Field and load relay

[Voeltagear]

My relay gave up the ghost today. Fortunately my battery had a high reserve capacity and I made it home. So my question is whether or not the relay is necessary. I replaced it with a Bosch style for now. Would there be any problem with just putting a fuse in line? I know other vehicles don't have one to begin with but I'm not sure if those run a switched power to the voltage regulator and alternator field.

 

[69-]

Relay for field and load??? Why?
Especially load - you need to be able to pass full alternator current through the connection, 65 amps or even 100, depending on your alternator.

The 'fuse' is a fusible link, a special cable acting as a slow fuse. It is necessary to protect the wires.

Look it up in the wiring diagramm.

 

[Voeltagear]

This is something only the cars with the heavy duty charging option had. It only supplies power to the voltage regulator and alternator field windings.

 

[69-]

Ah, OK, that makes sense. Alternator output is relay'd to The field side. 40amps Bosch relay should suffice.

 

[Mikes5thAve]

That relay was gone by the late 70s.

 

[Duke5A]

That relay was gone by the late 70s.

I was scratching my head on this. That makes sense why I've never seen one. lol.

 

[69-]

That relay then only came in the early 70s (have no such in my 69s) and was probably ditched by some bean counters because too expensive.

But it makes perfect sense. Its ground is via starter plus (!) According to the wiring diagram to avoid that the alternator draws current for its field coil while cranking. Pretty smart solution.

That little current of the relay through the alternator doesn't do anything wrong there (while driving)

 

[Mikes5thAve]

It's also one more thing to go wrong. Someone probably looked at it and figured it wasn't necessary

 

[69-]

True as well.

 

[Voeltagear]

It does also send power on a 12 ga wire to the fuse box, probably for optional equipment.

 

[69-]

Actually, it GETs its 12V supply via that plus splice (all fusible links connected) in the engine compartment.

It should work like this:
BRN is like the reference 12V for the alternator regulator (via TAN). The alternator's field is then regulated by the regulator via switched ground (DK GREEN). BRN is switched (output) by the relay.
BLK is directly connected to battery plus (via a fusible link, as a normal fuse was not used in the engine compartment). This is permanently available on the input side of the relay's switch.

DK BLUE and BRN are the two connectors for the relay's coil. They are:
DK BLUE -> switched by the ignition switch (same as e-choke)
BRN -> ground for the relay; the trick is, that they used the starter's plus for this, which in turn is switched to 12V plus only, if you're cranking (via starter relay). So if you're cranking you have 12V plus on both sides of the relay's coil - no short and coil not energized. If your're not cranking AND ignition is on, that wire basically has a ground connection through the starter. For those couple milli amps the starter does not care (it's wires are quite thick and need a couple more amps to turn), but for that small relay it's enough to switch.
That means, if you're cranking, the relay is not switched (open). Otherwise, if ignition is on (no cranking) the relay is switched (closed).

When it switches (closed switch), direct battery voltage is supplied to the alternator regulators input and you'll safely avoid any overcharging of the battery, as there are no further resistances between regulator input and battery. Without that relay, all those little resistances inbetween (connectors, e.g. twice the bulk head, starter switch, splices and so on) are out of the equation. And yes, this setup brings some other problems: the relay needs to work for charging and also yes, the relay and its connectors are a possible "connection-resistance-problem" by itself.

The wire to the fuse box is just ending at a dead fuse slot (nothing going on after the fuse).

At least, that's my theory, based on the above diagram.

But it si pretty close to my setup at my 69's cars: I put a relay on the firewall, switching input for the alternator regulator as well as to ignition. That's quite some amps (approx 5-7 amps for ignition and another 0-5 amps for the alternator) no longer being a hassle on the bulk head connector and interior wire harness.

And the wire harness being neglected dozens of years was one of the main reasons for such cars to light a fire... (too much resistance at connectors, splices and such by some deposits growing more and more until such resistance produces noteworthy heat...)

 

[Voeltagear]

That's what I see too. The Black and Brown wires are heavy gauge, 10 or 12ga. That's why I think it was also used for additional accessories like emergency lights/sirens. I'm not sure why they used the starter solenoid for the ground to the relay coil. A continuous ground with an accessory power source would have accomplished the same thing. It's hot in run and not in start. Maybe just to avoid another bulkhead connection? I don't have a bulkhead connector anyway, just a boot. There's connector under the dash but some of the circuits branch off before it.

 

[69-]

No alternator while cranking saves at least like 5-7 amps.
As voltage is pretty low while cranking, alternator would be field-fired all the way.

Thick wire to avoid voltage drop between battery and alternator regulator.

 

[Voeltagear]

No alternator while cranking saves at least like 5-7 amps.
As voltage is pretty low while cranking, alternator would be field-fired all the way.

Thick wire to avoid voltage drop between battery and alternator regulator.

I get your idea but the actual wire to the regulator is spliced off the heavy ga wire to maybe 16ga then spliced again to the field windings. As for shutting off the alt while cranking, I just thought it is a weird way of doing it. They could have used the accessory b+ circuit to power the relay. It's dead while cranking. Then the field relay wouldn't be grounded through the starter relay coil. Just me wondering, that's all.