Anyone know what the "Charge Temperature Switch" does on an '87 3.9L Dak?

Jack, The purpose of the switch is to alter the advance curve of the ignition, not operate the egr valve. The advance curve is also influenced by the engine coolant temperature as well. Some engines use detonation sensors in place of a CTS for the same purpose. Steve

Charge Temperature Switch (CTS) (Used only on 3.9L engine)

"The Charge Temperature Switch (CTS) is located on the intake manifold to the right rear of the carburetor. This switch monitors the intake charge (air fuel mixture) and feeds this information to the SCC (spark control computer) to prevent the EGR valve from opening when the intake charge temperature is below approximately 15°C (60°F)."

The switch is there all right. The only problem with this is that the EGR valve on this truck is controlled only by a "Coolant Vacuum Switch Cold Closed". This is a sensor that opens and closes an air passage based on coolant temperature. There is absolutely no electricity involved.

There is another coolant temperature sensor that feeds temp info to the computer to control the spark advance based on engine temperature.

Charge Temperature Switch (CTS) (Used only on 3.9L engine)

"The Charge Temperature Switch (CTS) is located on the intake manifold to the right rear of the carburetor. This switch monitors the intake charge (air fuel mixture) and feeds this information to the SCC (spark control computer) to prevent the EGR valve from opening when the intake charge temperature is below approximately 15°C (60°F)."

The switch is there all right. The only problem with this is that the EGR valve on this truck is controlled only by a "Coolant Vacuum Switch Cold Closed". This is a sensor that opens and closes an air passage based on coolant temperature. There is absolutely no electricity involved.

There is another coolant temperature sensor that feeds temp info to the computer to control the spark advance based on engine temperature.

Reply: There should be a solenoid on the passenger side valve cover that supplies vacuum to the EGR valve, the computer controls this solenoid (using that charge temp sensor as an input) to prevent EGR from working on a cold intake (to prevent EGR from causing a stumble). There may be other solenoids on the passenger valve cover, one of them is for the air pump switching valve (if equipped).

Tom from NY

The part that alters the spark advance is called the "Coolant Temperature Sensor" and is located in the front left of the carb. It measures the temp of the coolant.

The part I am inquiring about is referred to in the Ignition System section of the shop manual as the "Charge Temperature Switch" and is located in the intake manifold in the right rear of the carb. It measures the temp of the air/fuel mixture in the intake manifold.

After posting the question, I found another reference to the Charge Temperature Switch in the Emissions section of the shop manual in which it is described as sending temp info to the computer to 1) allow pumped air to be switched from the exhaust manifold to the catcon and 2) allow purging of the purge canister.

Tom, the EGR valve on this engine is controlled solely by vaccuum and is located on driver's side of the intake manifold. The EGR vacuum is supplied by a port on the carb and is routed through a "Coolant Vacuum Switch Cold Closed" that is closed when cold and open when warm. Refer to first answer on this thread for more info.

Bottom line is the shop manual got it wrong in the ignition system section and got it right in the emissions section.