5 amp fuse for Instrument Lamps keeps popping

[Krazy_Kowboy]

First off, first time on the forum (although I've read a ton of other posts over the years). So if this is messy, posted incorrectly, already been discussed and solved, or is confusing, I apologize in advance.


I have a 1979 Chevy C10 Bonanza that I recently inherited and started working on. Recent issue is that the 5 amp fuse for the instrument lamps keeps popping within seconds of pulling the knob (lights never come on). I have had this truck for about 5 months now and never had an issue. About 3 weeks ago while driving home at dusk, I pulled the light switch knob out and the lights (dash) went on then abruptly out. Headlights, taillights, license lights, etc. are all fine. Courtesy lights work fine as well.

So at this point I have replaced the headlight switch, all fuses, and replaced all the bulbs in the cluster. Printed circuit looks to be in good shape w/o any cracks to the circuits. Went out and bought a multimeter and now have NO clue where to start or how....

So to start, here's a pic of the fuse panel (if that's even helpful).

In advance, thanks for your time in this matter!

[ray_mcavoy]

That 5A instrument lamps fuse feeds a gray wire that leads to a factory splice in the harness. From there it branches off to 4 places:

(1) To the instrument cluster (which you have already checked).
(2) To the radio connector (to power the radio's dial light).
(3) To the heater control light.
(4) To the windshield wiper switch light.

Start by checking those other 3 branches of the gray wire leading to the radio, heater control, and wiper switch. Look for any rubbed, chafed, pinched, or otherwise damaged areas that could be shorting to ground.

[hatzie]

Grab the 1979 GM Factory wiring diagrams PDF from my signature. While your at it grab the rest of the 79 manuals.

Your panel lamps are labeled in the wiring manual as Circuit 8 which is usually several gray wires joined at a single splice labeled Splice 8 in the wiring diagrams.
The wires in the diagrams are labeled;
8A from INST LPS fuse to Splice 8
8B to the instrument cluster plug
8C to the radio dial lamp
8E to heater control panel lamp
8F to the wiper switch lamp

Sorry you bought a headlamp switch. There's no way that could be the problem. The dimmer Rheostat in the headlamp switch is prior to that fuse... it trims the resistance to the Dark Green wire feeding the INST LPS fuse.

The first place I'd look for butchery is the gray wire in the radio pigtail. I've found more than one gray radio lamp dimmer wire cut off and dragging on the back of the dash.

[hatzie]

Ray posted while I was writing...

[Krazy_Kowboy]

Where in this mess would that splice be located? And is it safe to assume the gray wire is a ground, right?

(I'll add pics of this adventure for future members looking for solutions)

[ray_mcavoy]

The splice is hidden inside the harness and is rarely ever an issue.

No, the gray wire is a power feed, not a ground.

Like hatzie said, the radio area would be the first place to look for a cut gray wire that could be shorting out on the dash. The radio wiring / plugs do seem to be a rather commonly hacked area ... especially if the truck has (or has ever had) an aftermarket radio installed.

[hatzie]

The gray wire is dimmed hot protected by the INST LPS fuse.
That fuse is on the output side of the dash lamp dimmer rheostat.

Battery Positive ---> Fusible Link to Circuit 2 ---> Headlamp Switch Instrument Dimmer Rheostat ----> 5A INST LPS fuse ----> Wire 8A ----> Circuit 8 Splice ---> Four Gray Circuit 8 Panel Lamp Wires;

Panel Lamps are in the Radio, Wiper Switch, Instrument Cluster, and Heater Control.

[Krazy_Kowboy]

So at this point I have yet to find any damage or anything suspicious with the wires discussed so far. However, I did notice that now my parking lights are out. Headlights are all I have left.

[Pontiac1976]

Maybe I wrong but did the 79 not have a glass fuses. from Factory maybe the something loose down there shorting.

MY 77 was, and I up graded to 80 - 87 fuse box. from a 84 set up.

[hatzie]

Originally Posted by Krazy_Kowboy

So at this point I have yet to find any damage or anything suspicious with the wires discussed so far. However, I did notice that now my parking lights are out. Headlights are all I have left.

T/L CTSY fuse in the middle of the panel blown or OK?

Originally Posted by Pontiac1976

Maybe I wrong but did the 79 not have a glass fuses. from Factory maybe the something loose down there shorting.

MY 77 was, and I up graded to 80 - 87 fuse box. from a 84 set up.

79 were the first ATC fuse panels 78 was the last AGC... Look at his picture. ATC panel.

[Pontiac1976]

Thanks I thought that 80 was the first year for the switch on the box.

[Krazy_Kowboy]

Hatzie, I didn't even think to look at that one again. It was blown.

Found a hidden (and loose) ground wire in the tow harness.... It was behind the bumper, so out of sight. Replaced the fuses, and I have lights again. I just hope they stay on this time! (And if not, I know where to come for help!)

Many thanks to everyone in this matter! This electrical stuff is frustrating so kudos to you who do this regularly.

[hatzie]

Originally Posted by Krazy_Kowboy

Hatzie, I didn't even think to look at that one again. It was blown.

Found a hidden (and loose) ground wire in the tow harness.... It was behind the bumper, so out of sight. Replaced the fuses, and I have lights again. I just hope they stay on this time! (And if not, I know where to come for help!)

Many thanks to everyone in this matter! This electrical stuff is frustrating so kudos to you who do this regularly.

Glad you found it...

I'm going to rant a little...

Some of the butcher jobs I've seen for trailer hookups are usually pretty stupendous. They are in the top three.
If you want your harness chopped to shreds get trailer brakes, trailer lights wired at U-Haul or Pennske and a Radio, and alarm system installed by your local 16 year old at Best Buy or...
Really no reason for it. Trying to do it fast instead of right so they can go out and have another smoke.

Brake and Battery power should be un-interrupted 12ga wires from the controller and a self resetting breaker on the firewall next to the J-Stud to the trailer plug with 12" service loops at both ends. Trailer ground should be a 12ga wire right to a bolt through the bumper or frame. Hopkins makes a real nice tee that plugs into the sealed tail lamp disconnect to connect your trailer plug turns/brakes, parking, & backup lamp power without butchering the vehicle wires with Scotchloks.

Metra makes inline jacks that plug right into cab harness radio pigtails just like the original radio. Solder the wires from the Metra pigtail to the aftermarket radio pigtail with heatshrink covers and plug her in. When you get rid of the vehicle the old radio plugs in and slides right back into the dash... And... you don't have dangling wires to short out.

Alarms can be wired without butchery but it takes time to fab proper taps and bundle the wires into harnesses with wire ties. Removal can be done without comprimising the vehicle too but most folks like to cut off the alarm wires and stuff them under the carpet.