Drive By Wire Throttle Body

My car has a GM 3800 Series II V6 motor that has a so called drive by wire system which is fairly common on vehicles today.  Drive by wire systems measure the position of the accelerator pedal. This reading is then sent to the car’s computer which sends signals to the throttle body’s motor which moves the throttle body’s blade.  The computer monitors the position of the blade via the appropriately named throttle position sensor which is also in the throttle body.   Prior to these systems a steel cable ran from the accelerator pedal to the throttle body to move the blade.  I should probably add that the throttle body meters the amount of air going into a gas motor.

The system in my car has been reliable for over 200k miles. Lately, though, I’ve been getting an intermittent trouble code and the car would drop into a reduced power mode.  The code indicated that there was disagreement between the 1st and 2nd throttle position sensors.  In other words, the car’s computer wasn’t sure where the throttle blade was and was unhappy.  I looked into replacing the throttle position sensor and quickly became unhappy myself.

I’ve replaced throttle position sensors before.  They’re pretty cheap at around $20-$30.  Remove two screws, swap sensors, put the screws back on and you’re done.  Well, on this car, the throttle position sensors cannot be replaced separately.  It turns out that you have to buy a whole new throttle body assembly because it is a “non-serviceable sealed system” or some BS like that.  The dealership wanted $800 for it and online parts warehouses wanted $360. No, thanks.  I found a used one from a junkyard car with 44k online and purchased it.  I swapped them out and all seems to be good again.

Since the old throttle body is now a paper weight I figured I’d look into what makes it so special.  Here’s my throttle body.  Air goes into the top, through the mesh, and then hits the blade inside the throttle body.  It this exits the throttle body into the motor.  The sensor in the middle of the screen is the MAF (mass air flow) which tells the computer how much air is going into the engine.  One the lower right is where the connector for the throttle body motor and throttle position sensors go.  Note that the black plastic sides are riveted on so you can’t mess with it.

tb1

A drill made quick work of the rivets and the cover was easily removed.  Here’s the throttle position sensor module.  The shaft that the throttle blade is attached to goes through the middle of the throttle position sensor module.  Note the throttle position sensor module is secured by regular screws.  More on it in a minute.

tb2

Under the cover on the opposite side, is the actuating motor and a bit of gearing.  There’s also a coil spring that closes the throttle blade mechanically.

tb3

The idler gear rides on the shaft and is easily removed.  Then just two Phillips screws hold the DC motor in.  Thats pretty much all there is to it electronically.      tb4

Let’s take a closer look at the throttle position sensor module.  The throttle position sensors are completely enclosed and connect to the motor through a plastic connector.  So, it is easily capable of being replaced.  Grr…

tb5

The back has a little panel that has been plastic welded on.  The weld can be scrapped off easily allowing access to the inside.

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Inside is pretty much what I expected.  A throttle position sensor is a potentiometer which is just a variable resistor.  A potentiometer has a resistive area and a piece of metal, called a wiper, moves along this area.  The total resistance of the area from end to end is a constant resistance.  The wiper can be moved which changes the resistance between the ends and the wiper.  The amount of resistance affects the voltage out of the potentiometer.   Anywhere you turn a knob on something electronic, that doesn’t have distinct positions, you’re probably dealing with a potentiometer.  In the picture below the resistive areas are around the inner walls and the wipers are on the middle piece.

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Here’s a close up of the resistive areas.  There’s two separate throttle position sensors in the module with one on each side of the inside of the module.  The resistive areas (the grayish areas) for each throttle position sensor are split in two connected by the wiper.  If you look closely, you can see three stripes inside the wide bands of resistive material.  This is where the wiper fingers have worn through the resistive material.

tb8The center piece holds the wipers and is rotated by the throttle blade’s shaft.  The separate fingers on each wiper add redundancy to the sensor

tb9

With the throttle position module removed, I was able to check it with my multimeter.  I connected one probe to the wiper pin and another to the end pin with the multimeter set to measure resistance.  Next, I slowly rotated the wiper while watching the multimeter.  I expected and saw the resistance linearly increasing.  At one spot the meter showed infinite resistance indicating that there was a break in the resistive material due to the wiper scratching it off over time.  This was my problem.  The sensors would agree for most of the time until one sensor hit this area and sent bad date to the computer.  The computer noted that the voltage value didn’t match and set the error code.  This check could have been performed without disassembling the throttle body.

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