Understanding Solenoids


Here I will explain how vacuum/pressure solenoids work and how to repair them.  A solenoid is basically just an electrically-controlled valve.  12V DC is applied to the terminals to redirect air flow.  It usually has three vacuum hose barbs on it:  the normally open (N.O.) barb, the normally closed (N.C.) barb, and the common barb.  Here, "normal" means "when power is not applied to the solenoid."  On Chryslers (and most other manufacturers), the barbs are arranged like this:

solenoid barbs

So, when power is not applied, air can flow between the common barb and the N.O. barb.  When power is applied, air can flow between the common barb and the N.C. barb.  In electronics, we call this a "single pole, double throw" (SPDT) arrangement because there is one (single) common barb (pole) that can be directed (thrown) in two (double) directions.

How They Work

An electromagnet moves a valve piston to direct air flow through the valve.  A spring causes the piston to block the N.C. barb until power is applied.  Then, the force of the magnet pulls the piston, closing the N.O. barb and opening the N.C. barb.  Here is a diagram to better explain:

transparent solenoid

Many of these solenoids are used to switch a vacuum-controlled device between manifold vacuum (or pressure) and the atmosphere.  For these solenoids, one barb will always have a foam block over it.  This is to prevent dust and dirt from entering the vacuum network and intake system when the solenoid is venting a vacuum line.  It is important that these foam block remain intact.  They tend to dry up over time (probably from gas fumes) and can fall apart when handled.  You should replace any torn or missing foam blocks with a new piece of foam or dense sponge.  Another way is to take a short piece of 5/32 inch vacuum line and stuff a piece of foam or cotton into one end and put it on the barb.  Try not to pack it too densely or air will not flow through it well enough.  Blow through it to test for too much restriction.

Where To Get Them

Probably the best place to get solenoids is from a salvage yard.  Most cars of any make or model will have some solenoids in them.  They are usually mounted on the fenders or firewall and will often still be in the engine compartment, even if the engine has been removed.  Since they basically never wear out, it is by far the most economical way to get them.  You can also buy them new from the dealerships or auto parts stores, but there are a lot more expensive than the $1 to $5 you'll pay at the salvage yard.

Other Types of Solenoids

Some solenoids do not have a N.O. or N.C. barb.  These are SPST (single pole, single throw) solenoids.  They are basically used as on/off valves for vacuum lines instead of vacuum line switches.  A solenoid without a N.O. barb is a normally closed valve (it is off until power is applied).  A solenoid without a N.C. barb is a normally open valve (it is on until power is applied).

Diagnosing and Repairing Solenoids

The Chrysler solenoids are a very rugged design and I have yet to have one fail personally.  The wastegate control solenoid, for instance, gets turned on and off several times per second whenever there is positive manifold pressure (boost).  Even so, if the system has been contaminated with dust or dirt (from a missing foam block, oil entering the vacuum network through the PCV, etc), sometimes the valves can get jammed up with dirt and grime.  Here is a simple procedure to test your solenoid and clean it out if necessary:

  1. Disconnect the solenoid and remove it.
  2. Blow through the common barb.  Air should come out of the N.O. barb.
  3. Block the N.O. barb with your finger and blow through the common barb again.  No air should come out of the N.C. barb and you should not be able to blow any air into the common barb.
  4. Apply 12V to the solenoid terminals (polarity does not matter).  You can use your battery for power, but be careful not to short out the wires you are using.  You should hear and feel a click.
  5. Blow through the common barb.  Air should come out of the N.C. barb.
  6. Block the N.C. barb with your finger and blow through the common barb again.  No air should come out of the N.O. barb and you should not be able to blow any air into the common barb.
  7. If everything checked out, then the solenoid is fine.  If the solenoid leaked out the wrong barb or did not switch, then it may be clogged or the magnetic coil may be open.  If you did not hear a click and did not see a small spark when you applied 12V to the terminals, then the coil is probably bad and the solenoid must be replaced.  You can verify this with an ohm meter.  The resistance across the terminals should be near or less than 50 ohms.  If you could blow into the common barb while blocking the appropriate barb and no air came out the other barb, then the solenoid casing may be cracked and the solenoid must be replaced.  Otherwise, you can try to clean out the solenoid to see if that helps:
  8. Connect two wires to the terminals.  You do not want to create sparks at the solenoid because you will be using solvents to clean it out.  There is a potential fire hazard here, so be careful.  I am not in any way responsible for anything that happens, so use your best judgment (see disclaimer).
  9. Squirt some alcohol or lighter fluid into the common barb.  These solvents do not ignite quite as easily, so they are the safest to use.
  10. Set the solenoid down and apply 12V to it from the other end of the wires (wherever you are getting power from).  Keep applying and removing power to work the solvent into the valve.
  11. Blow the solvent out (don't get it into your mouth) and test the solenoid again.  Repeat as necessary.
  12. If the solenoid still fails, then it may be corroded or rusted inside from water contamination or it may have some foreign object jammed inside of it.  If so, it will have to be replaced.

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Updated 11/11/2003.

Copyright © 1996-2003 Russ W. Knize.