1 Acre Warrior
Member
A lesson on solenoids:
A solenoid is simply a switch, that is operated by an electromagnet. Early vehicles with starters used a heavy-duty switch that was operated by the foot, to carry current from the battery to the starter.
The problems with this system are the longer cable leads needed (battery to switch to starter), and the requirement to keep those leads out of the passenger compartment, which usually meant mounting the switch through the floorboard.
The use of solenoids allows for a low-current switch to be mounted on the dash or as part of the ignition key switch, and the heavy cable needed goes straight from battery to starter solenoid, then another from solenoid to starter.
The solenoid must have at least three terminals. Two of these are big, for the cables. They are "normally open," disconnected from each other. Inside the solenoid, there is an electromagnet, a spring-loaded metal piston and some means of closing the connection between the two big contacts (usually a large metal disk).
Apply current to the third terminal, and the electromagnet energizes and pulls the piston until contact is made between the big contacts ("closed" position). Current flows between them.
Every time the solenoid operates with the battery and started connected, an arc passes between each contact and the moving disk. Eventually, there is enough arcing that the contacts burn or wear, and no longer pass enough current to turn the starter.
If convenient, simply replace it with a similar solenoid. If desired, a solenoid can often be fixed by opening it up and cleaning all of the contacts. WARNING : The return spring inside the solenoid is STRONG. The use of a vise or other device to remove spring compression in a controlled manner is necessary!
Solenoids are rated by VOLTAGE and CURRENT. There are actually two different volt/amp ratings for a solenoid. One is for "pull-in" (the electromagnet), which may require 12V at 2A to fully operate.
When you hear the familiar "clack-clack-clack-clack" from a low battery, you're getting enough to pull in, but the heavy load from the starter is dropping voltage too low to hold the starter, so it releases. Once the load drops, there is enough voltage to pull in the solenoid again, with another "clack." A single clack which doesn't release until you release the key may mean that contact is not being made between the big terminals. Check this with voltmeter or test light.
If contact is not being made, repeatedly hitting the starter switch may either clean the contacts enough to operate or allow the internal disk to rotate far enough to reach a good contact point. If this works, consider yourself lucky and replace or repair the solenoid ASAP, before it fails completely.
The other rating is for the switch itself, which may be hundreds or even thousands of volts and/or amps. So long as it is higher than the load (in this case, from the starter and any accessories), it will work. Any 12V, 400A solenoid will do the job of any other with the same rating, EXCEPT that some solenoids have three terminals and some have one or more additional small terminals, which may be used to provide a ground, or toggle such things as electrical accessories, lights, etc. Any of these can be adapted to the three-lead electrical system by simply finding the terminal which runs to the starter switch, and finding the necessary ground connection. Insulate the unnecessary terminals.
Solenoids are NOT polarity-dependent. The electromagnet will pull in the same way whether positive- or negative-grounded.
A solenoid can be bypassed simply by making contact between the two large terminals with a metal object which is heavy-duty enough to withstand the current. It should be kept in mind that it is not the (low) voltage which does the work in an arc welder, it's the (high) current, and select your bypass accordingly, and WEAR EYE PROTECTION.
A solenoid is simply a switch, that is operated by an electromagnet. Early vehicles with starters used a heavy-duty switch that was operated by the foot, to carry current from the battery to the starter.
The problems with this system are the longer cable leads needed (battery to switch to starter), and the requirement to keep those leads out of the passenger compartment, which usually meant mounting the switch through the floorboard.
The use of solenoids allows for a low-current switch to be mounted on the dash or as part of the ignition key switch, and the heavy cable needed goes straight from battery to starter solenoid, then another from solenoid to starter.
The solenoid must have at least three terminals. Two of these are big, for the cables. They are "normally open," disconnected from each other. Inside the solenoid, there is an electromagnet, a spring-loaded metal piston and some means of closing the connection between the two big contacts (usually a large metal disk).
Apply current to the third terminal, and the electromagnet energizes and pulls the piston until contact is made between the big contacts ("closed" position). Current flows between them.
Every time the solenoid operates with the battery and started connected, an arc passes between each contact and the moving disk. Eventually, there is enough arcing that the contacts burn or wear, and no longer pass enough current to turn the starter.
If convenient, simply replace it with a similar solenoid. If desired, a solenoid can often be fixed by opening it up and cleaning all of the contacts. WARNING : The return spring inside the solenoid is STRONG. The use of a vise or other device to remove spring compression in a controlled manner is necessary!
Solenoids are rated by VOLTAGE and CURRENT. There are actually two different volt/amp ratings for a solenoid. One is for "pull-in" (the electromagnet), which may require 12V at 2A to fully operate.
When you hear the familiar "clack-clack-clack-clack" from a low battery, you're getting enough to pull in, but the heavy load from the starter is dropping voltage too low to hold the starter, so it releases. Once the load drops, there is enough voltage to pull in the solenoid again, with another "clack." A single clack which doesn't release until you release the key may mean that contact is not being made between the big terminals. Check this with voltmeter or test light.
If contact is not being made, repeatedly hitting the starter switch may either clean the contacts enough to operate or allow the internal disk to rotate far enough to reach a good contact point. If this works, consider yourself lucky and replace or repair the solenoid ASAP, before it fails completely.
The other rating is for the switch itself, which may be hundreds or even thousands of volts and/or amps. So long as it is higher than the load (in this case, from the starter and any accessories), it will work. Any 12V, 400A solenoid will do the job of any other with the same rating, EXCEPT that some solenoids have three terminals and some have one or more additional small terminals, which may be used to provide a ground, or toggle such things as electrical accessories, lights, etc. Any of these can be adapted to the three-lead electrical system by simply finding the terminal which runs to the starter switch, and finding the necessary ground connection. Insulate the unnecessary terminals.
Solenoids are NOT polarity-dependent. The electromagnet will pull in the same way whether positive- or negative-grounded.
A solenoid can be bypassed simply by making contact between the two large terminals with a metal object which is heavy-duty enough to withstand the current. It should be kept in mind that it is not the (low) voltage which does the work in an arc welder, it's the (high) current, and select your bypass accordingly, and WEAR EYE PROTECTION.
