With a gauge equipped dash, the choke light also acts as a low lube oil pressure warning. There is a oil pressure switch (normally open) behind the distributor. It closes at 10 psi.
When the engine is started and oil pressure is established, it closes. With the contacts closed, 12 VDC is supplied to the choke heater. The choke heater assists in opening the choke plates. The longer the choke is closed, the more unburned hydrocarbons are produced by the engine.
So the idea is; get the choke open as quickly as possible after the engine is started. In the days before computer controlled engine management, this was how they decided to make it happen.
Here is what GM says about an illuminated choke light:
On most vehicles, an electric assist choke is incorporated onto the carburetor to aid in reducing the emissions of hydrocarbons (HC) and carbon monoxide (CO) during starting and warm-up (choke-on) period. The electric assist choke is designed to give a more rapid choke opening at temperatures of approximately 60-65°F., or greater and a slower choke opening at temperatures of approximately 60-65°F., or below. The electric assist choke system does not change any carburetor service procedures and cannot be adjusted. If the system is out of calibration, the heater control switch and/or choke unit must be replaced.
The main components of the electric assist choke system, Fig. 39, consist of a thermostatic coil, ceramic resistor, cover, bi-metal snap disc and contact spring. The electric actuated ceramic resistor heats the thermostatic coil, gradually relaxing coil tension and allowing the choke valve to open.
At air temperatures below 50°F., electric current applied to the small section of the ceramic resistor, allows slow opening of the choke valve for good engine warm-up. As the small section of the ceramic resistor continues to heat, a bi-metal disc causes the spring loaded contact to close and apply electric current to the large section of the ceramic resistor which increases the heat flow to the thermostatic coil for more rapid opening of the choke valve.
At air temperatures between 50-70°F., electric current applied to the small section, or both the small and large sections of the ceramic resistor, will produce the amount of heat required to control the choke valve position for good engine operation in these temperature ranges.
At air temperatures above 70°F., electric current applied to the small section of the ceramic resistor and through the spring contact to the large section of the ceramic resistor, provides rapid heating of the thermostatic coil for quicker choke valve opening when leaner air-fuel mixtures are required at warmer temperatures.
Diagnosis & Testing
Possible conditions for the electric choke not operating properly are listed as follows (use a voltmeter to check oil pressure and choke coil circuits):
1.Low or no engine oil pressure.
2.Faulty oil pressure switch.
3.No current to oil pressure switch due to:
a.Burned out fuse.
b.Broken wire to oil pressure switch.
4.No current between choke and oil pressure switch due to:
a.Broken lead wire.
b.Wire terminal not properly secured on coil terminal.
c.Faulty ground circuit between choke assembly and housing grounding plate.
5.Faulty choke coil assembly.
If it is determined that the choke coil assembly is inoperative, the following test must be made:
1.Remove choke coil from carburetor. After removal of choke coil, allow coil to cool to room temperature.
2.Connect a ``jumper'' wire between positive battery terminal and choke coil terminal assembly. Connect a second ``jumper'' wire between negative battery terminal and choke coil assembly grounding plate.
3.The tang on the choke coil should rotate 45°in 54-90 seconds.
4.If choke coil does not rotate or exceeds the 54-90 second time limit, replace choke coil.
5.If coil is within specification, check steps 1 thru 5 for possible cause.
But this is more important:
Many trucks utilize a warning light on the instrument panel in place of the conventional dash indicating gauge to warn the driver when the oil pressure is dangerously low. The warning light is wired in series with the ignition switch and the engine unit--which is an oil pressure switch.
The oil pressure switch contains a diaphragm and a set of contacts. When the ignition switch is turned on, the warning light circuit is energized and the circuit is completed through the closed contacts in the pressure switch. When the engine is started, build-up of oil pressure compresses the diaphragm, opening the contacts, thereby breaking the circuit and putting out the light.
TROUBLESHOOTING
On some models, the oil pressure indicator light also serves as the electric choke defect indicator. If Oil or ENG. indicator light does not light, check to ensure electric choke is not disconnected at carburetor. Also check for defect in electric choke heater, blown gauge fuse or defect in lamp or wiring circuit. If indicator light stays on with engine running possible causes are: oil pressure is low, switch to indicator light wiring has an open circuit, oil pressure switch wire connector has disconnected or on some models, gauge or radio fuse has blown.
The oil pressure warning light should go on when the ignition is turned on. If it does not light, disconnect the wire from the engine unit and ground the wire to the frame or cylinder block. Then if the warning light still does not go on with the ignition switch on, replace the bulb.
If the warning light goes on when the wire is grounded to the frame or cylinder block, the engine unit should be checked for being loose or poorly grounded. If the unit is found to be tight and properly grounded, it should be removed and a new one installed. (The presence of sealing compound on the threads of the engine unit will cause a poor ground.)
If the warning light remains lit when it normally should be out, replace the engine unit before proceeding further to determine the cause for a low pressure indication.
The warning light will sometimes light up or flicker when the engine is idling, even though the oil pressure is adequate. However, the light should go out when the engine speed is increased.
