'79 HVAC fan melting fusible link?? HELP!

[pismorat]

So i have been chasing an electrical issue for over a week now and am running out of patience and knowledge. The issue started with the fusible link at the junction block melting. I replaced it, then went hunting in the engine compartment for a short. I ended up replacing most the starter/alternator circuit wiring due to age and inconsistent readings. I also had the alternator tested to see if it was the voltage regulator, but it checked good. The test light illuminates when the ignition switch is in the 'on' or 'start' position, no where else, so I started ripping the gauges out and tracing wires from the fuse block, which is now disconnected. I think I've narrowed it down to the 'heater' 20amp fuse, which is a brown wire that goes up to the A/C/vent/heater control. If i unplug the fuse, the wire that comes from the bulkhead plug stops going to ground. I unplugged the resistor in the evaporator box (contains brown wire), and the test light only lights up on the plug behind the controls when its in the A/C range, heater, or defroster. If the resistor is plugged in, it lights up on all settings. Without just throwing parts at this thing, does anyone have any advice on where to look from here? I've not run the A/C or heater in years, but turned the vent fan on which in hindsight, is when this whole problem started. I've unwrapped almost all the harnesses, hoping to find a dead short, but nothing yet. HELP PLEASE!!

 

[Wes Harden]

Your fan may be the culprit. A test light can find ground through a complete circuit. So I'm am uncertain, if all your test are valid. If you were to disconnect the fan wire at the fan and redo the test, If you still have a path to ground then there is a wiring issue.
Blower motors have been know to draw to many amps, should have blown the fuse before fusible links. Might have been a combination of the old wiring and dirty connections.
Does the fuse link still blow ?

 

[pismorat]

I haven't hooked the positive cables up yet, just cause I don't want to blow a third link. And tracing the wires back, the fan system seems to be the issue. My neighbor's truck I've been checking doesn't 'light up' like mine is, so something is still wrong and I gotta keep hunting. I'll try out the fan next. Thanks!

 

[wazzabie]

I need to replace my fusible link. What guage wire did you use and what length?

 

[Wes Harden]

@6" of 14ga

 

[pismorat]

Yep, 14 ga. around 6". I thought about putting a fuse in, but I've back traced the wire connecting to ground to that brown wire attached to the fan resistor, which likely goes to the fan, which might be the issue. Been playing with the kid all morning, so I hope to get back after I this afternoon.

 

[pismorat]

OK, so I think the fan problem is solved (replaced fan and resistors), but now it looks like the gremlin has moved. The hot wire coming through the firewall to the junction block is still connected to ground with the key on, but now its the 20amp gauge fuse that is the culprit. I've removed the cluster, and its still grounding out. What else is on that circuit?

 

[pismorat]

In tracing it more today, the cluster is fine. It is things plugged into the IGNITION ports above the GAUGE fuse that are causing it to ground out. The major contributor looks like the tank selector switch. When on R.H. it is fine, when switched to L.H. it grounds out that circuit. Any thoughts if that is normal operation for that?

 

[pismorat]

In tracing it more today, the cluster is fine. It is things plugged into the IGNITION ports above the GAUGE fuse that are causing it to ground out. The major contributor looks like the tank selector switch. When on R.H. it is fine, when switched to L.H. it grounds out that circuit. Any thoughts if that is normal operation for that?

L.H. is supposed to kill power to the single wire solenoid on the tank selector. When cutting power, should it ground out - is the question. Can someone verify this on their truck?

 

[wazzabie]

I don't have a tank selector on the K5. Were you able to find a wiring diagram?

 

[pismorat]

Figured out the gauge cluster problem - faulty dual batt. solenoid that was plugged into the ignition circuit. Problem is I doubled checked the HEATER fuse and it was blown! Again, the fan and both resistors are brand new. I hooked a jumper wire from the battery to the fan and it turned on fine. I replaced the fuse and turned the key on (selector on VENT). No matter what position the fan switch is in (1,2,3, or 4), the fan doesn't turn on. When I go to high (4) it blows the fuse. I disconnected the A/C compressor plug, the carb. high idle solenoid wire, and tried again. Fuse blown again, but maybe not as quickly? I'm about ready to order a whole new temperature control assy., but that might be a wasted $90 from LMC if it keeps doing the same thing. Maybe I just give up on climate controls so I can drive it again, but I feel like I'm so close!

 

[Wes Harden]

did you check the high blower relay? is near the resistor block

 

[pismorat]

Both relays in the engine compartment have been replaced, as well as a new fan. I just chased the 10 ga. hot wire and its fine, and even with that disconnected, it blew the fuse when switched to high. Leaning more towards the control assy...