Author Topic: GM electric choke operation for Q-jet carbs  (Read 1287 times)

Offline Henry

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GM electric choke operation for Q-jet carbs
« on: October 08, 2018, 12:07:14 PM »
GM Electric Choke Operation
By Henry

Operation of the factory electric choke coil is actually quite sophisticated.  The choke coil, or thermostat, is a temperature sensitive, bi-metallic, coiled flat spring.  Controlled heating of the choke coil is achieved by the small black plastic heater assembly that the electric choke wire plugs onto (Fig. 1).  The plastic heater assembly is riveted shut.  Inside the heater assembly, there is a ceramic resistor, a bi-metallic disc, spring finger contacts, and other small parts that collectively manage the timing of choke warm-up and plate opening.  The choke heater is a precision assembly and should not be opened for servicing or immersed in any cleaning fluid.  Depending on the ambient air temperature surrounding the ceramic resistor, the heater assembly varies the RATE of coil heating.  Hence the choke coil tang, which mechanically engages the choke linkage inside the choke housing, moves quickly or slowly depending on the outside air temperature to provide the appropriate choke enrichment to the engine.  Opening of the electric choke mechanism occurs only while the engine is running and developing oil pressure. 

Figure 1.  GM electric choke assembly with encased integral heater and bi-metallic thermostatic coil spring.

Engines employing a factory GM electric choke use a 2-prong, normally open, non-grounding oil pressure switch (OPS) located either adjacent to the distributor (L6, plus some V6 and V8 engines) OR adjacent to the engine oil filter (some V6 and V8 engines) (Fig. 2).  Note that the choke OPS exists independently of the 1-prong, variable resistance, internally grounded oil pressure gauge sender (trucks with instrument cluster gauges) or the 1-prong, normally open, internally grounded oil pressure warning lamp switch (trucks with instrument cluster warning lights). 

Figure 2.  GM electric choke oil pressure switch.

How the OEM electric choke and indicator light circuit works is not as one might expect and is non-intuitive.  The choke light is designed to illuminate anytime the ignition is switched ON and the choke is not receiving power.  Referring to the circuit diagram above, switching the ignition ON provides +12V to both the instrument cluster choke light (via the fuse box INST fuse) and the choke OPS (via the fuse box CHOKE fuse).  With the ignition ON and engine not running (zero engine oil pressure), the choke OPS contacts are open, so no electric current passes through the choke heater from the CHOKE fuse.  At the same time, a small current supplied from the INST fuse flows through the dash choke light and the choke heater to ground.  The current passing through the choke light, though small, is sufficient to illuminate the choke light without increasing the temperature of the choke heater, thus the choke coil does not move the choke linkage and the choke plate position remains stationary.  The circuit is designed such that if the CHOKE fuse and/or the choke OPS fail, and the choke heater maintains its electrical integrity and connection to ground, the dash choke light will illuminate as long as the ignition is switched ON regardless of whether the engine is running, indicating to the driver that the choke is not functioning.

Once the engine starts and oil pressure rises above about 5 PSI, the choke OPS contacts close, passing current from the CHOKE fuse through the choke heater to ground.  The choke heater gradually heats the choke coil, which slowly relieves pressure on the choke linkage and the choke plate begins to open at a calibrated rate (typically, <4 minutes to fully open).  In the same timeframe, current flow through the dash choke light ceases since the choke light bulb has +12V applied to both ends of its filament and the choke light turns off.  Essentially, ignition ON, engine running and choke light OFF is the normal operating condition that causes the choke plate to gradually relax toward its fully open position while the engine warms. 
« Last Edit: August 03, 2020, 09:49:05 AM by bd »

Offline Stewart G Griffin

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Re: GM electric choke operation for Q-jet carbs
« Reply #1 on: October 09, 2018, 02:44:02 AM »
Thanks for posting.

Some of us still use the q-jet, for commuting no less.   And may for a while to come.......