73-87chevytrucks.com

73-87 Chevy _ GMC Trucks => Electrical => Topic started by: Mortyman on September 24, 2018, 06:32:15 PM

Title: Junction box fuseable link keeps blowing
Post by: Mortyman 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.
Title: Re: Junction box fuseable link keeps blowing
Post by: bd 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.
Title: Re: Junction box fuseable link keeps blowing
Post by: Rapid Roy on September 24, 2018, 08:41:40 PM
Welcome
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman 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?
Title: Re: Junction box fuseable link keeps blowing
Post by: bd 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?
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman 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.
Title: Re: Junction box fuseable link keeps blowing
Post by: bd 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?
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman 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.
Title: Re: Junction box fuseable link keeps blowing
Post by: bd 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 (http://forum.73-87chevytrucks.com/smforum/index.php?topic=11972.0) 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?
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman 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.
Title: Re: Junction box fuseable link keeps blowing
Post by: bd 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!
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman 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.
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman on October 02, 2018, 09:31:00 PM
Here's some pictures.
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman 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. 
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman on October 03, 2018, 04:02:06 PM
After looking at schematics, it's actually  brown/white wire.
Title: Re: Junction box fuseable link keeps blowing
Post by: bd on October 04, 2018, 05:12:47 PM
The brn/wht wire to which you refer is a nickel alloy resistance wire designed to prevent the alternator from attempting to charge the battery through the brown regulator "exciter" lead.  The resistance wire is used on truck models with gauges, only.  Trucks with idiot lights substitute a charge lamp for the resistance wire.  Its purpose is to provide a low current, 12-volt ignition signal to the regulator #1 terminal to turn the regulator on, allowing the alternator to generate charging current.  The current flow through that resistance wire should amount to milliamperes, only.

Unplug the two-wire regulator connector from the side of the alternator and run the engine.  Does the resistance wire continue to overheat?
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman on October 05, 2018, 10:54:50 AM
While I'm on the hunt for some 24/26 gauge connectors I'll ask this question. With this brown/white resistor wire, I would assume it needs to be a certain length to work properly? I cut out about 12" just to be sure I removed the damaged part when it started melting.
Title: Re: Junction box fuseable link keeps blowing
Post by: bd on October 05, 2018, 11:24:15 AM
Use an 18-gauge connector and fold the wire over to double its thickness.  Measure the resistance and overall length of the resistance wire that remains intact in the vehicle.  Post the results.


Don't neglect the following step...

Unplug the two-wire regulator connector from the side of the alternator and run the engine.  Does the resistance wire continue to overheat?
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman on October 05, 2018, 04:51:38 PM
I might have misunderstood your directions and jumped ahead. I spliced in all but the 3 inches of wire that melted. The overall length from steering column connector to the backside of the bulkhead is about 50 inches.  The part where I might have misunderstood was to measure the resistance before I spliced it all back together? I went ahead and unplugged the two-wire regulator connector, started the truck, and it ran without any wires heating up in the cab or in the engine compartment.
Title: Re: Junction box fuseable link keeps blowing
Post by: bd on October 05, 2018, 04:58:21 PM
Measure the resistance and length of the single strand resistance wire, only.  Exclude any multi-strand wire.
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman on October 06, 2018, 12:36:37 PM
Removing the wire completely from the harness and any shared connectors, it measures 54 inches long and has a resistance measurement of 0.01 @ the 20k ohm setting or 9.6 @ the 200 ohm setting.
Upon digging into the rear of the bulkhead, this brown/white wire does not share a spot with the 20 gauge brown wire going to the instrument cluster as previously indicated and as shown in a schematic. This truck has gauges and doesn't have that brown wire. The brown/white wire solely goes through the firewall/bulkhead to connect to the 16 gauge brown wire to the side connector of the alternator. And reversely, the brown/white wire goes to the connector on the steering column where it is crimped together in the plug with the 12 gauge orange wire.
Title: Re: Junction box fuseable link keeps blowing
Post by: bd on October 07, 2018, 02:26:54 PM
The resistance wire is within tolerance, manifesting only a slight deviation from the 10Ω, 6-watt factory specification.  Reconnect all of the wiring.  Connect a DC voltmeter between the battery negative post and the #1 regulator terminal (brown wire), positive voltmeter lead to the regulator terminal.  Start the engine and record the voltage measurement.  Check the resistance wire for overheating.  Post the results.
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman on October 08, 2018, 05:26:31 PM
Resistance wire did not overheat. No fuseable links have blown. When I probe the back side of the brown wire connector at the alternator I read 15.09 volts.
Title: Re: Junction box fuseable link keeps blowing
Post by: bd on October 08, 2018, 06:15:43 PM
What are the voltage measurements at the #2 regulator terminal and alternator B+ post relative to the battery negative post?  Then measure the voltage drop between the alternator B+ post and battery positive post, engine running, and the voltage drop between the alternator case and the battery negative post, engine running.  Post the results.
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman on October 08, 2018, 06:57:40 PM
Voltage measurements at the #2 regulator terminal relative to the negative battery post is 14.7 volts.
Voltage measurements at the alternator B+ post relative to the battery negative post is 14.9 volts.
The voltage drop between the alternator B+ post and battery positive post was .2 volts.
The voltage drop between the alternator case and the battery negative post was zero (both measured 14.7).
Title: Re: Junction box fuseable link keeps blowing
Post by: bd on October 08, 2018, 07:37:18 PM
The full significance of the voltage measurements isn't really known because the current output of the alternator during those measurements isn't known.  Nonetheless, although there are minor inconsistencies in the measurements, a hard failure is not outwardly evident based on those measurements.  If a charging system issue exists, I suspect it is intermittent.  Place the vehicle back into service and be mindful of any symptoms that may indicate an unresolved problem.

Did you happen to notice where the #2 regulator wire ties into the B+ charge lead?
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman on October 09, 2018, 04:24:39 PM
This is good news. The #2 regulator wire goes to a spliced 6" fuseable link at the starter. The shared wire from the starter fuseable link goes back to the bulkhead. The b+ charger lead goes to the spliced fuseable link at the junction block. So I'm not sure where they would tie into each other. :-\
Title: Re: Junction box fuseable link keeps blowing
Post by: bd on October 09, 2018, 05:05:57 PM
The #2 regulator wire is the "voltage sensing" lead used for feedback to the regulator to control alternator output.  In other words, alternator output depends on the voltage that the regulator senses on the #2 wire.  The 12-gauge red wires that pass through the bulkhead connector and the alternator B+ lead connected to the firewall junction block all converge at the starter solenoid 3/8" battery cable stud.  If excessive resistance, such as a loose or burned connection, exists anywhere between the #2 regulator terminal and the alternator B+ post, the regulator will boost alternator output voltage to compensate.  Typically, the voltage that the regulator wants to see on the #2 terminal is in the range of 14.0 - 15.0 volts, with 14.2 - 14.7 volts being the sweet spot.

With this in mind, inspect all wire connections to the 3/8" battery cable stud at the starter solenoid to make sure no wires are loose or appear burned from formerly being loose.  If all appears well then you are probably finished with this particular endeavor.
Title: Re: Junction box fuseable link keeps blowing
Post by: Mortyman on October 09, 2018, 06:06:06 PM
This all makes sense to me now. Your explanations are very thorough and easy for a novice like me to understand. You're a great contributor and I thank you very much!
Title: Re: Junction box fuseable link keeps blowing
Post by: bd on October 09, 2018, 06:49:33 PM
Thank you for the kind words.  You are quite welcome!