73-87 Chevy _ GMC Trucks > Electrical

Diagnosing a few things...No power, a burnt wire, fusible link removed


I am a newbie on this site and by no means a mechanic.  I'm a professional in the medical realm, but love my 1985 C10.  Unfortunately, its been at the paint shop for coming on 4 years.  I have a good mechanic, who I actually bought the truck from.  The truck has a 350 with Holley carb and has needed no mechanical work during its stint at the paint shop. It ran fine, just not enough.  Its a long story, but the paint is complete and I'm in the process of working on it a couple of times a week after I get off work.
The issue that I am having trouble with is that painters have been moving my truck in and out of the shop routinely to work around it.  About 4 months ago there was no power at the ignition.  Thought it was the ignition switch, but discovered a burnt wire on the protected side of the fusible link that leaves the flat busbar on the firewall.  The fusible link was replaced with a 15 amp fuse ( which after reading several posts was a mistake) and both the alternator and distributor were replaced new.  Afterwards, the truck would start, the fuse would blow after of few seconds, and would die and would not start unless the fuse was replaced. I wondered if there was short that blowing the fuse, so I spent several afternoons unwrapping the wiring in the engine bay and inspecting for obvious shorts or burnt wires and found none. Most recently, we took down the bulkhead and fusebox in the cab to inspect for any problems and found none. We (or I should say my mechanic) took down the steering column and replaced the ignition in case there was a short in it. We were able to get the truck running and stay running.  A new fuse is required to start, blows quickly and the truck runs like crap, but it does not die.  The last thing to happen was while we were running the truck after the fuse blew, after a minute, we noticed smoke from inside the dash. I believe it was the cruise control motherboard and a burnt component?....I guess its fried, but wasn't planning to use, didn't know that was even on the truck.

All this to say, we still don't know why one of the wires , the one with the fusible link burnt and not the link.  I thought that the link was protecting the downstream wires in case of overload. And secondly, if the fusible link is replaced and/or the fuse that we put in its place is blown and the circuit open, what may be causing high current in the dash to overload computer chips. Fi

I know this is a lot to sort through, but I have turned to this group for suggestions and a method for me to systematically figure out where my problem is. I have the wiring diagram for the 85 and I'm sure I will be getting to know my B+ test light very well. And replacing out fuse repair with an appropriate fusible link.  Thanks for any help.

Start with the wire that was fried, replace it and the fusbile link. Not sure what motherboard you have under your dash but try to find out what that is.

Thanks for your reply.  Finally got to replace the fusible link at the busbar.  The truck started and we were able to adjust the timing and it ran well.  As I was still wary of melting the link, I closely monitored how hot the link and wire got.  According to my infrared thermometer, it reached near 120 degrees F.  Still trying to figure out what may have burnt the link previously, we check the operating temps of several wires.  The wire from the alternator that is connected to the fusible link was hot as well.  Unsure of exactly, how hot is too hot, we checked the connection at the stud on the alternator and when we jiggled it a bit, the idle picked up and down somewhat.  After the jiggle, we thought maybe the ring connector could use changing.  For good measure we also replaced the alternator pigtail switch connector.  We never could repeat any idle variations with attempts to move the wire.  The truck ran and idled well.  Out of curiosity, I measured how many amps we in that wire both at the alternator and its connection with the fusible link, and both were between 40-45 amps. Is this the normal? Aren't alternators rated near 80-100 amps?  Any discussion on the power flow through that harness would be appreciated.  Thanks for your help.

Hi Remi, the output from the alternator will vary according to the load on the electrical system and the condition of the battery, if you measure the output from the alternator just after starting it will be high then will slowly drop back as your battery is recharged to replace the power used to start your engine.



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