Zanussi ZUF1056 Frost Free - Frosting up / LED's flashing

[Leftnothing]

Hi,

I'm trying to diagnose the fault on the above freezer. The symptoms are after a period of a few months, the freezer get's frosted up (behind the plastic case around the evaporator)

The LED's flash alternatively between Green/Red on Orange off --- Green/Red off, Orange ON.

Can anyone tell me what the codes indidcate?

I assume it's either the thermal cut out and/or the defrost heater

Is there a way I can diagnose if either is faulty, perhaps with a multi-meter?

Any help appreciated!!

LN

[Leftnothing]

The LED's flash alternatively between Green/Red on Orange off --- Green/Red off, Orange ON.

Oops, looking at the freezer again (it's not doing it at the moment), I think the above was incorrect.

The sequence is Orange/Red ON (Green off) --- then Green ON (Orange/Red OFF)

LN

[Leftnothing]

I've just tested the thermal cut-out(s) and defrost heater with my multi-meter and get the following results:

Thermal Cut-out
- Grey block/wires = no continuity (closed circuit)
- White block/wires = continuity (open circuit)

Defrost Heater
- I get a continuity reading of 365 on my multi-meter, so I assume this is okay?

Looking at this circuit diagram , '26' & '27' look to be the thermal cut-outs and '9' the defrost heater ('24' is the fan which does spin so there is power getting there).

If '26' AND '27' close at the same time, then the heater will come one looking at the diagram.. so does it look like the thermal cut-out is faulty?

I 'assume' that the thermal cut-out's are preset at a certain temperature, one for high temp, one for low? (I may have that completly wrong lol) If so, is there an easy way to test if they are 'working' (switching). I applied some heat with a hair-dryer to the grey one, but it didn't appear to open. (didn't spend much time on it though.. I can test again another time) ... I tested it as soon as I got it opened (it was closed circuit) and after I'd finished defrosting the ice from the evaporator and it was still closed. The white one remained open from the start.

(After looking at the circuit diagram, it looks like I could just remove the thermal cut-out for further testing whilst keeping the freezer working?)

Any thoughts/advise appreciated on the above and where to go from here?

L.N.

[bill88]

I don't fix fridges for a living, but is it possible Zanussi just use two identical thermal cutouts for redundency/safety purposes ?

I would think the cutouts have to be closed circuit at sub zero freezing temperatures, otherwise the defrost heater cannot be energised when it is needed.


Is there any other temperature sensor attached to the evaporator?

[bill88]

ps. I would concur the white cutout is most probably open circuit faulty.

[Leftnothing]

I don't fix fridges for a living, but is it possible Zanussi just use two identical thermal cutouts for redundency/safety purposes ?

I would think the cutouts have to be closed circuit at sub zero freezing temperatures, otherwise the defrost heater cannot be energised when it is needed.

Hi Bill,

Looking at this circuit diagram there are two thermal cutouts (which look like this) to control the operation of the defrost heater.

I too was wondering what the purpose of the two cut-out's were.. both need to be closed circuit in order for the heater to work. The grey one being open circuit stops the heater from operating, the white one being closed allowing operation.

Unfortunetly the writing on the thermal cut-outs has worn off in places, can anyone identify what these are/specs?

Grey-TCO.jpg
White-TCO.jpg

I assume they are to cut off the heater at a certain temperature, one possibly a redundant cut-off for safety as you suggest, perhaps set at a different (higher) temp? (hence why I was looking for the specifications of the above pics/components)

LN

[Leftnothing]

ps. I would concur the white cutout is most probably open circuit faulty.

The grey one was open circuit, however I too think that is faulty.

[Leftnothing]

Acually, a possibility is that the white one is a thermal cut-out and the grey one a thermal fuse!?... logically then if the TCO fails, the TF blows at a higher temp!? - If I put a hair dryer on the white one (currently closed), would a hair dryer typically provide enough heat to open the TCO?

LN

[bill88]

I'm a bit confused as to which component is permanently open circuit after rereading your earlier postings. The 3rd and 8th posting appear to contradict each other.


Anyway, after studying your two jpegs of the thermal cutout devices, the markings suggest to me they are probably both identical in characteristics.

Do the two thermal cutouts clip onto the evaporator pipes?


Did you observe whether there a separate temperature sensor attached to the evaporator?

[bill88]

Correction, I meant to say the 3rd and 7th postings seem to contradict each other.

[bill88]

fwiw, I read where bimetallic defrost thermostats are employed, they apparently go open circuit a few degrees above freezing point. ie. they would be naturally open circuit at room temperature.

I would personally just verify your thermal cutouts are both closed circuit at 'freezing' temperatures.

[Leftnothing]

Correction, I meant to say the 3rd and 7th postings seem to contradict each other.

Sorry, post 3 was incorrect, it should have said:

Thermal Cut-out
- Grey block/wires = no continuity (open circuit)
- White block/wires = continuity (closed circuit)

I have just removed both of them for further testing (after reading they were IP67 sealed (mine looks like the top right pic)) I placed each one in some hot water and then cold water...

The white one goes open circuit when in hot water (i.e. no continuity), and close circuit when it cools down (I placed it in cold water for speed). So this looks to be designed as a cut-out for when the temperature is too high (hot).

The grey one stays open circuit during both tests. I therefore conclude that the grey one is faulty.

Do the two thermal cutouts clip onto the evaporator pipes?

Yes. (see the top right pic on this PDF)

Did you observe whether there a separate temperature sensor attached to the evaporator?

There is a Electronic Thermostat with a NTC attached (see number 3 and 48 in this drawing), which I assume is to control how cold it gets.

The question I now have, is it safe for me to bypass the grey TCO (for testing purposes) to make sure there are no other faults with the defrost heater / controller before I spend £40-50 on a new TCO? (I was thinking of joining the two grey wires together, then operate the freezer (full of food) for a day or two)

The only thing I'm not sure on is if the defrost heater is 'regulated' by the TCO's i.e. is one of the TCO's purpose to act as a 'thermostat' for the defrost heater OR are they just redundant safety devices to prevent overheating in the event the controller fails? (the controller sends the Live to the TCO/heater circuit and 'operates' the defrost cycle)????

LN

[bill88]

fwiw, looking at the new drawing, I can't see any other NTC attached to the evaporator. It does suggest one of the TCOs is used as a 'thermostat' as you suspect. I'd be hesitant about bypassing it.

I presume items 3A in the drawing represent the thermal cutouts.



I wonder if there is a way of forcing a manual defrost cycle to commence on the controller board?

[bill88]

fwiw, a glass of cold water possibly may not necessarily be 'cold' enough to verify the grey TCO is faulty. a glass of water & crushed ice would be closer to freezing.

Thermal Cut-out
- Grey block/wires = no continuity (open circuit)
- White block/wires = continuity (closed circuit)

But I presume the above results was when the TCOs were earlier tested in situ at freezing temperatures, which confirms the grey TCO is defective.

[Leftnothing]

fwiw, looking at the new drawing, I can't see any other NTC attached to the evaporator. It does suggest one of the TCOs is used as a 'thermostat' as you suspect. I'd be hesitant about bypassing it.

Hmm... I'm also hesitant as I don't know the set temp for the TCOs (the PDF suggests there are two temp settings)... and therfore which one is the 'thermostat'..

I presume items 3A in the drawing represent the thermal cutouts.

Yes, 3A on the "Cooling System" diagram and 26/27 on the "Functional diagram" are the TCOs.

I wonder if there is a way of forcing a manual defrost cycle to commence on the controller board?

Logically, you would think there could be, but I can't find a service manual for it (if anyone has one they could email/provide URL, that would be fantastic! or if anyone knows a way to force a defrost cycle? - I could then confidently test the defrost heater (by-passing the faulty TCO), whilst watching! (I've just tried a few combinations of holding down the (only) button whilst switching on the power, but didn't seem to work.. well, it didn't cause the flashing LED's which display when the defrost fails to operate.. and looking on the controller board doesn't reveal anything obvious..)

LN

[Leftnothing]

fwiw, a glass of cold water possibly may not necessarily be 'cold' enough to verify the grey TCO is faulty. a glass of water & crushed ice would be closer to freezing.


But I presume the above results was when the TCOs were earlier tested in situ at freezing temperatures, which confirms the grey TCO is defective.

Yes, I tested in situ and also since removing earlier, I put the grey TCO in the freezer and tested again after a while and still got no continuity on the grey TCO.

[mikeref]

One sensor will be for defrost termination and the other will be for fan delay. So one will open circuit when above zero degrees C to turn off defrost heater regardless of PCB action and the other will not allow evaporator fan to start untill evap coil is several degrees below zero C. Heater resistance sounds fine.. Mike.

[Leftnothing]

One sensor will be for defrost termination and the other will be for fan delay. So one will open circuit when above zero degrees C to turn off defrost heater regardless of PCB action and the other will not allow evaporator fan to start untill evap coil is several degrees below zero C. Heater resistance sounds fine.. Mike.

Hi Mike,

Thanks for the input! - That sounds very logical and if I hadn't seen the wiring diagram I too would have assumed they were for different operations as you suggest. However, in the wiring diagram (parts 26/27) you will see both TCO's are wired in series, so they must both be for the defrost heater!? As soon as the first one in the series goes open circuit, there is no communication with the second TCO electrically so would be unable to control the fan.

Or am I missing something in my thought process?

L.N.

[mikeref]

Yeh, your right. No fan delay on this circuit. Can only be over-current and over temp. I'd be checking if defrost heater is leaking to earth before attempting to bypass fault, but don't leave it in this condition.

[Leftnothing]

Yeh, your right. No fan delay on this circuit. Can only be over-current and over temp. I'd be checking if defrost heater is leaking to earth before attempting to bypass fault, but don't leave it in this condition.

Currently I've removed the TCO's and disconnected the defrost heater and the freezer is 'operating'.

Out of interest, why do you think it might be leaking to earth? - I'll do a continuity test between wires 'b' / 'e' to wire 'a' (earth) with my multi-meter (if that's what you mean?)

There is some discolouration of the defrost heater wires for about 3-4cm near to the internal connection. It looks like the wires have got hot at some point, the sleeve has a fine 'burnt' coating on both wires for 3-4cm but after scratching off with finger nails, it reveals the insulation looks to be sound and intact... This is however a reason why I want to test the freezer more before just replacing the TCO's.

I guess I should also test for a permanent live on wire 'e' to make sure the controller hasn't 'stuck' (failed) ON. I'm a bit hessitant on testing live electrics though with my multi-meter (I've only ever used it for low voltage & continuity checks), especially as there may be a further fault with the controller.

Could I wire in a 230v 60W light bulb between wire 'e' and 'N' in the junction box (1 in the wiring diagram) as an alternative test to a multi-meter? (I can then stand back when I switch it on... just in case... - Essentially, it looks like wire 'e' should only be live during the defrost cycle; so if the bulb comes on when I initially turn on the freezer, there may be an underlying fault with the controller board!? (i.e. the light should only come on when entering it's defrost cycle)

LN

[Leftnothing]

Update:

I left the grey TCO in the freezer for several hours and before going to bed, I thought I'd just do a quick continuity test. Whilst it was freezing cold, I had a closed circuit, I held it in my hands for a few minutes to warm it up and it clicked over to closed circuit.

So, either I 'fixed it' (by warming it up, banging it a few times (to see if it would click over during tests), then freezing it earlier) or my fault lies elsewhere!

In summary, it looks like the Grey TCO cut's out somewhere just above freezing (regulating the defrost thermostat?) and the white TCO cut's out at a much higher temperature (e.g. when I placed it in hot water) as a safety cut-out?

With that in mind, are there any other checks I should make besides those mentioned in post 20 before fitting the TCO's back in?

The symptoms of the original fault are the flashing LED's (see post 2) after a few hours of operation; then after a period of months, the evaporator unit freezing up with solid ice along the top & bottom (suggesting to me the heater is not working correctly).

LN

[Leftnothing]

I held it in my hands for a few minutes to warm it up and it clicked over to closed circuit.

Sorry, that should have said "..clicked over to OPEN circuit!"

[mikeref]

Sorry, that should have said "..clicked over to OPEN circuit!"

Grey one is then the defrost termination thermostat and the white is the over-current thermister. Sometimes they just fizzle out but for this one to fail, there could be some problem with the heater. 375 ohms seems fine for a resistance reading though if power leaks to earth and your home is not protected by earth leakage circuit breaker, than something has to give as in live frame of fridge before regular circuit breaker kicks out. I would test heater with a megga to test insulation resistance. Some defrost heaters have been known to be o.k. when cold but fail insulation test when hot.. Mike.

[Leftnothing]

Grey one is then the defrost termination thermostat and the white is the over-current thermister.

Why do you think the white one is for over-current? - During my tests (see above), it consistently reacts to (hot) temperature and goes open circuit when measured with a meter.

375 ohms seems fine for a resistance reading though if power leaks to earth and your home is not protected by earth leakage circuit breaker, than something has to give as in live frame of fridge before regular circuit breaker kicks out. I would test heater with a megga to test insulation resistance. Some defrost heaters have been known to be o.k. when cold but fail insulation test when hot.. Mike.

I have now tested the defrost heater for earth leakage (it was okay) and the circuits at my home are protected by a earth leakage CB (I replaced it a year ago and was independently tested a few months ago). I don't currently have access to a IR tester (I've used them professionally in the past), how common are the heaters to fail in that way?

Thinking more about the 'brown' coating around a few cm's of the heater wire, I think there is a high probability that occured when the evaporator was all frozen up, including around those wires upto (around) the point where there is discolouration (i.e. the rest of the wire was covered in ice, therefore 'cooling' it down and preventing it from overheating.)

When it froze up the first time (and yesterday when I defrosted it again), there was solid ice around both TCO's and above/below the evaporator. If the heater was operating during that time, there would be no way it would switch off with the temp of the grey TCO below 0 degrees... Without the temperature regulation of the grey TCO, the heater would continue to draw current until I assume the end of the time period allocated by the controller. This is what I presume has caused the wires to get hot.

If the above premise is true, the question remains as to why it frosted up in the first place; heater not working correctly? The grey TCO failed and is now fixed? Abnormal temperatures? The kids leaving the freezer door open too many times? Blocked drain plug (bottom of evaporator)? Anything else? (It had been working correctly for a couple of years prior to the fault developing last summer)

Typically on freezers and/or Zanussi/Electrolux in particular, is there a method to force a defrost cycle? Or does anyone know how often the defrost cycle typically happens? (time in hrs or door opening's etc?) Would this information be in a service manual?

I've now reassembled the freezer with the TCO's back in place, it's been operating normally for 6hrs now with no warning LEDs showing.... {Yesterday, the warning LED's came on the same day... fingers crossed}

LN

[mikeref]

Grey to be the thermal cut out as you said it is closed circuit when below zero celcius and open when held in hands. White would have to be over current as putting it in " hot water" simulates excess power consumption... Getting back to origional question, as to why freezer ices up over several months. This comes down to the age of appliance, wear and tear, ambient temperatures/ humidity and duty cycle. Sometimes newer fridges and freezers are "lemons" right from the start. If the defrost heater did not function at all, then freezer would fail within, say 4 days, not months. Check compliance sticker on freezer and see what heater is rated at. using your figure, 375 ohms, and formula: V= IR, then, at 240 volts, "I" would have to be .64 amps. Therefore volts times amps (240 by .64) = 153.6 Watts... mike.

[Leftnothing]

Well, after working okay for 35hrs, the warning LED's have just come on again.

Without opening the freezer, I did a continuity test from the junction box at the rear on the connections with the heater/TCO's and got open circuit. To double check it was the grey TCO that was open circuit (faulty?) I removed the rear cover and checked (it was), the white TCO was closed circuit.

I measured the TCO temperature with my IR thermostat and got a reading of 1 degree C, however, this was after several minutes of the door being open, me removing the screws, and testing the TCO's.

I'll check for continuity again in a few hours (now i've re-assembled and it's back on again) and if it's still open circuit, it looks like the grey TCO is intermittently faulty and the likely cause of my problems.

BTW, I measured the heater again and got a reading of 385 ohms. The rating plate says 138.5 watts and 230v for the compressor. So assuming 230v and 385 ohms gives me 137.4 watts which is about right then!?