Choke Light!!??

[jason]

Ok.. so I'm new to the forum here to the forum.  Thanks for having me!!

I just purchased a 1985 gmc siera 1500 4x4.  It is the best vehicle I've ever owned.

With that said, the choke light has come on and I'm having some trouble diagnosing the issue.  After some careful research it appears that the oil pressure switch may be to blame for the light being on and the choke not engaging during cold weather.

I'm not sure though... I found what appears to be an oil pressure switch on the passenger side of the engine block in front of the starter.  It is a single prong switch with a darker blue wire

There is also what appears to be an oil pressure switch (3 wires) alongside the oil pressure sending unit behind the distributor off a brass tee ( i could barely get my camera in that space to snap a photo)

I couldn't check the oil pressure switch behind the distributor but tried testing the switch on the block... and didn't get any voltage when a test light was hooked up...

Even when the wire was run to ground.

The light turns off when I disconnect the wire from the choke heater on the carb.

Do you just disconnect the distributor and replace the oil pressure switch in at the back of the engine??

Ahh.. its rather annoying.  Any advice would be appreciated.  I included some pics as references for anyone else having the same issue!!

Thanks, Jason

[bd]

Hello Jason.  Welcome to the site from California!

The switch with the dark blue wire you found in front of the starter is the detonation (knock) sensor for the Electronic Spark Control (ESC).

The oil pressure switch (OPS) you found behind the distributor with the pink, dark-blue, and light-blue wires connected controls power to the choke heater on the right side of the carburetor. 

Checking the choke circuit...
With the ignition switched ON, use a test light and probe for 12 volts on the pink wire at the OPS.  Assuming you find 12-volt power on the pink wire, probe the light-blue wire connected to the choke heater on the side of the carburetor; the test light may glow dimly or not at all.  Now, start the engine and re-probe the choke heater.  If there's no change in the brightness of the test light, shut the engine OFF then turn the ignition back ON.  Pull the connector off the OPS and jump the harness plug with a paper clip, so that 'pink' is jumpered to 'blue.'  Re-probe the choke heater and you should have full 12 volts.  In addition, the Choke Light should be OFF in the dash.  Watch the choke for 2-3 minutes and it should relax and open.  If the choke functions with the OPS bypassed, replace the OPS.  If not, post an update.

If you don't find 12 volts on the pink wire at the OPS with the ignition switched ON, check your fuses under the dash.

Let us know how it turns out.

[jason]

OK.. I will try that.

Am I jumping pink to light blue or dark blue? 

Also.. the switch has 3 connections.  If the pink is the hot lead and then there are 2 paths for electricity to follow... How does the switch work when closed?

Obviously the electricity goes into the switch and then back out.  ... this is my guess as to the operation (correct me if I'm wrong).

1 switch lead> pink hot into switch.. are the other two paths for the electricity out of the switch  (when closed)..

2nd switch lead> to dash light to turn it off and

3rd switch lead> to choke heater to energize it and close the choke?

Also, thanks A LOT for clarifying that the blue wire on the block was a knock sensor!!!  I was beyond confused.

[bd]

Am I jumping pink to light blue or dark blue? 

Also.. the switch has 3 connections.  If the pink is the hot lead and then there are 2 paths for electricity to follow... How does the switch work when closed?

Obviously the electricity goes into the switch and then back out.  ... this is my guess as to the operation (correct me if I'm wrong).

1 switch lead> pink hot into switch.. are the other two paths for the electricity out of the switch  (when closed)..

2nd switch lead> to dash light to turn it off and

3rd switch lead> to choke heater to energize it and close the choke?

Addressing your questions in order...

• Light-blue and dark-blue are connected to the same terminal of the OPS.  So... jump pink to blue.
  
  
• The OPS has three wires, but two switch connections - pink is connected to one switch terminal, both blue wires are connected to the second switch terminal.
  
  
• Pink is 12 volts (protected by a fuse).
  
  
• Dark-blue is choke light ground.
  
  
• Light-blue is choke heater feed.

When there's no oil pressure, the OPS is 'open' and the instrument cluster choke light 'grounds' through the choke heater.  The choke light illuminates.  However, when the engine starts, oil pressure 'closes' the OPS and 12 volts is fed directly to the choke heater.  The choke heater heats and the choke butterfly opens.  But, at the same time, 12 volts also is fed to the 'choke light ground.'  With 12 volts applied to both sides of the choke light at the same time, current flow through the bulb ceases and the light goes out.

Refer to the 1985 Wiring Manual in our Technical Pages for circuit details.