Gas Pedal diagnostics –
 From idle to full throttle


    By all accounts BMW was the first production 
vehicle equipped with a true electronic throttle 
control (ETC) system back in 1998. Although, in 
1997 GM installed a very similar system with the 
nomenclature of (TAC) throttle actuator control 
system on the C5 models. But, I’ve also found, 
through various sources that even earlier attempts 
were made to use an electronic throttle system on 
engine systems with multiple throttle plates and 
intake sources such as the V12 engines. Whatever 
the true “first” is, it really doesn’t matter. 
 They’re here to stay.

    Most articles and text books on the subject of the ETC system focus a lot of time on the throttle body side of the equation. That being said, I thought it might be time to put together some information on the actual gas pedal and the various problems associated with them. So let’s put the pedal to the metal and get started.

The gas pedal feel

    The first thing that you’ll notice, or should I say, you don’t notice, is the effort it takes to push on the gas pedal. The idea of course, is to simulate the pressure needed to apply the tension on an old cable type driven throttle plate. This is accomplished with a large spring encased into the plastic housing of the pedal assembly. The spring also acts as the return spring for the electronic sensors mounted on the pedal assembly. (We’ll get into those later). The big thing is of course, the feel. Hop in any car, any model, any type (with electronic throttle) and the gas pedal effort is identical. 

    No more squeaky throttle cables, cables pulling the carburetor plates to wide open throttle because of a broken motor mount, the ends of the cables getting frayed or pulling apart leaving you stuck with an idling car, the guide plates for the cable wearing thin and allowing the cable to get hung up or sluggish to return to the idle position, gas pedals falling off leaving you with this little nub to press your foot against, and hundreds of other problems all related to nothing more than the gas pedal itself.

    So, in that sense alone we should all be grateful the ETC as part of the modern car. But, with every new technology there are new problems that crop up. ETC systems are no different. As with any electronically controlled system, there is a certain amount of learning curve technicians have to endure in order to repair them, and there is also a learning curve that the driving public need to deal with when it comes to these failures. 

Fail Safe Mode

    The very first thing the driver will notice if and when there is a problem with the throttle control, is the service light popping on. On some cars it’s the good old MIL or check engine light, while on other models such as the Jeeps and some Chrysler products there can be an actual ETC failure light. Lately on newer models, I’ve seen warning messages scroll across the screen stating the electronic throttle is not responding.  

    Whatever way the message is received the big thing is that it’s almost always associated with the dreaded “Limp” mode. I’m still taken back by the various circumstances people get into after they have encountered the limp mode for the first time. It ranges from running along just fine, then suddenly finding themselves not being able to go faster than idle speed . . . in traffic, on a busy street, taking the kids to school, late for work and dozens of other reasons. The frustration is usually boiling over by the time they’re at the service counter. All I can do is sympathize with their plight and informed them on why this is such a good thing.

    If a problem occurred with the pedal assembly (electrically) without a failsafe limp mode, the whole incident could end up like the old cable drive systems and leave a customer with the throttle stuck in one position or worse yet with the throttle wide open. It’s a lot safer to have the failure point fall back to an idle position and not anywhere else. 

The A-B-C’s of the APP’s

    The APP (Accelerator Pedal Position) sensors is the real key to the whole operation of the electronic throttle assembly. With the throttle cable gone we need to have some method of connecting the driver’s responses to the engine throttle. This is where the APP sensors and the various computer systems redundancy checks and balances come into the play.  

    All throttle assemblies use a minimum of two APP sensors, some vehicles will use three APP’s or more. Two of the sensors work entirely backwards from each other. They’ll mirror the others response. One will go from idle to WOT, while the other APP sensor reads from WOT to idle. It is these two signals that are used by the ECM to calculate the mean voltage used to establish the actual throttle position. The third APP checks for the proper voltage change as the throttle opens and closes.

    The pedal position signal is converted into degrees of travel (rotary angle) by the PCM. Then the PCM software converts these degrees into counts, which is used as output signals to verify throttle position as the throttle control motor drives the throttle plate to the desired position.

Typical problems that you’ll encounter

    These systems are not without their problems. A lot of times I’ve seen more parts thrown at a repair than in other types of system failures. I’m not sure that’s from a lack of understanding in regards to the basic system setups or the idea of doing the best for their customers by not leaving any stone unturned in the diagnostic process.  

    By far I’ve seen just as many failures of APP’s as I have seen of wiring or wiring connectors. Testing the APP is pretty straight forward, but testing the wiring and connectors can be a bit tricky. Just like the problems of the throttle cable getting stuck, or rubbing against something and causing a problem the same can be said about the wires and harnesses. Extra care has to be taken to check for any rubbed spots, scrapped leads and high tension (secondary ignition) leads affecting the reference voltages and in some instances the actual resistance values.

    Connections on the other hand can be a nightmare to solve. A good example of this type of problem is the dreaded intermittent failure. I’ve seen countless times when a car has come in with every part involved with the electronic throttle has been changed (including the PCM) but the telltale light is still coming on. 9 chances out of 10 the problem is the connectors. They may look perfectly normal, they might even feel like they’ve snapped solidly into place, but I’ll take a bet the problem is right there.  

    What I’ve found to be the culprit is the weather packing in the connector, or the actual connector leads are causing the connector to slightly back out of the connector housing. Which causes a momentary loss of signal either from a bump in the road, movement of the engine and harness, moisture, or a hot/cold condition. It’s almost impossible to physically see the problem, and in some rare instances a replacement connector doesn’t solve the issue either. Because the replacement connector has an even better weather packing seal than the original. 

    Sometimes, if everything else has failed a couple of wire ties “hog tying” the connectors together does the trick. But, even then it’s just a matter of time until that will also work loose. Keep in mind, it just takes a second for a loss of signal on any one of the leads for the entire system to go stupid. Either going into limp mode or merely turning on the service light. Testing and retesting is still the only method I’ve found to ensure a good result. 

Basic Testing

    A good scope is very valuable when diagnosing electronic throttle control problems. By looking at the TPS voltage and the servomotor square wave duty cycle, the actual operation can be viewed. When the TPS voltages change, there should be a corresponding change in the duty cycle of throttle servomotor. This relationship will show you if the system is working properly or if there are problems in the feedback or control circuits. Depending on where in the circuit that you’re tapped into.

    An analog multi-meter is a good choice to check the actual movement of the potentiometer in the APP’s. Hook your meter up just as you would for any other type of potentiometer sweep test. Now you’ll be able to follow the action and range on the meter as you press and release the accelerator pedal. However a digital multi-meter may not change smoothly enough to see any faulty spots on the APP potentiometer. A multi-meter that has a graphing bar would be a better choice, since the graph acts very similar to the sweeping action of the needle on an analog meter.

    If the needle jerks (does not have a fluid upward movement), that would be a clear indication of a problem with the APP.

Test result examples

    Here’s a few examples of test results. (Use the appropriate factory information to check your APP voltages and resistances. These results are for demonstration only. Your values maybe different)

Ford using three APP sensors with a total of 7 circuit leads
 2 reference voltage circuits (5 volts).
2 signal return (ground) circuits.
APP1 voltage with negative voltage slope (5-0 volts)
APP2 voltage with positive voltage slope (0-5 volts)
APP3 voltage with positive voltage slope (0-5 volts)

GM - 4.8L, 5.3L, 6.0L
APP Sensor 1 Resistance Specs. (By pin location numbers)
G & F 5.03 K Ω - Pedal at Rest (not depressed)
G & F2.98 K Ω - Pedal completely depressed
E & F2.58 K Ω - Pedal at Rest (not depressed)
E & F4.58 K Ω - Pedal at Rest (not depressed)
APP Sensor 2 Resistance Specs. 
C & D1.34 K Ω - Pedal at Rest (not depressed)
C & D3.23 K Ω - Pedal completely depressed
B & C3.73 K Ω - Pedal at Rest (not depressed)
B & C1.75 K Ω - Pedal at Rest (not depressed)

 As you manually depress and release the accelerator pedal, your multimeter should swing its needle (or bar graph) smoothly and without stops or gaps in the movement.

Something to consider

    I for one, really like the electronic throttle systems and for the most part prefer it to the old standards of using cables and levers. The possibilities in arrangements for the hot rodders and custom car folks is endless. In fact, there really isn’t a need for a gas pedal. You could literally take the APP’s and mount them in any fashion (minus the heavy return spring) anywhere you’d like. It doesn’t have to be a foot controlled throttle at all.

    New concepts such as self-driving, self-parking, and the latest in cruise control or ABS systems can take advantage of the electronically controlled throttle. Which I’m sure, in the future, even more control will be passed onto the latest technology too. Older generations may say something derogatory about the drive by wire phase of the automotive world but it’s a change that’s progressively changing how the average person on the road gets from point A to B. 

    As for the future mechanics and technicians, APP’s and electronic throttles are going to be a part of your future too. Because I seriously doubt we’ll be going back to a cable for throttle control any time soon.  



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