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

From the 87 Dakota shop manual:

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.

 

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

From the 87 Dakota shop manual:

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.

 

From the 87 Dakota shop manual:

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.