Pin F8 is used to tell the PCM when 4WD low has been selected. Is this only used for this? Or, is it also used for the power steering pressure switch in some applications?

 

Everything I have seen says that F8 is only used for 4WD Lo input. I have not seen any applications that actually used the P/S pressure switch input. From what I understand it may have been used on some of the earlier 4 cyl. ECM applications though I am not sure. HTH

 

In the computer there appears to also be an option for P/S load compensation. It adds airflow when the 'power steering' switch closes. I cant see any need for adding airflow when placing the transfer case in 4WD low. Im assuming that it was a little used option in the trucks. I dont recall ever seeing any change in idle speed the few times that I could get the 4WD low to engage on my blazer.

 

I wonder if it limited RPM or maybe limited timing and fuel to cut down on torque applied to the driveline that seems more probable to me, seems I read about that somewhere on something to cut down warranty work. Dont know if GM actually used it at all in there code.

 

Some of the older 4cyl. applications had a pressure switch in the power steering system. When the switch was on, this meant the power steering pump was loading the engine, much like an A/C compressor. The extra load on the small engine (2.2L) would cause the idle to drop so the IAC motor was commanded to open a step or two to compensate. This was only ever used on 4 cyl. setups and has nothing to do with 4WD. HTH

 

The P/S pressure switch was used on at least some of the V8 thirdgens. I dont recall how much was done with it.

The alternate use for pin F8 definatly appears to be some form of load compensation, and is an option and seperate from the 4WD stuffas far as I can tell. It basically adds IAC flow for a period of time whenever the switch connected is closed.

 

The code is definetly set up for using a P/S switch, however L0036 b5 is the switch input and L0096 b5 is the 4wd lo input and they are both enabled by different mode words. How are they both using pin F8? Maybe I missed something.

 

There is a seperate routine that is accessed if the assumed power steering switch is in use. If the option is selected, the short routine is run that sets/clears the carry depending on the state of the switch, which is given by an I/O word (L004D in the $0D). Havnt looked real close at the rest of the stuff but the status bit in L0096 is set/cleared by reading in the state of the switch from the same I/O word. I would also assume there is logic in the VSS logic as well to distinguish which one is in use by which option is set.

I tested pin F8 on the bench and it defintly looks like they use the same pin.

 

The reason for modified IAC routines in a 4wd, is to keep you from going through the windshield when you let off the gas in 4wd low.

 

With this routine, though, its only done once when there is a change in state. The switch closes, and added airflow is initially added in, duration timers set, and idle integrator updates blocked.

 

It could be related to the 4WD low, but I dont see any reason to have something like that, unless there is something Im missing.