Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
-1996 GMC Savana 1500 5.9l
-No DTCs
-Slightly high idle 1100 RPM
-Scan tool reading for absolute throttle position bounces around erratically from 0% to 9% with Engine running and at idle.
-Used voltmeter to confirm 5v reference signal (key on engine off)
-Confirmed 1v Throttle position at closed throttle, with smooth transition up to 5v at WOT (key on engine off)
-TPS is new, I put one on after a garage recommended it last month.
Any thoughts on this? Seems TPS is good and sending proper signal to PCM, but PCM is interpreting incorrectly. Maybe PCM grounding issue with engine running?
Last edit: 5 years 6 days ago by csipes1983... Reason: Need to add detail
From what I have read it goes from 1-5 or so. Note: my multimeter doesn’t read decimals. It only reads whole numbers. So it might be less than 1 volt but over .5 volt. Yeah I need a better one.
Well the percentage is what my scan tool reads, and it is very erratic, never staying at a consistent percent reading for more than a second. That’s not normal.
The PCM will idle the engine at a normal speed when the TPS voltage returns to the learned base idle value. This is also called 0% throttle angle. If this throttle angle is 1% or more at idle, the PCM will assume the engine is actually off idle and raise the engine speed to prevent a decel stall. Note that another sensor using the same 5 volt reference as the TPS can momentarily glitch the reference voltage low. This will cause the TPS to send a lower base idle voltage to the PCM, which the PCM quickly records as the new learned base idle voltage. When the reference voltage returns to 5 volts, the base TPS signal voltage increases to it's previous value. The PCM now interprets this as being off idle. This will cause the throttle angle to increase to 1% (or more) and idle the engine at a fast speed. This "glitch" can occur very quickly and may not be visible on a scan tool. IF you can, try to use a pico or similar tool.
So an oscilloscope would allow me to confirm the irregular 5v reference signal? How Would I determine what is causing the glitch? Start unplugging sensors?
Thanks
Yes a scope will confirm and pull a schematic and see what shares 5v ref with the TPS. If there is a glitch, unplugging the sensors will help you narrow down to the problematic sensor
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