What is this cable for?

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Loww

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Hi,

I bought a 79 GMC K25. It seems, that it was once equipped with a dual battery setup. However someone removed one of the batteries and some of the cables. But there is still a "Divi-Charge"-Device left, which is connected to an unfused red wire that comes directly out of the alternator and to a wire that runs "wildly" through the engine compartment and disappears in a hose below the distributor, which then goes to the cabin.

I don't want a big, unfused wire to run close to hot engine parts. Do you think I can remove the "Divi-Charge"-Device and connect the two cables? Where did that red cable from the alternator go to before the installation of the Divi-Charge?

See pictures...

Thanks

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chengny

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That is a battery isolator.

When two or more batteries are installed in your vehicle (and are powering different systems), use of a battery isolator allows them to share a common charging source while being independently discharged.

Battery isolators are commonly used to maintain the starting battery charge while using a second battery to power accessory items (sound systems, emergency lights, etc.) eliminating the fear of having a dead battery when trying to start the engine.


If you are currently running just one battery, you can remove the isolator and the lead that runs through the firewall. You will however still need to have the red wire going to the alternator. Connect it to the firewall junction block and it will be protected by one of the two fusible links down at the starter solenoid.
 

Loww

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That is a battery isolator.
Connect it to the firewall junction block and it will be protected by one of the two fusible links down at the starter solenoid.

Thank you!

Do you mean the left junction block on the firewall in the picture?

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chengny

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Thank you!

Do you mean the left junction block on the firewall in the picture?

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Yeah, that's it -you can connect to either stud. This shows how the red lead connects to the battery schematically:

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It normally runs in the loom traced in yellow:

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75gmck25

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My '75 had the factory dual battery, and it was set up differently.
- The charge wire for the 2nd battery runs over to a relay on the fenderwell on the driver's side fender.
- There is a power wire to the relay that runs over to the junction near the brake booster.
- The relay is activated by a key-on brown wire that comes out of the fuse box.

If the ignition is on, the relay connects the 2nd battery to the power junction and it is charged off of that junction (which has the stock power wire running to the starter). If the ignition is off, the relay disconnects the 2nd battery so it can be used to power a camper without running down the main battery.

The batter isolator you have was probably dealer-added, and they used a more complicated wiring scheme. I'd pull it all out and avoid any possible shorts from worn wires.

Bruce
 

Loww

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Yeah, that's it -you can connect to either stud. This shows how the red lead connects to the battery schematically:

Thanks! In the schematic it seems there was a fusible link connected between the wire and the junction block. Can I omit this fusible link or should I replace it with a fuse?

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Loww

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My '75 had the factory dual battery, and it was set up differently.
- The charge wire for the 2nd battery runs over to a relay on the fenderwell on the driver's side fender.
- There is a power wire to the relay that runs over to the junction near the brake booster.
- The relay is activated by a key-on brown wire that comes out of the fuse box.

If the ignition is on, the relay connects the 2nd battery to the power junction and it is charged off of that junction (which has the stock power wire running to the starter). If the ignition is off, the relay disconnects the 2nd battery so it can be used to power a camper without running down the main battery.

The batter isolator you have was probably dealer-added, and they used a more complicated wiring scheme. I'd pull it all out and avoid any possible shorts from worn wires.

Bruce

Thanks Bruce for the explanation!
 

chengny

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Thanks! In the schematic it seems there was a fusible link connected between the wire and the junction block. Can I omit this fusible link or should I replace it with a fuse?

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It should neither be omitted or replaced with a fuse - there should be a fusible link inserted into that section of wire. Just cut the 2F lead at a point a few inches from the JB - but before the split - and splice in a new link. It's easy.

First determine the size (amp rating) of the link. If yours was still installed, you would only need to look closely at your links and note the 2 digit number on the plastic collar (e.g. 14,16,18). That is the AWG size of the copper conductors inside the insulation.

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But since yours is gone, you will pull that information from the wiring diagram. GM indicates the cross-sectional diameter of wire using the metric style (mm2 - mils). Fusible links are rated by the diameter of the conductor within them - and that diameter is represented using the AWG (American Wire Gauge) number.

So, to know which link to buy requires getting the wire size of the link (in mils) from the wiring diagram and consulting a conversion chart to get the equivalent AWG size.

Hey look, somebody already did it for us:

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The image and dwg agree that it is a 16 AWG link.

A fusible link is just an undersized length of wire with special insulation that is designed to puff up as an indication the copper conductors inside it have burned. Read on for more info on fusible links:


In addition to circuit breakers and fuses, some circuits use fusible links to protect the wiring. Like fuses, fusible links are "one-time" protection devices that will melt and create an open circuit.

Not all fusible link open circuits can be detected by observation. A blown link often, but not always, has "bubbly" appearing insulation making troubleshooting easier. Always inspect that there is battery voltage past the fusible link to verify continuity.

Fusible links are used instead of a fuse in wiring circuits that are not normally fused, such as the ignition circuit. For AWG sizes (wire gauges), each fusible link is four wire gauge sizes smaller than the wire it is designed to protect. For example: to protect a 10 gauge wire, use a 14 gauge link (or for metric, to protect a 5 mm wire, use a 2 mm link). Links are marked on the insulation with wire-gauge size because the heavy insulation makes the link appear to be a heavier gauge than it actually is. The same wire size fusible link must be used when replacing a blown fusible link.

Good and damaged fusible links:

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Double wire feed fusible link repair:

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Choose the shortest length that is available. A fusible link should NEVER be longer than nine inches. Fusible links longer than this will not provide sufficient overload protection.

To replace a damaged fusible link, cut it off beyond the splice. Replace with a repair link. When connecting the repair link, strip the wire and use staking-type pliers to crimp the splice securely in two places. To replace a damaged fusible link which feeds two harness wires, cut them both off beyond the splice. Use two repair links, one spliced to each wire harness.

One final note: Butt splices are often hincluded with aftermarket replacement links, but they should be avoided. The preferred method of inserting a link is by soldering it inline and insulating the soldered joints with heat shrink tubing.
 

Loww

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It should neither be omitted or replaced ...

Thank you so much for the detailed explanation Chengny :happy107:!

I took a closer look yesterday and found, that the cable that seems to run through the firewall in fact goes up to the junction block (where it belongs) and, that there is a fusible link still installed, that looks exactly like the one in your picture! So I just have to get rid of the "Divi-Charge", connect the two cable ends and place the cable back on top of the engine (even the original cable tube is still there, but empty)...

Thanks again!
 

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