Anatomy of APPS:

This thread addresses the construction, possible dis-assembly, and possible repair of erratic or failed APPS units.

These throttle control units, APPS (Accelerator Pedal Position Sensor), most frequently seem to fail or become erratic in operation due to deteriorating contact between internal rotating fingers and the resistance track they ride on inside the APPS unit, causing surging, jerking or other erratic engine RPM deviations at varying speeds.

These APPS units are NOT intended to be disassembled, or repaired - they are considered a replace-only item - and should only be worked upon as a last resort, with the assumption that efforts to repair them MIGHT only make things worse!

BE FOREWARNED!

BUT, since they cost about $300+, it might be worth the gamble to at least attempt to restore proper function by use of injection of a contact cleaner to restore proper internal contact of the fingers to their respective resistance tracks, if that IS actually the cause of erratic engine behavior.

Here's what they consist of internally - this is the solid-state circuitry under the label cover:



There is absolutely NOTHING the average owner can diagnose or repair in there, so no use even messing with it!

Here is the other side, where the APPS control shaft is visible:



As can be seen, I have made 4 cuts down about 1/8 inch with a hacksaw to gain access to the internals - but truth is, if your problem is serious enough to require this step, you're probably better of simply replacing the unit!

Here's what's inside:



Visible are the internal printed circuit carbon tracks - and to the right, the removed cover with contact fingers shown.

Best approach to repair, seems to be drilling about a 1/8 inch hole HERE for spraying with a good contact cleaner:



Another, overall view:



The hole should be 1/4 inch from the top surface - the white gizmo seen in the hole is just there to show where the hole is, and how the hole lines up with the innards of the case, and will be about 1/8 thru the outer case.

A sleeve or other limiter to prevent the drill from extending too far inside the case is a good idea - there's not anything close to that point to be damaged, but caution is advised. It's unlikely that residue from drilling would present any problem, and the hole can be covered with tape or silicone sealant after application of the contact cleaner.

In applying the cleaner, only insert the tube (assuming you use something like what is displayed below) about 1/8 inch inside the case for widest spray pattern and best dispersal - after application, rotate the APPS control briskly thru it's range of travel to clean and disperse the cleaner for best effect.

Here's the stuff I use for this sort of work:



All this can be done without removing the APPS unit from it's mounting bracket, so no adjustment or calibration should be needed.

Good luck, hope this helps!

 

Excellent!

 

Just want to make sure nobody gets confused about what procedures there are to do with the APPS and the difference between them.

This article discusses the actuall disection and cleaning of a faulty sensor which is the most extreme of the APPS fixes and should be used only when the other options have been tried and failed.

You can also do an APPS reset procedure that recalibrates the accellerator pedal with the sensor.

The last APPS fix is the recalibration of the sensor to the computer, by removing the sensor and getting the number off the tag on the bottom of it and setting it back to that factory setting.


I just want to make sure that when we suggest that sombody try resetting their APPS as a fix that they don't get the saw and drill out by mistake.

Gary, feel free to correct me if I misrepresented anything here. I've done all three procedures but claim no fame to knowing exactly what they are all doing inside the electronics of the truck.

 

NOPE - no problem at all - and as I pointed out STRONGLY at the start of my thread, what I picture and suggest should be only as a LAST RESORT, when all else has failed and there's nothing to lose...

 

We'll put all three of these procedures here in one thread to make it easy for everybody. They should be tried in the order that they are listed here, trying the simplest precedure first.

1. APPS Reset procedure.

"Resetting the APPS causes the ECM to 're-learn' where the throttle is at all times when something happens to cause the ECM to not understand what the APPS is telling it like a dirty spot or something like that. What you do is disconnect BOTH batteries (yes you will need to reprogram your radio), turn the key on and wait for atleast 10 minutes, then turn the key off and hook up the batteries. Once that is done turn the key on but don't start it yet. SLOWLY and steadily push the throttle to the floor and SLOWLY and steadily release it. Turn the key off and then you can start it. You have now re-set the APPS.


2. APPS Recalibration procedure.

This is different than resetting it. I did it to mine today and it made a nice difference. Boost comes up faster and smoke is down. You have to measure the voltage that the APPS is putting out then compare that to the voltage that it's supposed to be set at. For example. My apps was putting out .450 volts and it was supposed to be putting out .584 volts. I went and adjusted it and now I'm putting out .580 volts. It's not a major difference but it is nice. I've only put maybe 5 miles on it so far but come wednesday I'll be doing 500 miles a week so I'll give yall a better idea on how it drives.

Instructions:

Tools: Digital Volt Meter, T-20 Torx bit, #1 phillips, 10mm socket

With the key on, engine off you need to probe the APPS wire to see where the voltage is currently set at.
The best location to probe the wire is on the PCM (Power Control Module) which is located on the passenger firewall behind the air intake.

* You want the C1 connector, this is the connector closest to the engine
* You want the orange wire with a dark blue tracer which is pin #23
* Voltage should be somewhere around 0.5 volts,
* At this point the voltage reading does not matter, it just has to be accurately written down for later reference

TURN THE IGNITION OFF ON THE TRUCK

Remove the black plastic cover off of your APPS. There are two plastic screws. One on the top and one on the front facing the radiator. It takes a little prying to pull it off.

Undo the 6-10mm bolts that hold the bracketru in place but DO NOT REMOVE THE CABLES!!

You will notice the APPS is on the back of the bracketry that you just removed, it is held in place by 2-T20 torx bit screws. These screws have a little bit of locktite on them so make sure you have a good socket and gently give a little tap with a hammer before attempting to loosen. Be very careful because these screws strip VERY easily and you only get one shot at them (VERY VERY VERY IMPORTANT; They are VERY tight).

On the APPS you will find a white tag that gives you the information on what the APPS voltage should be.

* After loosening the screws (do not remove) you can rotate the APPS clockwise or counter clockwise to get the adjustment (towards the front of the truck increases voltage and towards the rear of the truck decreases voltage)
* The reading you took at the beginning of the process on pin #23 should match the white tag on your APPS. If it doesn't the adjust accordingly.

For more info on this check this thread: https://www.dieselbombers.com/98-5-02-cummins-24v-5-9l-vp-44-tech-talk/911-adjusted-apps-i-bored.html



3. Apps Disassembly and cleaning

See post number one of this thread.