Diode at AC compressor

[watglen]

Volvo EC240B
Air conditioner clutch blowing fuse and kills the whole cab ventilation system.
I diagnosed it as a shorted out ac compressor clutch. It happens, and the machine has 5000 hrs on it. Makes sense.
Have the guy out to replace the compressor/dryer/r134 etc. He did but didn't fix the problem.
For whatever reason, he opened the loom at the compressor clutch and found a power diode.
I checked the wiring schematic and found it shows a diode around the AC compressor.
Odd thing, unplug the diode, fuse blowing goes away. AC is working well.
I don't know anything about diodes, but I tested with my tester. 72 kohms in one polarity, infinite ohms in the other.
Can anyone explain what is going on here? What is the diode for, what happens when you remove it?

 

[Vetech63]

Someone can probably explain this better than me but a diode is usually like a one way check valve. Electrical current will pass through one direction and not the other. They are usually installed to prevent a electrical backfeed to a connecting circuit. Sometimes when a diode is removed, it will cause a current draw that will put a drain on the battery system.

 

[uffex]

Good day
The clutch of the compressor can cause a kick when it opens the diode prevents the kick from damage to other electronics, you can often see a resistor used for a similar purpose.
kind regards
Uffex

 

[Nige]

Diodes are pretty standard across AC compressor clutches because, with the clutch being magnetic, they engage with a bang as Uffex pointed out above. I'm a little surprised that the diode was actually inside the loom, usually they are a plug-in to a connector somewhere right by the compressor.

 

[Ronsii]

The diode 'around' the compressor is called a Clamping Diode, When you release the voltage to the electromagnetic clutch on the compressor(turn it off) it will kick back a voltage spike... the diode is there to 'clamp down' the voltage spike or short it out... when the diode goes bad or shorts out it is essentially a full time short across the compressor circuit and hence the blowing fuses. you need a new diode.


Edit : when you get rid of the diode the full time short goes away and all works well... but this will allow the voltage spikes to cause harm to other parts of the system... like the relay that switches on/off the compressor or the electronics that do the job.... so while it will work it will be short lived before other issues pop up.

 

[rodnro]

When replacing,make sure the cathode( the end with a ring around it) goes to the supply to the clutch and the other end to chassis,if wrong it will blow the fuse.

 

[John C.]

My understanding of diodes pretty much is the same as above with one other concept. Computers work in digital formats of zeros and ones. A one is a voltage and a zero is no voltage. The spike from a collapsing coil can travel either way along the wire path since the voltages are very high and system voltage is low. Computers control operations using the ground circuits so you don't want the high voltage going into the computer components. The high voltage can substitute a zero with a one and if that doesn't work with the software you get some crazy issues showing up in operation of the machine. The high voltage spike can also damage components on the mother board of the computer.

The diode is set up so a spike can start down the negative side and then cycle through the diode to the positive side and run the circle until the energy is dissipated. The check valve type action prevents current from flowing directly from the positive side to the negative side and shorting out. Diodes can fail in two ways. They can short through causing blown fuses or can burn out and always be open, thus doing nothing. When I hear complaints about a machine doing odd things like computers going blank after several short cycles of an auxiliary function or something just quit working all together I start to think about diodes. Most all the electronically controlled excavators I've worked on have diodes in some part of the machine and nearly all are the same part with a different color on the ends. I usually find out who makes the least expensive part and use that.

 

[CatKC]

There are various types of diodes, triodes, and quadrodes, but it is likely a -Zener Diode.-

A -regular diode- is a one way valve only. A Zener diode is a DC -voltage dependent valve-.
A Zener diode does nothing -unless- the volts reach the peak level of it's rating, then it 'shorts out' the circuit (usually to ground) until the voltage lowers.
A Zener diode cannot be verified with a forward-reverse OHM check like a diode.
When a Zener Diode goes bad it -generally- starts acting like a diode and causes a short.
Removing it would allow the circuit to work but it would not prevent a spike, the purpose of the Zener diode.

 

[Nige]

In most Cat machines the diode that is in parallel across the A/C compressor clutch solenoid is often referred to as an "Arc Suppressor". Not sure it's a Zener diode though, or even whether the icon would give any indication. See illustration.

 

[heymccall]

It is NOT a zener diode. Zener diodes allow reverse electron flow when a given voltage is exceeded.
It is a standard diode. And is used for "clamping" as stated above.

 

[CatKC]

In most Cat machines the diode that is in parallel across the A/C compressor clutch solenoid is often referred to as an "Arc Suppressor". Not sure it's a Zener diode though, or even whether the icon would give any indication. See illustration.
View attachment 184224

Yep Nige. . . . . that is exactly how a Zener Diode is diagrammed. The little bent ends on the center line give it away.
As 'standard diode' is just a 'solid state rectifier' that allows current 'one way'. They are commonly used to turn AC into DC, like in an Alternator.
In the case of a clutch or any ferro inductive (like a motor) device when the circuit is 'shut off' there is still 'inductive voltage/current' in the coils (and with a motor being nothing but a 'reverse generator') that has to go somewhere. The Zener 'shorts it out'. In this case + to - of the coil.
The -clue- was when watglen said he removed the Zener diode and the fuse quit blowing. The Zener is defective.

 

[John C.]

A Zener diode that I am familiar with has three legs. All these diodes have two. The diode in the diagram shows clamps both ways while the type in the machines that I am familiar with have only one triangle in the diagram.

What I am familiar with on Zener diodes and that they are used in alternator regulator circuits.

 

[funwithfuel]

Volvo is very fond of quenching diodes. Sometimes just a simple direct path to ground, but as machines became more electronic dependent the use of zeners became more prolific. You have to be careful with Volvo when replacing relays as well. Some are resistor suppressed, some are diode suppressed. Swapping one for the other can sometimes have negative effects.
CatKC explained it pretty clearly

 

[heymccall]

Again, not a zener diode. If someone were to engineer a zener diode in there, it would have to be in series with a silicon diode.

 

[Wes J]

These are called "flyback" diodes.

A coil, such as the coil of wire in a compressor clutch is basically a large inductor. Inductors resist changes in current, similar to how a capacitor resists changes in voltage. When you switch the coil off, the coil holds current. That current is still there when the coil is switched back on. The sudden surge of that stored current can damage the switch or relay that turns on the coil. This is basically how an ignition coil works.

To prevent that surge, they use a flyback diode. The diode is oriented so that no current passes through it when the coil is energized. But, when the coil is turned off, current can flow the opposite direction through the diode and basically close the circuit of the coil. That allows the stored current to be dissipated as heat.

It's common to find a flyback diode anywhere you find a coil. Starter solenoids, hydraulic solenoid valves, etc.


FWIW, in electric motor circuits these diodes are called "freewheel" diodes. Similar idea, but when a motor is switched off, it can become a generator and the current generated wants to go somewhere. The diode keeps if from damaging the switch gear.

 

[Birken Vogt]

The Cat diagram shows 2 Zener diodes pointing into each other. This will clamp voltage either direction with no respect to polarity but is a somewhat more exotic solution, I guess they did it so it will not be polarity sensitive. It also has to be set somewhat higher than any system voltage that would ever be encountered so there will still be some spike but probably only 35 or 40 volts. But this will be enough to save switch contacts or whatever.

No, a Zener diode does not have 3 legs, no diode does. DI means 2, a diode is always a 2 ended device.

A Zener conducts in the forward direction at very low voltage drop like any regular diode but in the reverse direction at a specified voltage precisely, that is why 2 of them pointing together works like that either way.

A regular silicon diode is most commonly used for flyback because it is cheaper, more common, less sensitive to spikes and surges, etc. but it is sensitive to polarity and if this type of system is reversed it will blow fuses. There will be no rise in voltage whatsoever when the circuit is opened, it just allows it to free wheel down to zero on its own.

 

[watglen]

Thanks guys. Sounds like i need a new diode.

Uffex, can you tell me what the code A25 is? On the ventilation control panel. How do you clear it.
Sn 81066

 

[John C.]

I like the thread because it brings in different terminology and perspectives from different manufacturers. When I started working on Komatsu excavators diodes were all over the wiring harnesses and no one knew why. I worked for the Link-Belt dealer who found out when you install a thumb without a diode in a coil circuit the computer would just blank out if you hit the button too many times too fast. Now from what is stated here there are all kinds of diodes all over the place being ever more complicated and adding more complexity, cost and things to fail and stop production.

When I started in the business there was no need for all the electronics and dirt got moved, land got cleared, material got handled. What really is a micro processor system doing that a human can't?

 

[Ronsii]

snip``

What really is a micro processor system doing that a human can't?

Blank out when you hit him with a couple hundred volts at next to no amps

 

[Nige]

The Cat diagram shows 2 Zener diodes pointing into each other. This will clamp voltage either direction with no respect to polarity but is a somewhat more exotic solution, I guess they did it so it will not be polarity sensitive.

There's actually quite an interesting story to that one. I went into the Electrical Schematic from which I took the first snip because I knew it was there. When that particular model was first introduced we got one of the first production machines and after assembly had loads of issues with the A/C. After a while we tracked it down to an incorrectly-manufactured machine wiring harness with the 521 wire being in Pin 1 and the 513 wire being in Pin 2 of the connector for the diode, not the other way round as shown on the schematic. The connectors are Deutsch DT and so they only connect one way. Simple solution was to disassemble the connector and switch the wires - problem solved. The original pre-production electrical schematic shows a much more simple diode in that location.


Then we were told to swap the original 106-8704 diode for a 218-4935. I guess Cat thought it was easier to use the more complex diode as a final solution rather than put time and effort into making sure the machine wiring harness was being manufactured correctly .........