Question for the Electrical Gurus--Alternator Switched Circuit

[vwwestyman]

On my TDI-Converted Bus, I've changed a lot of the original wiring. Recently I discovered an unintended consequence and am wondering the best way to sort it.

I have a small fuse/relay box in the engine compartment. In there, there is a relay that is triggered by the alternator, very similar to the charging circuit with a Westy house battery.

When that relay switches on, it provides power to 3 fuses. One fuse leads to the original Westy refrigerator, one to the reverse lights, and one to the intercooler coolant pump.

I did not have a "house" battery.

That system worked just fine for a long time. It allowed the fridge to be on either when the engine was running or by shore power. It did not allow the fridge to run on a battery because there was no house battery. I wasn't concerned with this. One time on a whim, I flipped on the Westy battery charger and noted that the pump came on, but that wasn't a big deal.

Now, I've added a house battery because I want to eventually hook up my Espar heater to it, and also run my new fridge on it. (Via a cigarette lighter to the house battery.)

The side-effect of this is, power is back feeding through the wires and causing the pump to run full-time unless I unplug it. (It also allows the reverse lights to come on with the engine off, but that is no big deal.)

Question is, what is the simplest/best way to stop that from happening? I have two ideas:

1) A diode in-line of the wire that goes up to the fridge and the battery. Then, power would only go one-way, right?

2) Split up the circuit at the fuse box back there and have two relays triggered by the alternator. One relay would go to the fridge and house battery, and the other relay would go to the reverse lights and pump. Seems like that wouldn't allow power to back-feed, right?

3) Something different? Open to suggestions.

Assuming I go with option #1, what type of diode would be recommended? I googled it and found lots of options.

Here's a diagram of the positive side of the circuit as it stands.

Thanks!

_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[ImAddicted]

I vote splitting. Now that you have a house battery, I’d isolate it. Use a split charge relay or one of the fancy-schamcy isolator-charge-switch things. This way you can run and charge your house while driving but be completely isolated from the bus side when motor is off.
_________________
1979 Transporter (sold)
KC1MUR

[vwwestyman]

Currently, I do have a split charge relay setup. But, I've also connected the circulation pump and reverse lights are also connected to the output side along with the fridge/battery circuit.

Your comment, though, did make me think of a relay with dual output. Some of those have both outputs connected internally, which would just move the problem. But with others, the outputs (terminal 87) aren't connected until the relay is energized which would solve the problem.

That would separate the circuits otherwise and would involve only moving one wire.

Time to go digging through my spare relays box!
_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[vwwestyman]

Had a chance to wire in the new relay. This relay is marked with 87 and 87b. Those two terminals are not connected when the relay is not triggered (as opposed to one that has two 87 terminals), and are both connected when the relay is triggered (as opposed to one that has an 87 and 87a terminal).

Now it works exactly as I'd hoped, and I didn't have to take up the spare relay spot in my box.

Engine off, the pump doesn't run because it's not being supplied power by either battery and engine on, the house battery gets charged and the pump turns on. Perfect.

For the benefit of anyone who might want to create such a circuit for their own accessories, here's the updated wiring diagram. Note I "X-ed" out the wire I removed in the fuse box.

_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing