ETI4K

sweatybetty: Yep. Looking at #2AWG. AZ: I was trying to find an OEM type (rather than one that's been amped up - pun intended) to ensure a long and healthy life. I did just read that Megasquirt 3 provides control signals for about any alternator, so suddenly the process gets a lot easier. But, as you said, it's down to finding one that fits, and I believe the higher output alternators have larger cases. Finding the right one is tough to do over the internet.

ensys, I applaud your effort for trying to make this improvement without substantially altering the harness. There's never been only one way to solve a problem. I look forward to seeing your ideas.

Wow, that's weird. My wiring diagram ('76) has the fusible link feeding pin 7 and pin 6 feeding headlights. Something's screwy here. We must have been trained to different conventions as to illustrating relays. The way I've always shown it is as though the coil would draw the contact arm towards it when energized. So, the de-energized state would look like below. Gee, this is like when two NASA locations were working in different units (imperial vs metric) and did not tell each other. ? The suspenders and belt idea sounds smart. Make them all easy to get to and document your additions for posterity (and troubleshooting!).

I'm not clear on how the primary relay is controlled. My understanding is 12V (always hot) comes into the switch on pin 7, and 12V is supplied to the headlamp fuses on pin 6 when the switch is ON. You've interrupted that connection with the insertion on the primary relay. So if the relay is not energized and the switch is OFF, 30-87 is closed providing power to the headlamps and pin 6, with pin 7 essentially at ground potential. If the switch is ON, pin 7 is hot which energizes the primary coil, opening 30-87. But as soon as that happens, power is removed from pins 6 and 7 de-energizing the coil. I feel like I'm missing something.

I'm in the process of making wire harness changes that include upgrades and additions that will increase the total current requirement from the alternator/battery. Ultimately, besides wire gauge changes, the alternator will need to be upgraded. For this calculation, I started with the belief that winter and summer might be very different. Turns out, maybe not so much. Loads are computed at minimum charging voltage: 13.5 (arguably). Lighting current is based on using incandescent bulbs which also will see "persistent" and "transient" use. The persistent load comes out to about 17A, while transients total around 16A. I took a WAG and chose 25A for both. Some other transient loads, such as power windows, are not included. Audio equipment is not included. The highest load case in this example is 134A. If you believe in safety factor, and I most certainly do, it looks like I'll need at least a 150A alternator. Has anyone successfully used an alternator rated in the 150 - 200A range?

I agree re headlights not inducing much arcing, especially not a concern with the very low switching frequency - a few times a night, maybe? Although, I have seen relay contacts weld up on a motorcycle headlamp flasher (a safety add-on that had a small market) for that very reason (but that was about 100 years ago, I think). But couldn't the same thing be said of the flyback energy on a coil being controlled, not by a solid-state device, but a mechanical switch? There's no real switching noise, and the flyback energy from a 200mA(?) coil load couldn't have any negative impact on the 12V power circuit. Although I suppose it could make its way through to an ECU or solid-state regulator. Questions, not assertions. Comments always welcome

Well if that won't teach me to stop thinking while on pain meds (recovering from shoulder surgery) - It certainly seemed okay yesterday. ? CO: I was looking at the diodes to suppress arcing from the load when the relay opens which would reduce contact damage. Not sure I ever used one across a coil, but I certainly agree coil inductance can cause trouble in certain circuits. My oversight on the 20A feeding both - I agree with you. Thanks for the corrections!

Just saw this, so my comments may be moot. Interesting design. Nice graphics. What year is your car? It looks like your shunt diodes are across the coil, while they should be across the contacts. The arc you are trying to suppress will occur at the contacts. The 20A fuses are probably too high. OEM are 10A, IIRC. If you're planning to use LEDs, then the current will be even less. What are you trying to protect: relays, wiring, bulbs? While a 20A relay will run to 20A, it shouldn't ever. Ideally, you'd like max load <80% of rating so you can have some margin of protection - call it safety factor. A 55W halogen would run just over 4A at low battery charging voltage (13.5V), so a 10A fuse should be adequate. As for wire gauges, as a general guideline, 350CMA (circular mil area) per amp will provide good service with acceptable IR loss. A 12AWG wire will certainly easily handle 4A, and if you are into the wire harness anyway, maybe it's no big deal. It's just not necessary for your intended purpose. That is as long as you're not wiring against what we used to call the gorilla factor - you're trying to prevent some monkey who might be working on your car from damaging a small wire through lack of care. In such cases, then 12AWG might be just right. As an example, the 12AWG wire is about 6500CM which should be good for more than 18A. Also, it looks like you trying to energize the coil of your HB relay through the indicator lamp? Not sure if that's actually the plan, but you may find the relay is jumpy without a good, solid connection to ground. Some relays don't need quite as much coil current to throw as others might. I'd suggest grounding one side of the coil, and using your stalk switch HB signal on the other. Tie the HB indicator to pin 87 on your HB relay. Have fun.

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