Modifying Extended Idle behaviour

The idea I'm kicking around here really only applies to anyone who has done the Extended Idle mod on their XJ, but I'd be happy to hear from anyone who may have useful input on this. If you're not familiar with it, see this thread and this thread for details.

Basically, I'd like to modify the Extended Idle behaviour in such a way that it raises the idle in all gear lever positions except possibly reverse. While I realise that its intended function is to keep the alternator and water pump spinning at higher-than-normal RPMs in Park and Neutral, a trail run today caused me to realise that this could also make a useful 'crawler' mode switch for situations where brakes may more useful than throttle for low-speed control.

We already know how the switch ties in to the PCM - but what I'm wondering is what happens from there. Knowing that Extended Idle normally only operates in Park and Neutral has me thinking that it checks the state of the NSS somehow. But what's not clear is how it samples this information - is it fed by the TCM, the ECU, or both? I can't figure it out from the FSM at this point, and, if it is NSS-dependent, all it should need is a bogus NSS signal sent somewhere to make it work the way I'd like it to.

Note that I'm not oblivious to the fact that this would cause transmission slam if you went straight from, say, Park to Reverse or Drive with it engaged. My thought is that there could possibly be a bypass switch installed somewhere that moves between 'normal' and 'all-position' Extended Idle operation. For right now, though, I'm just curious to know how it figures out when it's in something other than Park or Neutral - and this seems like something that may be useful to build into an AW4 shifting modification.

i may be wrong.... but, i believe you could remove or disconect the safety on the shifter, and then the idle switch wouldnt be able to read what gear it is in.
it would bypass the shifter and turn on whenever you turn the switch on! once again. i may be wrong, but it should work!

I did it with mechanical parts.

I wanted a fast idle for my on board air to fill tires faster. So not wanting to spend much money on it, I just used some old Jeep parts. On the early XJs, there is a vacuum actuated high idle. It consists of an electrically operated valve and vacuum motor on the carb. The valve is found on the inner driver side fender right behind the washer bottle. It will have two wires that disappear into the harness and go to a pressure switch on the high side power steering hose. The purpose of the system is to give a 200 rpm increase when the power steering pump is pumping pressure.



The valve is a special valve and is directional. The hose that points the same direction as the wires in the photo goes to the vacuum motor. It is normally open, allowing atmospheric air pressure to the vacuum motor and thus allow it to be in a normal state of rest. When closed, the atmospheric pressure is sealed off and vacuum goes to the motor and the plunger is extended. The plunger in the photo has been extended almost all the way allowing the full range of the motor to be utilized, about 1/2"





Here is the vacuum motor attached to the throttle cable bracket on my Chevy engine. I used JB weld to attach the back of the motor. However there is a nut that goes on the large threaded portion that can be used if a bracket is fabricated. I welded a small tab on my throttle linkage to give the motor something to push against.

So how well does it work? It is wired to my compressor clutch circuit. I get an engine speed of about 3000 rpms from it which allows my Sanden compressor to fill four 31" tires from flat to 35 psi in about 3.5 minutes

This system can be adjusted to work any amount of idle you want.