Yet more charging problems

[SaltyDaze]

I have a problem with the charging system of my newly aquired Autotrail Merc 307 of 1988 vintage. I bought it privately and the vendor assured me all was working well. It had it's normal share of problems' leaking water heater, fuel guage U/S, toilet not flushing, ect ect, but i worked through these, did a full engine service, and went off for a first few days camping down on the south coast to see how things went. I didn't really understand the "Mac control unit" and why nothing worked in the van position but all the electrics worked in the car position so it seemed ok. When i checked in the battery box under the passenger seat to check the condition of the Leisure battery suprise suprise the leads went into the box but no battery! £85 later, new leisure battery but it's not charging either from mains power or from the alerenator when the engines running. There's a regulator or switching unit of some sort under the drivers seat next to the starting battery which would appear to have possibly overheated at some time and an inline fuse holder with no fuse. I replaced the fuse but it made no difference still no charge to the leisure battery. The starting battery is charging well from engine and mains. The motorhome electronics run fine from the starting battery with the Mac unit in the car position, and from the leisure battery if i charge it with an external charger with the Mac unit in the van position. Can anyone help please, Salty

 

[Firefox]

I would rewire with a new split charge relay or if you want to do it simply, just take a 100 A cable from the the +ve terminal of the van battery and connect it to the +ve terminal of the leisure battery with a heavy duty battery isolator switch in the cable. Make sure the leisure battery has a decent earth.

When you drive, just put the switch to "ON". When you stop, just put it to "OFF". This is the system I use on my van. It is simpler and more robust than wiring a new charge relay, but if you are forgetful, I'd go for the charge relay route. The thing about relays is that they can burn out or stick and that is probably what has happened to yours, judging by your description.

 

[NicknClair]

Hello Salty,
Reading carefully through the post, it seems to me that there is a link gone down/wiring fault between the leisure Battery positive and the MAC switch.
If the Engine battery is receiving charge on mains hook up (from the original Autotrail Charger) when the switch is in Car, then it clearly shows the charger working. All the switch does is feed 2 x wires, one to engine one to leisure. The reason you have 12V electrics when hooked up on mains with it switched to Van (but no charge is getting to the leisure batteries), the charger is acting like a 12V transformer merely supplying the electrics with 12v. I would guess that when unplugged and switched to Van, none of the 12V in the habitation is working??
I would also add that the reason the leisure battery is not receiving charge is that this same link from the MAC switch is also the same link shared by the factory split Charge relay.
Try tracing the positive wire from the leisure battery, checking fuses along the way. The worrying thing you also mentioned is that you found the leisure battery terminals just sat there under a steel seat base. More than likely the exposed positive terminal had shorted out something whilst unprotected I'm sure (as the moment you had started the van, power would have been applied to the leisure battery circuit from the engagement of the Split Charge relay).
My money is on an in-line fuse situated along the leisure battery positive wire. Being an early Autotrail, I would start taking a look under the seat initially, then under the bonnet near the engine battery (where you should find your split charge and fridge relays). There is normally a bank of 3 x fuses. 1 x engine to MAC switch, 1 x Fridge Supply, 1 x leisure positive.

Good Luck