PRELIMINARY DRAFT!
Last updated: 05/19/2006
Mustang 5.0L & 4.6L TPS FAQ
By stangPlus2Birds
Many thanks to the people on the Corral, Stangnet, Tweeer Forums, EEC mailing list, and the many others!
Quick Summary:
You never have to adjust a Ford TPS unit on a Ford TB (that has the idle set to spec)- NEVER!!
For people that adjust their idle higher than spec to keep their engine running with their setup, or people that use a NON-Ford TPS,
then they may have to adapt the TPS unit (by making slots) so that it reads from ~0.6v to ~1.1v at idle.
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Long Version:
A Ford TPS on a Ford TB with the idle screw set properly is plug and play. There is no adjustment at all for the TPS. The TPS is non-adjustable. It never has been, and never will be.
People will adapt the TPS when they use a non-Ford TB or they adjust the idle screw up high. You can't adjust a non-adjustable part.
Basic grade school grammar tells us that.
No Stang TPS has ever been adjustable! People adapt the TPS by elongating the holes so that the TPS is between ~0.6v an ~1.2V at startup. All Stang EEC (86+) "Zero-out" the TPS at startup. They also look for "invalid start-up voltages" for the TPS. That's why it has to be between ~0.6v an ~1.1V. So, ~1v is easy to remember. However, as long as the TPS is between ~0.6v an ~1.1V, it won't make any difference at all.
So, if you have a Ford TPS, a Ford TB, and the idle screw is set properly, Vref to the TPS is ~5V, then the TPS should be under ~1.1v at idle. If it is not, then you have a bad TPS. Get a new Ford TPS. Don't waste your time or money on a non-Ford TPS - they're all junk.
For all Mustangs with EFI from the factory, the TPS is zeroed out at start up. That *includes* the SD EEC-IV's. Most Stang EEC's set WOT at ~2.7 volts over the turn-on voltage of the TPS.
The main purpose of the TPS, in normal driving, is to add "accelerator pump" or the dash-pot function when there is a "quick" change in the throttle. The MAF (vacuum for SD) is slow to respond, and the O2's are super slow. So, the EEC uses the rate of change of the TPS along with the RPM to "estimate" the proper amount of additional/reduced fuel for the desired A/F ratio.
When the MAF (vacuum for SD) catches up, the EEC can "better estimate" the desired injector pulse width for the desired A/F. However, it's really the O2's that determine a ton of stuff.
At WOT, the EEC uses the trim, the MAF output (vacuum & RPM for SD), and special WOT tables to calculate the fuel. Of course, those tables are often set to be on the rich side in order to prevent pre-detonation. But, being too rich does cost HP. So, some tuners can adjust those WOT tables for a specific engine and set up and get you "a few more HP".
The sensors run off of ~5v from the EEC (Vref). Vref to the sensors can vary by ~+/- 5%. So, you can't set the TPS at the "max spec" that Ford allows, because Vref varies over temperature, load, battery/alternator load, etc. The TPS resistance and the tang on the throttle vary over temperature and age. The EEC ADC (Analog to Digital Converter) has a couple of "counts" of noise and is accurate to only a few "counts". Then, there are many more real-life issues that go into doing a full analysis. For something like the TPS, it would take about total of man month of time (from all of the different departments) to do a full and accurate analysis. Most big companies take that time. Some companies will do a "quick estimate" and make sure they leave plenty of room for error.
You can't set your TPS to 1.20 volts at idle and just see if the EEC doesn't give an error code. Over time, temperature, age, and battery voltage, that setting will vary. There is also the consideration of how accurate your meter is and how accurately you get a reading (by measuring the TPS output and TPS ground wire directly). So, that's why it's suggested to make sure your TPS is between ~0.6V and ~1.1V at idle. But, saying ~1V is easy to remember. So, that's where that "magic" value came from.
See pages 88 & 89 in Probst. All of the rotary TPS (sensors) with an EEC IV/V have had the EEC zero out the TPS. That includes all stang EFIs. So, from 86+, all stang EEC's have had the EEC zero out the TPS.
The 94/95 EEC's add in more error checking on the TPS. They had bigger proms. So, the algorithms, and diags are much more complete/ involved/ robust/ complicated. With 94/95's, you can also read the sensor and trim values with a scanner via the EEC extended diag protocol.
BTW: The quote from Probst:
Recent Ford systems, all EEC since 1988, use a Rotary TP. The potentiometer increases resistance as the throttle shaft rotates. It is not adjustable, but the control-module programming compensates for the differences in sensors, readjusting to a base voltage when the throttle is closed.
See the book by Probst. The TPS does basically Four things:
o Tells the EEC the engine is at idle. Compares start-up voltage with current voltage and the rate of change of change of the TPS.
o Determines WOT (Wide Open Throttle - open loop - "Max HP" no emissions needed). For most EEC's it's ~2.7v over the start-up voltage of the EEC. So, if your TPS was 0.8V at start-up, WOT is at 3.5v. If your TPS was 1.0V at start-up, WOT is at 3.7v.
o Tells the EEC that the driver wants to accelerate. The MAF, and RPMs lag. The TPS is the first indication. The EEC will add fuel (accel pump) depending on rate of change, RPM, etc for a Short time until the MAF catches up to the new air flow.
o Tells the EEC that the driver wants to decelerate. Similar to above. Except, that the EEC adds "dashpot" to the fuel calculation for a smooth transition.
What is does not do:
o No Stang TPS sets the idle speed or mixture.
o It does not effect the A/F (rich/lean) mixture in any way other than for the accel/decel conditions that I described above.
o It does not matter at all what the TPS voltage is set at as long as it's always between ~0.6v and ~1.1v at idle. The +5V Vref can vary by ~5%. So, when the FORD TPS is adapted to a Non-Ford TB, it's "suggested" that you set the TPS to below 1v.
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There are absolutely no slots in TPS for a Stang (type D/RD).
One hole is a very slightly larger than the screw. The other is "oblong" to help in mounting. Some people mistake that "oblong hole" for a "slot". That's fair. The difference could be considered "semantics". But, for a technical answer/ paper/ article semantics are critical! The same is true of the terms adjusting and adapting. If I say that something is a circle and someone else calls it a square, then there's a misunderstanding somewhere.
There are tolerances in the plastic of the TPS and the screw holes in the TB. The oblong hole is there on the TPS to allow for looser tolerances on those parts. Therefore, they are cheaper to make. However, regardless of how you put a Ford TPS on a Ford TB, it's always plug and play. That's part of the job of the EEC-IV & EEC-V. Having the "computer/ software/ firmware/ ASIC/ FPGA" zero out sensors is a very common practice in hardware.
You will never ever find or see anything from Ford that even remotely hints at adjusting that type of TPS.
Ford did have an older style TPS (Level C) that was used mostly with the POS EEC-III's computers. Those "Level C" TPS units had to be adjusted. The EEC-III was such a POS if the temp sensor went bad, or if the wire to the temp sensor broke, then the engine would die. Today, if Ford or another car company had a ECM that did that, they would be sued out of business. When Ford switched to the EEC-IV's they still left the old TB's on some models for a while until the next update of the engine components. Also, until ~1989, Ford had problems making the D/RD TPS units with a proper seal from water. So, they kept using the Level C units until they where sure they could make the new TPS units reliable and also update the all of the needed TB's.
From a Ford dealer Service manual:
http://www.veryuseful.com/mustang/tech/engine/images/tpsAdjustment.jpg
http://www.veryuseful.com/mustang/tech/engine/images/tpsInfo1.pdf
http://www.veryuseful.com/mustang/tech/engine/images/tpsInfo2.pdf
In the second one, notice the note that says that the sensor value may vary by +/-15% due to sensor and Vref variations. As mentioned above in the TPS FAQ, the sensor value changes with age and temp. The Vref changes with temp. All real-life sensors and voltage regulators do that. Component accuracy and "TempCo"'s (Temperature coefficient) are one of the important factors in selecting a specific part.
