Fusible link issue 70 challenger

I replaced the link because it was hanging by a few strands. I put a fuse holder in place with a 20 amp fuse. It has blown the fuse a couple of times in about 300 miles. While out for a drive yesterday it quit while I was driving,now it blows the fuse as soon as I plug it in. With digital meter I am seeing 000.0 at first then goes to 000.8 or so after a second or so on the red wire and a couple of others on middle plug on bulkhead. Seems dumb but I'm not sure if I'm reading resistance across a winding or something when it goes to 000.8? If I unplug the bulkhead connector it goes open on the meter.

Based on your description it sounds like you are trying to measure resistance while the circuit is hot. Generally not advised. If so, what you may be seeing is the resistance increase slightly as the fuse gets hot. If you are measuring votlage drop, that's OK and the explanation is the same.

Take a look at this overview of the charging system power flow.
I think what has happened is:
1. the battery did not get sufficiently recharged.
2. as the battery voltage decreases, the voltage regulator tries to have the alternator supply more current for recharge.
3. 20 amp quick fuse blows immediately under the larger load.

If so, then everything should be back to normal after recharging your battery with a charger and installing a new fusible link. You'll have to dig through the service and parts books for the correct amperage for your car's fusible link.

A fusible link is in place for a reason. It's a SLOW blow circuit. Your modification can't survive short duration loads in excess of 20A like a link can.

The factory used a fusible link because the initial load is too much for a fuse. Replace the fuse with a fusible link,which is available at any parts store,and you will solve your problem.

Sorry guys, sometimes difficult to put into words. The tests with the meter were done with the battery + side disconnected. I did use the negative side for ground for the meter lead.

The battery is not dead. This is happening driving down the road.

I have no issue putting another fusible link in. My concern is I have a short. As soon as I put a fuse into the holder it blows it if the battery is connected. The other 2 times this happened I replaced the fuse and it was fine for some time.

Trying to isolate the issue with a meter and the readings are confusing me.

If I'm reading 000.0 when I first test and it goes to 000.8 on the meter a second later or so I'm wondering if I'm reading resistance on something inside the car on that circuit or if I have a short on that wire?

Where I'm taking this reading is at the bulkhead connector where the red wire that goes to the amp meter is. I'm confused if the 000.8 to ground is a short or reading resistance in the amp meter or the rest of that circuit?

The fusible link can and does handle 50-60 amps from time to time by original design. No way a 20 amp fuse will hold up.

It sounds like the original problem was fusible looked in poor shape. Suggest you replace with another fusible link. Fusible link is available in 16 gauge (original I believe) or 14 gauge at about any auto parts store.

I had to replace the link in a fairly new premium repop harness because the small spade connector was failing and overheating it. I put the new link (from Autozone) in with soldered ring connectors at each end and a small bolt. Plus electrical tape. Not elegant but no overheating since then and still easy to service if needed.

Agreed, I would slo charge the battery back up (if it ain't already) & replace the fusible link then (eng off) with all lights etc off including doors shut pull off a batt cable from its post after dark & see if there is a spark which if so would confirm a short somewhere that is drawing current & with everything off nothing should be "on" & drawing current or in the daytime switch your meter to the highest DC amp scale & work the meter amp scale setting downward with the probes on the batt post & the cable & should be no flow. Holler back.

A. A 20amp fuse is too little. The Alternator can put out a minimum of 40amps, more if you have upgraded it. Additionally if you add all the fuses together and add the headlights (they usually have a built in circuit breaker in the switch IIRC), that is the potential load your fuse has to handle. Either way you look at it, the voltage potential is too great for a 20amp fuse. Most modern cars use multiple 40 to 60 amp fuses or a single 100 to 150 amp fuse on the main lead.

B. How much have you done to the rest of the harness? Is it new, used and untouched, or used and repaired?

C. I often use a circuit breaker in place of the fuse during diagnostics while I am looking for the problem.


Since it seems you have a short try this.

1. Pull all the fuses from under dash and check to see if the 20amp fuse you are using blows.
1a.If it doesn't blow then replace the fuses 1 at a time until it does. Remove the last fuse before placing the next one in place to prevent erroneous/cumulative current problems. When it blows, that circuit is the problem.

2. If it blows without any fuses installed, disconnect the wires to the alternator and make sure they don't short. Does it still blow?

3. If it still blows check the bolt on connections to the ammeter, a common problem. Does it still blow?

4. If the ammeter is not the problem disconnect the ignition switch wires, another common problem. Does it still blow?

5. Disconnect each switch and connector under the dash one at a time. Pay close attention to the fan switch and wiring. Does it still blow?

6. If everything is disconnected and it's still blowing fuses then it's probably a short in the harness. If you repair the harness it is most likely the alternator/ammeter wiring circuit since it feeds everything in the harness.

I make the following modifications when I rewire a harness.
1. Solder all connections for the Ammeter circuit.
2. Place a fusible link at the bulkhead and the alternator. A runaway alternator will burn up a harness just as fast as a short.
3. Check the Ammeter connections extremely well. NO PLASTIC WASHERS. Ammeters build up heat and require HI-TEMP insulators.
4. Don't run the ammeter wires through the bulkhead connector. Either run continuous long loops of wire through a hole in the firewall to eliminate the connectors OR use bolt on, through the firewall, terminals for a better connection. I have drilled holes through the bulkhead connector and pushed a loop of wire through the hole to eliminate the spade terminals in the bulkhead connector. Not convenient to disconnect that way though.
5. Solder the large wires going to the ignition switch under the column to eliminate the bullet connectors that fail often.

Hope it helps.

^^^ good info. #1 pin down the short. EDIT This has been bothering me, the Fl only had a few strands left/you replace with a fuse/fuse wouldn't cut it. any possibility that there is no short?? & when you replace the FL it'll all be good. MORE EDIT I did not read back thru everything but did you say that there was no problem other than the FL was hanging on by a thread (the several strands)?

Thanks for all the info guys. history, I bought the car last year, had been sitting for years but still ran and drove when I bought it. I decided to freshen up the engine, clutch etc after I went through the car after purchase.

I removed everything from the engine compartment when the engine was out so I could clean everything. When cleaning the harness I noticed the fusible link only had a few strands of wire holding it on. I replaced that with a fuse holder and selected a 20a fuse (no idea why, seemed like a good place to start lol).I replaced the voltage regulator with an NOS one and the starter relay, left everything else as it was.

I got the car running again and drove it this way for about 200 miles, mostly short in town trips but one 60 mile trip. The fuse stayed intact until recently. The first time the fuse blew I had just installed a pertronix ignition in the original distributor, While I was driving it the fuse blew.I found I forgot to tighten the wire that goes to the positive side of the coil. I replaced the fuse and away I went. Monday I drove the car about 10 miles, when I left to go home I'm driving down the road and the car just quits. I pull over and the fuse is blown again. I go to replace the fuse and it blows the fuse as soon as I go to install it in the fuse holder. I get the car home and Test the fuse holder to ground and that is where the meter is showing 000.0 when I first test it, hold the lead in place and after a very short time ( a second or 2) the meter will climb to 000.8. My confusion is the 000.8 a short or resistance from the alt or something in the car?

I have not disconnected the power wire from the alt to bypass that yet.

All of the meter testing I've done was with the battery disconnected. When the fuse blows I have the key off and battery connected.

You are overthinking this. All of the individual accessory circuits, with the exception of the charging circuit, are protected with either fuses or circuit breakers. A fusible link is designed to protect against a gross overload of the entire electrical system. Replace your fuse with the correct fusible link and then start troubleshooting. Having the fuse in there is only muddying the waters.

I don't want to ruin the $3.00 fusible link if I have a short

I'll grab a parts store fusible link and install and see what happens.

Thanks again for the help.

6pak is right about the accessory circuits are probably not the problem.

If you insist on a fuse go to at least a 40 amp. I am currently running a 30amp in my /six No AC/Radio/Washer pump barracuda and it works OK but I don't drive at night when I would need headlights.

I lay odds the fusible link was just a victim of age, every spike weakens them and eventually the individual wires inside start to break one buy one until a catastrophic failure occurs. If it's an unmolested or improperly repaired harness I lay odds the problem lies in the harness itself. It is very common for the main power wire to slowly melt over time and short into other wires. Eventually it will find a ground through a circuit and the fusible link goes bye bye. I had one melt into the headlight switch wiring. I couldn't turn off my parking lights but I could dim them with the dash panel dimmer.

The wiring from the factory is minimal at best. I bet if you peel back the cover on the harness behind the fuse block you will find the black or red large gauge wires are melting.

Rebuilding a harness is pretty easy. Pull the whole dash and lay it on a bench. Pull the harness and unwrap it using plastic ties to keep the bundles loosely configured so it will be easier to re-wrap. Inspect the wires and if you find a bad one SOLDER a repair/replacement wire in it's place. Do not use crimp connections alone. Those are a bandaid that will eventually fail. Re-place the main power wires with a wire at least 1 gauge larger and don't use the factory bulkhead/harness connectors for the main power (ammeter/Alternator) and main ignition switch wiring.

That's why I used the term "troubleshoot". If you suspect a problem, and I believe you do, don't just replace the link, hook up the battery, fry the link, and say "Gee, I think I have a problem". Do your homework.

You can buy a 10 foot roll of Fusible link at Napa for about $10. Cut to length and add ends. Napa even sells the connectors that go into the bulkhead connectors if needed but I don't suggest you run the main power through the factory bulkhead... I think I may have mentioned that in my wordy previous posts. LOL

I bought two rolls, one 14 and one 12 gauge, years ago and still have most left.

Thanks guys, I did pull the fuse panel off the bulkhead and look at the wires inside the car. I was actually surprised how well everything looked, no discoloration or melted insulation etc. I'll update the post when I have results.

Yes you need to trouble shoot this, but to do so you need to have a plan and understand the circuits.

Go to the function diagrams in my first post.
Then make a wire diagram for your car and identify places you can access to isolate sections of the circuit. Feel free to use the modify the wire diagram at the bottom of the page to create one for your 70 E.

The functional diagram will make it easier to see that the fusible link only protects against high current from the battery or extreme charging of a dead battery.

In addition to the possibilty of a short to ground, it is possible that your alternator is failing, that the pertronix has been wired to battery side of the charge circuit, or the pertronix is draining the battery more during start up than the points system. ---> The lower the battery's charge, the more current the alternator will supply through the fusible link. The fusible link can handle 40 to 60 amps depending on vehicle, at least for a few minutes.

Update, had a little time last night. Made up a 14 gauge fusible link and installed. There is a short. The short is on the charging circuit shown in your diagram Mattax. I checked the alternator 12v terminal to ground and it is not shorted.

Hum, I powered the Pertronic to the + and - side of the coil. I'm wondering if I powered the coil with the wrong wire from the charging circuit when I put the engine back in? This issue did start after I changed over to the pertronic ignition.

I'll get my service manual out and start chasing wires to see what's going on.

Thanks again for the help.


Mike

Hmmm. Did you disconnect the battery lead from the alternator and then check each one separately? It seems odd that the charging circuit is the culprit but the lead at the alternator doesn't indicate that. The only time I have ever personally seen something like this was on a 360 where the harness to the alternator had rubbed on the intake manifold and there was an intermittent short to ground. Took a long time to find that one.

Ok. You're getting there.
If its a short as you suspect
By taking the connectors off (as you did the middle) you can narrow down the location. It sounds like you've checked to see if there was a reading between the connector and the alternator's output and there was none.
Do the same for the battery charge/supply wire.
Then check between one of the matching terminals on the bulkhead and ground.
-> If there is reading, then it must be inside.
-> If its open, check again with the ignition switch on 'RUN'. If its still open, then its one of the circuits that got disconnected when you removed the connectors.


The coil gets power from the ignition circuit.
During Start power from the starter switch goes directly to the coil.
When running power from the Run position of the switch goes through the resistor before going to the coil.

With points, the negative wire on the coil grounds and opens to build and collapse the charge. My guess is that an Ignitor I or II does the same. The positive is always hot (when the switch is in Run or Start).

Check the connections on the back of the ammeter.

Here is the wiring diagram for your car.
http://www.mymopar.com/downloads/1970/70_Dart_Challenger_Wiring_Full.pdf
If you look on Page 11 and 12 in the file (number 8-94 & 8-95 on the page header) You will notice that everything is tied into the Ammeter either directly or indirectly. Page 6 is the engine compartment (header #8-89) diagram.

First disconnect the pertronix ignition system and check for a short to be sure it isn't that.

Start by isolating where the short is. Battery side (probably not since it blows the fusible link), under dash, or alternator side of the engine compartment.

Note: There are 4 bulkhead connectors. 1 Large connector to one side of the bulkhead, 1 large in the middle of the bulkhead, and 2 small to the other side. I will refer to these in the following steps.


1. Close all doors, turn off all accessories, remove any underhood lights, Disconnect aftermarket radio's, and disconnect the battery. This prevents any erroneous readings from lights/devices being energized. Now connect an Ohmmeter to the large alternator terminal.

a. If it says there is a short disconnect the large bulkhead connector to the side. This separates the charging circuit from the under dash circuits. If the short goes away then it is probably under the dash but it could be a bad or chaffed wire under the hood. Moving the wires around will sometimes help you find it.
If it still says short disconnect the voltage regulator, ballast resistor and alternator checking for shorts after each. Still shorted? start unwrapping the wires to look for burns.

b. If goes away by disconnecting the large bulkhead connector to the side then start under the dash. Pull all the fuses and connect the ohmmeter to the back of the ammeter. Pulling the fuses prevents the erroneous reading from door open chimes lights etc. Disconnect both large bulkhead connectors. Does the ohmmeter read a short? If so disconnect the ignition switch wiring and the headlight switch connector. Both connect to the main harness without fuses. Still got a short? Double check the ammeter by disconnecting the wires and check each wire for a short separately without the ammeter connected at all. If the short goes away it is probably the ammeter insulators. The hands down most common problem in old mopars. That is why people are afraid to run them and switch them over the Voltage gauges. Check the ammeter terminals for short to ground.

c. Still got a short after disconnecting the ammeter? Then check the wires to the ammeter. If the red wire shows short then disconnect the middle large bulkhead connector to separate the battery side of the harness. Red is a straight shot the the bulkhead and fusible link if it's shorted you got problems in the harness. It will probably be on the black wire of the short is anywhere other than the ammeter or ignition switch because it feeds everything.

d. Black wire shows short? Disconnect every connector under the dash one at a time. Don't forget the wires to the AC/Heat. Still a short? probably a burnt wire in the harness. If the short goes away when you disconnect a particular device let us know which one.

Hope it helps

Power flows from higher voltage to lower voltage.
The Alternator supplies power around 14 Volts.
The Battery supplies power around 12.5 Volts when the alternator is not running.

When the alternator is running the battery floats closer to 14 and pretty much all the power is drawn from the alternator.

Wire R6 goes from the Alternator to the Bulkhead connector to the Main Splice.
Distribution occurs at the Main Splice.

Power distribution is hooked to the ammeter. It is wired to the ammeter from the Main Splice; the exact same way as the headlights, the fuse box and other circuits.

Thanks vey much guys, I'll have time Saturday to work on it. I will update then.

Update, my 47 year old alternator didn't want to play anymore. The short was in the alternator. Replaced the fusible link and put another alternator I had on.

Thanks again for the help.

Mike

Glad you found it. That's why I recommended disconnecting the lead at the alternator and checking each section separately. When it comes to trouble shooting electrical problems, divide and conquer usually is the best approach.

You guys help made it much easier. Thanks again.