1969 Roadrunner Fusible Link Burning Hot to Touch When Running / Fine With Engine Off / Help Troubleshoot

[dedline]

Hello everyone. I’ve got a 1969 Plymouth Roadrunner with an upgraded 440. My son does the electrical for me and is the one writing up this post.



I have a problem with the fusible link getting too hot, so hot you can’t even touch it. It only gets hot when the engine is running. I can turn the key to the on position with the engine off and everything is fine. The fusible link appears fine and doesn’t look like it’s being damaged.



The car has this problem about 90% of the time. The other 10%, the fusible link stays reasonably cool. I ran it the other day for a short time and the fusible link stayed comfortable to hold. So, the problem appears slightly intermittent.



Recently, I’ve added two electric fans and a vacuum booster for the brakes. The fans run off of relays that get their positive directly from the battery and the booster is wired into the fuse box so it gets power when the key is on.



The car is running an upgraded alternator, a Powermaster 100 amp unit. The bulkhead connector is clean and properly snapped into place.



Any ideas where to start troubleshooting? Thanks

 

[Canadian1968]

The fans wired directly to the battery is your problem .

When those things turn on they are sucking a ton of amps. You don't mention any upgrade to your wiring , so all that current is going threw the orginal wiring to power your fans which is running off the battery.

My guess is your ammeter is maxed out charging all the time?

@72RoadrunnerGTX has some GREAT videos ..it's a sticky in electrical forum.


Check out his videos they will explain everything you need and tells you how to properly load a factory style system.

 

[pnora]

You are building up excess resistance. Most likely the alternator is overworking the underrated wiring. The system was never designed for the larger output alternator. You are going to have to make some changes. This always brings on debates. Myself I would run a dedicated alternator battery feed direct from the alternator to the starter relay. Painless makes a kit. I also would be looking at ammeter and bulkhead wiring. My self I would consider an ammeter bypass and installing a voltmeter.

 

[72RoadrunnerGTX]

Any ideas where to start troubleshooting?

As mentioned by @Canadian1968, get those added loads (fans) off the battery, connect them to the alternator output stud. The alternator is the primary power source while the engine is running, not the battery if still running the stock charge path, load placement matters.

 

[j-c-c-62]

I don't know how hot a fusible link operates at to touch, but before it opens/fails, it has to get hot enough to melt the enclosed wire conductor. It intentionally has an undersized wire that adds needed resistance so that when enough current flows it heats up just enough to melt and open the circuit. The fans connected directly to the battery operated by a relay only add to the fusible link the amount the relays draw to turn on, insignificant. Is the fusible link in the circuit between the battery and the alternator output?
It was not clear here how the electric vacuum pump is connected. That is likely? an intermittent draw depending on number of times and how long brakes are applied, which could account for the different temps the fusible link is showing, and indicating maybe you are starting to see amps closer to the links rating. A fusible link by design is a more forgiving current limiter, Ie slow to heat and open. It has to get rather hot to open, way beyond what you can touch unless under load for a long continuous time with little surrounding air movement and high ambient temps.

 

[Canadian1968]

I don't know how hot a fusible link operates at to touch, but before it opens/fails, it has to get hot enough to melt the enclosed wire conductor. It intentionally has an undersized wire that adds needed resistance so that when enough current flows it heats up just enough to melt and open the circuit. The fans connected directly to the battery operated by a relay only add to the fusible link the amount the relays draw to turn on, insignificant. Is the fusible link in the circuit between the battery and the alternator output?
It was not clear here how the electric vacuum pump is connected. That is likely? an intermittent draw depending on number of times and how long brakes are applied, which could account for the different temps the fusible link is showing, and indicating maybe you are starting to see amps closer to the links rating. A fusible link by design is a more forgiving current limiter, Ie slow to heat and open. It has to get rather hot to open, way beyond what you can touch unless under load for a long continuous time with little surrounding air movement and high ambient temps.

The relays hooked to the battery , will be triggered ( turned on ) by a 12v ignition source ( low current ) at pin # 86

Once closed , the relay allows high current draw from pin #30 , In this case it's from the battery. It should be from the alternator.

IF all other wiring is factory

 

[j-c-c-62]

Yes, I agree with your summation, but the OP in his opening comment only states where and how the fans receive their power, he is not clear on where/how the vacuum pump gets its power, which might account for the "extra" heat in the fusible link.

 

[Nacho-RT74]

Same reason why bulkhead connectors tipically burn… sourcing from wrong side. Specially on a stage with a batt not fully charged

 

[pnora]

As mentioned by @Canadian1968, get those added loads (fans) off the battery, connect them to the alternator output stud. The alternator is the primary power source while the engine is running, not the battery if still running the stock charge path, load placement matters.

It still falls back on wiring size. The 12 gauge wire and connections are under size and do not slice it no matter where the loads are placed. The systems maximum load is 30 to 40 amps coming or going. If you want big electrical loads than you should set up for it. You need a central point for charging and loads to be sourced from. Modern day we use a power distribution center. It supports alternator, loads and battery with proper wire size and circuit protection. The one problem with sourcing these heavy load accessories off the alternator stud is what happens when the alternator is not working? Examples key on engine off or when the alternator quits charging. All loads are placed on a under size 12 gauge wire with multiple connections and ammeter. 72RoadrunnerGTX has some excellent posts and well worth reading. I firmly believe if you want to add all these modern loads on an old under rated system do it right.

 

[Bird 426]

Seems like not a week goes by, week after week, same thing. All of these self inflicted electrical issues. How did our cars run and function just fine way back when? I don't add all of that on and my cars run and function just like they always did. All that I can do is shake my head and say why? If you stab yourself with a knife, it's going to hurt. Sorry, but this just gets so old.

 

[Nacho-RT74]

It still falls back on wiring size. The 12 gauge wire and connections are under size and do not slice it no matter where the loads are placed. The systems maximum load is 30 to 40 amps coming or going. If you want big electrical loads than you should set up for it. You need a central point for charging and loads to be sourced from. Modern day we use a power distribution center. It supports alternator, loads and battery with proper wire size and circuit protection. The one problem with sourcing these heavy load accessories off the alternator stud is what happens when the alternator is not working? Examples key on engine off or when the alternator quits charging. All loads are placed on an under size 12 gauge wire with multiple connections and ammeter. 72RoadrunnerGTX has some excellent posts and well worth reading. I firmly believe if you want to add all these modern loads on an old under rated system do it right.

True! When alternator quits working or you turn on accs with engine off then becomes on a problem HOWEVER that’s not the regular stage, while sourcing from batt it becomes on a problem still on regular use

 

[72RoadrunnerGTX]

It still falls back on wiring size. The 12 gauge wire and connections are under size and do not slice it no matter where the loads are placed. The systems maximum load is 30 to 40 amps coming or going. If you want big electrical loads than you should set up for it. You need a central point for charging and loads to be sourced from. Modern day we use a power distribution center. It supports alternator, loads and battery with proper wire size and circuit protection. The one problem with sourcing these heavy load accessories off the alternator stud is what happens when the alternator is not working? Examples key on engine off or when the alternator quits charging. All loads are placed on a under size 12 gauge wire with multiple connections and ammeter. 72RoadrunnerGTX has some excellent posts and well worth reading. I firmly believe if you want to add all these modern loads on an old under rated system do it right.

Agreed, this level added loads along with a high powered alternator should really have complete up-sized charge path wiring bypassing the bulkhead Packard terminals, a fleet bypass at a minimum.

 

[72RoadrunnerGTX]

... The fans connected directly to the battery operated by a relay only add to the fusible link the amount the relays draw to turn on, insignificant.

This statement is wrong, assuming a stock charge path and wired as pictured, this is not about the 150ma of current it takes to activate the relay, it’s where you source the secondary contacts/load power from that matters. A fusible link hot to the touch indicates there is current flowing across it and the rest of the stock charge path close to the level of its designed protection, too much for the stock charge path wiring/components over a period of time.

The OP indicates the vacuum pump is powered from a keyed source at the fuse box, all power there is sourced from splice 1, switched or not, on the alternator side of the ammeter, would not contribute to high current at the fusible link while running. Not familiar with what level of current those draw but it too needs to be factored into the entire picture to be considered safe.

 

[j-c-c-62]

j-c-c-62 said:
... The fans connected directly to the battery operated by a relay only add to the fusible link the amount the relays draw to turn on, insignificant.

This statement is wrong, assuming a stock charge path and wired as pictured, this is not about the 150ma of current it takes to activate the relay, it’s where you source the secondary contacts/load power from that matters.

The op said in opening comment the "fans" are powered directly from the battery.
Please explain how my comment is 'wrong".

 

[72RoadrunnerGTX]

j-c-c-62 said:
... The fans connected directly to the battery operated by a relay only add to the fusible link the amount the relays draw to turn on, insignificant.



The op said in opening comment the "fans" are powered directly from the battery.
Please explain how my comment is 'wrong".

Pretty sure my reply explains why that statement is wrong, any and all loads connected at the battery on this stock charging system, whether it’s the primary side of a relay (yes, insignificant) or the secondary contacts/loads, will draw their current from the alternator through the entire stock charge path, including the fusible link, while the engine is running. The alternator is the primary source of power when in operation not the battery.

 

[j-c-c-62]

Why would the output of the alternator need to be protected by a fusible link in normal conditions, vs the alternator directly feeding the battery and all the accessories other than the starter run thru the fusible link directly to the battery? From a safety standpoint I personally would prefer not to have my brake vacuum pump protected by a master fusible link.
And yes, my statement is incorrect based on your description of the charging circuit.

 

[72RoadrunnerGTX]

Why would the output of the alternator need to be protected by a fusible link in normal conditions, vs the alternator directly feeding the battery and all the accessories other than the starter run thru the fusible link directly to the battery? From a safety standpoint I personally would prefer not to have my brake vacuum pump protected by a master fusible link.
And yes, my statement is incorrect based on your description of the charging circuit.

Not sure I am following that question. I would suggest you take a few minutes and watch the first part of the linked video in post #4, it is about how this original ammeter-based charging system was designed to function and what happens when added running loads are misplaced at the battery. Happen to have exampled a cooling fan added load and related relay.