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Fusible link fried - what to check before replacing

mr_spock

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Hi all, looking for some guidance. Car is a '66 Coronet with a 383 conversion (was a 318). Everything electrically worked, but the speedo had failed. I removed the cluster, replaced the speedo, put it all back. However, being an idiot, for the first time I forgot to check the ammeter wiring and connected the wires the wrong way round. Put the cluster back, hooked up the battery and FZZZZT there was smoke immediately. Disconnected the battery straight away. I'm confused as that shouldn't matter, the ammeter would just read backwards, right?

Pulled the cluster again to test. So far I've confirmed:
- Fusible link is fried
- Bulkhead connectors are all good, no sign of any damage, heat, smoke, melting or anything
- Everything in the cluster works so far, circuit board and instrument voltage regulator are fine, lights work etc. I have not tested the ammeter yet.

Note - I did not attempt to start the car, I didn't even turn on the key. It just blew immediately I connected the battery.

Now, I know it's a good idea to modify wiring in a few ways but right now I just want it to be a car again. So - what else should I check before replacing the link and trying again? They're $35 or so, don't want to experiment at that cost! The only thing I can see is that one of the ammeter posts seems a little loose. I have a spare.

Thanks!
 
Sounds like you had a straight to ground short. Trace the wiring back to find it.
 
I thought that - maybe one of the ammeter posts shorted on something loose in the dash, but wouldn't I see some burn evidence?
 
You are correct swapping the wires would just make the ammeter read reverse (charging when discharging and vice-versa).

The posts for the ammeter need to be isolated from the dash frame.
Did you maybe loose/damage an insulating ring or washer on the ammeter itself?

As a test you can try disconnecting the wires from the ammeter and checking the posts to chassis ground with an ohm-meter.
Should be infinite resistance if it's low then you need to correct.

Similarly you can bolt the two ammeter wires together and insulate (tape) over the connection.
The car should function normally but without the ammeter of course.
 
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Oh yes, cool, will check that. Thanks!
 
Oh yes, cool, will check that. Thanks!
1698698110569.png
 
I would start by disconnecting the alternator, and make sure the alternator output is not grounding. Then check if the alternator wire to the bulkhead is grounded/shorted.
If it is shorted, then disconnect the center bulkhead connector and re-check to see if short still exists in the engine compartment. If no short, then start checking under the dash, maybe at the altmeter connections (and isolators.)
 
Sure will do. Why would something like that break in just a month of sitting there?
 
You are correct swapping the wires would just make the ammeter read reverse (charging when discharging and vice-versa).

The posts for the ammeter need to be isolated from the dash frame.
Did you maybe loose/damage an insulating ring or washer on the ammeter itself?

As a test you can try disconnecting the wires from the ammeter and checking the posts to chassis ground with an ohm-meter.
Should be infinite resistance if it's low then you need to correct.

Similarly you can bolt the two ammeter wires together and insulate (tape) over the connection.
The car should function normally but without the ammeter of course.
I concur.....it was highly likely that when you removed the ammeter wires to remove the dash panel that one or both of the ammeter connections studs caused the stud insulator(s) to become in contact with the frame (ground). The origional insulation was just a thin fiber board piece, a paper like material, to allow the connection stud to contact the frame, causing a short to cround. This fiber board is over 50+ years old and is probably very fragile. The instrument can be repaired using nylon, pvc, plastic, or anything non metallic on both sides of the case where the studs pass thru. Small non metallic washers could also be used on the studs. The insulation is to prevent movement of the studs when connecting the wires. Consider using shake proof (srar) lock washers and double nut the connection to prevent the studs from turning when tightening the wires. There are some pix of a typical Mopar ammeter that shows the assembly.....it's a very simple yet fragile design.....not one of Mopar's best design.......just my opinion of course.......
BOB RENTON
 
Your fusible link did just what it was designed to do. Now, before you replace it, hook your digital ohm meter up to the wire going to the firewall and test to ground. If it reads Zero or close to it you still have something shorted to ground. You will need to trace circuits until you find where the short is. Start by pulling fuses one at a time to see if the ground goes away. If you still don't find it, you have a bare wire going to ground somewhere. You definitely need to find it before replacing the fusible link and hooking the battery up. Diligence and patience are required here. Good Luck!
 
Thanks for everyone’s input! Did some testing today. All bulkhead connectors unplugged, red wire, no connection to ground. Good. Black wire, just under 3 ohms to ground.

I traced possible paths (apart from something being fried), pulled the stop/tail fuse and the black wire was now not grounded at all. From that fuse, the only things that are paths to ground are the stop switch at the brake pedal, the cigar lighter and (I think) the glovebox light. Also a map light if you have one.

Long story short, I disconnected the cigar lighter and the meter read infinite again for the black wire. When I reconnected it and pushed in the lighter, back to around 3 ohms.

I can’t quite believe that it could cause my problem, but it’s the only path to ground so far. I expect I’d see something similar with the headlamps on or the brake pedal depressed.

Now to reconnect the bulkhead connectors and see if that changes anything.

Incidentally, the ammeter reads zero ohms (good) and the loose stud just needed the bottom nut tightening a tad.

More later
 
You're gaining bro...........It looks like you have cleared the hurdle of something under the dash being the culprit. Once you plug in a firewall plug, check again.
 
Plastic or any similar material able to melt no matter if SLOOOWLY or fast with a small ammount of heat is a wrong decision on ammeter studs isolation JUST IN CASE some heat is produced on assembly by the current going throught (for any reason). Must be a heat resistant material. If not the original one, bakelite for example (if can be molded/cut to the shape) or an old PCB sheet without the metal layer.
 
I should have said - I did check the stud isolation, it’s fine both visually and tested with a meter.
 
I should have said - I did check the stud isolation, it’s fine both visually and tested with a meter.
Cool… the ammeter gets two, one outside visible and the other one inside between amm and housing.

however I said that because was mentioned the use of plastic and similar materials for that, which could be a wrong decision.

sure the alt gets a “melteable” isolation on stud, but alternator gets a vented cooling system by default and a wider surface contact offering less resistance to the current.
 
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So - what else should I check before replacing the link and trying again? They're $35 or so, don't want to experiment at that cost! The only thing I can see is that one of the ammeter posts seems a little loose. I have a spare.

Thanks!
$35 for a fusible link??
You can get them at a NAPA store for under ten bucks.
 
The only time I ever blew a fusible link was when the alternator hot wire found ground and the 'link' protected the car! After I got it on a trailer and got it home, I found the wiring harness on the back side of the valve cover (/6 car) had chafed the insulation. Typical old age stuff. Repaired the harness and was off to the races once again. That was the only time my 66 let me down in the 10 years I drove it.
 
$35 for a fusible link??
You can get them at a NAPA store for under ten bucks.
A full roll in fact for that ammount. At least on eBay.

sure if wishing a perfect reproduction or NOS including the flag and correct terminals won’t be cheap
 
Oh! BTW! which wiring to check ? If the short came out without activate the ign switch then every wire network ilustrated on this diagram.

1699129566392.jpeg

all those lines are hot and without a fuse protection on line no matter the ign switch position.

(bulkhead based on E-71/74 B body)
 
Did a bit more diags yesterday. The alternator has a Carquest sticker on, code seems to be a 100A alternator. No idea why, there’s nothing that would need that capacity, but whatever. The resistance from the output stud to ground is about 14 ohms. I have no idea if that’s ok, but it’s not a short to ground. I have an alternator (a 1 wire GM) on my 57 Plymouth and the resistance on that is about 38 ohms, so same order of magnitude. Any thoughts on that?

The reason I ordered the repro fusible link, even at that price, is that it does indeed come with the correct blade connector, and there are 2 wires anyway, one being the fused link and the other is just a wire. So by the time I’d pulled the old one apart, tried to detach the connector, sworn at it, cleaned it etc. it would be another couple of weeks and I’d like to drive it before the weather makes that unpleasant.

So I don’t think I’ve found an obvious cause except the cigar lighter, which I can’t imagine is it. When I get the new link installed I’ll meter it all again and see if I can find anything. Last think I guess could be the starter relay, but that checks out OK too.
 
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