Parking Light Circuit - Heavy Current Draw

gmcnet1

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Apr 23, 2019
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Looks like it's electrical gremlin day on GMCforum! Here's another one: While on an excursion this weekend I turned on my parking lights and noticed a
significant voltage drop (.5 to 1 volt below the normal 13.5). I haven't driven the coach much at night since acquiring it four months ago, so I don't
know if this is a new problem. However, I did notice something new the other day -- my left front clearance light isn't working. The final symptom is
that the headlight switch rod (that the knob attaches to) is getting very hot -- consistent with high current draw. I won't be using the lights any
more until this problem is addressed!

My first suspicion is a partial short in the clearance light due to corrosion that draws a lot of current but not enough to blow a fuse. I consulted
the service manual and it looks like it would be straightforward to unplug the connector to the front clearance lights to see if this isolates the
problem. Another potential trouble spot is that I recently pulled down the inside rear cap to install a backup camera, and as luck would have it I
chose to wire it into the rear clearance light circuit. As suspicious as that sounds, I don't believe it's the root cause. The camera and rear
clearance lights are working fine and the camera doesn't draw much current. Nevertheless, the wiring back there did get disturbed.

I would welcome any advice about common problems that these symptoms point to, diagnostic approach, etc. Thanks!
--
Christo Darsch
1977 Eleganza II - "Komet"
3.50 Power Drive, Disc Brakes, Alcoas
Weymouth, MA
 
I think your approach is right on. Every parking light problem I have ever chased on a GMC ended up being a corroded / rusted clearance light(s).
Obviously it still could be a tail light, or side marker, or parking light.

As a side note every GMC clearance light I have worked on had the + and minus reversed. So the outside frame was at +12 volts instead of ground.
That will not cause your immediate problem, but should be noted if you choose to change the clearance lights to an LED type.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
Or license plate light.

> I think your approach is right on. Every parking light problem I have ever chased on a GMC ended up being a corroded / rusted clearance light(s).
> Obviously it still could be a tail light, or side marker, or parking light.
>
> As a side note every GMC clearance light I have worked on had the + and minus reversed. So the outside frame was at +12 volts instead of ground.
> That will not cause your immediate problem, but should be noted if you choose to change the clearance lights to an LED type.

--
C. Boyd
76 Crestmont
East Tennessee
 
> Or license plate light.

Thanks Chuck. That is one I forgot about.
--
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
 
I'd look at bulbs first. Sometimes they burn out and the filament rewelds across the terminals but at a shorter spread creating a much lower Ohm
resistor.
--
John Lebetski
Woodstock, IL
77 Eleganza II
 
Great tip, John! By the way, if my apparently burned out bulb/fixture isn't the source of the problem, I'd still like to unplug the main connector to
the front clearance lights to narrow down the problem area. Where specifically do I find that connector?
--
Christo Darsch
1977 Eleganza II - "Komet"
3.50 Power Drive, Disc Brakes, Alcoas
Weymouth, MA
 
> Great tip, John! By the way, if my apparently burned out bulb/fixture isn't the source of the problem, I'd still like to unplug the main connector
> to the front clearance lights to narrow down the problem area. Where specifically do I find that connector?

According to the wiring diagram it is a 8 pin connector "Under the instrument Panel Upper LH side"

--
Tom Lins
St Augustine, FL
77 GM Rear Twin, Dry Bath, 455, FI-Tech EFI, Aluminum Radiator Quad-Bag Suspension
Manuals on DVD
GMC Dealer Training Tapes
http://www.bdub.net/tomlins/
 
Well, I couldn't find the 8 pin connector to the front clearance lights by looking under the dash, and I didn't want to pull the instrument panel, so
I went ahead and opened up the recently inoperative left front clearance light. It's an LED unit from 2010 and there was about 2.5 inches of corroded
ground wire wrapped around one of the mounting screws. The high current draw mystery is solved; welcome to Jankyville. Replacement lights are on the
way.
--
Christo Darsch
1977 Eleganza II - "Komet"
3.50 Power Drive, Disc Brakes, Alcoas
Weymouth, MA