Split charge relay - any risk in shorting -ve terminals?

[tallbongo]

Hi all,

I've been having some problems with the split charging system on our newly acquired Bongo. The main symptom isn't serious - the radio keeps resetting when the engine is off, but it's something I want to sort before we set out for a couple of weeks in it.

I checked the leisure battery and it only measured 12.3V after a 2 hour motorway drive. I expected it to be nearer 12.8V. The starter battery was at 12.8V.

I removed and charged the leisure battery overnight and it seems to have stabilised at ~12.75V.

So I reckon there's a fault with the split charging system. The circuit seems similar to the one at the top of this page but I don't want to undo all the insulating tape to be 100% sure unless it's absolutely necessary. All the fuses are ok, although I can't find any fuse between the alternator ground and terminal 86.

The one fault I have noticed is that I cannot find a short from either battery's -ve terminal to the chassis, so the ground appears to be floating on both batteries. At one point I measured 3V between the two -ve terminals. I'm sure this is not right - shouldn't both batteries should be grounded to the chassis and there should be a short between them?

So after all that, my question is as follows:

"Is there any potential risk in making a new connection from both the negative terminals to a new ground point on the chassis, e.g. next to the original ground on the leisure battery."

Many thanks,

David.

[dave_aber]

Sounds like you are on the right track to me. Both -ve terminals should be electrically connected, and should be electrically connected directly to the chassis. The engine / gearbox should be bonded to the chassis / -ve terminals as well.

I can't imagine that there would be any risk in providing a new earth connection, as long as the split charge system is wired as per the diagram you posted the link to. I can't imagine a scenario where it could be wired wrongly in fact.

Is it possible that the leisure battery earth is connected to the chassis, and the main battery earth is connected to the engine (otherwise it wouldn't be charging up when running), but the link engine -> chassis is broken? Not sure where this is on a Bongo, usually a thick copper braid.

[g8dhe]

Fully agree.
The fuse on pin 86 of the relay goes to the Alternator Charge or Ignition circuit not earth, its not that clear on the diagram from the first link.

[tallbongo]

Thanks. The bit below fits the scenario and I'll check it at the weekend. Bongo is in for a service today.

...

Is it possible that the leisure battery earth is connected to the chassis, and the main battery earth is connected to the engine (otherwise it wouldn't be charging up when running), but the link engine -> chassis is broken? Not sure where this is on a Bongo, usually a thick copper braid.

The fuse on pin 86 of the relay goes to the Alternator Charge or Ignition circuit not earth, its not that clear on the diagram from the first link.

.

You're quite correct. I think the diagram was clearer than my brain, but thanks for wording it to give me an excuse Regardless, I've not located the fuse to pin 86 yet.

[g8dhe]

The fuse probably doesn't exist its not essential to connect one at that point as its not considered a supply source, there are three ways of switching the relay,

• Simply driving it from the IG2 connection on the Ignition switch, so that it operates when you turn the ignition on, but not when you are actually turning over the starter motor.
• Connecting it as shown from the alternator Charge wire which means it only operates when the alternator is generating power.
• Using a Voltage Sensitive Relay (VSR) which means it only operates when the voltage on the S/B exceeds the voltage set normally around 13-14 volts

[tallbongo]

Is it possible that the leisure battery earth is connected to the chassis, and the main battery earth is connected to the engine (otherwise it wouldn't be charging up when running), but the link engine -> chassis is broken? Not sure where this is on a Bongo, usually a thick copper braid.

I found this cable (it's on the underside and probably normally hidden by the plate that the garage didn't refit) and right enough could not measure a short circuit from the engine side to the chassis. I was unable to clean it up sufficiently to sort this (old waxoyl amongst other deposits).

I did however eventually find some points on the chassis that measured short circuit from the main battery -ve terminal so have run a fairly heavy cable from the L.B. -ve to this. It seems to have fixed the problem. I've not given the Bongo a long run yet to see if the L.B. charges and holds it, but was getting over 14V to the L.B. with the engine running so it looks promising.