Hotpoint fridge freezer repair

Thought I'd write up a repair I did last weekend. This is a frost-free fridge freezer, single compressor, bought 1997 IIRC. Symptom is that the fridge stopped being cooled, intermittently at first, and then permanently.
First, I had to work out how it worked. The evaporator (inside cold element) is in the freezer only, behind a panel, with a fan to blow air around the freezer compartment. The evaporator has a heating element intertwined to run defrost cycles on the evaporator. There are two air pipes from behind the freezer panel up to the fridge compartment. They form a flow and return air path to the fridge, and there's a motorised flap on one of these to open/close it.
On the web, failure of this design to cool the fridge is not uncommon, the two reasons typically being the fan motor burns out, or the air pipe gets blocked with ice. Neither applied in this case.
A few weeks ago I had a go at fixing it, but I had almost no tools with me at the time. Having established that the fan was not burned out, and that there was no ice in the freezer, my suspicion was a thermister in series with the fan. I shorted it out, and the fridge started working. However, within days, it became intermittent again, so that wasn't the problem.
At this point, armed with soldering iron and some resistors/LEDs, I added some lights to show when the microcontroller was driving the compressor, the fan, and some other things. It became obvious that the fan output from the microprocessor was not operating most of the time I expected it to. Indeed, it was mostly off, but flickered occasionally.
Without the fan, it seems there is just enough convection cooling to keep the freezer cold, but nothing to drive cold air into the fridge. As a temporary measure, I disconnected the fan output from the microcontroller, and jumpered it across to the compressor output signal. I wasn't really expecting this to work too well, as that would mean the fridge would only cool when the freezer was calling for the compressor, but thought it might keep the family going a few days until they can get a replacement.
However, much to my surprise, it seems to be working perfectly like this. both fridge and freezer are keeping rock stable temperature. It may be that at some extreme of ambient temperature, the fridge could drift from its setting - we'll see.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Friday, April 18, 2014 9:27:52 PM UTC+1, Andrew Gabriel wrote:

Might be worth putting on the wiki.
NT
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Kind of makes you wonder, though, what it is in the controller which is causing this. It sounds more like a dry joint as chips tend to just fail one way or the other, not now you see it now you don't. Incidentally something similar to this finished off an old Hitachi from the 80s a few years back, but it was beginning to degenerate insulation wise as well so I got a new one in the end. Brian
--
From the Sofa of Brian Gaff Reply address is active
"Andrew Gabriel" < snipped-for-privacy@cucumber.demon.co.uk> wrote in message
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Fri, 18 Apr 2014 20:27:52 +0000 (UTC), snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

This is a feature of frost-free freezers. They all do this. I'd rather defrost the freezer every so often.
--snip--

It might work but I would expect the fridge temperature to vary unpredictably in this case.
--
(\__/) M.
(='.'=) If a man stands in a forest and no woman is around
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wednesday, April 23, 2014 10:23:48 AM UTC+1, Mark wrote:

.
,
p
r,

y
ht

.
It means fridge cooling is proportional to freezer cooling. This will be fi ne within a limited ambient temp range. Since average ambient temp changes gradually (after the machine's thermal inertia) it wouldnt be hard to tweak the dial a bit between summer & winter.
NT
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
On Wed, 23 Apr 2014 02:58:25 -0700 (PDT), snipped-for-privacy@care2.com wrote:

I guess a lot depends on how often you open the fridge compared to the freezer. The compressor may not need to run for the freezer but the fridge temperature may rise significantly if the kids keep raiding the fridge ;-)
--
(\__/) M.
(='.'=) If a man stands in a forest and no woman is around
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Indeed. I suspect this one has gone wrong about the same number of times as my own (non-frost-free) has needed defrosting over the same period (about 3 times in ~15 years).

So did I, but so far, it's stayed on its setting except for a few mins after the door has been open a lot.
Frost-free fridge freezers often have several fall-back paths in their microprogram to cope with common component failures, such as thermisters used by the defrost cycles, which they will try to work around by changing to a fixed defrost duty cycle, but at temperature extremes, this may fail to fully defrost and cause icing up.
--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]
  Click to see the full signature.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.