Coolant Temperature Sensor (CTS)


The CTS is one of the most vital sensors used by the ECM.  It tells the ECM what the temperature of the engine is (or at least the temperature of the coolant running thru the engine).  In most applications, the ECM uses this sensor to calculate ignition spark advance, fuel delivery to the engine, engine idle speed, EGR operation, and whether or not to permit closed loop fuel operation (which will be discussed in another article) among other things.


The CTS is a simple thermistor which means its resistance changes based on its temperature.  The ECM supplies the CTS with a ground and a small voltage reference signal.  The CTS pulls down (towards ground) the voltage reference signal based on temperature and the ECM looks at the amount of voltage pulled down and uses this to calculate temperature.  High resistance equates to less reference voltage pull down which the computer interprets as low temperature while low resistance equates to more reference voltage pull down which the computer interprets as high temperature.


In OBD1 applications, two trouble codes are associated with the CTS.  A code 15 indicates the coolant temp reading is lower than expected.  If this code is present, before replacing the sensor you should check for an open circuit to the CTS wiring.  A code 14 indicates the coolant temp reading is higher than expected.  If this code is present, check the CTS signal wire for a short to ground before replacing the sensor.  If either code is set, or there is a problem with the CTS itself, it is very likely you will notice a running change in the engine.  In cases if the ECM is not getting the correct reading from the CTS, it may be altering the spark advance and fuel delivery to the engine incorrectly which will probably cause a lot of drivability issues such as spark knock (detonation), loss of power, or exhaust odor because of incorrect fuel mixture.


The CTS can be tested using a simple ohm meter.  In order to test this sensor, unplug it from the wiring harness and measure the resistance across its two terminals.  The temperature vs. resistance chart is below…


°F         °C        OHMS

210       100       185

160       70        450

100       38        1,800

70        20        3,400

40        4          7,500

20        -7         13,500

0          -18       25,000

-40       -40       100,700


On other computer systems, the information provided by the CTS to the ECM may also used by the computer to calculate other output functions such as coolant fan on/off control, electronic transmission shift points, air conditioning compressor clutch operation, evap purge solenoid duty cycle as well as various other functions.  As you can see, if there is a problem with the CTS itself or the coolant temp signal going to the computer, a lot of vehicle systems can be affected in addition to engine operation and performance.



Ignition Control Module (ICM)


The ignition control module (ICM) replaced the points and condenser used in early spark ignition systems.  The ICM consists of solid state technology which means there are no moving mechanical switches or parts inside the module to trigger the ignition coil.  Instead, modern electronic components such as transistors (and/or a small computer) are used to switch an electrical power signal output on and off in order to trigger the ignition coil so it initiates a high voltage spark discharge at the correct time.  On earlier non-computer controlled carbureted cars the ignition control module served this very basic function without computer control.  Ignition timing advance was handled mechanically via a vacuum diaphragm and/or centrifugal weights.  When GM introduced the engine computer in the early 80’s, the function of timing advance was handled electronically instead of mechanically.


There are two types of ignition systems used on OBD1 (and some OBD2) GM engines (exc LT1, Vortec Truck, and LS engines).  One was standard computer controlled distributor ignition module while some later engines used the Distributorless Ignition System (DIS).  Regardless of what type of ignition system your engine has, the essential functions of the ignition module remain pretty much the same.  The ignition module accepts inputs from the ECM and distributor pickup coil/crank sensor and outputs switching (on/off) voltage used to activate the ignition coil or coils.


When cranking the engine over to start it, a reference pulse is generated by the pickup coil inside the distributor or the crank sensor in the block and is sent to the ignition module.  The ignition module interprets this input signal and generates an output signal to the ECM that is used to determine engine RPM speed (known as distributor reference pulse).  Any time the engine RPM is below about 400 rpm, the ignition control module is directly controlling the amount of spark timing advance.  When the ECM sees the engine RPM go above a preset level (usually 400rpm), it considers the engine to be running and supplies a 5 volt signal which goes to the bypass circuit in the ignition module.  When this happens, the ignition timing advance function is transferred from the ignition module to the ECM.  The ECM supplies a pulse signal along the EST (electronic spark timing) circuit which commands the ICM to fire the ignition coil(s) accordingly.  When setting base timing on a distributor based system, the ECM supplied voltage is removed from the bypass circuit so the ignition module assumes direct control of spark timing advance.  This puts the engine into “base timing mode” so you can set initial timing by turning the distributor.


The ECM uses the distributor reference pulses to determine engine RPM which it uses to determine when to fire the injectors, as well as a basis for most other calculations needed to determine fuel and spark delivery to the engine.  In OBD1 systems, there are two basic trouble codes associated with the ICM, codes 12 and 42.  Code 12 is used during the Diagnostic Circuit Check procedure to test the code display ability of the ECM thru the check engine light (series of flashes indicating code numbers).  Code 12 indicates that the ECM is not receiving the distributor reference pulse (engine RPM) signal from the ICM.  Without a distributor reference pulse, the engine WILL NOT RUN since this signal is used for determining when to fire the injectors.


A code 42 indicates there is a problem with the bypass circuit or wiring.  When the engine RPM is below 400, the ICM grounds the bypass line and thus the ECM expects to see this circuit grounded during these conditions.  When the engine rpm goes above about 400 rpm, the ECM applies 5 volts to the bypass line.  At this point the ignition module should no longer be grounding the bypass line.  If the bypass line is open or grounded, the ICM will not switch to EST mode (computer control) and a code 42 will set.  If you have a code 42 present in the ECM, you should check the connections at the ECM and ICM.  If the connections are good and free of corrosion, you should check the bypass wire for an open circuit or short to ground.


There are three other wires that connect the ICM to the ECM besides the bypass wire (which is tan w/ black stripe).  One is either red w/ black stripe or black w/ red stripe - which is a ground supplied to the ignition module by the ECM.  Another is a white wire, which is the EST circuit.  And the last is a purple w/ white stripe which is the distributor reference pulse generated by the ICM that the ECM uses to determine engine RPM.  If there is a short to ground or open circuit in any one of these three wires, the engine may not run at all, or it may fire while cranking but immediately cut off as soon as the engine RPMs climb above 400rpm.  These four basic wire colors have been used by GM for the same functions since computer controlled timing and the ECM were introduced in the early 80’s up until modern model year vehicles.



Testing the Ignition Module


The ignition module can only be tested using specialized test equipment.  But even then, a faulty ignition module may not test bad if it is not hot.  Most of the time when ignition modules start to go bad, the number one symptom is the engine stops running when it gets hot and will not restart (no spark while cranking) until the engine cools off again.  If the ICM is at fault, this is most likely caused by a break of one of the internal circuit connections inside the ignition module due to heat expansion.  By the time you remove the ignition module from the car and take it to a facility that can test it off the car, the module cools down enough for the internal break in the circuit to reconnect – and this may result in the module testing “good”.  It should be noted that this same scenario can also be caused by a faulty pickup coil or crank sensor.  Pickup coils and most crank sensors can be tested using an ohm-meter, consult your vehicle repair or service manual for resistance specifications and testing procedures for these components.



Gen1 Distributorless Ignition System (DIS) Overview


As explained earlier, the DIS module functions in much the same way as the standard ignition module used in distributor based, computer-controlled ignition systems.  The main difference is in a DIS setup, there is no distributor and no moving parts, but rather the ignition module’s circuitry is more complex and has the ability to control multiple ignition coils directly.  But computer-controlled timing advance (EST), bypass, and output reference pulses work the same as they do in a distributor-based ignition system.  Instead of using a distributor-mounted pickup coil, the DIS module instead uses a crankshaft position sensor to “read” notches on the crankshaft or balancer (known as a reluctor wheel).  Unlike a distributor’s ICM, the DIS module needs to know where the crankshaft position is in relation to cylinder no.1 TDC in order to trigger each ignition coil to fire in the proper sequence, so there is usually an extra notch in the reluctor wheel that tells the DIS module where/when this occurs (in some cases there is not an extra notch but rather the notches are spaced irregularly to accomplish the same task).  Therefore the internal circuitry of the DIS module needs to be more complex in order to work correctly in this type of triggering system.  Testing and diagnosis of the DIS module is the same as the distributor ICM for the most part – in that specialized test equipment must be used.  The DIS module can also fail due to heat just like the distributor’s ICM.  So the same engine shut-off and no start (no spark) issues can be the result of a faulty DIS module just like what can happen with the Distributor’s ICM as discussed earlier.


NOTE: When replacing the ignition module, be sure to use dielectric grease or heat sink compound between the module and surface it mounts to.  It is also a good idea to make sure these surfaces are clean of corrosion before applying the grease/compound.  The grease/compound is needed so proper heat transfer can be achieved from the ignition module to the mounting surface.  Otherwise the ignition module could overheat and fail.


Hopefully this article gives you a better idea of how these devices work, what their purposes are, and how to diagnose problems associated with them.  As always, consult your vehicle repair or service manual for proper testing procedures and specifications for your particular application.

Back to Tech Page