Cyrix ct 120a not opening?

[mattoesaurito]

Hey there. Weird situation happening here, would like to know your ideas!

Setup is a simple little van, starter battery, cyrix ct and a leisure battery. Got a sensor error and suspected a low voltage of starter battery, did some tests few days and weirdly I always noticed that the voltage of starter battery and leisure battery was always the same after few hours.

The cyrix is closing correctly when engine on and correctly charging the leisure battery btw.

So I’m thinking the problem could be the cyrix not opening while engine off. Did another test, testing voltage when engine off of starter battery. When the fridge connected to leisure battery started drawing power, the voltage of starter battery was going down!!! So I’m supposing cyrix is faulty. Disconnected from system, and did a continuity test but was negative? How can I properly diagnose it?

[MikeD (Mike)]

So : With the test that you are doing, loading the leisure batter with fridge, test to see if both battery voltages are less than 13.2V.
Also test the behavior of the control input: If low or disconnected, then the relay should open - even with the battery voltage higher than 13.2V. In this case “low” probably means less than 6 - 8 V. If this remains high, then the batteries will stay connected until <13.2V.

check also that it’s wired according to diagram.