Intro Don’t like the response of your OBS’s accelerator pedal? Here’s one approach to “fixing” it. Please note this information is based on data from our 1996 truck so your details may vary. What’s all this stuff on the accelerator? As far as I can tell, the OBS 7.3’s represent Ford’s first use of ETC – Electronic Throttle Control (which is used on all the newer SD’s). There’s no cable as the 7.3 injectors are controlled electronically. Beside the pedal’s mechanical parts it has these electronic sensors: an APS – accelerator position sensor (some refer to this at the TPS – throttle position sensor) and an IVS – Idle Validation Switch. The APS is nothing more than a variable resistor. The ECU (aka PCM) supplies it with a +5VDC reference voltage and a ground. The ECU “reads” the pedal position by sensing the voltage developed at the wiper of the variable resistor. The IVS informs the ECU when the pedal is in the idle state – foot off the pedal (switch = open) or pressure applied (switch = closed). The Symptoms and the Problem Details It takes a lot of pedal pressure to get the engine speed to increase. When it does, the rpm jump is rather abrupt and can be a major PITA, especially when towing. Sometimes the Check Engine Lamp (CEL) will turn on at startup. Here’s what’s happening: The ECU ‘ignores’ the APS output until the IVS closes. When the switch closes the ECU “reads” the APS output and adjusts engine RPM accordingly. The chart below may help explain this. (Vaps is the voltage measured at the wiper of the APS; IVSgwot is accelerating, IVSgbi is decelerating – the green arrow pointed down was lost in the pasting.) Follow the red line as the accelerator pedal goes from 0 to 100% open. As long as the IVS is open, the ECU uses its default IDLE rpm setting. When the IVS closes, the ECU ‘reads’ the APS voltage and its value is used to control engine RPM. The other item to note is that about 30% of the throttle pedal’s travel is “lost” due to the ECU ignoring the APS when the IVS is open. FIX #1: Make the IVS adjustable! The mod to make the IVS switching points adjustable was inspired by RÖENTGEEP’s MicroAdjuster. Simply drill and tap the tab that actives the IVS plunger for a 10-32 machine screw. I used an inverted nylon locking nut as a jamb nut to ensure the adjusting screw doesn’t move. Some suggest bending the tab. I think the machine screw with jamb nut is a much better approach. The 10-32 screw size was selected as I had a box full, literally. Here’s this mod all done and ready for final adjustment: Another item to note is that the APS voltage, at idle, can vary from 0.37 VDC up to 1.4VDC. The lower in this range the more “useable” output from the APS is possible but, you need to get the IVS switch closed “early” to make use of it. Trial and error says try to get the APS to provide 0.5VDC, at idle, and get the IVS switch to close when the APS gets to 0.7VDC. Some “real values”, from my pre-mod system: APS at idle 1.14; APS going off-idle (IVS closes) 1.62; APS going back to idle (IVS opens) 1.39. The difference between the IVS switching points is known as hysteresis (his-ter-ree-sis) and is a desired ‘feature’ in this application. Comparing these to the preferred values leads to FIX #2 – modify the APS sensor to bring it’s output voltage to the lower end of its range. Previous attempts to do FIX #2 lead to drilling an alternate set of mounting holes for the APS in the throttle bracket. When the APS is viewed “from the brake pedal”, the tang that rotates the APS’s wiper moves CCW as the accelerator pedal is depressed. To lower the APS voltage at idle, its body needs to be rotated in the same direction (CCW). Turned out it was pretty easy to drill and tap a set of “rotated” holes. Sadly, the rotation (with the new holes) must have reduced the APS voltage too low as the CEL was on at startup and there was no throttle response (typically, the ECU ignores ALL APS inputs if it finds the APS or the IVS “idle values” out of tolerance at startup). (This was verified right before the mod – 0.31v.) The APS mod method that works best is to “egg out” the mounting holes in the body of the APS. Here, a picture is worth many words: (Post mod edit – the modified hole shown in the picture has been elongated about twice as much as necessary. Also, note the extension of the relief for the head of the mounting screw.) The desire is to have the ECU recognize OFF-IDLE with minimal pedal depression and to bring the the engine’s rpm up gradually. I think it’s better for the drive train and makes trailer maneuvering a good bit easier. I found out when adjusting the APS, post-mod, that the ECU maintains idle until the APS voltage is ~0.95 V (one of the drawbacks of digital meters – measuring any changing value with accuracy). Based on this new piece of info, I’d suggest adjusting the IVS closing point to around 0.9v (or below). Also, because the factory data says IVS should switch closed no earlier than when the base idle voltage increases by 0.2 v, any base idle below 0.7v should be acceptable. I also went back to the “manual,” after the mod and final adjustments, and found this chart: Note that if the IVS closes at 1.5V and the base idle APS voltage is say 0.7, the engine rpm is likely to jump considerably when the pedal is depressed and the ECU recognizes “off-idle”. Taking the lessons learned from the mod means the need to do the IVS adjuster is a must. It also infers that minimal ‘egging’ of the APS mounting holes should be required. The big variable here is the manufacturing tolerances applied to the APS as well as the quality control used. The last item for consideration is to increase the pedal travel for WOT. Inboard of the APS, you’ll see a thick horizontal pin that also functions as the WOT stop. The suggested mod is to replace it with a thinner pin or bolt. We don’t got WOT too often and don’t have any issues, so this area wasn’t touched. This link shows one way of achieving this: http://outdoors.webshots.com/album/563093673uvXqaM The last page has a few pictures of the pedal assembly during the modifications. Part numbers: Throttle pedal assembly F6TA-9F836-AA, APS – DORMAN 699-200 Thanks to FTE members Talyn, c00nhunterjoe, RÖENTGEEP, superduty4x4, tjbeggs, and Robbgt for previous posts covering this area and their feedback on this topic!!! Special thanks to RÖENTGEEP for providing a peer review of this document. This shows the APS in the ‘normal position’. Note the connector’s reinforcing ribs are aligned with the top of the bracket. There is a slight amount of adjustment with the stock holes. This shot shows the APS after the mounting holes have been egged out. The holes were increased about 100%, but all in the clockwise orientation. The APS has been rotated for minimum output here. Turns out enlarging the diameter ~ 50% would have been adequate.
