Pulling switched power from factory fuse block

mshred

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1977 Aspen, looking for a slot to pull switched 12v power, but there are no empty slots. What slot should I piggyback from, and how are you guys doing it?

IMG_20240606_090557221.jpg
 

Aspen500

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First look up behind the dash for a blue triple bullet style connector. The soft rubber type. Check it with a test light but that should be switched power. Don't remember off hand if it's hot in only run, or both start and run.

Best thing to do is probe the fuse panel with a test light or DVOM. Some fuses are hot in accy and run, run only, or hot at all times.
 

mshred

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First look up behind the dash for a blue triple bullet style connector. The soft rubber type. Check it with a test light but that should be switched power. Don't remember off hand if it's hot in only run, or both start and run.

Best thing to do is probe the fuse panel with a test light or DVOM. Some fuses are hot in accy and run, run only, or hot at all times.

What is that triple bullet connector for?
 

Ark

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Discovered these little guys last time I did a dash cam install. No more stripping and soldering, no more shoving bare wire ends into fuse terminals.

Plug in and use a DVOM to check the lead is hot under the ignition switch conditions you desire. Only real limitation is to make sure you don't set it up to draw more amps through that fuse socket than it was designed for.
 

mshred

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View attachment 53295

Discovered these little guys last time I did a dash cam install. No more stripping and soldering, no more shoving bare wire ends into fuse terminals.

Plug in and use a DVOM to check the lead is hot under the ignition switch conditions you desire. Only real limitation is to make sure you don't set it up to draw more amps through that fuse socket than it was designed for.
Aren't those for blade style fuse blocks only though? If I recall my 77 would use glass fuses still
 

Ark

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Aren't those for blade style fuse blocks only though? If I recall my 77 would use glass fuses still
There are little brass pieces that can add 1/4" spade terminals to glass fuses, which you can then connect a wire to.

1718127420598.png


There are also these adapters that you can use to replace the glass fuse, then plug a blade style fuse tap into.

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Might depend on the clearance in your fuse area, though. If you use the 1/4" brass terminal type, be sure to put it on the negative side of the fuse, so power will be lost if the fuse blows. Or, put it on the positive side, and splice in a fuse holder and a separate fuse for the accessory.
 

XfbodyX

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You can also pull the back cover and stick your phone up for a pic or simply remove the two small fasteners and see what your actually pulling from and the gauge of the wire and whats split up and whats not. I know what I always pull from but others may decide differently. You can stealthy connect in from the backside if you have a reason to.

Really as crap as our wiring is and what your doing and may do a clean heavy gauge wire to positive side (of course fused) to a aux switch panel is a consideration. When ya start to smoke some of the oem wiring or when it is inadequate for the job things become a mess quickly and not easily fixed.

There are many different configs but the base layout is the same.

Two examples.

1a.jpg


2.jpg
 

mshred

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So my radio wire going to the fuse box is not being used. This is 5 amps, and shared with a 20 amp fuse that powers the a/c clutch, turn signal, and back up lamps. Is this wire and 5 amp fuse in its place sufficient to provide switched power to a relay that will then send power to a small clean auxilliary fuse block?
 

Mikes5thAve

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What you're probably going to find is the accessory power inside works the same as the dark blue switched power under the hood. It goes to the same positions of the ignition switch.

A relay doesn't take too much power so it can go back to the radio circuit or the connector that Aspen500 mentioned.
 

Aspen500

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A 12 volt relay coil draws maybe 100 mA at the most to operate. More common is less than 50 mA. The 100 mA is 0.1 amps, so they take basically nothing to operate.
 

Ark

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So my radio wire going to the fuse box is not being used. This is 5 amps, and shared with a 20 amp fuse that powers the a/c clutch, turn signal, and back up lamps. Is this wire and 5 amp fuse in its place sufficient to provide switched power to a relay that will then send power to a small clean auxilliary fuse block?
Should be no problem running the coil side of a relay. They pull very little amperage.
 

mshred

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Has anybody in here that has responded so far adapted a CD ignition box like an MSD in place of the factory electronic ignition in an F-body 1977 or later?
 

XfbodyX

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Check your mfg. website as most have instructions for what your doing with there products.
 
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mshred

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Check your mfg. website as most have instructions for what your doing with there products.
The instructions only show for older chryslers while KEEPING the factory electronic ignition box, which I am wanting to eliminate.
 

Mikes5thAve

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All I've seen online is you combine the start and run wire together off the starter relay to power those newer ignition boxes from. You might aswell try that and see what happens. It's the same as when you connect both sides of the ballast resistor together to get it running short term when that fails.
 

mshred

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All I've seen online is you combine the start and run wire together off the starter relay to power those newer ignition boxes from. You might aswell try that and see what happens. It's the same as when you connect both sides of the ballast resistor together to get it running short term when that fails.
Yes, that is correct. On a 77 aspen J2 *Blue which is IGN 1 comes from the ignition switch, but J3 *Brown begins at the starter relay, not at the actual ignition switch...thus, my concern is if I tie them together, then there may be a backfeed to the starter when key is in run. I have to test for this, but the harness is out of the car and I wanted to get it all sorted and put together neatly before reinstalling into the car.
 

Mikes5thAve

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I think J3 is seperate from the starter part of the relay. Its already getting some voltage backfeeding it anyway thru the ballast resistor albeit probably not enough to do much but like I said connecting them together is the same as bypassing the ballast and people used to do that all the time when it failed with no issues.
I wouldn't do anything permanent until you are able to test it first. It seems like this always ends up being the sort of thing that when you plan ahead it rarely works as intended.
 

XfbodyX

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Are you doing something like this? Sorry to ask so many questions but is there a chance your overthinking things? Ive msd boxes on several, one with a oem mopar dist, one with a billet mallory dist, and one with a matched msd dist. Never really thought much about it all just put them in.

Anything in this link helpful?

https://documents.holley.com/6425.pdf

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Screenshot 2024-06-26 at 15-48-03 6425.pdf.png


Screenshot 2024-06-26 at 15-46-38 6425.pdf.png
 
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MARCH374

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There are little brass pieces that can add 1/4" spade terminals to glass fuses, which you can then connect a wire to.

View attachment 53298

There are also these adapters that you can use to replace the glass fuse, then plug a blade style fuse tap into.

View attachment 53297

Might depend on the clearance in your fuse area, though. If you use the 1/4" brass terminal type, be sure to put it on the negative side of the fuse, so power will be lost if the fuse blows. Or, put it on the positive side, and splice in a fuse holder and a separate fuse for the accessory.
Where did you get these from ?
 
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