Help- 87 R20 voltage keeps dropping intermittently while driving

[87 Wrangler]

Hi guys,
Hoping someone has run across this in the past. My 87 R20 with tbi350 voltage keeps dropping then intermittently coming back up while driving. I have changed the battery and alternator from my parts truck, but last night it acted up again. Seems to be fine until I turn the lights on. Drops to about 12.6v when I stop at a light, then takes a while to come back up after if it does. I have load tested the battery, checked alternator and wiring, and I am stumped. There does seem to be a small inline fuse on the power wire exiting the back of the alternator. I am questioning this.
Anyone have any ideas? Thanks.

[LTZ C20]

Could be intermittent high resistance in a fusible link when a high load is placed on the system, such as turning the head lights on.

[frotosride]

There may actually be an electrical problem but there are a few things you can do to make the system more efficient. First is with the alternator wiring. Remember that the alternator only puts out positive voltage thought the lug so all the negative voltage is through the body of the alternator. With that said,grounding is paramount! There should be a ground straight from the nearest mounting bolt to the chassis of the truck. Make sure the chassis/frame is clean of all rust and paint for a good ground. The reason for this is resistance. If you are relying on any other ground the (-) voltage has to travel through the boo ck to the ground wire and so on throughout the try k. Secondly, if you are using the original 3 wire design (SI) style alternator there is a voltage sensing wire which when relocated to a more distant point ie, the cab, will allow the alternator to actually sense voltage drops better and not just what the battery is putting out. If you look at the back of the alternator there will be one connector with two wires. The Brown or sometimes Black wire is for field flashing. The other wire is really short and simply goes from the connector to the positive output lug. This is the voltage sensing wire. It tells the rectifier to kick up the output based on load on the electrical system. Lengthening this wire and placing it on a large load inside the cab will make a big difference in electrical performance of the entire system.

[bd]

...There does seem to be a small inline fuse on the power wire exiting the back of the alternator....

          Post a pic...


I agree - there is excessive resistance somewhere in the battery positive cabling that feeds into the cab off the starter. 

Begin by disconnecting the ground cable from the battery, so you don't accidentally arc to ground through a wrench while working.  Inspect/clean/tighten the battery positive cable and two fusible link connections to the 3/8" stud on the starter solenoid.  Loose cable connections and resulting heat damage to the wire terminal ends and the starter solenoid end cap are not uncommon.  Make sure that the battery cable and fusible link attachments to their respective terminal ends are clean and securely crimped.  Give the fusible links a gentle tug to see if they stretch.  Perform a similar inspection of the 12-gauge red wire(s) and fusible link connections to the firewall junction block located above the left valve cover.  Correct any problems you find before continuing.  If you need to replace a fusible link, see How to make a Fusible Link in our Technical Pages.

If the connections and cable terminations described above are sound, disconnect the firewall bulkhead plug located below the windshield wiper motor and inspect the terminals for the 12-gauge red wires that feed into the cab.  Don't be alarmed by the black tar-like antioxidant that seals the bulkhead connector.  The sealant helps prevent water intrusion w/o compromising electrical connections.  Look closely for telltale signs of heat stress, including distortion or melting of the plastic connector.

And, by all means, make sure the alternator and engine are directly grounded to the battery through clean, tight connections.  Then take a moment to inventory and check all of the remaining primary grounds: battery-to-frame, battery-to-radiator support, back of right cylinder head-to-cab, and cab-to-instrument panel.  If any cables appear to have been replaced, verify that they are of adequate gauge (factory spec or larger).

Be cautious about modifying the factory wiring when a suspected electrical problem exists, if you aren't resolved to find and repair the electrical problem at the same time.  Intentionally masking over an existing problem can cause damage to related systems at the very least and potentially burn a vehicle (and any adjacent structures) to the ground!

[87 Wrangler]

Wow thanks for all the info guys!!! Next time I go out to the garage il take a pic of this fuse thing. Be a few days before I can give it a once over, I really appreciate the step by step info!
I did try to clean the alternator plug when I swapped them out. Was pretty crappy looking but connectors were decent, and I lubed with dielectric grease. I would not be surprised if I have a bad connection, has been sitting all winter. Just seems strange all of a sudden its acting up. When I left Home Depot I pulled the lights on and could hear the alternator "whining." Got home and it was Hot! Smelly hot! Swapped it and no different, threw in a different battery and was good for 350KM or more until I pulled the lights on leaving Home D again! Think im cursed.. Last year a mouse chewed the headlight plug wires on my sled, maybe he went for the truck this time...
     

[87 Wrangler]

Hes a few.

[87 Wrangler]

The fuse thing?