Split charge relay - fuse blows

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Split charge relay - fuse blows

Postby adrianbutter » 07 Jan 2018, 21:19

The fuse blows on my SPlit charge relay too often, and especially when I drive with a load attached - like a camping fridge plugged in to the Aux battery - so I thought I would take a proper look.

Firstly I found it odd that the Autosleeper wiring (for the internal lights and other bots and bobs came off the wire from the Relay to the Aux battery - but this is the same positive on the aux, so I guess makes no difference - you'd just expect it to be a connection to the Aux itself, rather than in the main battery well.

But here's the weird bit - the relay has been wired up wrong - for years? The earth is connected to terminal 86 (where the switched live should be) and the switched live is connected to 85 - the earth. Looking at the relay diagram - https://www.amazon.co.uk/Hella-4RA-003- ... B00H8UDS6S
- shows however this should not matter ? As 85 / 86 are the low current side of the relay, and the connections have been simply reversed.

Am I right / does any of this explain the frequent fuse blows? (Fuse is in a fuse-holder directly next to the positive terminal of the main battery) I have used a 15A glass fuse. The relay has a 20A fuse.
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Re: Split charge relay - fuse blows

Postby AngeloEvs » 07 Jan 2018, 23:04

Not clear how your two fuses are connected, is one in line with the relay coil (86or 85) and the other in lne with the main battery charge feed (30) and if so which is doing what?

I use a 30A amp fuse from Main battery to Split Charge relay.

Some prefer to have the fridge connected to the ignition live side due to the high current draw of absorption type fridge elements.

Assuming your leisure battery is in generally good condition and not heavily discharged it is worth checking that the relay is disconnecting the leisure battery from the starter battery when the ignition is off. If the two batteries remain connected via the relay with ignition off then a high current will be drawn from the leisure battery when the starter is engaged.
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Re: Split charge relay - fuse blows

Postby 937carrera » 07 Jan 2018, 23:28

This is a good starter for how your Autosleeper should be wired - depending on year variations.

https://vwt25.blog/2016/05/13/zig-mc-2000-wiring/

All the relays and fuses are in the left/ aux battery compartment on mine, so yours could have been "amended"
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Re: Split charge relay - fuse blows

Postby adrianbutter » 07 Jan 2018, 23:42

Good evening - thanks for the reply.

The blown fuse is on the cable from the battery - (in line fuse holder), the other fuse is part of the relay iteslf - standard JustKampers Hella unit. I've been using a 20A, as the relays are sold with a 15A fuse?

Is there a chance the charge indicator is not from the right place? I assume the charge should only begin to trickle to the Aux once the engine is started - but I note that whilst the link is dead when the key is out - it is 'connected' when I turn the key initially (before turning the starter)...
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Re: Split charge relay - fuse blows

Postby adrianbutter » 07 Jan 2018, 23:48

Thanks '937' - I see the autosleeper diagram (my bus is 1985 - so older) has 30A fuses - so I will try this.

Just need to work out if the 'From Vehicle Ignition Switch' is correct - as surely this should only send current to Aux once engine is started. Meanwhile I'll test the van again - to see if this really is the case.
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Re: Split charge relay - fuse blows

Postby 937carrera » 07 Jan 2018, 23:57

https://www.12voltplanet.co.uk/split-charging.html

You should have a split charge system of variant "2" on the above page.

I suggest you go back to the basics of checking how your van is wired against the AS diagrams. It sounds as if the fuse you are using is smaller than AS fitted as standard, so it could simply be blowing the fuse for that reason, so step 1 could be to simply uprate the fuse to standard (note you would not be putting a bigger fuse in as a bodge in this case). I wouldn't pay any attention to what JK say.

It is of course possible that someone has modified (amended :wink: ) the wiring at the ignition switch end so that current can flow when the engine is not running (see comment in the 12vplanet regarding using the alternator output as the feed for the relay).

Swap the terminal 86/87 connections over as well - shouldn't make any difference as you say, but better to have it right.
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Re: Split charge relay - fuse blows

Postby adrianbutter » 08 Jan 2018, 00:11

Great, thank you. I'm going to change fuses tomorrow, and then take a look at 'following' that cable to see what it is using...
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Re: Split charge relay - fuse blows

Postby 937carrera » 08 Jan 2018, 00:28

If you have a multimeter, just put it across the leisure battery and see when the voltage changes as you go through the process of starting the van.

If it drops when cranking that would likely be bad. If it doesn't increase until the van is started that would be good :)
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Re: Split charge relay - fuse blows

Postby bigherb » 08 Jan 2018, 07:20

adrianbutter wrote: (Fuse is in a fuse-holder directly next to the positive terminal of the main battery) I have used a 15A glass fuse. The relay has a 20A fuse.

Glass fuses are rated at the current they will blow at, continous current is half the fuse rating, e.g a 15A glass fuse = 7.5A blade/ceramic fuses which are rated at their continous capacity (they blow at twice their rating).
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Re: Split charge relay - fuse blows

Postby Mocki » 08 Jan 2018, 07:40

it could just be your leisure battery is too flat and the system is trying to draw too big a current that the megre split charge can deliver
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Re: Split charge relay - fuse blows

Postby adrianbutter » 08 Jan 2018, 09:01

Think I need a new multimeter.... My VDO gauge on the dash measures around 12 pre start, and 13.5 after start. Meter at the battery terminals has 10V, and the leisure batt exactly the same 10V..

Anyway, what I did get is that the line to the split charge relay has been badly fitted - it is live when the ignition key is turned, but before the cranking position (is there a name for that state - ignition on I guess, but not yet engine).

On a T25 1985 - is there a good wire to tap in to under the dash / near the fuse board? I see the advice is to take off the altenator - buit presume there is a connection at the front..
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Re: Split charge relay - fuse blows

Postby bigherb » 08 Jan 2018, 09:28

adrianbutter wrote:
Anyway, what I did get is that the line to the split charge relay has been badly fitted - it is live when the ignition key is turned, but before the cranking position (is there a name for that state - ignition on I guess, but not yet engine).

That is how VW wired them. It should switch off whilst you are cranking the engine to start, then back on again when the engine has started.
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Re: Split charge relay - fuse blows

Postby 937carrera » 08 Jan 2018, 23:18

adrianbutter wrote:Think I need a new multimeter.... My VDO gauge on the dash measures around 12 pre start, and 13.5 after start. Meter at the battery terminals has 10V, and the leisure batt exactly the same 10V..


Yep, I think so too :)

Looks like yours is wired as VW intended, so change the fuse (and fuseholder to accept blade fuses, same as the vehicle fusebox) and see how you go having re-run the tests.

bigherb wrote:Glass fuses are rated at the current they will blow at, continous current is half the fuse rating, e.g a 15A glass fuse = 7.5A blade/ceramic fuses which are rated at their continous capacity (they blow at twice their rating).


I didn't know that - applies to the ceramic fuses in common use through to the '80's too 8)
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Re: Split charge relay - fuse blows

Postby adrianbutter » 12 Jan 2018, 23:38

Think I may have used poor english - or I am confused.

On my bus the signal wire is live to the split charge relay the moment I turn the key to position one; engine not started... That's not right is it? If I understand it correctly, this will connect the 2 batteries togeher via the relay. When I crank the engine the starter will pull max current through the relay, from both batteries - no wonder the fuse keeps on blowing...

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Re: Split charge relay - fuse blows

Postby Oldiebut goodie » 13 Jan 2018, 00:27

The X-relay should cut that power to the relay along with headlights etc whilst cranking. ( As Bigherb has already stated)
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