Battery cable replacement

[Rough75C20]

Hello,

Simple question just to double check. I want to replace the positive battery cable on my truck. The new cable doesn’t have the splice with the smaller cable that splits off to go to the back of the alternator. Can I just run a separate wire from the positive terminal to the back of the alternator instead of splicing it like the original cable with the red plastic box? Any idea what gauge this smaller wire needs to be? Should I add a fuse? The splice in the red box currently doesn’t have a fuse. The new setup would be just a large positive cable to the starter and a smaller wire going to the alternator directly from the positive terminal.

Thank you


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[bd]

It is a simple enough question, but I need more information to provide a viable answer.  Does your question refer to your 1975 C20?  The '75 didn't originate from the factory with a B+ cable between the battery and alternator.  What do you mean by, "the original cable with the red plastic box?"  What is the current output rating of your alternator?  Providing year/model information, whether the charging system has been upgraded or modified, and posting a pic of how the truck is presently configured should help.

[Rough75C20]

Hello bd,

The engine is an ‘82 305. Not sure amperage on the alternator. I’ll try to get a picture of it and post here shortly. It is some sort of junction block that splices the battery cable to the back of the alternator.

Thank you


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[Rough75C20]

Here are some pictures of the setup


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[bd]

None of the battery cable or the spliced jumper to alternator is original to the truck.  How many wires connect to the output post on the back of the alternator?  Is it just the one from the splice in the "red box?"

[Rough75C20]

There appears to be two wires on the output terminal. One from the battery cable splice and one that appears to run to this terminal block up on the firewall


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[bd]

AND ... the plot thickens....  The vehicle wiring has been modified from the original configuration.  I can provide an effective repair strategy, but not without further investigation. 

Disconnect the jumper wire between the battery cable 'red box' and the alternator output stud, but leave the wire between the alternator output stud and the firewall junction block connected.  Reconnect the battery cable to battery.  With the ignition switched OFF (i.e., engine not running), probe the alternator output stud with an incandescent test light that is clipped to a shiny clean ground.  Does the test light illuminate?

[Rough75C20]

Hello bd,

Yes I will try this tomorrow. I assume by testing this way we are trying to determine if there is redundant wiring here? Perhaps the jumper wire from the red box isn’t needed if the light illuminates without the connection from the jumper wire? Am I following correctly?

Thank you


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[bd]

Hello bd,

Yes I will try this tomorrow. I assume by testing this way we are trying to determine if there is redundant wiring here? Perhaps the jumper wire from the red box isn’t needed if the light illuminates without the connection from the jumper wire? Am I following correctly?

Thank you

You are.   

[Rough75C20]

Hello bd,

From what I’m looking at in the manual and going back and forth with ChatGPT, what I have looks to be potentially unsafe. The smaller cable coming out of the back of the alternator doesn’t seem like it would be large enough gauge wire to carry a charge current even if it makes its way down to the starter battery terminal. My guess would be that is some sort of voltage sensor wire or some sort of indicator wire based on what I’m reading. Is this a thing on these trucks normally? The larger gauge wire that comes from the spliced battery which should be the main charge cable is unfused. this should normally run to the starter battery terminal and charge the battery from there correct? If this is the case, should a new wire be run from the back of the alternator stud to the starter battery terminal with an inline fuse? My guess would be someone got lazy and spliced that cable from the alternator stud into the positive battery cable to save wire and not have to mess with the starter connections. Still doesn’t make sense though cause the diagram from the 77 - 87 shows a cable going directly from alternator to battery.
 

Thank you


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[bd]

A major weakness of aftermarket wiring diagrams is that they are generic, attempting to broadly cover too many vehicles.  When available, use the factory wiring diagrams that are specific to the year and model vehicle on which you are working.  It just so happens that we provide the factory repair and wiring manuals in our Technical Forum (1982 Factory GM Repair and Wiring Manual; the wiring diagrams are near the back of the manual).

The factory charge lead is 12-gauge, which is adequate for the original alternator capacity and electrical loads.  All B+ busses are protected by specifically sized fusible links, not modular fuses.  In a properly functioning system, an alternator will charge at high current output (e.g., 75 - 90% of alternator capacity) for only a short time (typically, < 1 minute), quickly tapering down to just compensate for disposable loads plus battery charging (e.g., 15 - 25 amps is typical).  The momentary high current output does not last long enough to overheat the alternator or wiring. 

Any battery that is severely discharged should NEVER be recharged solely by the vehicle alternator, as this CAN damage both the alternator and wiring through excessive heat production!  Use a suitable shop charger for severely discharged batteries.

Because they are still in their infancy, online AI tools may give you wrong indications and raise false alarms.  Take AI recommendations with a grain of salt.  Once we define the unknowns, I'll recommend proven repair actions that will provide proper and safe function, allowing room for future upgrades should you so choose.  The process is methodical.

From what I've observed so far, I think the wiring changes were an attempt to compensate for voltage losses across suspect wiring connections and a slightly increased alternator capacity.  Investigation should determine whether the approach taken was sane or SWAG.  Only later years used a direct-to-battery charge lead.

[bd]

Addressing your specific questions, the red and brown wires that attach to the alternator via the flat two-wire connector are discrete regulator circuits.  The small-gauge brown wire supplies an initial, current-limited trigger voltage via switched ignition power (I+) to energize or "excite" the regulator and initiate charging of the vehicle battery.  To block unwanted battery charging through the brown 'excitor' wire and prevent damage to the small-gauge cab wiring, trucks with the "base level" idiot light dash cluster supply the regulator trigger voltage through a charge indicator lamp, limiting current flow to ~0.3 amp.  When the ignition is first switched ON, the charge indicator lamp grounds through the regulator and illuminates.  Once the alternator begins charging, output voltage from the alternator backfeeds the charge indicator lamp via the brown wire, balancing the voltage supplied to the lamp by the ignition switch, and the lamp extinguishes.  Trucks having the instrument panel "gauge" option substitute a single-strand nichrome resistance wire (~7 ohms) for the 'base level' 18-gauge brown wire and charge indicator lamp.

The red regulator wire is a voltage-sensing circuit connected to B+.  The regulator uses the voltage present on this wire to regulate the alternator's output voltage.  Intelligent positioning of the red wire in the vehicle harness can compensate for inherent voltage losses across connections.  Consequently, all of the battery charging current from the alternator is conducted through the 12-gauge output lead from the alternator output stud; none passes through the regulator wiring.  If the alternator is upgraded to greater output than the factory design, the 12-gauge charge lead should be upgraded accordingly and possibly rerouted to shorten the path to battery.

[Rough75C20]

AND ... the plot thickens....  The vehicle wiring has been modified from the original configuration.  I can provide an effective repair strategy, but not without further investigation. 

Disconnect the jumper wire between the battery cable 'red box' and the alternator output stud, but leave the wire between the alternator output stud and the firewall junction block connected.  Reconnect the battery cable to battery.  With the ignition switched OFF (i.e., engine not running), probe the alternator output stud with an incandescent test light that is clipped to a shiny clean ground.  Does the test light illuminate?

Hello bd,

I did this test. After removing the jumper, reconnecting battery, turning ignition to “on” but not engine running the test light does illuminate when touching he output stud with the test light. With the switch in the off position, (key not in ignition) the light remains off. Let me know what I should try next.

Thank you


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[Rough75C20]

My thought would be maybe to reconnect the jumper and possibly take a multimeter to the alternator output and see what kind of amperage we are getting from the alternator to determine if the increased size wiring is required. Let me know your thoughts

Thank you


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[bd]

Disconnect the jumper wire between the battery cable 'red box' and the alternator output stud, but leave the wire between the alternator output stud and the firewall junction block connected.  Reconnect the battery cable to battery.  With the ignition switched OFF (i.e., engine not running), probe the alternator output stud with an incandescent test light that is clipped to a shiny clean ground.  Does the test light illuminate?

I did this test. After removing the jumper, reconnecting battery, turning ignition to “on” but not engine running the test light does illuminate when touching he output stud with the test light. With the switch in the off position, (key not in ignition) the light remains off. Let me know what I should try next.

You added an extra step by probing the alternator post with the ignition ON...

Repeat the test exactly as you did before, including your extra step, but with both wires disconnected from the alternator output post.