Why control an electric choke with engine oil pressure?

[mcintyrederek]

Sorry for being somewhat ignorant.. but.. I've always wondered what was the reason for running the electric choke through the oil pressure switch?  What purpose does that serve?

Before I converted to manual, I just ran the electric choke lead to the same circuit that the B+ going to the distributor went to.  The electric choke element only draws a couple of amps at 12 volts so I didn't see any problem with that and it worked well.

Please educate me, I've always wondered.

Thanks,
Mack.

[VileZambonie]

You don't want the choke to turn on just because the key is in the on position. If the engine is running the oil pressure switch is an easy way to energize the choke. You don't have oil pressure with the engine not running therefore the choke stays closed.

[VileZambonie]

http://forum.73-87chevytrucks.com/smforum/index.php/topic,6838.msg38009.html#msg38009

[mcintyrederek]

Gottcha..  I understand that concept and it makes sense.  But who sits around with the key switch in the "ON" position without starting the engine for any length of time and why?  The ACC switch maybe, if you want to listen to the radio while you're waiting in the parking lot for your wife at the mall..  But not the ignition ON position. 

Regardless, point taken, oil switch sensor does make sense.

Thanks,
Mack.

[mcintyrederek]

Well if that's the case, then just use a vacuum switch attached to a manifold vacuum port on the carb if you don't want to mess with the oil pressure switch.  It might be physically easier to get to the front of the carb than the back of the intake where the pressure port for the oil is found.

Lots of vacuum switches found in junkyards on most any vehicle.

Mack.

[rich weyand]

The issue is that if you have trouble getting the thing started in the winter, especially if it is parked outside in the cold, all the while you are trying to start it the choke is heating up.  If the choke heats enough to open, then you really will have a heck of a time starting it in the cold.  So the idea of running it through the oil pressure switch is to keep the choke closed until the engine is actually running, then start heating it.

[jaredts]

There is a port behind the distributor and one near the oil filter.  Usually the choke's oil pressure switch is the one by the oil filter.  It's not that hard to get to.

[Greybeard]

My druthers would be to keep the manual choke. I have always had trouble with so called automatic chokes, especially the electric heated type. The stovepipe (bimetal spring) heated ones worked much better if kept lubed properly. But nothing beats a smooth manual choke that's kept in good adjustment and experience with the engine and what it needs and when it needs it. Maybe the new aftermarket choke preheat kits are made better than the ones I got ten years ago. 

[mcintyrederek]

100% agreed.  Manual choke = total control..

That is, if you remember to pull it off before driving..  I converted my 86 C10 to manual choke and several times pulled out of the driveway and couldn't understand for the life of me why the engine ran like crap, and then it dawned on me..

My Quadrajet had electric choke and it took WAY too long to open up.  It was sitting there for several minutes blowing out black smoke before it opened up enough to breathe.  I had it calibrated so that the choke plate just did barely close when it was cold.  The vacuum operated choke pull-off valve and fast idle cam was working properly, so evidently I had a bi-metal element that was bad because it didn't expand fast enough when 12 volts was applied.

The Edelbrock 1405 with manual choke that I just bought even has a fast idle cam that raises the RPM up a few hundred when you put the choke on, to keep it running in cold weather, especially with a big cam.

Mack.

[Dr_Snooz]

x3. Manual choke is > all.