Battery wire fire + alternator arcing

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AuroraGirl

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It's my truck. I take full responsibility. But I mentioned "they went heavy on "cheap"!"

Was about to pull the alternator to have it tested when I noticed this. The body shop didn't install the bolt from the engine to alternator. Is my understanding correct that the alternator gets its ground through the engine and the vibration was causing the alternator housing to separate from the engine which led to the arcing & fire due to intermittent loss of ground?

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that is specifically the ground of the alternator case ,yes. it could be handled by the other 2 ears, but they are prob corroded just like that back face is. I would suggest you clean the aluminum oxide layer (insulating) off the 3 points and properly secure it to the front 2 then do the rear one, and then carry on from there. if you wanted to increase the grounding of this engine, you could attach a lugged cable to the hole below it on the bracket and then run it to the chassis ground or the battery ground, I would do battery.
 

AuroraGirl

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I appreciate all of the feedback and assistance thus far and hope I don't become a nuisance with my questions :)
I keep thinking I'm doing this multi-meter testing wrong. In my mind I think any non-grounded wire should be open (hope that's correct term) and not visible by the conductivity test until a switch/relay closes the loop. If any of those wires have failed by making contact with the truck then I'd expect to see conductivity similar to if a switch/relay closed the loop.

I've removed all terminals from the junction block. I then touched one probe to the smaller + battery cable and the other end to the same wire's ring terminal. I got a loud constant chirp from the multi-meter as I'd expect. It's a straight wire with no other connections. I then moved the probe from the smaller + battery cable to the negative battery cable (battery is not on the truck) while maintaining the connection of the other probe to the ring terminal. Surprisingly, to me, I got a split-second "grunt" sound from the multi-meter. Okay I yanked that wire completely out of the truck. Next I did the same test with the alternator wire by touching one probe to the end that connects to the alternator and the other probe to the wire terminal that goes to the junction box, but it's disconnected. As expected I received the loud constant chirp confirming conductivity. I then moved the probe from the alternator side and touched it to a bolt going into the firewall with the other probe still attached to the ring terminal. Surprisingly I'm still getting a constant chirp. I've not pulled back the tape/loom that houses the cabling from the alternator to the junction block. But the service manual shows it to be a direct connection with no other splices/junctions.

I probably need a lesson on 12v or meter testing :) I can't understand why there would be a chirp from the terminal to the firewall if those 2 aren't physically connected anywhere. There are more wires from the junction block that are also chirping when I touch a probe to them and another to the firewall. But I'm going from left to right on the junction block trying to isolate each one and reading the wire diagram to dig deeper.

Any ideas of what I'm doing wrong or what I'm not understanding? Thanks!

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idk what logic is but i would assume autoranging of the values displayed? The one below it should be audible, at least to me that makes sense
 

basscat

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Thanks. Yeah I have the replacement bolt to complete that installation. I may run a tap through to clean those aluminum threads.
I think that logic mode is continuity with pulses. Something like that. Both provide audible chirps.

Still not sure there’s an actual electrical issue or just my incompetence. Would hate to spend $$ on all these new cables & terminals and have a repeat. If I was confident it was a grounding issue that can be corrected with new cables that would be great. But this continuity thing has me baffled and others confirmed I shouldn’t see continuity if everything is ok.
 

Frank Enstein

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If the wiring is compromised, the alternator can be damaged trying to charge what it thinks is a dead battery.
Put that new battery on a slow overnight charge. If you have a fancy battery charger that can de-sulphate/repair the battery it is a good idea to do that while you are fixing the wiring.
Alternators are TERRIBLE battery CHARGERS. They however are GREAT battery MAINTAINERS!
 

Frank Enstein

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A continuity test will show connection as either yes or no. It has it's place but it is not the only test. It cannot show resistance. In my experience, continuity tests will show good even with very high resistance. Also understand that sometimes the ground is switched i.e. interior courtesy lights and windshield wiper/washers.
 

basscat

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Thanks for the feedback. Am I likely testing & going about this wrong? I set out to determine if something other than the alternator losing its ground to the engine or an improperly seated terminal on the alternator or finally the failing ground wire from the passenger fender was the cause of the arc and fire. Using the multi-meter with the truck battery removed (no specific reason other than waiting for new fusible link to arrive) and simply checking if the terminals in the junction block produce a continuity signal when placing the 2nd probe at various locations such as block, frame & firewall. When I started getting continuity I assumed this meant there’s a short in the system somewhere. This is not a test where the light bulb says I’m getting power since the battery is disconnected. I’ve found no blown fuses or any signs or arcing or discoloration or melted wires. Maybe there is no short and I’m diagnosing incorrectly?
 

basscat

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A little more progress. Confirmed short (after continuity tests) by hooking up - battery terminal to - battery post and connecting a test light between + battery terminal and + battery post. One by one I pulled fuses from inside the truck and then walked around to the front and connected the test light. All fuse removals still illuminated the test light. I then pulled the 30 amp ACC/BAT fuse (Wagner 30A 12V) and the test light no longer illuminated. There are 2 identical fuses stacked in the fuse box. The 2nd is ACC/IGN. I swapped fuses to ensure it wasn't the actual fuse that was causing the issue.

Looking at the diagram I've traced the path. It appears the parking lamps and courtesty lights are involved in the circuit too. Does anyone have suggestions on the next best steps to narrow the search?
I've identified that 6294641 to be the ignition switch connector.
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basscat

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For anyone interested in how this story ended….

Turns out the alternator internally shorted. I guess it became a welding machine until the + battery cable burned in half. Good ‘ol O’Reilley’s put the alternator in their machine & it spit out a big green PASS message. I guess they only test whether an alternator is capable of welding in either direction. Anyhow, I wasn’t educated enough at the time to bench test it with a multimeter. I installed it and attempted to connect battery cables & got zaps & sizzles & immediately pulled cables away from post. Researched how to test for an alternator short & learned that testing for resistance between B+ alt post & alternator case is appropriate & there should be resistance. Mine had 0.2 ohms regardless of which probe was touching case vs post (virtually no resistance). Bad alternator is no big deal. Buy a new one & move on.

But my situation was unique. Recall the truck had no fusible link at the battery. Would that have made a difference? I don’t know. But prior to determining the alternator was bad I was testing the electrical system the best I knew how. I was searching to determine if the welding created damage in the form of a short. I utilized a simple test light connected in series with the - battery post & - battery cable. It was lightning up like a Christmas tree with the doors closed & hood lamp bulb removed. From my research this indicates a short since everything is theoretically turned off & open.

I started pulling fuses 1 by 1 & repeated the test until finally finding one that terminated the Christmas lights. Earlier in the thread I showed my findings. For whatever reason I tested again in full sunlight and at first I concluded again that the test light wasn’t illuminating. But for whatever reason I cupped my hand around it to create a shadow and noticed an ever so slight glow from the filament. What?! (If you’re screaming “use a multimeter to test for amperage” keep reading). At this point I yanked all fuses. Fuse box is empty. Tested again. Still a slight filament glow. Good grief!

I used the meter to test for voltage at the junction block. The battery is disconnected. It doesn’t really make sense to test for voltage but for whatever reason I did. With one probe touching junction block terminal and 2nd touching random bolt in firewall the meter read 2.5 volts!

For whatever reason I started probing areas inside the truck with the volt meter. There’s a pair of old stereo amplifiers under each seat. These were competition Lanzar amplifiers 30 years ago when I installed them. :)

But. But. But. The power wires are disconnected under the hood & the individual glass fuses in the distribution block next to amplifiers were already removed. The amplifiers were still grounded to the frame & speaker wires were still connected & the remote turn on wire from the radio was still connected. Guess what? I touched one probe to the amp’s disconnected dangling power wire & other probe to the ground wire & the meter read 2.5 volts! Apparently the frame became energized & sent current into the unpowered amp and the voltage was being stored in the capacitors.

The 2nd amp had no voltage.
I completely removed the amps and the voltage in the truck disappeared. I had never heard of that before.

Now why didn’t I test for a short using the multimeter test for amperage? I did! But it always read 0 amps. Inexperienced user … So kept returning to test light. Turns out my meter has 2 fuses for amperage testing & both were blown. I remembered I had a 2nd one today and hooked it up and it read amperage when I opened the doors.

If you’ve read this far I have a favor to ask. This 1990 model has less going on electrically vs the 1995+ models. With no radio in the truck I tested amperage and I get 30 milliamps for about 3 seconds then it goes to 0 amps. In other words the battery is disconnected then I attach meter probes between - battery post & - battery cable. I know 30 milliamps is small and on new trucks I think they draw that 24/7. But what is happening for those 3 seconds to read 30 milliamps then 0? Trying to conclusively rule out any remnants of potential damage :)
 
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