The ONLY reason there's a voltage reference referred to and stamped on the OEM DC APPS, is because it uses transistorized/electronic IVS switching, and requires a specific voltage threshold to activate that electronic circuit. Manufacturing tolerances for various components commonly affect that specific activating voltage, thus the slightly differing stamped voltages from one APPS to another - as well as the need to slightly adjust the mounting position of each APPS so that it's IVS open/close is in spec with mechanical throttle position.
A manual, mechanical switch, such as the Williams uses, does NOT have an "electronic threshold", so is not voltage sensitive or referenced, or even have one marked on it's outer case as the DC one does - and requires ONLY a mechanical adjustment that sets it to open/close reliably down close to the resting idle accelerator position. Of course, that setting must fall within the broad overall range of APPS spec, as set forth by the SAE, and displayed earlier in this thread.
If the setting is too far away from the idle stop position, there will be too much pedal travel before the IVS triggers, and engine acceleration begins, which might be annoying - and if WAY too far, will be outside the "window" the ECM expects, and set a code. If too CLOSE to the idle stop, and the IVS switching between open/close is not reliable, the ECM will again set a code, and the engine most likely will not respond to accelerator movement. The specific mechanical adjustment Tim refers to will normally, easily take care of establishing proper pedal travel on the idle side.
To a great degree, the Williams APPS is NOT voltage sensitive or needing a voltage adjustment - it's pretty much purely mechanical, unless it's WAY out of tolerance, or other APPS bracket stop/WOT adjustments are pretty screwed up.
The Williams APPS setup can probably easily be done OFF the truck, with the APPS module installed in it's mounting bracket - with NO truck connections to it at all - that's the way I have been doing the similar setups I am running and sent to others, so far with perfect success, using only a simple ohmmeter. Typically, I set the at-rest idle resistance of the potentiometer to between 350-400 ohms, using the mechanical stop screws - and then adjust the MS I use for IVS function, to open/close reliably just off resting idle. I have little doubt that same setup procedure will work much the same way with the Williams.
There is only *ONE* characteristic I have noted with the single Williams APPS I have - the potentiometer section wiper seems to travel off the internal resistance track at each end - apparently a poor, or no internal mechanical stop, unlike the DC APPPS modules I have tested - which means the adjustment of the idle/WOT stops on the DC APPS bracket really needs to be checked, otherwise, the ECM will see what appears to be an open circuit at idle or WOT, and likely set an error code.
Timbo might have a comment on that, since he has worked with lots more of the Williams APPS's than I have.
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