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Last Update 8-20-23
Tech Tip #4- Park/Neutral Signal Wiring for GM TPI, TBI, LT1, and LSx Conversions

One of the keys to getting converted computer controlled Jaguars to run properly (and be emissions legal, as required in some states), is to be sure the new engineís computer is receiving all of the correct input signals. For computers used on GM TBI, TPI, LT1, and LSx conversions, the park/neutral input tells the computer that the car is not moving, and shifts its control strategy to an alternate mode (different control of idle, etc.,.). For those not familiar with the signal required here, the computer is supposed to see chassis ground when the shifter (and transmission) is in park or neutral.

The Jaguar shifter on 70-87 XJ6 and all XJS cars already has a park/neutral switch on the shifter that is used as a safety for the starter (the starter will not engage if the shifter is not in park or neutral). The problem is that due to eccentricities with the Jag wiring, hooking the park/neutral line from the new GM computer directly to this switch will cause several dash lights to come on in shifter positions other than park and neutral (when the car is running)-very annoying. This is doe to voltage from the GM computer traveling back up the Jag wiring past the park/neutral switch.

A simple way to overcome this is to use a diode (an electrical device that permits current flow only in one direction) in the park/neutral signal wiring. This modification costs about $2 from your local electronics supply store, and can be completed in about 30 minutes. The diode (sometimes also called a rectifier) that I always use carries part number NT5812. There are 2 wires on the Jaguar park/neutral switch- a black that goes to ground, and a black/green that goes to the starter rely. The black/green is removed from the switch, and the diode is installed between this wire and its original connection on the park/neutral switch. Then the park neutral signal wire from the mew GM computer (usually orange/black) is also connected at this original back/green connection on the park/neutral switch. The diode should be installed pointing down towards the park/neutral switch (see the labeling on the unit). Attached is a wiring diagram of the correct installation. Iíve run this setup on converted Jags for over 10 years with no problems.

Note:- If after installation the starter will not work even in park or neutral, check the installation of the diode- it is likely installed backwards, and just needs to be turned around.