Got this off a machine the other day when the owner wanted a new one instead of fixing this one. I brought it home and dismantled it to show what goes wrong with them. The first photo shows the components with the plunger with contacts still in place. You can see some of the black marks from the sparking when engaging and disengaging.
The next photo shows the plunger removed and turned upside down. A lot of melting is going on and the copper is rough and sharp to the touch.
The third image shows the contact surfaces in the body of the switch. They are in line with the copper terminals . One terminal is the positive lug to the battery the other lug sends power to the machine systems.
Final photo shows the plunger removed from the coil tunnel. There are some rough areas that were sticking in the bore not allowing quick snap in of the contacts. It also was getting stuck and not dropping out when the key was turned off. The was the big problem that showed up. The batteries would be dead overnight.
Changed 2 or 3 , never took one apart. Thanks for this. Now I might feel confident in going in there to polish and smooth surfaces till a replacement can be had.
Thats a good informative post John C
I cleaned the contacts up as well as possible. Used a flat file on the copper on the plunger but had to smooth the high points on the blades with a carbide burr. Used a scotch brite pad on the bar and bore for the plunger. Took a wire wheel to the casings and mating surface of the center section. I used a little silicone seal on the components and resembled the mess. Check the small connectors with an ohm meter to read the coil and got 5 ohms of resistance. Wrapped up in the replacement switch box and will hand it back to the owner when I get up there next.
A Guy I know said he use to build up the worn out copper sections with silver solder. Have not tried it myself but will try it one day. Probably cheaper than new for some older machines. Simon
One thing I have wondered about was the two electrical components soldered across the terminals. I suspect they are to prevent the sparking the does the burning and erosion on the disc and contact points. Does anyone recognize them.
They look like a diode and a resistor, this setup is typically used on solenoid coils to provide rapid residual voltage drain to ground on the positive wire so when you turn off the circuit that's powering up the coil the coil will then snap off quickly.
I was kind of thinking that but they are on the main leads and not across the coil. That's why I was thinking they are along the lines of a capacitor being used for a set of points in an old ignition distributor. Maybe I should do some more testing with an ohm meter and read them out both ways. This switch is on the positive side of the batteries and the down stream side connects directly to the starter terminal. The two small leads are always hot even when the switch is open. Turning on the key ignition switch grounds that circuit which allows amperage flow through the coil pulling the plunger down and the disc making contact on both plates. Sounds kind of Rube Goldberg when you describe the operation in words.
Hmm, on the Kommie CD110R that diode and resistor show to be on the coil circuit, not the main contact circuit. Curious.
Now that I look at it closer I see you are right. The wires dive through the rubber mounting and come out underneath and into the coil. So there is nothing to prevent sparking when the switch is closed and opened.
They use the same relay on the older Volvo excavators. The doide/resistor combo was typically used to protect the electronics from voltage spikes resulting from collapsing magnetic fields. So Willie 59 was on point, it quickly kills the circuit, but squashes any spikes straight to ground.
Good day JC please see attached Kind regards Uffex
I believe this is called a snubbing circuit and serves two functions. One, the resistor is to help keep the coil from "bouncing or chattering" and two, the diode is to shunt voltage spike to ground when magnetic field collapses.
Thanks for all the responses. After Willie59 posted the diagram it set me to thinking more about the component and the system itself. First of all, there is no grounding on this component. The base is insulated from the machine frame and you have to ground one leg of the coil for amps to flow to close the switch. When you turn off the key the ground goes away but there is still potential in the coil that has no where to go. What I envision in my head is that the energy just runs in circles backward around the coil circuit. Another way to think of it is there are amps flowing when the key switch is turned on to ground. When the key it turned off that flow stops and backs up on the coil. The diode allows the flowing amperage to runs circles until the potential on both sides of the coil is the same. All this further makes sense in the scheme of computer controlled components. All I have worked on put the electronic control circuits on the ground side of a component. Since the components aren't grounded in any way, there is nowhere to drain off the voltage spikes when the ground control is opened to de-energize the solenoid coil. Basically the only place to drain off that voltage was the fragile computer circuit. It's kind of funny in that I've known about diodes since they were first used in heavy equipment and knew what happened when they didn't work. But I never had the time to just stop and think about their necessity within the systems. Thanks for helping me finally understand the concept.
Controlling electricity is similar and just as fascinating as controlling fluids John C. One of the simplest things in electronics that still amazes me is the bridge rectifier used to convert AC power into DC power. The world we live in couldn't exist without that simple electronic component. https://en.wikipedia.org/wiki/Diode_bridge#/media/File:Diodebridge-eng.gif
You mean there are no more selenium rectifiers? How am I ever to keep my Letourneau scrapers running
JC, you can use a flux capacitor instead!
Wouldn't that make my scrapers go back in time?
What he said.
I started learning the ropes of electronics just a few weeks ago and my question may sound stupid for a pro like you, but I’m still going to test my luck and - hopefully - somebody here will help me out. Thin is, I’ve been trying to figure out the difference between a bridge rectifier and a rheostat. When it comes to bridge rectifiers I guess I mostly understand how it works thanks to the video on this page https://www.derf.com/how-a-bridge-rectifier-works-step-by-step-tutorial/ , but it’s more complicated with rheostats for me. What’s the biggest conceptual difference between the two? Thanks
A bridge rectifier is 4 diodes used typically to turn an AC source into a DC source. Rheostat is an old fashioned way to say variable resistor, the two are nothing alike.
The diode and resistor connected to the coil, if somebody mentioned this in so many words I missed it and apologize, but these are common on relays and solenoids, the coils act exactly like the ignition coil on a gas engine, except they don't have a secondary winding. When switch contacts or a solid state device disconnect the power to the coil the magnetic field quickly collapses and were it not for the diode intense arcing would occur, just like ignition points with a failed capacitor, or in the case of a solid state device the junction would be destroyed. My way of explaining it.
I have a 1998 pc200-6 komatsu excavator, need wiring diagram for battery relay