1995 Jeep 4.0 - coolant temp reference voltage drop

1995 Jeep 4.0 - main issue is idle control.  idle is low and irratic, no codes, smells rich, - in working on trying to find problem, I found the coolant temp reference voltage from PCM drops from 4.95v to about 1.9v every 1.4 seconds... voltage drop stays for 33ms.  Is this a normal thing for a Jeep/Chrysler?????  Any thoughts on what might cause?     FYI: The reference voltage to the Intake Air Temp sensor has a steady 5v.

Hey 04300EXC! I'm assuming you're looking at your ECT signal circuit with a lab scope? Would you be able to post the capture (or a screenshot)?

Does the ECM/PCM scan data reflect the same dropout that you're seeing in the scope capture?

My first instinct is that you're seeing secondary ignition interference in your ECT signal waveform, depending on your scope and how it's set up.

Thank you for response - Yes, using a 2 channel Pico scope. I will try to post screen shots.
Note - the voltage drop happens with AND without engine running. It still happens with sensor connect unplugged. But the IAT sensor has a steady 5v signal. All readings taken at ECM main connector with probe.

   

Blue scope reading is Coolant temp ref voltage taken at ECM main connector, key on engine off and connector at sensor unplugged
Red scope reading is IAT sensor (same conditions as Coolant - I did this to see if this sensor also got the voltage drop...but it did not)

As I mentioned in my initial posting - I am trying to resolve an idle control problem - I found the weird voltage issue with the coolant temp signal to ecm and thought it might be causing an issue with the ECM sending appropriate signals to injectors

second blue scope is the coolant temp signal alone with shorter time to show more detail.


I pondered if the 1.4s voltage drop pattern is being caused by Heated O2 ???I'm guessing.... the O2 is the only thing I can think of that switches at about this frequency??? note that the O2 sensor and coolant temp sensor (and the IAT sensor) all share the same ground input at the ECM (pin 4).