Ignition Switch Terminal 15

[Pazuzu]

I'm designing a new fuse assembly for my car, using bussed fuse blocks. I've found that one would need at least 3, along with several 1-1 fuse blocks.

That's not the point.
The point is, am I correct in seeing that the Ignition switch terminal 15 runs something like 100 amps of equipment???

Right off the bat, it feeds fuse 11 and 12, 16/25amps. They feed fuse 16, 25 amps. So, 70 amps of fuses right there.

THEN, it secondarily feeds all of the gauge power, triggers the main A/C relay, feeds Fuse 22 in the back (25 AMPS), feeds the CDI box directly, maybe more?


That's not even taking into account terminal X ,which has it's own collection of power feeds (fuses 13/14/15 at 55amps total, power window relay trigger, speedometer power, and feeding the low beam part of the headlight switch (16 amps)!


Now, I know that adding up fuses is not the same as actual load, but even if it's running at 50%, that means a possible 80 or so amps through that switch.

[ischmitz]

I'd say 10 - 20 Amps is more realistic and yes on the 911 there is no dedicated T15 bus relay as in newer designs. That's why the electrical part of that switch is prone to failure. To get an accurate reading you'd need a current clamp.

In the next iteration of designs (still with real ignition switch) most of the T15 stuff is controlled through a T15 relay that itself is powered from the ignition switch or a control module. Then only the ignition coils are supplied directly from the mechanical switch to ensure a frozen relay doesn't keep the car running. With push-start it gets even more complicated.

Ingo

[Pazuzu]

Here is a new fuse assembly using 3 bussed panels...



The panels are 40amp each circuit, 150 amp total so we're no where near the limit for them.

I know I'm missing the brake lights, I think they'll hang on Fuse 12a for now, since my cruise control is disabled.

This takes away the difficulties of doing new fuses with all of the jumpers that are on the factory setup, It should also simplify the look of things, since all but 4 or 5 wires will be exiting the bottom of the bussed panels. I have an extra fuse in each case available, and some of the more annoying things are separated to separate fuses.

Now, this still doesn't take the load off of the ignition switch, unfortunately. For that, I would have to add a large relay above each bussed panel, triggered by "X" or "15" from the ignition switch and fed from an 8 gauge wire right to the battery. That would reduce the load on the ignition switch to a few amps total.

Would a Bosch 75 amp relay like this be enough?
12 Volt DC 75 Amp Continuous Duty Relay Bosch Relays 0 332 002 156
75 amp continuous, NO, with a diode.

2 of those, one for "X" and one for "15" would then feed everything.

So, what have I missed? I run almost all LED lighting, the headlights are on relays now shown there. The A/C is custom and does have 3 relays and extra condenser fans.

[Pazuzu]

Originally Posted by ischmitz

I'd say 10 - 20 Amps is more realistic and yes on the 911 there is no dedicated T15 bus relay as in newer designs. That's why the electrical part of that switch is prone to failure. To get an accurate reading you'd need a current clamp.

In the next iteration of designs (still with real ignition switch) most of the T15 stuff is controlled through a T15 relay that itself is powered from the ignition switch or a control module. Then only the ignition coils are supplied directly from the mechanical switch to ensure a frozen relay doesn't keep the car running. With push-start it gets even more complicated.

Ingo

Tell me about T15 relays, what are they and are they better for continuous, ignition triggered scenarios than a monolithic Bosch style relay? What about the solenoid style relays?

[ischmitz]

As far as I know most cars use a simple Bosch relay (solenoid style). As long as it takes the amperage you are ok. And it's easier to replace than the ignition switch.

[SpyderMike]

Not sure if you will find anything interesting here. I redistributed my power to relays and busses like you are thinking.

understanding the ignition switch

battery relocate = fuse box relocate?

[Pazuzu]

Originally Posted by SpyderMike

Not sure if you will find anything interesting here. I redistributed my power to relays and busses like you are thinking.

understanding the ignition switch

battery relocate = fuse box relocate?

Thanks! Lots of good info in there.

I'm well into the planning and design of a fully bused setup, using lots of relays, bused fuse panels, and terminal strips for +12V and ground sources for everything. It'll be larger than the factory setup, but will be stronger, easier to diagnose and repair, and easier to upgrade.

Once you step past the "I need this to fit on the factory panel" stage and give in to size, then things get more interesting!