[chengny]

Choke light on while the engine is running - that's bad medicine!

The choke light can be more thought of as an low oil pressure warning. Seriously who really cares whether on not their choke heater is getting power. GM designed the choke heater to start pulling the choke open as soon as oil pressure was established - this was in the days before ECM's and stuff.

When you put the ignition switch in the run position the choke light illuminates. As soon as you start the engine, oil pressure (10 psi for that switch) develops and closes the switch this extinguishes the choke light and power is then directed to the choke heater.

If you have gauges and the oil pressure looks okay you can safely assume that the switch is bad or a wire has failed. Fix it when you get a chance.

If you don't have gauges I would make sure that you actually have oil pressure before running the engine again.

From the GM shop manual for 1986 C/K trucks:

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.


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.I can't think of any connection between the vent fan failing and the choke light.