Electrical testing on Frigoboat 12/24v systems

voltage meterWe handle many calls for troubleshooting help, and the vast majority end up being electric problems, mostly with the boat’s power supply to the Frigoboat unit. While the voltmeter on the distribution panel might show more than adequate power, it is what comes out of the end of the two wires at the Frigoboat unit that counts, and there is a lot of potential in the wiring for gremlins to creep in and spoil the party.

Power supply testing on Danfoss DC powered refrigeration systems

To properly test the power supply to a Danfoss/Secop powered 12v or 24v system, the following testing procedure must be carried out. This will establish whether the power supply feeding the system is free of bad, loose and/or high-resistance connections. Reading the voltage on the panel or at the batteries is meaningless,

as is the fact of a new installation or new batteries. Size and the capacity of the battery bank is irrelevant.

1. Turn off the breaker (or remove the fuse) supplying DC power to the system.

2. Unplug one of the thermostat leads at the controller.

3. Using a multi-meter, read the DC voltage at the battery terminal(s).

4. Connect the multi-meter reading DC voltage to the power terminals (+ and -) on the controller so that it can be left connected and monitored.

5. Turn on the breaker (or install the fuse) to the system.

6. Check that the voltage is the same as the voltage seen at the battery terminals.

7. Whilst watching the multi-meter, reconnect the thermostat lead and monitor the voltage continuously before, during, and after the compressor starts or attempts to start.

Interpreting results

▪ If the power supply is free of loose, bad, and/or high resistance connections, the voltage reading at “5” above will stay very stable and only drop slightly when the compressor starts. As a general rule, on a 12v system the reading should not drop below 12v.

▪ If, when the compressor attempts to start, the voltage reading drops significantly, a bad electrical connection should be suspected. If the voltage drop is sufficient to fall below the 10.5v (23v) cut-off built in to the controller, the compressor will stop. (At this point the voltage may return to it’s original reading.) The fan or pump will continue to run for approx. 45 seconds and then the compressor will attempt a re-start. If the voltage is then above 11.5v (23.5v) the compressor will start or attempt to re-start again. WARNING If the multi-meter being used is a digital model that is slow to react, the voltage may drop below 10.5v (23v) and then recover too quickly to register on the meter. This can lead to the situation where the compressor starts then stops from low voltage, the voltage returns to it’s original value, and there being no significant drop on the meter.

▪ If the compressor starts and runs OK but stops after a short while, the voltage may be gradually dropping towards and below the 10.5v (23v) cut-off point. This should be easily identified on the meter.

▪ If the nature of the fault is such that the voltage reading at “5” above drops below 10.5v (23v) even before the compressor attempts to start, a very bad electrical connection must be suspected. This is because even the small load of the fan or pump relay, both less that 0.5 amp (0.25 amp), is seemingly sufficient to reduce the voltage considerably.


What to look for

A loose and/or high-resistance connection can be anywhere in the supply between the batteries and the controller. i.e. a bad breaker or fuse, a loose or corroded screw connection, a poorly made or corroded crimp connection, a damaged section of wire, etc. HINT A good place to look first is the negative (ground) connection, especially on a European-built boat. These tend to be multiple, common connections that are added to over time.


A note on controllers

1. Older (pre-1995ish) four-pin controllers for BD2.0, 2.5, and 3.0. The Danfoss version of these controllers had no voltage alarm. The Frigoboat version has an LED that flashes when the voltage reaches 11.5v (23.5v) and then glows steadily and stops the compressor when the voltage reaches 10.5v (23v). The compressor will attempt to start again when the voltage rises above 11.5v (23v).

2. Current three-pin controllers for BD35 and BD50. These controllers have provision for a 12v diagnostic LED to be connected, and this is a standard feature on the Frigoboat Smart Speed Control (SSC). This will flash a fault code in the event of low voltage, high fan/pump-relay current draw, low compressor RPM, compressor non-start, or electronics overheat. Diodes are available from Coastal Climate Control, Inc.


Resistance (Ohms) readings for compressor windings

Older (pre-1995ish) four-pin controllers for BD2.0, 2.5, and 3.0 - The cable from the controller connects to the compressor by means of a four-way plug that is a friction-fit onto four pins on the compressor, which are arranged in a diamond pattern. The resistance reading from the bottom pin to either of the two side pins should read approximately 0.2 ohms. The resistance reading from the bottom pin to the top pin should read approximately 3-4 ohms. There should be no continuity between any of the pins and the compressor shell.

Current three-pin controllers for BD35 and BD50 – The controller is mounted on the compressor and is secured by a screw. Removing the screw allows the controller to be levered back to expose three wires that connect to the compressor via a three-way plug that is a friction-fit onto three pins on the compressor. The reading between any two of the three pins should all be the same; approximately 1.5-3 ohms. There should be no continuity between any of the pins and the compressor shell.

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