Vauxhall Combo 1.3 cdti - Car Battery isolator - positive or negative post?

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Wed 19 Feb 2025 18:42
Vauxhall Combo 1.3 cdti - Car Battery isolator - positive or negative post? - Cornish Lass

Hi All,

We have had a parasitic drain on our 2005 Combo van, which has proved difficult to trace. Had to resort to disconnecting the battery if van parked so as not to drain it. As a last resort, we fitted an isolator switch to make it easier to cut the battery circuit when van is parked. Installed this via the negative terminal on the battery. All seemed OK for a few days, then found battery flat after van had been parked over the weekend. Wouldn't hold charge and was fitted in 2018, so time for a new one. Garage replaced it today, but mentioned the isolator in the negative circuit may have led to the battery's demise, as the battery is "searching" for earth/ground so leaching charge when switched off. They also mentioned the switch should be closer to the battery, rather than inside the cab (cables around 2m but size/capacity was recommended by the supplier). Don't want to damage my new battery, so should I switch the isolator to the positive circuit, or relocate it closer to the battery? We thought the negative post on the battery was the right way to go, but if this is not the correct way to fit an isolator, please could someone advise.

Any advice appreciated, thanks :)

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Wed 19 Feb 2025 19:01
Vauxhall Combo 1.3 cdti - Car Battery isolator - positive or negative post? - elekie&a/c doctor
Battery isolation always on negative side ( unless it’s an old positive earth car ) . If you’ve used the correct size cabling for the re route , then I can’t see any problem. If the battery is fully isolated with this switch, then it can’t be searching for anything. Think the garage have made up that story . I’ve fitted one of those remote control battery shut off systems to my little used bmw . Works fine , with no extension of any cabling .
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Wed 19 Feb 2025 19:32
Vauxhall Combo 1.3 cdti - Car Battery isolator - positive or negative post? - Big John

When there is a full break in the circuit between a battery terminal and THE incoming cable then the battery is disconnected so will not pass current however battery terminals may have multiple cables emerging from them so all have to be isolated.

Frequently (but not always) the negative terminal is a single thick cable (usually routing towards the body and then towards the engine) that would be the easiest to insert an isolator. However because the currents involved when starting the car are high I wouldn't recommend long cables too/from the isolator as voltage drops can be higher(V=IR). The cables would need to be vey thick.

Isolating would technically work on the +ve side but frequently there are numerous cables emanating from the battery terminal and having an unfused +ve terminal in the car could be dangerous if it shorts out anywhere.

Re the original parasitic drain they can be difficult to pin down but measuring voltage drops across fuses can be useful. Still difficult as cars frequently fire things up when you are in and about the car - ie opening drivers door etc. Lookup table:-caravanchronicles.com/wp-content/uploads/2018/10/f...f

Edited by Big John on 19/02/2025 at 21:14

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Wed 19 Feb 2025 23:26
Vauxhall Combo 1.3 cdti - Car Battery isolator - positive or negative post? - Andrew-T

With my simple mind I would have thought removing one of the main battery connections should be OK for a 20-year-old vehicle - assuming the battery is reasonably accessible. It is naturally more convenient to arrange something in the cab, but a diesel takes a high starting current, hence the need for heavy cabling - is that worth it ?

It's what I do with my 1991 Pug 205, but that is a simpler petrol car ... ! Takes about a minute to connect or disconnect when needed.

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