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Old 11-05-2010, 10:24 AM
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Chevy wiring question

Was working on the 75 Monte Carlo today and have a couple questions. May be easy to answer though. sbc 400

The ground wire coming from the battery says to connect to an engine ground. Is this to the block itself or the alternator or something else?

Second question has to do with a wire off of the distributor. There's a thick red wire coming from the distrib. that connects to a connector that then has a black wire splitting off of it that goes to what looked like one of those tube fuses. I opened it up and the fuse shattered instantly. What kind of fuse goes in there? And then I followed the wire and it goes through a loom on the firewall and then its not connected to anything. It's just cut. Any ideas on where this connects to?

If you need pics I can get some later on. Thanks

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Old 11-05-2010, 03:51 PM
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The black cable from the battery - terminal goes to the engine block. The "ground" is really engine, chassis, body ground where all metal parts of the car are bonded (connected) together to provide a good electrical path for current to flow unimpeded.
The red unused wire was likely for a tachometer. Remove it and tape any bare conductor.

vicrod
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Old 11-05-2010, 04:10 PM
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Quote:
Originally Posted by vicrod
The black cable from the battery - terminal goes to the engine block. The "ground" is really engine, chassis, body ground where all metal parts of the car are bonded (connected) together to provide a good electrical path for current to flow unimpeded.
The red unused wire was likely for a tachometer. Remove it and tape any bare conductor.

vicrod
Thank you for the response.

So basically for the ground, it should be connected close on the block to a point where all the other grounds (the braided lines) are closely connected to?

Also, I wasn't too clear on the red wire. That one is thick and connects perfectly to the distrubator. When you follow that wire, it goes to a connector that's mounted on the firewall, which continues on red to a harness. However, at that connector, it splits to the black wire with a fuse. My question was pertaining to that black wire, and where that goes. Its a much smaller gauge than the red wire as well.

Again, thanks.
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Old 11-05-2010, 04:36 PM
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The large cable from the battery is connected to the block because the starter is by far the largest load on the car. The braided jumpers can be connected at convenient locations on the block as long as they are solid connections.

If my memory is correct the '75 Monte had an HEI distributor. If this is the case you only need one wire to the distributor. It is ignition switch 12 volt power. Should be a pink wire. All other components for ignition are inside the distributor.
Are there any other wires, other than the plug wires, connected to the distributor? It could be non stock.

vicrod
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Old 11-05-2010, 05:44 PM
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On the stock setup the large negative cable is grounded to the alternator bracket, which is bolted to the engine. There is a 2nd smaller ground wire that runs from the battery negative terminal to the body.

The stock tach wire on the HEI should have had a tach filter connected to it. Maybe that's the "fuse" that you found.

Bruce
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Old 11-08-2010, 10:17 AM
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Thanks for the responses everyone. Sorry I couldn't get a pic sooner, but here is what I'm talking about...


You can see the red wire on the left side of the connector connects to the distributor. It sits on the firewall, and then that black wire comes from the top going to that black connector that opens up, what I thought was a fuse inside. It shattered when I opened it up. The other end of that black wire is not connected to anywhere.

Also since I'm here, not exactly electrical, but it's coming from the harness, I have this orange/tan little vacuum line. Anyone know what this is? Not finding at all where this goes to.



Thanks
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Old 11-09-2010, 11:24 AM
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Anyone? Really need this all squared away before this weekend. Looking at an answer for the orange vacuum line and the black wire coming from the harness. Thanks
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Old 11-09-2010, 02:36 PM
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Ok, you need a hot feed with the key on to the distributor + or BATT terminal. You other wire could be something someone tapped into to feed something else.
As far as the vacuum it could be the feed for the HVAC controls. the picture is pretty cluttered so it is hard to tell whats what.
The wiring has obviously been messed with so this will usually require some tracing of wires.

As far as your ground, I like the Negative cable to the block and a ground strap from the back of the head to the fire wall. You should also run one from the block to the frame. Always use star washers on these connections.
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Old 11-09-2010, 03:19 PM
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Quote:
Originally Posted by T-bucket23
Ok, you need a hot feed with the key on to the distributor + or BATT terminal. You other wire could be something someone tapped into to feed something else.
As far as the vacuum it could be the feed for the HVAC controls. the picture is pretty cluttered so it is hard to tell whats what.
The wiring has obviously been messed with so this will usually require some tracing of wires.

As far as your ground, I like the Negative cable to the block and a ground strap from the back of the head to the fire wall. You should also run one from the block to the frame. Always use star washers on these connections.
Ok so I guess I'll leave the black wire disconnected for the time being and out of the way. The vacuum line goes into the loom and can't trace it to where it ends. And I really don't see any place that it can go. If it were coming from the HVAC controls, where would that generally connect to? All the hoses and wires on the AC side are all accounted for.
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Old 11-09-2010, 03:26 PM
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Quote:
Originally Posted by david-b
Ok so I guess I'll leave the black wire disconnected for the time being and out of the way. The vacuum line goes into the loom and can't trace it to where it ends. And I really don't see any place that it can go. If it were coming from the HVAC controls, where would that generally connect to? All the hoses and wires on the AC side are all accounted for.
usually comes through the firewall and connects to direct vacuum. You are not running the original manifold so there may not be a port for it. You may need to tee off of something
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Old 11-09-2010, 05:30 PM
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im not high jacking the thread here,but on my 70 truck i ran the ground to the frame, is that not ok? factory was on the alt bracket i believe but i put alt on drivers bottom side,as thats the only belt im running right now. i forgot to mention i also have a ground ran from head to frame.runs fine?id rather be right now than burnt up wiring later
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Old 11-09-2010, 05:59 PM
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As stated earlier up thread, it is best to connect the large cable from the battery directly to the engine because the starter is the biggest load in the car.
The alternator bracket has nothing to do with it except maybe saving the cost of one bolt for GM. Hey $.05 x 1,000,000 cars/year=$50,000.

vicrod
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Old 11-09-2010, 07:20 PM
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Quote:
The vacuum line goes into the loom and can't trace it to where it ends. And I really don't see any place that it can go. If it were coming from the HVAC controls, where would that generally connect to? All the hoses and wires on the AC side are all accounted for.
One end should be inside the cabin, attached to the vacuum switching panel that controls the vents and doors, etc. for the HVAV system.

The engine end should go to a black plastic ball-shaped vacuum reservoir. The reservoir will have another vacuum line running to a manifold vacuum source.

If the reservoir is gone, you can run it straight to a manifold vacuum source, but you may find the heater/vent doors, etc. doing strange things when you hit the gas and the vacuum drops.
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Old 11-10-2010, 11:22 AM
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Above I mentioned the doors and etc. doing "doing strange things when you hit the gas and the vacuum drops". To expand on this:

IIRC, the vacuum controlled doors, etc. go into a "default" position if the vacuum to the control panel were to be lost.

It seems the default position would have airflow through the defroster, which makes sense- at least from a safety standpoint.

So, if you have no vacuum reservoir under the hood anywhere, you may find the HVAC system going to this default position when you accelerate. This is because accelerating or rapidly opening the throttle causes the manifold vacuum to drop to near nothing. Once your speed levels out and the engine is back to making good vacuum, the controls would resume to operate as they were originally set.

I also said it was a black, round unit. It is- at least on many GM vehicles in the late 70's - early 80's, like my '81 Malibu.

Mine is mounted on the passenger side in clear view. Yours may or may not be black, round- OR in plain view. But after reading all this, I believe you should be able to ID it once you se it.

Just don't mistake the evap canister for it. The evap can will have several larger hoses going to it and will be shaped like a can, not a ball. On my '81, the evap can is on the driver side, up near the back side of the headlight.

Hope this helps...
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Old 11-10-2010, 12:40 PM
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Quote:
Originally Posted by david-b
Thanks for the responses everyone. Sorry I couldn't get a pic sooner, but here is what I'm talking about...


You can see the red wire on the left side of the connector connects to the distributor. It sits on the firewall, and then that black wire comes from the top going to that black connector that opens up, what I thought was a fuse inside. It shattered when I opened it up. The other end of that black wire is not connected to anywhere.
Thanks
That black wire is connected to the red wire
There are 3 pieces of connector bodies.
Male---Female----male/female tap (in the center with the black wire)
Not really sure where that black wire is supposed to go, but it is powering some option that is only powered with the ignition circuit.
Follow that black wire to see where it goes. could very well be a jumper to provide 12 volts during cranking---dunno if the ignition circuit drops out during cranking on a 75 ignition switch (does not on a 79).

At any rate----that black wire is an option (dealer or factory) as I have seen this same connector/tap on the 69 Olds I have been problem solving on.

EDIT---and that black single wire connector is just a weather resistant connecor-----if the other end of the wire goes nowhere---disconnect it
and don't be concerned until some option in the car don't work.
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