Author Topic: Junction box fuseable link keeps blowing  (Read 5956 times)

Offline Mortyman

  • Newbie
  • Posts: 16
  • Newbie
Junction box fuseable link keeps blowing
« on: September 24, 2018, 06:32:15 PM »
New member here.  I have a 79 K10 that recently blew the fuseable link on the firewall.  It happened the first time when it was at the paint shop.  And it has happened several times since.  I replaced the fuseable link with a fuse holder so I wouldn't have to splice over and over again.  I can start the truck and it will run for a second or so and then that fuse will blow.  The truck will run very rough at that point and the dome light under the dash comes on and pulsates/flickers. This is intermittent. I was able to take it for a test drive for about 10 minutes the other night. I came back and parked it, half an hour later (not having done anything to it) I started it up to put it in the garage and it popped again.  I have new wiring from the bulkhead, to the starter, to the alternator, new fuseable links at the starter, etc... I just can't figure out why it blows 20/25 amp fuses very quickly when it does and then occasionally it will run for several minutes. Thank you.

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Junction box fuseable link keeps blowing
« Reply #1 on: September 24, 2018, 07:51:45 PM »
Welcome to the site!  Disconnect and tape up the 12-gauge, red, charge wire connected to the B+ post on the back of the alternator and unplug the two-wire regulator connector located next to the B+ post, then try it again.  Trace and closely inspect the entire length of the charge wire between the alternator B+ post to the junction block on the firewall, looking for abrasions, cuts or heat damage to the wire insulation.  Don't make any assumptions while doing this.
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Online Rapid Roy

  • Junior Member
  • ***
  • Posts: 635
  • Euless, Texas
Re: Junction box fuseable link keeps blowing
« Reply #2 on: September 24, 2018, 08:41:40 PM »
Welcome
1974 Cheyenne 10 LWB STOCK 350 W HEI /TH350/AC/4 BBL Quadrajet
Mopar by Birth
Chevy by Choice

Offline Mortyman

  • Newbie
  • Posts: 16
  • Newbie
Re: Junction box fuseable link keeps blowing
« Reply #3 on: October 01, 2018, 12:11:03 PM »
Thanks for the welcome! I finally got around to playing with the working this weekend. I took your advice and checked all wiring. Everything looks and feels normal.  I have found that the problem is only when I start the truck and it's intermittent. I was able to start and drive the truck a couple times this weekend and the fuse never blows after it has been started. But if I turn it off, there's a 50/50 chance that when I go to start it again it might blow the fuse at the junction block. Key is off, battery and fuse read just over 12 volts. Key on, same thing. Truck running, the battery and fuse read about 14.5 volts. Suggestions?

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Junction box fuseable link keeps blowing
« Reply #4 on: October 01, 2018, 01:21:04 PM »
Does it pop the fuse while cranking or only after the engine starts?  Since you substituted a fuse for the link, what is the fuse rating and style of fuse that you installed?  What is the alternator rated current output?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Mortyman

  • Newbie
  • Posts: 16
  • Newbie
Re: Junction box fuseable link keeps blowing
« Reply #5 on: October 01, 2018, 04:08:08 PM »
I believe it pops the fuse while cranking. The truck doesn't take more than a second to crank and fire up. It's quite simultaneous. The in line fuse has 12 gauge wire with a blade fuse. I was using 20 amp as well as 25 amp fuses and both amperages will blow. As far as the alternator rated current output it's at 63 amp.

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Junction box fuseable link keeps blowing
« Reply #6 on: October 01, 2018, 05:55:01 PM »
All of the charging current passes through that 20/25-amp fast-blow fuse, so immediately after startup that fuse will need to withstand ~63 amps for several seconds, tapering quickly to perhaps 25/30 amps.  That's partly why I recommended disconnecting the B+ alternator lead and regulator connector.  Did you do that?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Mortyman

  • Newbie
  • Posts: 16
  • Newbie
Re: Junction box fuseable link keeps blowing
« Reply #7 on: October 01, 2018, 06:07:49 PM »
I ran out of 20/25 amp fuses. I unplugged the 2 wire connector, removed and taped the b+ wire. It currently has a 30 amp fuse in which it started up and ran. I have started it a dozen times and the 30 amp fuse does not blow.

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Junction box fuseable link keeps blowing
« Reply #8 on: October 01, 2018, 07:01:37 PM »
Replace the 30-amp inline fuse with a 6" long, 16-gauge fusible link.  Don't substitute any other type of wire.  Refer to How to make a Fusible Link for details.  I recommend that you mechanically crimp and solder the terminal ends then use marine grade (adhesive lined, dual wall, polyolefin) heat shrink.  The link should insert between the junction block on one end and two 12-gauge red wires on the other (one to the alternator, the other to the firewall bulkhead connector).  Make up a spare link for future use while you are at it. 

With the engine off, ignition off, do you see an arc as you reattach the B+ and regulator leads to the alternator?
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Mortyman

  • Newbie
  • Posts: 16
  • Newbie
Re: Junction box fuseable link keeps blowing
« Reply #9 on: October 02, 2018, 05:10:47 PM »
I made the 6" fuseable link like you suggested. I hooked it up to the two red wires on one end and the junction box on the other end. Slapped the positive cable back on the battery and then with the engine off, key off, reattached the B+ and regulator leads with no arc/spark.

Offline bd

  • Global Moderator
  • Senior Member
  • *****
  • Posts: 6438
Re: Junction box fuseable link keeps blowing
« Reply #10 on: October 02, 2018, 06:21:54 PM »
If the symptoms recur, concentrate your inspection on the B+ wiring between the junction block, through the firewall bulkhead connector into the cab.  Good luck!
Rich
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Mortyman

  • Newbie
  • Posts: 16
  • Newbie
Re: Junction box fuseable link keeps blowing
« Reply #11 on: October 02, 2018, 09:16:39 PM »
Thanks for all of your help! I hooked the fusible link up, started the truck, and it ran without that fusible link blowing or heating up. However while looking at the wires in the cab I noticed there was some smoke developing. I discovered a 16/18 gauge gray with white stripe wire that was heating up. Following that wire in one direction, it goes up to a connector underneath the dash mounted on top of the steering column and splices in with a 10/ 12 gauge orange wire. That orange wire goes down to the back of the fuse box that ultimately gives power to the heater fuse and a blank one to the best of my knowledge. Following the grey with white stripe wire in the other direction it goes to the back of the bulkhead box comes out through that and is actually the brown wire that goes to the connector on the side of the alternator. I don't know if I'm transferring the problem to another wire or if indeed this gray with white stripe wire is the problem altogether.

Offline Mortyman

  • Newbie
  • Posts: 16
  • Newbie
Re: Junction box fuseable link keeps blowing
« Reply #12 on: October 02, 2018, 09:31:00 PM »
Here's some pictures.

Offline Mortyman

  • Newbie
  • Posts: 16
  • Newbie
Re: Junction box fuseable link keeps blowing
« Reply #13 on: October 03, 2018, 03:55:19 PM »
The gray/white stripe wire is asingle solid silver wire and is shown next to a 16 gauge copper wire. 

Offline Mortyman

  • Newbie
  • Posts: 16
  • Newbie
Re: Junction box fuseable link keeps blowing
« Reply #14 on: October 03, 2018, 04:02:06 PM »
After looking at schematics, it's actually  brown/white wire.