Battery isolator stops van starting

[Bewick]

Yes, the isolator is supposed to stop the van from starting, but not when it's in the de-isolated position!

My Mk6 Transit is often sitting still for a little while and sometimes has a flat battery when I come to turn her over. I started disconnecting the battery when I knew the van would be sitting for a prolonged period and found that when I reconnected the batter when I came to start her again it was fine. Disconnecting and reconnecting the battery all the time is a bit annoying so I fitted an isolator switch which initially at least worked. Then when I came to start her she wouldn't turn over. I checked the voltage on the battery and it was fine (12.3 or something, enough that she should have been starting). So I took the isolator out of the equation, connected the battery up as normal again and she started.

This was a bit weird. The isolator was fitted properly (as far as I'm aware! - on the negative battery terminal), the battery was definitely isolated (central locking etc wasn't working as you'd expect) and when I de-isolated the central locking and so on worked again, it just wouldn't start the van.

I wondered if the switch might be faulty, so I tried a different one - same problem.

I'm back to disconnecting and reconnecting the battery for the time being but it would be good to know if this has happened to anyone else and if anyone knows why, or how I could fit something which will protect the battery from being drained if she has to sit still for a while.

Thanks!

[Altransit]

Anything in the circuit that offers any resistance, even a tiny amount, to current flow, will stop it starting. It could be that the switch is barely capable of passing the large current needed during starting, or it could be possible that the glow plugs have gone open circuit, causing a large current drain at starting, and the switch is just that bit too much resistance

Are there any current ratings printed on the switch

[Bewick]

I'll have a look when I get home. I got it online and I'm pretty sure the listing said it would be okay for a van. Would something a bit chunkier potentially be in order then?

[Altransit]

It's hard to say without knowing what the switch is, but the fact that bypassing the switch allows the van to start normally makes me think that the switch isn't man enough for the job. It would be worth checking if the glow plugs are shorted out, and drawing too much current when starting, it's a common problem with Mk6's

[v8dave]

These type of isolators have been used in motorsport for years with no apparent effect on starting.

[the dutch guy]

12.2v is borderline. If you have a spent battery it will not start. Float voltage is not a indication of capacity or capabillity. Depending on temperature it can mean that 12.2v is about 20~30% capacity left at 20c. If it is colder it is even less.That is not enough to start.

[Bewick]

12.2v is borderline. If you have a spent battery it will not start. Float voltage is not a indication of capacity or capabillity. Depending on temperature it can mean that 12.2v is about 20~30% capacity left at 20c. If it is colder it is even less.That is not enough to start.

I can't remember the exact voltage, she did start once I removed the isolator switch from the equasion so it's not a battery issue.

[dumper]

My brothers doblo has been slow At starting for a bit let him down last week I have a HD battery tester and a starter motor tester after jumping off and driving home starter amp draw ok nearly melted battery tester it says it’s ok and chargeing so put it down to one of them things two days later same again so new battery fitted sorted no more problems.at work they have one of the high tech tester often it will tell you battery ok when it’s not sometimes you have to just fit new battery

[Bewick]

As I say, the battery will start the van when the switch isn't fitted, so I don't think it's the battery. It sounds like the switch might be a touch too much resistance and I'll check the amp rating when I get a chance. Any other ideas as to why the switch might be causing this problem would be much appreciated.

[Bewick]

It's hard to say without knowing what the switch is, but the fact that bypassing the switch allows the van to start normally makes me think that the switch isn't man enough for the job. It would be worth checking if the glow plugs are shorted out, and drawing too much current when starting, it's a common problem with Mk6's

If the glow plugs were shorting would the van still start? But just take more from the battery to do so?

[dumper]

I assume you have unbolted the earth wire from the body and fitted it to one pole on the master switch have you used to small a wire from master switch to the earth point on the body and then getting a high resistance in the earth circuit

[dumper]

Just another thought the mk 6 has a fuseable link in the earth lead when you rerouted the lead to fit it’s not putting a strain on it and giving a bad connection

[Bewick]

Right, I've finally had time to check the amp rating. It's 200! That's probably too low isn't it? Could a switch with a higher rating sort out the issue?

[dumper]

If you can get hold of a inductive amp meeter and crank the engine over with starting it you may have to pull a fuse or relay when I do it i use a snap on remote starter it will tell you what amps the starter is drawing if starter is good it should not draw more the 120 to 150 amps if it goes over 200 the starter may be past it’s best

[the dutch guy]

Right, I've finally had time to check the amp rating. It's 200! That's probably too low isn't it? Could a switch with a higher rating sort out the issue?

it should remain under 120A at all times. 100ish during the first half-second and then drop to 80ish amps. anything above 120 means something is wrong. usually it means the battery cant sustain the voltage so the current rises until it cannot supply that anymore and then it dies.

the main fuse should be 300A. a fresh battery should be able to blow a 200A fuse instantly if you tried starting while in gear or something.

if you have a solid 14v during engine on and below 12.5v when you measure after leaving it for the night then my first reaction would be to replace all the batteries you have. mixing old and new batteries is not smart, the worn one will kill the new one in a few weeks or months.
by replacing them both you can use a manual swich to use both batteries for starting making them live much longer as you spread the load over both batteries.

with lead acid: 1+1=3 batteries.