Continuity
/
By Michael Teahan
Basics
Espresso machines are evolving so rapidly now that simple concepts like tracing wires and circuits could become obsolete. But not yet. Machines with sophisticated low-voltage systems still have to interface with line voltage devices and the simple process of tracing back circuits can eliminate a lot of basic failures when diagnosing problems.
While continuity testing is the simplest test — listening for a tone to determine if one end of a wire is connected to another — a digital meter that measures ohms along with a tone provides better information. As machines age and connectors expand, contract and oxidize with heat, the quality of a connection can become an issue and any resistance in the circuit becomes its own tiny heating element.
On conventional machines without solid-state relays or low amperage main switches, you can check few things without even taking a panel off of the machine. Merely unplugging the cord, setting the switch to heat and measuring the resistance between the two prongs of the plug can tell you if the element is okay. You can even tell of the element has melted or simply expired and evaluate a switch to a degree because the pilot light will offer some resistance with a dead element. Even the transformer in the CPU will show resistance, so the bottom line is that if you have an open circuit, the switch is likely dead.
Blue Wires
Electrical systems in Europe and North America are little different in how they provide power to coffee machines, but the equipment itself can’t tell the difference. Most machines assume a live line and a neutral for reference. Because the neutral isn’t protected with a circuit breaker and doesn’t carry any current unless a component is active, they leave that neutral wire connected to everything all the time. They do pass the neutral through the main switch, thankfully, but from there on it connects directly to just about everything except the heating element – and that wire is always blue.
The blue wire will go into an electronic box but it won’t come out, they only switch the L1 leg. Because that blue wire meanders through the machine, it is usually connected in series from one solenoid to another and on to the pump. You should get continuity between any two points in the machine on the blue line; from the cord cap, through the switch, and every blue connection in the machine. The resistance will be a few tenths of an ohm, enough to trigger a tone. If the resistance is much more than that, you may have a poor connection in the loom and you will have to trace it back.
Other Wires
Most of the time, when something is dead, you simply go straight to the device and check it. But because of the common blue wire, you can also check everything at the CPU. Measuring resistance between the blue wire and each of the outputs on the board will give the resistance of all the coils and even the windings of the motor. Sometimes it’s easier to check at the CPU than it is to pull solenoid coils on a hot machine or open up a remote pump motor.
Some CPU’s check the resistance of the coils on startup to diagnose for failures.
North American Problems
In North America, that blue wire isn’t a benign neutral. It carries current and is live all the time on 220V machines. While no one should check continuity on a live machine, the live blue wire presents a couple of challenges. If a component shorts to ground, there isn’t a fuse to protect the common. If a solenoid shorts to ground, the connection may not be enough to blow the fuse but enough current will pass through the coil from common that the coil won’t release.
When inlet solenoids short, it may blow the fuse on the board but the autofill light will often be glowing faintly as the common wire passes current through the light to ground. It’s how we used to diagnose bad inlet coils on Brasilia’s, which was not an uncommon condition.
A Beep isn’t Enough
Continuity can provide a point-to-point diagnosis of the strength of a connection, but to really understand what’s happening you need to know the resistance of a circuit. Because every component of a machine is wired in parallel between the two legs, you will always get voltage to ground on both sides of a heating element, for example, even with one leg disconnected. The same is true of checking the voltage on the output side of an electronic box. The blue wire will feed voltage to a coil and feedback loop it to the output connection of the box — if you are merely checking voltage to ground, it won’t give you real information.
Disconnecting a machine and checking resistance provides much better information. Because everything is parallel, the more components and coils active in the circuit reduces resistance. Three element coils together will have less resistance than any one coil on its own, for example. You should be able to determine if an element has a dead coil without having to disconnect it.
Some Important Numbers to Consider
Elements will always register ohms in double digits. Depending upon the voltage and number of coils, they should be in the low teens to perhaps the low 40’s. Higher voltage and amperage machines have lower coil resistance.
Coils are usually around 1200 ohms. They will also vary a bit, but if it’s infinite, in double digits, or less, it’s dead.
An element that registers 500 to 700 ohms to ground is melted or fractured because it’s measuring the resistance of water. The same goes for probes to ground. If the resistance of the water from the tap in the restaurant is 600 but the probe is 700 or more, it’s a sign of calcium buildup on the probe or in the boiler.
If you are getting mega ohms from an element connection to ground, it’s probably nothing but dirt, calcium or residual acid on the insulators from a rebuild and not really an issue.
Lastly, most straight connections will register 0.1 to 0.5 ohms. Test leads and surface oxidation rarely make for perfect readings and that’s normal – much more than that and you will have to check connections. It’s important to note that if the resistance hits 10 or 20 ohms, it is no longer a connection; it is now a heating element.