light on and off? In order to pass car inspection, it is a requirement that the check engine light come on initially with the ignition switch and then shut off shortly afterwards with no state conditions recorded.
The mind reels at the thought of getting a GM computer logged onto the Porsche CANBUS and speaking anything other than gibberish... You'll see our solution in the photo below
Yep, that's an LED we are manually inserting/installed into the Porsche instrument cluster that the GM computer to turn on and off under its own control - and hopefully pass inspection!!!
P.S. would not believe what it took to get the instrument cluster dismantled, much less take it in and out of the the dash
Now it's off to Amelia Island for Concours - so no work on FrankenBoxster for another week or two.
Then the Pinehurst Concours will be here before you know it!
That's it for this installment... thanks for reading and viewing.