Question: Idle Stop Solenoid

[Lib]

74 big block 454 with stock Quadrajet. Can someone tell me the function of the idle stop solenoid mounted on the throttle side front corner of the carb? Also, how to correctly test the solenoid for functionality?

 

[JohnZ]

Others will be more familiar with the year-to-year differences, but most of those were set up so they're energized when the key is in the "On" position, so the plunger extends and sets the idle speed. When you turn the key off, the plunger retracts, fully closing the throttle butterflies, to minimize "run-on" or "dieseling" at shutdown.

 

[2VettesMike]

I agree with John on the function.

To test pretty simply, use a test light and connect 12V. It will extend itself until power's removed.

Thanks,

Mike

 

[Peer81]

I thought the solenoid is there to raise the idle rpm a little when the A/C pump is on, otherwise the idle rpm will be on the edge of to low and shut down. But I could be wrong?

Greetings Peter

 

[Lib]

Thanks all! I put a 12v jumper on it and the plunger extended just as it should. (One less part to replace )

 

[Hib Halverson]

I thought the solenoid is there to raise the idle rpm a little when the A/C pump is on, otherwise the idle rpm will be on the edge of to low and shut down. But I could be wrong?

Greetings Peter

It's an anti-dieseling solenoid not a idle control solenoid for the hvac.

Back in ancient times, retarded spark and lean air/fuel ratios to reduce emissions meant a lot of curb idle throttle opening to maintain idle speed. Problem was, when you'd shut the engine down, it would tend to "run-on" or "diesel". That solenoid exists to fully close the throttle plates upon shutdown.

 

[2VettesMike]

It is on auto, manual and non-AC cars as well.

I thought the solenoid is there to raise the idle rpm a little when the A/C pump is on, otherwise the idle rpm will be on the edge of to low and shut down. But I could be wrong?

Greetings Peter

 

[Peer81]

Problem solved I think. Mine tends to diesel on to from time to time. Now I know to also take a look at the solenoid

Groeten Peter

 

[Vettehead Mikey]

I thought the solenoid is there to raise the idle rpm a little when the A/C pump is on, otherwise the idle rpm will be on the edge of to low and shut down. But I could be wrong?

Greetings Peter

It depends very much on the year of the car. Some had a solenoid to bump the idle for AC as you mentioned, others were simply for the anti-dieseling feature as mentioned, others still had a combined solenoid for vacuum advance.

On the '74 in question, it was simply for anti-dieseling. Note also that the hot idle speed is to be adjusted with the solenoid and not with the carb screw.