Fusible link trouble

-
Duster_71

Duster_71

Well-Known Member
Joined
Jul 22, 2013
Messages
284
Reaction score
19
Location
PA
A couple weeks ago I had the 1974 Duster out to a cruise-in. On the trip home I lost all electrical power. I made a quick check of all the connectors back to the bulk head and everything seemed in order. Didn’t have the tools to dig further so I called a tow truck. Once home I started some research which led me to re-inspect the fusible link and the ammeter. The Fusible link seemed ok. I removed the wires from the ammeter and temporarily bypassed it and wallah everything was right with the world again. I even drove the car to another cruise-in, with no problems, until the new ammeter came in. Received the new ammeter and removed, cleaned up the instrument cluster, replaced new meter and hooked everything back up. I then hooked the battery up and in pretty short order the fusible link melted away. Continuity seems to be fine so now I’m scratching my head. I’m starting over now and rechecking continuity am I missing something else? Thanks, Ron
 
What he said ^^^.
Duster_71 said:
Continuity seems to be fine so now I’m scratching my head. I’m starting over now and rechecking continuity am I missing something else? Thanks, Ron
Click to expand...
What he's saying is the problem is not an open in the circuit.
The fusible link melts when there is too much current through it. That happens when the electricity finds a shortcut to the negative post.
see this:Fusible Links in Charging Systems with Ammeter

Either there's an internal short in some equipment (alternator, ammeter, etc), wire insulation has chafed off and is touching. You prob know that some (all?) Chrysler ammeters have an insulating fiber washer on the studs to insure the housing (if metal) doesn't ground. In any event the link above should help you trouble shoot.

A short could have left you stranded just as easily as an open circuit. Based on everything you've mentioned it's possible moving the wires to one stud on the old ammeter shifted wiring so it wasn't shorting.
Tou could check both ammeters for continuity. Should be no continuity between either stud and the housing. Should be infinite contuity (no resistance) between studs.
 
Mattax said:
Should be no continuity between either stud and the housing. Should be infinite contuity (no resistance) between studs.
Click to expand...
Just to clarify...
Housing to either post should show "OL" on most digital Multimeters, resistance would be greater than 100s of millions of ohms

Post to post should show 0 ohms or VERY close to it.
 
In the first instance, that short probably blew the ammeter, leaving just the shunt working. That shunt is just big enough to run the ignition system, and maybe a couple of running lights. But as soon as you turn the headlights on, the current all goes to them and the engine stalls.
In the second instance, the ammeter seems to have survived, because the F-link did it's job.
I would go straight back to the ammeter and check your work there; those posts come very close to the column support, and perhaps you "missed it by that much"
 
AJ, The shunt can handle normal loads fine. The portion of current diverted to move the needle is tiny. There's nothing to 'blow' on the shunt. 67Dart273 has posted pictures showing failures from heat. That takes time, and what happens on some is the the connecting studs become loose. Of course loose studs mean more heat when current is flowing and so it only gets worse.

Anyway, the entire feed/charge line should and can handle ignition, field, and all lights. I've driven A-bodies at night on battery power only quite a few times. It's only around 17 amps, a couple more with brakes on.

edit: For visual comparison, look at the size of the shunt and attaching studs.
Much bigger than the 12 gage wire this particular gage was attached to.
upload_2019-6-26_22-7-2.png
 
Last edited:
If I was looking for a short circuit in a 74 model I would start with 2 white plastic harness connectors. One is below the steering column, links ignition switch to cars harness. The other is right side of firewall, links engine to cars harness. Both are prone to melting, intermittent short circuit. Good hunting
 
Hello All,
While replacing the ammeter I decided to clean everything up in the instrument cluster. After I reassembled the cluster I did some continuity checks on the cluster body and cluster board had continuity to the cluster body. Removed a couple of cluster lights and continuity gone, replaced lights and continuity fine. No continuity between ammeter and cluster body. Checked continuity between inside bulkhead passage to the red connector for ammeter and had good continuity. Checked red connector at ammeter to ground and no continuity (good right?). Checked black lead off ammeter to ground and had good continuity (should that be?). Additionally, placed new headlight switch while everything was apart. New switch looked identical to old switch with the exception of a tab coming off the back, looked to be for additional ground. I’m going to do some more checks but is there an area I should concentrate on?
v/r
Ron
 
The ammeter is in the battery feed.
When the battery is connected all of the main wires are at battery potential.
upload_2019-6-27_11-42-23.png


Duster_71 said:
Checked black lead off ammeter to ground and had good continuity (should that be?)
Click to expand...
Danger , Danger! Will Rogers

Headed to a scenario like the second example here:
Fusible Links in Charging Systems with Ammeter

Find where/what connects that lead to ground. Then it should be good.
 
And just to be sure its not in the ammeter, check them as dana clarified.
Fusible link trouble

It sounds like your housing is at ground. If so, make sure the leads do not touch it when attached to the studs. There's should be insulating washers. @RedFish may know if thats how it is on a '74 Duster.
 
Inst' housing and it bolted to dash is the ground path for majority of lamps and the limiter. With panel hanging loose from the dash, the only lamps that would work are oil lamp and brake lamp.
All gauges mount isolated from ground.
I'm not convinced the fault is/was at the amp gauge. A hundred or so examples of failed amp gauge are found in a forum such as this but... considering the thousands of these gauges in service, research here can be misleading.
Let us know what you find.
 
Found a picture showing the back of a '74 that was posted a few years.
mms_img1708453986-jpg.jpg
 
Last edited:
Mattax said:
AJ, The shunt can handle normal loads fine. The portion of current diverted to move the needle is tiny. There's nothing to 'blow' on the shunt. 67Dart273 has posted pictures showing failures from heat. That takes time, and what happens on some is the the connecting studs become loose. Of course loose studs mean more heat when current is flowing and so it only gets worse.

Anyway, the entire feed/charge line should and can handle ignition, field, and all lights. I've driven A-bodies at night on battery power only quite a few times. It's only around 17 amps, a couple more with brakes on.

edit: For visual comparison, look at the size of the shunt and attaching studs.
Much bigger than the 12 gage wire this particular gage was attached to.
View attachment 1715355664
Click to expand...

I have seen them melted in old Dodge vans. The first one took me a long long time to find
 
I recall an insulator strip needed on the back of the ammeter to keep it from shorting to the housing. Goes over both terminal studs.
 
I replaced my fusible link with a fuse. I blows much faster than the link, but the fuses are so much less expensive and readily available
 
AJ/FormS said:
That's all good until one day....... 30 miles from no where, late at night, and yur out of fuses.
Click to expand...
I am never out of fuses. Plus that is what I have a wife for, I have called her at night and had her pick up a hose and bring it to me so I could get the car going

I keep quite a few spares and make sure I have them on hand. Who carries a spare fusible link?
 
You missed the point.
A fuse blows asap.
A fusible link will allow quite a bit of current to passthru it, in a short time,
or a lotta time to pass at a small overload;
either, without blowing.
You probably know all this, but
So if you have an intermittent short, or a low-draw short, on the run circuit, which is an unfused circuit,feeding the ignition, the choke heater, the entire fuse-box, the VR, and anything else you mighta wired directly to that circuit instead of isolating it with a relay; you might never even know it. Whereas the fuse just pops instantly, and there you are in the dark trying to diagnose this.... and running out of fuses, while your dear sweet wife waits, and waits, and waits.
 
You can also buy slow blow fuses that are used in motor starting applications that will take an over current for awhile before blowing. I have not done any detailed comparisons, but I think the performance would be similar to a fusible link. Maybe someone out there has some experience along these lines?
 
Mike69cuda said:
You can also buy slow blow fuses that are used in motor starting applications that will take an over current for awhile before blowing. I have not done any detailed comparisons, but I think the performance would be similar to a fusible link. Maybe someone out there has some experience along these lines?
Click to expand...
The problem I have using them is that much damage can be done while the fuse is waiting to blow. Fuses are not that expensive. I converted the five or six glass fuses on my Duster. When I rewired the car, I split things into 20 fuse protected circuits. The largest fuse I use is a 10 amp, and down to a 3 amp.
The main fuse is a 30 amp fast blow. If the problem gets past my branch circuit fuses, I want the main fuse to open quickly
 
Pawned said:
The problem I have using them is that much damage can be done while the fuse is waiting to blow. Fuses are not that expensive. I converted the five or six glass fuses on my Duster. When I rewired the car, I split things into 20 fuse protected circuits. The largest fuse I use is a 10 amp, and down to a 3 amp.
The main fuse is a 30 amp fast blow. If the problem gets past my branch circuit fuses, I want the main fuse to open quickly
Click to expand...
If I may make a suggestion. Instead of continually replacing fuses, remove the fuse and connect a headlight bulb with jumper wires where the fuse link or now fuse is located. The bulb connects in place of the fuse link, not to ground. When the short to ground is present the headlight bulb will light. When you find and remove the short the light goes out. Much easier than poking around at random.
 
Rustedwrench said:
If I may make a suggestion. Instead of continually replacing fuses, remove the fuse and connect a headlight bulb with jumper wires where the fuse link or now fuse is located. The bulb connects in place of the fuse link, not to ground. When the short to ground is present the headlight bulb will light. When you find and remove the short the light goes out. Much easier than poking around at random.
Click to expand...
There is noting stock about my wiring. I rewired the entire car with my wires. Did not pay someone to make a wiring harness, I built it myself. My setup is perfect for my car. The only time my main fuse has blow, is when I screw thing up while working on the wiring. If I unintentionally drop a wire and it grounds out or something similar. Nothing has ever blown when while driving the car
 
Hello All, Did some more tracing and have come up with the following. Remember I found continuity from the black lead off the ammeter to ground. I have the service manual and started tracing the harness. I've found that block 7 & 8 in the fuse block seem to be troublesome. First block 8 and the light switch, lets see if I can explain, with fuse in I checked the light switch socket and found that R read 1.0 to ground and D read .2 to ground, I don't know if that is normal. I tested two light switches and found the R to I post read .2 with dash lights off and it went up as you turn the switch. Then I checked post with just marker lights on and found post R to B2 read .1 and R to P read .1 then I checked with head lights on and found post R to B2 read .1 and B1 to H read .1. After all that If the light switch is off I do not get continuity from black lead off ammeter to ground. On the other hand if I put the fuse back in block 7 I get the continuity back. Am I just flapping out there or am I getting somewhere?
 
If the fault was beyond one of the smaller glass bar fuses that fuse would blow. Fusible link protects the larger supply wires. Headlight switch has 2 supply wires, B1 and B2. Neither should have a path to ground with switch in off position. So if the wiring to your dimmer switch or that switch was shorted to ground, your fusible link would stand until the headlight switch is pulled to on.
Some wires in that harness connector would show the resistance in bulb filaments path to ground. Dimmer rheostat would show the variable resistance to ground that you found.
If the dome lamp fuse and bulb are in place, both red and black at the amp gauge would have a small path to ground while the door is open. I don't know what ohms values to expect to find. I've typed pretty much all I do know. Hope it helps.
 
You must log in or register to reply here.
-

Back
Top