Want to know what's in a solenoid?

[Morich]

We get lots of trouble and discussion of the mode solenoid on our RV. The name may be confusing so maybe a small point there will help. Many know what a relay does but the difference is simply the shape and how the contacts move. For simple speak, relay contacts move left/ right while solenoid contacts move up/down and that is why the different names! That Is the way simple idea of the name! Nothing else to matter.
There is nothing complex inside the solenoid, so lets take a look at one I've been hauling around for way too long.
This is a basic Roadmaster solenoid much like the common Cole-Hersy on many RV.
It is one that uses two small lugs for the control wiring while other designs use the metal case as a path to ground through the mounting screw. Three lugs or four, both operate the same!

When we cut the top off, we find the center metal shaft with the big metal "washer" that makes contact between the two lugs at each side. The coil of the solenoid pulls the shaft down and the spring pushes it back up when power is removed from the coil.

Down inside we find the coil that acts as the electro-magnet to pull the contact shaft down.

See how simple this is? No big mystery to confuse us but a couple points that I might point out.
Note how new and shiny the washer as well as the big contacts on each side and the spring that is nearly perfect.
That is not true when the solenoid has been used for years in our RV.
These contacts open and close every time we start/stop the engine and they arc every time, making things wear out over the years.
One big thing is the burns on the metal contact that are like a mini arc welder. They eventually get burned and fail to make contact or they get welded together and won't open! But that center shaft and washer are free to rotate around and that can mess with our mind. If it lands on a burned spot it may not make contact one time and the next it may move around slightly and land on a good spot to work!
That little spring can break and become a wild card in the operation. A little piece of broken spring can move all around at random and make for all kinds of stop and start operation!
Or the plastic holding the shaft can get a bit of extra loose movement and the contacts may find a path to ground. Maybe a small broken spring part lands between the big lug and the metal of the can? Every good bump can make it move around and cause all kinds of chaos! That path can lead to a massive battery drain or do lots of other weird stuff to confuse us!

This is what a good one should look like. No burns on any of the contacts, no wear or broken springs to get into trouble!
But the problem is that we can't look without tearing it apart, so we need to test how the simple plan gets power from one side to the other, and that is not hard to do.
Test there is good 12VDC on each big lug from the two battery groups. Touch a wire between one of those big lugs to the small lug that brings battery to operate the coil and you should hear or feel it close, "thump"!
While it is closed the voltage on the big lugs should match as they are connected!
If the solenoid is doing that, move to looking at things like dirty cables, etc. because the solenoid is working!
No big mysteries in there to get hard to figure!

 

[Bnystrom]

My understanding is that a solenoid is just a particular type of relay, which is just an electrical switch. Here are Webster's definitions:

Solenoid:
A coil of wire usually in cylindrical form that when carrying a current acts like a magnet so that a movable core is drawn into the coil when a current flows and that is used especially as a switch or control for a mechanical device (such as a valve).

Relay:
An electromagnetic device for remote or automatic control that is actuated by variation in conditions of an electric circuit and that operates in turn other devices (such as switches) in the same or a different circuit.

The most significant difference in the definitions is that a solenoid may be designed to control a mechanical device, but a relay as they define it is electrical only. That seems to contradict my understanding, but frankly, I've seen the terms used interchangeably. The "boost solenoid" in RVs is strictly an electrical switch, so is it a solenoid, a relay, or both? Realistically, does it even matter as long as we know what we're talking about and understand what it does?

 

[Morich]

To most who don't deal with either very often, it may be practical to call them both by the same name but that will throw things into a problem if we tell them to check their mode relay! It'll be a long time before they figure out to look behind the panel that tells them where the solenoid is located!

 

[Bnystrom]

Consistency in terminology is really the key. I had never seen the term "mode" solenoid until this discussion. The term I've seen most often is "boost" solenoid, which probably come from the "Battery Boost" switch on the dash.

 

[kamphiker2]

Trombetta is a major supplier to Winnebago for this contact solenoid. My experience is these fail within 4 - 5 years as they are not a true 100% duty cycle. I have always replaced mine with a much better 100% Continuous duty cycle Trombetta "Bear DC Contactor"

 

[Bnystrom]

Most people have been replacing the stock Trombetta solenoid with a Cole Hersee 24213 Continuous Solenoid, though the "Bear" above seems like a good alternative.

Just a reminder: If you install a DC-DC charger as part of an electrical upgrade, you're typically going to bypass the solenoid. It's not going to be used unless you install a switch to manually activate it, so you can still use the "battery boost" function. The stock Trombetta solenoid will be fine for occasional use in such an installation.

 

[Morich]

My use of the term "mode solenoid" is strictly from the Winnebago info as that is what they call it in most of their info but they are not really very consistent in what they do!
But this is the only place where I've noticed them being called mode solenoids. Pretty much the only place I see them used for those two purposes, too.

I assume the mode p[art of their name is due to different mods of using it, sometimes to charge the coach or sometimes to boost a weak chassis.

What we call them is only important until we get the idea of what they do and where to locate them! Mostly, I wanted to point out how little there is and why they can fail.

 

[Bones2003]

Richard, thanks for taking the time to show the inside of that solenoid and explain its operation!

 

[Morich]

I have looked at solenoids in other uses more than the RV use and one of the things we have set around and considered was how we might find one that might give us less trouble if it were made differently. This was not in RV use but with the same general problem of contacts that get burned and pit ed to the point they fail to make contacts or welded together so they don't open.
Setting around talking to the guys who tear things apart just to see what is in there, we have come around to thinking there really isn't much way to get a better longer lasting solenoid until we move into better metals to resist the arcing.
First idea was to get solenoids with ratings for higher current but then we decided the current was not actually the problem but just the simple pitting at the surface of the contacts!
first look is the lugs are heavy duty, thick stuff but the washer seems thin, so we talked about getting a type with thicker metal for the washer like maybe 1/8 inch instead of 1/16?
that seemed easy but we talked ourself out of that as it is not a problem of the washer warping or deforming but just the surface getting burned and pitted.
Thicker metal still leaves the same amount of surface with the same burning, so we backed off the idea that higher rated solenoid would slow the decay in any way.
Same type problem as many places where metal to metal makes electrical contacts but this is one where cheap has to come down as a major factor before making the expensive changes to help it last longer.
We decided we could make solenoids that lasted years longer but nobody would buy them for the costs involved!

 

[RayIN]

It simply uses low amperes to pull higher (DC or AC ampere contacts together.