As a diagnostic tech, there are a few things I really don't like. One is intermittant faults, two is diagnosis by substitution. Yet the two often go hand in hand. If you can't find a fault, you can't show-prove what the cause is. So you can either send it out NTF, or try replacing the "most likely cause". My thought is that if you change nothing on a vehicle, then there is no reason for the symptom to NOT return- eventually. But I really want to get a "smoking gun" if I can.

Now it might be an educated guess for replacing a part, but it's still a guess. That seems to be the case too often on parts such as the electronic throttle body assemblies. Most are fairly "simple"- a reversible motor with dual TPS sensors. Yet I seldom have had any luck capturing proof of a glitch- be it TPS or motor codes. So I have replaced the TB units ( they ARE high wear item) & sent them on their way- so far all have been no further trouble.

A few weeks back a 2010 Mercury Milan 3.0L came in with a MIL & complaint of intermittant no throttle response- car would idle-only until restarted. Pcm had a P2111- Throttle Actuator stuck open."the throttle angle does not match the angle commanded"- Possible causes are TB (bind), pcm, wiring open, short, etc.

Of coarse it won't act up for me. The code flow chart has you perform a bunch of continuity tests that might help if there was a hard fault- all tests pass. Pin drag tests were OK, Since a couple tests were for the TPS's, I swept them on the scope- both looked fine. (towards the end of the chart it says to replace the TB assembly & have a nice day)

I thought about the code description & tests- this wasn't a TPS fault code, it was a motor circuit code. I reasoned that the pcm could only be detecting a hi or low current condition. I threw the low amp clamp over a motor wire & got current spikes that varied & inverted as the pcm supplied PWM +/- to open-hold-close the throttle blade. see the first waveform. [2010 Mercury Milan Premier, ECM/Inputs/Outputs Waveform]

Looking closer, I found a couple spots where the spikes were cut short & looked ragged. Something was bad, but I wasn't 100% sure what- motor? pcm driver? (didn't look quite right for a wiring fault, but I didn't have time right then to look at the pcm-motor voltage patterns on the scope) see second waveform. [2010 Mercury Milan Premier, ECM/Inputs/Outputs Waveform]

Since I wanted to test the throttle body without the pcm & wiring, it finally dawned on me to treat it like any other motor- feed it power & ground then look at the pattern on the scope. I hooked test leads to the TB connector, but fed it through a #912 bulb- that allows a enough current to open the blade, but limits it to about 1 amp when it reaches full travel so I don't hurt the motor or geartrain. I got the third waveform- which shows the bad commutator bar in the motor very clearly. [2010 Mercury Milan Premier, ECM/Inputs/Outputs Waveform]

So here is an easy test to check the health of this direct pcm controlled type TB motor. Like fuel pump waveforms, this isn't 100% yes-or-no, but if it looks really bad then you know (& can show) that the part does need to be replaced.

Chris from New Jersey

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