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?
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.
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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
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?
Blank out when you hit him with a couple hundred volts at next to no amps
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 .........
The device you guys have been discussing is depicted as back to back zener, probably in a common package. When the machines were upgraded to these it probably corrected more than the wiring issue Niege experienced. When the current flow stops from the control device the magnetic field collapses across the winding in the clutch causing a back EMF in the order of 200V and several amps. The problem using a convention diode across the coil to dissipate this current is that the magnetic field doesn't just collapse to zero. It actually oscillates like AC with a sine wave shaped current many times on the way down to zero. So a zener being bidirectional would suppress the reverse surge in both directions and have the effect much quicker suppression. A conventional diode will probably be ok as a replacement but you may well hear a noise in your radio when the clutch disengages, and system will likely have a little more back EMF going back in the wiring. If it is back to back zener it is testing faulty with your meter as should be high ohms in both directions.
I liked Niges story. . . the last diagram also shows a standard diode. John C. wrote that the ones he is familiar with has 'three legs'. That is called a 'Triode'. It acts like a 'solid state switch'. When a specific level of voltage is applied to the third leg the "Triode" allows current to pass. You are correct about the collapse of a magnetic field and the back to back reference, a 'basic' Zener is a 'one way short' but the function is the same. In addition when a magnetic field collapses it can produce 'induced voltage' much higher than the 'applied voltage'. Think of an ignition coil where the spark plug does not 'spark' when voltage is 'applied', but does as soon as the voltage is 'removed'. The ignition 'points' also experience a higher voltage which is 'absorbed' by the condenser/capacitor. The 'control relay' of a 'magnetic field/coil' probably also has a capacitor but with so many unknown variables (unlike a single wire ignition coil) the voltage spike could exceed the rated voltage of the capacitor and short it out, hence the Zener Diode. Personally I would not try and replace a Zener with a standard diode because a control relay contact can 'arc' if a 'reverse volt spike' occurs. Now if you want to talk about the same type device for AC, that would be a MOV (Metal Oxide Varistor) which 'could' be used in a similar circuit but they are not quite as 'sturdy'.
Sorry CatKC, i don't agree with a lot of your references. The basic zener function has a forward conduction voltage across the junction in one direction and begins to slowly avalanche current as you near the reverse voltage rating in the other direction. Modern ignition systems work totally different, but referring to the old points system you referenced. A standard coil would produce around 500V back EMF but this was not wasted across the capacitor. The 12V primary winding is in series with the secondary high voltage winding and that voltage is added to the HT voltage to help increase spark performance. The capacitor across the points does help with arcing points but its main purpose is to absorb the oscillations i spoke of to keep the speed of the magnetic collapse as fast as possible. Most manufactures don't give a crap about arcing relay points or thermostat points wearing out due to arcing. With the increase of electronics and microprocessors in the system the suppression of arcing has become important, to remove high voltage transients coming back through the DC and interfering with logic cards. You are correct about the MOV although good at clamping high voltage transients can not deal with the current which will be in the order of several amps and will occur every time the clutch is disengaged. As long as the wiring was checked to not have the original error in polarity a conventional diode is a way more robust device to replace his zeners. The OEM would have had to select a zener over 30V reverse in both directions where a diode will be .6V in one direction and maybe 60V in the other direction as the coil collapses. So the zenner would be roughly same ripple appearing at device i would wager. OEM prob chose zener as it fixed there wiring issue, so win win.
I'm not sure can I agree with this statement. Lets say the Zener suppressor is set for 40 V. If the contacts open and the back EMF rises to conduct the suppressor, it will conduct and dissipate energy until the voltage across it drops to where it doesn't conduct any more. Then the field will collapse and may pull the other way but I can't see how the field is going to jump up so high in the opposite direction to make it conduct again, since all that energy was dissipated in the first go-round. It MIGHT come high enough to maybe TOUCH that point but I doubt even that. After that the remaining cycles are not going to come near the Zener point. I have seen a hundreds or thousands of silicon diodes in this service and I always knew you could use a double Zener but never seen it done until now as a coil clamper. One other point to make is that the silicon diode does not dissipate much energy because its forward voltage drop is only around 0.5, where the Zener assembly will be dissipating 80 watts whenever it clamps a 2 amp coil for a fraction of a second. Probably not much but it is something.
I was making a reference to an ignition coil only to make things simpler, not to be technical . . . an analogy. I am quite aware of the dynamics of such devices . . . back as far as the Selenium Rectifiers used IN TV'S when you knew they went out because they smelled like 'rotten eggs'. I only designed/worked on solid state devices and I am not posting here to argue about semantics or anything else. I only post here to try and help the -non technical workers- and help them understand how to repair equipment. End result is. . . . . the Zener is bad.
A technical response only: Just my opinion - you are correct. Once the forward field voltage/current collapses (and with to the -short- the Zener provides) there is little or no reverse voltage/current -in a coil-. . . . UNLESS like in a motor (a reverse generator) there is a -force injected- , like a -rotating magnetic impulse-. A 'coil' (without external influence) only has so much 'pent up energy', which when 'shorted' is gone. And -anything below the threshold- of the Zener is 'absorbed' by the capacitance of the connected devices. Your last point is very good (of course the voltage drop would be dependent on the applied volts) as (even though a Zener can be made of silicon, germanium or gallium arsenide) that is -one reason- to use a Zener, it transfers/dissipates a -lot of volts-*amps/watts-.
Well, I DAM sure didn't expect the thread to go like this! Let me know whos right and Ill go with it.
I completely understand that -the -thread- is getting DEEP (digressing) into technical electronics, and there is a lot of technical give and take, right snd wrong comments. The technical BS is not relevant. End result, the diode (whatever type) is defective.
I'll bet it is just a straight silicon diode and not the Zener contraption that Cat came up with. Show us a picture with dimensions and we can give some idea of a replacement.
I wish to thank all ye who posted the information about this topic ... since there will likely always be coils and on/off control actions with these items. Understanding the various strategies and theories is very helpful. ~ An image comes to mind of an aerial lift with a literal array of these types of contrivances.
I am in agreement with you. It interests me in how various devices are used in different applications and the reasons. Sometimes you just scratch your head and say 'why'? Seeing other peoples opinions sometimes helps explain the 'why'. And maybe this discussion will help prevent someone from paying for unnecessary parts and repairs.
I build my own controls and such, and on any relay or anything with a coil, AC or DC, I will always apply some sort of snubber to protect the control upstream.
You know- I like to think of myself as a fairly sharp guy. I don't drool at the table, can carry on a fairly decent conversation about a wide array of subjects. But I've read this whole thread through, a couple of times, really really slowly. It's like a different language. I get a few words here and there, and I get that this is being used as a switch (kind of). But my hat is off to you guys that really, really, understand how this all works, because I'm willing to admit that I don't. I just appreciate the knowledge you guys have. Thanks for the time spent posting, I'm always up for learning something- and actually learning how little I really know, is never a bad thing either.
Flyback! Magnetic lines of force cutting copper windings! Inductance! PN junction! Silicon! Zener! Diode! Triode! Forward/reverse bias! Power dissipation! Avalanche! I don't know how they work inside either. (Some people know how the electrons go.) I just know how they behave when voltage and/or current are applied to them.
The issue is why does an air conditioning system need to be controlled by a micro processor which requires the added cost of the diode and associated wiring. How many Red Dot systems have been installed world wide with simple switches and cable controls for the ducting? The only conclusion to be made about the solid state systems is it means vertical integration of all parts of the machine. The functions are not changed. It is likely detrimental to the reliability. It only adds cost to the purchase, operation and maintenance of the machine.
Air conditioning systems, even with only mechanical systems, still have diodes on compressor clutches (sometimes). It saves the contacts of the switch upstream and adds to reliability. I install them across any coil on systems I build be it a relay or a solenoid coil. I agree an air conditioning system does not need a microprocessor. It needs an activation switch, a clutch cycling switch, and a blower speed switch.
It's not the same thing (maybe sort of...) I remember the hydraulic steer Wabco scrapers had a pile of capacitors and a resistor to drain off the alternator field when the elevator was switched off. If the resistor went bad (open circuit) they would blow the blocking diode that supplied the initial excitation from the batteries due to the reverse voltage spike.
What you and John C. sy is 100% correct. Unfortunately "electronics" integration and controls have gone to the point of being ridiculous. Some controls of cars and airplanes are 'control/fly by wire' where the motor/control/function (like steering or brakes) is operated by a wire connected to a control computer. While this saves a mechanical connection it adds complexity to the point to where they are no longer 'serviceable' to the average person and many times are not even by a trained person, except by 'replace parts till it works'. Heavy equipment is headed in the same direction.
The pic below is the ac connector from a gm vehicle. The various US (don't know about foreign) manufactures used Zener diodes over the years. I am not sure how they get rid of the magnetic field spike these days on cars. A real good site to understand how the Zener diode works. https://www.homemade-circuits.com/zener-diode-circuits-characteristics-calculations/
Revitalizing an old thread, my 289c a/c system is not getting power to the clutch, found everything good up to the device (17) CONT AC PROTECTION 212-2204. Does anyone know what the function of this device?
It's a delay timer. About 30 seconds.
Yup. Read here...down halfway, to "note" How to Test A/C Compressor Condition. Refrigerant Compressor - Test Identifying compressors that function is the purpose of this instruction. Compressors that function should be ... caterpillar-beginners.blogspot.com
Thanks,,, no protection function? Is seems expensive for a delay timer $199!
Doesn't that 212-2204 also have a diode bridge in it, to prevent "flyback" when the A/C compressor clutch is engaged/disengaged.? See Post #15 on Page 1.