Author Topic: HELP!!! FIREWALL fusible link repeat offender  (Read 3117 times)

Offline JamesMcHale

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HELP!!! FIREWALL fusible link repeat offender
« on: February 06, 2020, 05:18:13 PM »
In brief:

78 Chevy C20. Had for a smog check, had to get a new cat put on, retested for smog, passed. Driving home engine died. No lights, no ignition, no nada. Towed home :(

Me not a genius. Bought a test light.

Found burned wire at firewall. Chopped it out and put in a fuse connector from Autozone. Started up, had lights, and etc....

Scratched my head.

Went online to Dr. Google and discovered the fusible link. Not too sure what I cut out if it was 1 sq mm or 2 sq mm. Ordered a 1 sq mm. Put it in. It lasted a week and burned out. I put the fuse connector from Autozone back in and drove home. Ordered the 2 sq mm fusible link. While waiting the 30 amp fuse failed. Put in a new 30 amp fuse and it immediately blew out. 2 sq mm fusible link shows. I put it in. It catches fire immediately when battery terminal reattached.

So, I see that wire goes behind the block and disappears into the wiring loom. I guess those wire head for the starter area and maybe it splits off toward the alternator area.

My question: should I start disconnecting the wiring so I can pull the loom up and check for a direct short? Is there a known pathology that includes the stater, solenoid, alternator, or something else that I could do prior to exploring the wiring?

Guidance wanted!

Please HELP!!!

I’ve been walking a lot.

Thanks,

James

Offline bd

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Re: HELP!!! FIREWALL fusible link repeat offender
« Reply #1 on: February 06, 2020, 08:40:45 PM »
First off, NEVER substitute an inline quick-blow fuse for an ultra-slow-blow fusible link.

On your 1978 C20 the fusible link at the firewall junction block was 16 gauge (1.0 (mm)2) x 6" long as delivered from the factory.  The three wires protected by the fusible link are each 12 gauge red.  Two of the three 12-gauge wires run to the alternator B+ post and the regulator #2 terminal.  The third 12-gauge wire runs to a splice inside the cab through the firewall bulkhead connector, feeding various circuits.  One of the three 12-gauge primary feeds connecting to the fusible link is shorted to ground.

To diagnose the cause of the short, closely inspect the wires and loom running between the alternator and the firewall junction block and between the firewall junction block and the firewall bulkhead connector for chafing/pinching/etc.  If you don't discover anything obvious, disconnect and tape the red wire attached to the alternator B+ post and unplug the two-wire connector from the regulator.  Procure a 15- to 20-amp auto-resetting circuit breaker (image) and fit it with two ~18" long 14-gauge leads with alligator clips.  Install the circuit breaker between the firewall junction block and the three 12-gauge red wires, substituting the circuit breaker for the melted fusible link. 
  • If the circuit breaker doesn't trip, try reconnecting one-at-a-time the regulator plug and then the B+ lead to the alternator.  If the circuit breaker trips as you reconnect the alternator, the alternator is shorted internally.  Replace the alternator.

  • On the other hand, if the circuit breaker begins tripping the instant it is connected between the junction block and the three-wire harness while the alternator is still disconnected, unplug the engine harness from the firewall bulkhead connector and retry.  If the circuit breaker continues to trip with the bulkhead connector and the alternator unplugged, one of the 12-gauge red wires is shorted within a few feet from the junction block.  Reinspect it carefully.  If the circuit breaker stops tripping when you unplug the bulkhead connector, the short is probably inside the cab - more than likely a pinched or chaffed wire.

The process is simple elimination by trial.  The trick is to protect the wiring using the circuit breaker so that nothing catches fire. 
« Last Edit: February 06, 2020, 08:43:24 PM by bd »
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 JamesMcHale

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Re: HELP!!! FIREWALL fusible link repeat offender
« Reply #2 on: February 07, 2020, 07:36:51 AM »
Okay, thank you. That sounds doable.

Offline 75gmck25

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Re: HELP!!! FIREWALL fusible link repeat offender
« Reply #3 on: February 11, 2020, 06:27:12 AM »
This type of GM replacement fusible link used to be available in the Dorman/Help! section of most parts stores, but I haven't checked lately. .https://www.repairconnector.com/products/-GM-Starter-Solenoid-Fusible-Link-14-Gauge-.html  You can just splice it in to replace the burned fusible link.

Bruce

Offline bd

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Re: HELP!!! FIREWALL fusible link repeat offender
« Reply #4 on: February 11, 2020, 08:03:00 AM »
The gauge of a fusible link is specific to the gauge of the wiring it protects; see Table 1 in Fuses and Circuit Protection.  Be mindful that you comply with the guidelines to maintain proper wiring protection.  Refer to the 1978 Factory Wiring Manual for the OEM wire sizes used.
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 75gmck25

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Re: HELP!!! FIREWALL fusible link repeat offender
« Reply #5 on: February 19, 2020, 05:38:03 AM »
If you look at the wiring diagram, the main power wire from the starter lug to the junction block on the firewall is a 10 gauge red wire with a 14 gauge fusible link.  AFAIK this is a common gauge for this vintage of GM vehicle with a 10si alternator, and I assume that's the reason it's often packaged as a "GM fusible link."

On my '75 GMC the fusible link was in the end of the wire that connects to the junction block next to the brake booster, but the wiring diagram seems to show it in the end that connects to the starter (which make more sense to protect the wire).  My fusible link started to smoke when I accidently hooked the battery terminals up backwards, but I pulled power quickly.  However, the link burned up completely one day when I was on the highway at about 3k RPM and then flipped on the headlights. The alternator was charging and the extra current draw for the lights was apparently too much for the old fusible link.

Bruce