Last month we discussed the differences between a conventional carburetor and the Computer Command Control carburetor, and we covered how the mixture control solenoid worked.
We also discussed how the mixture control is set using dwell. Well, I should say we teased you. We actually did not adjust the dwell because the dwell is the last setting we will make on the carburetor, but since it’s one of the most important concepts to understand on the Computer Command Control carburetor that is why we discussed it first.
This month we are going to cover removing the carburetor air horn, removing the tamper-resistant plugs and adjusting the throttle position sensor (TPS).
The TPS is an input device that is used to monitor throttle angle. This information is used to help make fuel demand decisions. This sensor uses an electrical signal supplied by the engine control module (ECM).
The TPS is simply a variable resistor (voltage diverter) signaling the ECM of a change in position. Think of this sensor like a dimmer switch in your home, as you move the dimmer switch it allows more or less voltage to travel through the switch and in turn the lights become brighter or dimmer.
The TPS voltage is adjustable and there is a specific voltage for each carburetor application. (This voltage will usually be around 0.50 volt). You will need to check your service manual for the correct closed throttle voltage setting. This setting is always done when the choke is in the open position and the air conditioning off.
The throttle position sensor has three wires: ground, voltage supplied to the sensor from the ECM (approximately 5 volts) and voltage returning to the ECM from the sensor (approximately 0.50 to 5 volts, depending on throttle position).
Removing the Tamper Resistant Plug
If adjustment of the TPS is necessary there is a tamper-resistant plug covering the TPS adjustment screw that will need to be removed.
Drill a small hole in the aluminum plug covering TPS adjustment screw. Only drill deep enough to start a self-tapping screw. Start a self-tapping screw into the plug, turning the screw in only enough to ensure good thread engagement in the hole so you do not damage the adjustment screw.
Pry against screw head to remove plug.
Whenever a tamper-resistant plug has been removed in the Computer Command Control carburetor be sure to install a new tamper-resistant plug to help lock in your final adjustments and keep debris from getting into the carburetor. Most Computer Command Control carburetor kits sold today come with new plugs.
Throttle Position Sensor Adjustment Procedure
TPS voltage can be monitored with a scan tool or with a digital volt-ohm meter (DVOM, also known as a multimeter). The multimeter must have overload protection against overcurrent capability.
I like to set the TPS voltage after most of the other tune-up settings have been adjusted, such as timing, fuel mixture, choke, base idle adjustment, etc.
1. Turn the key to the Run position, do not start the vehicle.
2. Connect the multimeter or scan tool to the vehicle.
2a. If using a scan tool monitor the TPS voltage, this parameter identification (PID) can usually be found in the sensor section of the scan tool.
2b. If using a multimeter connect it to the TPS connector center terminal and the bottom terminal. You can make a jumper for this task or back probe into the connector. If back probing into the connector take care not to damage any wires or short them together or against any medal surfaces.
3. With the carburetor choke in the open position and the air conditioning off, the base voltage can be set with the correct TPS tool by turning the screw until the correct TPS voltage is obtained.
4. After setting the base voltage with the engine off and the key in the Run position, slowly rotate the throttle smoothly to the wide-open throttle (WOT) position. Monitor the TPS voltage, which should progressively climb without dropping and be between 4.5 and 5.0 volts. If the voltage drops during this test suspect a faulty TPS or bad wiring.
A common problem is that the TPS plunger has been known to stick from dirt and debris and may need to be cleaned or replaced.
Air Horn Removal
If the TPS or mixture control solenoid must be replaced the top of the carburetor will need to be removed. It is safest to remove the carburetor from the vehicle for this operation. Note: the air horn gasket may need to be replaced if the air horn is removed.
Make a holding fixture to keep from bending the throttle valves.
Remove the upper choke lever from the end of choke throttle valve shaft by removing the retaining screw.
Remove the choke rod from the lower lever inside the float bowl casting. It’s best to remove the rod by holding lower lever out with small screwdriver and twisting rod counterclockwise.
One trick is the removal of the accelerator pump roll pin. Using a drift punch, drive the accelerator roll pin in just until pump lever can be removed from air horn. When installing simply pry the accelerator pump roll pin back into place.
Remove the vacuum hoses.
Remove the seven air horn-to-bowl screws and lock washers.
Remove the two countersunk attaching screws located in the carburetor venturis.
Remove the air horn from the float bowl by lifting straight up. Do not remove the air horn gasket.
Notice the location of the metering rod solenoid plunger assembly (on top of the air horn gasket).
When replacing the TPS sensor, slightly stretch the spring underneath the TPS plunger to ensure it has the proper tension.
When reinstalling the air horn use a small screwdriver to lightly depress the TPS sensor down against the spring tension.
Next month we will try to finish this segment up by covering the base rich stop and lean stop settings, the air bleed screw in the top of the air horn, removing the plugs to gain access for adjusting the air/fuel mixture screws, what order you should use when making the settings and how to use dwell to fine-tune the carburetor.
Photography by James Berry