A fridge/freezer question for 'Pamow69'!

Hi Pamow69, I have a problem with an F&P N500B fridge/freezer.
The fridge is working fine, and appears to be defrosting itself fine. The temperature is correct at about 4°C.
The freezer however is not cooling down the the correct temperature.

Some weeks ago, I first noticed that the items in the freezer were not as frozen as they should be (The icecream is a dead giveaway as it is used regularly. ).
I thought that the door may have been left ajar at some stage so ran a temperature check over a period - found that the temperature was only down to about -5°C instead of the expected -18°C.
Removed all items, removed the fan panel and found that the evaporator fins were well iced up - defrosted thoroughly and reassembled.
The freezer then worked fine with temperature stable at -18° until the last few days when the problem recurred.
Same icing on the evaporator so defrosted again.

This leads me to believe that the auto-defrost system on the freezer evaporator is not working.

I have the schematic to hand so I've worked out how it's all supposed to work and what's probably not working.
I've measured the resistance of the defrost heater element at the connector behind the fan panel and that seems to be a sensible value so I'm assuming it's intact.
As I see it, the most likely suspects are the thermostat on the freezer evaporator or the defrost timer motor switch.
However, because of the way these 2 items work, I will have to make some sort of watchdog monitor that I can attach and check the operation of the circuit elements over a period of time! (Days, I presume for the full defrost cycle to run).

Before I do this, it would help if you could please answer a couple of questions based on your extensive experience in the business?
1) How long is the typical defrost cycle? Is it hours or days and is it based on compressor run-time or does the timer run independently? (It's not clear from the schematic).
2) In your opinion, which of the 2 devices (Freezer thermostat or defrost timer) is the most likely to be at fault and deserves the most scrutiny in long-term monitoring?

Cheers.

Pretty sure this is the model with the mechanical defrost timer, they defrost every 12hrs for about 30 minutes roughly. It is not based on compressor run time.

You can test the defrost timer motor connections with a multimeter. Not sure of the resistance of the motor coil, but probably somewhere in the range of 2-10kOhm at a GUESS. Also check the switch contacts.

Or you can manually turn it to defrost, and then see if it turns itself off defrost after about 30min, to check that the timer is running. At the same time, check if the heater is drawing current.

I don't know if they have an over temp cutout for the heater element. Maybe you can see from the diagram. Obviously that would be closed at normal temperatures.

I haven't seen Pamow around for a while but maybe they will turn up for your special request ;)

Post a photo of the wiring diagram if you want more information.

Thanks johotech, that's useful info. The timer is a mechanical motor driven one - at present it's just an anonymous grey box on a bracket alongside the compressor, covered with years of dust!
I'll dig it out, clean it up and do some multimeter checks on it.
I didn't realise it could be advanced manually - I thought I would have to wait out the cycle - that'll make it much easier to diagnose.

The freezer thermostat has upper and lower switch points of +9°C and -8°C. (with an over temperature fuse point of 66°C!)
I can see that waiting for the system to shift between the 2 switch points could be a bit tedious so I am expecting to have to cobble up a custom monitoring breadboard device that can be left attached unattended and will light up some 'flag' LEDs to show what has or hasn't happened!

Cheers.

https://trademe.tmcdn.co.nz/photoserver/full/400283478.jpg

It shouldn't be complicated to fix. The timer is the usual suspect.

If you can measure the resistance of the heater element at terminals 1 & 2 (when the freezer is cold), then the "thermostat and fuse" is probably fine.

The simple way to test it is connect through a amp meter, run it until it's cold, (1hr or so), manually turn the timer to defrost, check the heater current, then wait to see if the defrost timer turns back around to normal cooling.

Thing is with those timers, is that it might not be the motor burnt out, it could be that the little plastic gears driving the switch are stuffed.

Defrost timer is probably around $50-60

Hi tmenz I don't have a lot of experience with fridge repairs. I prefer to stick with dryers,wash machines, dishwashers & ovens. I do have a manual for that model so the following may help:
DEFROST TIMER - NO FROST MODELS
The finned coil located behind the back wall of the freezer compartment will defrost every 12 hours of compressor running time. On initiation of the defrost cycle, the compressor and fan switch off and the defrost heater (attached to the finned coil) is energised. The compressor and fan will not restart for 20 minutes after initiation of the defrost cycle. Should the coil defrost and reach 8deg C+/-3deg C beforethe 20 minutes has elapsed, the thermostat will open circuit the element. The defrost cycle can be initiated manually by turning the Manual Advance screw located on the back of the timer. TURN CLOCKWISE TO ADVANCE.

Also this test procedure :
PARAGON DEFROST TIMER 12 HR
Part No 873253
Timer motor across terminals 1-2.Normal operation. Contacts 1 -3 closed.
Defrost operation. Contacts 1 -4 closed.
The defrost timer is located in the compressor unit compartment and is mounted on the terminal block bracket with two screws.
DEFROST TIMER TEST PROCEDURE
The following procedure is recommended to all servicemen before they are replaced.
1. Measure the resistance of the timer motor between terminals 1 and 2. This should be in the range 27K -30K ohms.
2. Rotate the cam on the timer (clockwise) to align the marks on the cam with the line on the housing. You should hear the internal contacts break as these marks align. Slowly rotate the Cam 8 'clicks' again, and you should hear the contacts break again.
3. If you don't hear them break, the switch contacts may be welded. With the cam mark somewhere between the two positions noted above, check the continuity between terminals 1 and 3, then 1and 4. Terminals 1 and 3 should be open circuit and terminals 1 and 4 should be closed circuit.
4. If the above checks are correct, connect (230V) phase lead to terminal 1 and the neutral to terminal 2. Take care to insulate.
5. Looking at the back of the timer (on the motor body) you will see 2 holes covered by a clear plastic sticker. Looking down these holes you should see a "dot" on the motor rotor revolving. Note: On a jammed timer the "dots" will not be revolving as the motors power to rotate is very
small and can stall the motor.
6. If all these tests check out OK, then the timer is expected to be in good condition (and should not be replaced or returned for a warranty claim). These tests can be carried out in the home. Often the PC evaporator coil being iced up is due to the FC door not being closed correctly.
The Paragon Defrost Timer 873253 is a 12-hour timer with 1 defrost every 12 hours of accumulated compressor running time. The defrost time lasts 20+/- 3 mins. (8 clicks).

Johotech and Pamow69, thanks very much for all the advice and info - it's always much easier when service info is to hand rather than have to work everything out from first principles!
Now it should be just a matter of systematically working through it!
Cheers.

The freezer thermostat is only there to terminate the heating of element if the temperature of evaporator coil reaches 8 deg before the defrost timer clicks back to normal run. Should only be a problem if it is permanently open circuit. Simple test with your meter. For the defrost timer you could put a small white paint dot on the cam so you can check and see if it moves around over a period of time. If it does then manually turn it until it clicks and then check if fan & compressor stop and also if element is getting warm. ( Defrost element is 171 ohms )

Thanks Pamow69. Today's the day for flexing the muscles and then getting down and dirty behind the fridge and grovelling in the dim dark recesses of the compressor compartment! I reckon I'm getting too old for this sort of thing though!

Problem sorted (I hope).
Timer not turning - motor winding open circuit.
New one on order from F&P for $50 incl. courier. (p/n 813516)
(Defrost element measured at 167 Ohms).
Thanks again for your info and pointers.
Cheers.

Hmmm, update!
F&P Parts dept choked on the order for p/n 813516!
Has to be p/n 813516P which is the latest substitute and is, of course, more expensive at $55 +GST!

Do you want me to get a price for you?

Checked my supplier. Only $1 cheaper than the price you got.

Thanks, I've ordered one - should be here tomorrow.

Hi tmenz good to hear you have solved the problem. If you ever need a part let me know & I can order it on my trade account with F&P or Simpson/Westinghouse. Might save you a few dollars. Trade on that part is showing as $48-13 plus GST.

Thanks Pamow69.
In deference to the increased price, they waived the courier fee so $63.25 is what I paid - I'm happy with that!
The replacement slotted straight in and all functions are now good.
I took the opportunity to take measurements of all the component resistances and I now have a good understanding of how the whole system works.
Cheers.