Bodged Fridge Thermostat?

[Baggywrinkle]

Fridge packed up at the weekend, investigation revealed a 1KOhm resistor soldered into the thermostat wire, cause of failure was the solder joint.

Anyone care to hazard a guess as to why this was done?

 

[nigelmercier]

Soldered to where? Is it a Danfoss? There is a resistor you can add to change the cut off voltage, it's in the manual.

 

[Baggywrinkle]

Don't know what make the compressor is ... it appears to be the original as delivered from Bavaria in 1999.

The resistor was soldered in series in one of the 2 wires connecting the thermostat to the compressor, effectively increasing the resistance of the thermostat circuit by 1 kOhm.

Symptoms were, the pump would run briefly for a few seconds then stop. A couple of minutes later it would run again for a few seconds and stop.

I disconnected the thermostat (at the compressor) and shorted the terminals on the compressor together - it ran continuously.

I reconnected the cables and tried shorting at the thermostat end, nothing happened. Strange I thought, then I noticed some insulation tape on one of the wires - removing it revealled the resistor which had broken at one end.

Dilemma is do I just replace the cable or should I put the resistor back?

 

[FlyingDutchman]

Are you sure the batteries are sufficiently charged and the connections are all ok? The Danfoss compresors do as you describe when they think the batteries are not charges enough. A bad connection can also lead to this effect.

 

[Baggywrinkle]

I thought that, but a direct short of the thermostat terminals had it running continuously which is why I suspected the thermostat/thermostat wiring.

The boat was on shore power and had been charging for 24 hours, leisure and starter batteries were all fine but I didn't measure the voltage at the compressor. Cables certainly looked thick enough (and the connections healthy enough) to cause minimal voltage drop.

I removed some redundant cabling - it really was redundant - and in the process I think we disturbed the thermostat wiring and broke the soldered resistor in the cable - it looked like it was hanging on by a thread anyway.

Hence the question ... why the resistor ?

 

[little_roundtop]

Soldered to where? Is it a Danfoss? There is a resistor you can add to change the cut off voltage, it's in the manual.

+1

I've had the same problem with my Danfoss BD35F. The resistor is there to control the compressor motor speed. See here. A 1.5kohm resistor (on mine) makes the compressor run at full speed.

 

[Billjratt]

The resistor is there to boost the compressor when there is lots of power available.
It's a mod I found on a website concerning motorhomes in the States. I've got more info on the boat, but not there now.
If you short it out you are back to normal operation IIRC.
I never got round to doing it but was going to fit a "boost" switch to the circuit.

 

[nigelmercier]

+1

I've had the same problem with my Danfoss BD35F. The resistor is there to control the compressor motor speed...

Yes, this is it (R1), I didn't have the data sheet handy. But there is another one (R2) to change the cut-out voltage as I said above. Well worth doing, as the default is something like 10.5V

To the OP, I would try it without resistor R1, then replace it if you want the pump to run faster.

 

[Baggywrinkle]

Thanks for all the answers, I'll take a couple of resistors with me and see if it's a Danfoss.

 

[little_roundtop]

Thanks for all the answers, I'll take a couple of resistors with me and see if it's a Danfoss.

Don't leave the resistor "hanging" just soldered in to the wire. After mine broke I cable tied the resistor to a handy spot and ran the wires to there. Now there is no (physical) load on the resistor or the soldered joints and it's lasted four years so far......

 

[CharlesSwallow]

+1

I've had the same problem with my Danfoss BD35F. The resistor is there to control the compressor motor speed. See here. A 1.5kohm resistor (on mine) makes the compressor run at full speed.

If you look at the wiring diagram and the associated tables you will see that "R1" is a VARIABLE resistor (that's what the arrow means) to reduce compressor speed and that "R2" is a fixed value one - although several values are given in the "Battery protection" table to allow various minimum cut out voltages to be arranged.

Now my compressor (same type) has only R2 and not the variable one and I suspect that is true of most. It certainly applies to units fitted to Bavarias.

When buying a replacement you need to check if there is a resistor in place and what it's value is and whether this fits your requirements.

The bit of insulation tape is just a sophisticated Bavaria modification. Mine was yellow!

Chas

 

[nigelmercier]

If you look at the wiring diagram and the associated tables you will see that "R1" is a VARIABLE resistor (that's what the arrow means) to reduce compressor speed ...

No, adding R1 increases the compressor speed. It is shown as a variable type, but this is only so it can be adjusted to find a suitable value.

 

[CharlesSwallow]

No, adding R1 increases the compressor speed. It is shown as a variable type, but this is only so it can be adjusted to find a suitable value.

depends which way you turn it, surely? Are you just out to score?

Chas

 

[nigelmercier]

depends which way you turn it, surely?

No, adding the resistor will never drop the revs, only increase them.