GM Prizm/Nova 1985-1993 Repair Guide

Cold Mixture Heater (CMH)



See Figures 1 and 2

The cold-mixture heater (CMH) system reduces cold engine emissions and improves driveability during engine warm-up. The intake manifold is heated during cold engine warm-up to accelerate vaporization of the liquid fuel.

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Fig. Fig. 1: Cold mixture heater system

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Fig. Fig. 2: Cold mixture heater operation chart

The computer looks at alternator terminal L to determine if the engine is running and also watches the engine's coolant temperature. If the engine is running and the coolant temperature is below 109°F (43°C), the computer energizes the cold mixture heater relay, which in turn applies battery voltage to the cold mixture heater. The CMH is a multi-element heater ring that is mounted between the carburetor base and the intake manifold. Once the coolant temperature exceeds 131°F (55°C) the CMH relay is de-energized and the heater elements turn off.


Cold Mixture System Operation
  1. Start the engine. The coolant temperature must be below 109°F (43°C).
  3. Using a voltmeter, check that there is voltage between the positive (+) terminal (white/red wire) and the ground.

The voltmeter probe should be inserted from the rear side of the connector.

  1. Allow the engine to warm up and check the OHM with the engine warm. The coolant temperature should be above 131°F (55°C).
  3. Using a voltmeter, check that there is NO voltage. If no problem is found with this inspection, the system is working properly.

Mixture Heater
  1. Unplug the wiring connector.
  3. Using an ohmmeter, check the resistance between the heater terminals. The resistance should be 0.5-2.0 -. Readings outside this range require replacement of the heater element.
  5. Replug the wiring connector.

Cold Mixture Heater Relay
  1. Check that there is continuity between the No. 1 and 2 terminals. Check that there is NO continuity between the No. 3 and 4 terminals.

The relay is located under the air intake hose behind the battery.

  1. Apply battery voltage to terminal No. 1 and ground terminal No. 2. Use the ohmmeter to check for continuity between terminals 3 and 4.