HT4100 4.1, 4.5, 4.9 Discussion, Weird 4.9l issues in Cadillac Engine Technical Discussion; It is the "Lo" fan speed indicator that needs to be monitored.
1. Press "OFF" and "Warmer" (i.e. red arrow ...
It is the "Lo" fan speed indicator that needs to be monitored.
1. Press "OFF" and "Warmer" (i.e. red arrow up button) at the same time for 5 seconds.
2. Allow the diagnostics to cycle thru the PCM, IPC, SIR, etc errors, until the "PCM?" message appears.
3. Press "Fan Up" arrow to enter the pcm functions...you'll notice several icons illuminate on the Climate Control Readout.
4. With your foot off the gas pedal (i.e. gas pedal at "Idle") the "Lo" icon should be illuminated indicating that the ISC's internal switch is closed.
5. With your foot on the gas pedal (i.e. gas pedal NOT at "Idle") the "Lo" icon should NOT be illuminated indicating that the ISC's internal switch is open.
The above procedure should 1st be done key in ignition, ignition set to accesory, engine off. If the "Lo" icon acts correctly, then try the above procedure while driving the vehicle....you will be able to sense when the engine is "sailing" when your foot is off the gas (i.e. engine speed at higher then expected speeds) at that moment, you can glance over and see if the "Lo" is off. I've seen it on more than 1 occassion where the internal switch initially closes when the driver releases the gas, but then the internal switch opens up a second or 2 later).
As a final note, if the ISC's internal switch is found to not work correctly, it does not necessarily mean that it needs to be replaced. The internal switch looks very much like point contacts (i.e. contact from a modern industrial relay or similar to ignition point contacts from a typical 1960's automotive distributor). The point contacts can become corroded (on a microscopic level) and they may be easily cleaned up with emory cloth or the wife's old/used emory board. If you choose to clean the contacts, be careful when opening the ISC as there are 3 gears, a spring and a washer that will likely separate as the ISC is opened up. All of the parts easily be repositioned to close the ISC unit, you just need to be careful that the parts are not lost.