electrical short

[wibilly]

irecently purchesed a 91 gmc slx. it has a short and blows the gauge fuse instantly upon turning the key on. it does it with the cluster remove and the brown wire off the alt as well.i'm a 67-72 guy so i'm pretty green around these trucks, anybody have any ideas as to where to start looking. thanks for all your help.

[speedygonzales]

Going to be a little more difficult without having a schematic. But here's something that can help without one. At the fuse holder with that particular fuse out, measure the contacts for 12 volts. One side will have 12 volts when the key is on, the other side will not. It is the side that does NOT have 12 volts with the key on that you want to focus on.

Turn the key off and set your meter to ohms. Be sure that anything you know of on that circuit is disconnected. You had mentioned the gauges, that should at least be disconnected. Now put one of the meter leads into the non-12 volt fuse contact and the other meter lead to a GOOD ground.

If everything on that circuit is disconnected, the meter should be reading open, infinite or OL. Yours will most like be reading very low (close to 0) ohms. Now you want to be wiggling wires until you see the meter change to most likely a high reading.

Focus first where wires go through or near metal. Keep moving wires and hopefully you will find the point that touching ground.

Remember, if there are electronic devices still connected to that circuit, it may make your reading on the ohm meter low. So it's important the circuit be isolated with no connections when you try this.

[ChevyTech]

If it has cruise control, try unplugging the cruise switch and the cruise module.

If it has a hot fuel handling module, you could try unplugging it.

Is it a diesel or gasoline engine?

[wibilly]

it is gas. i was wondering about the cruise and speedometer too. not sure how all that is tied into gauge circute but all must be. the harness on these are so concieled that it's hard to trace anything. another reason i,m stuck on my old trucks

[ChevyTech]

The diesel engine trucks have much more connected to the gauge fuse.

If the cruise control switch wires get pinched in the tilt column (if it has tilt) it can blow the gauge fuse. This is not a common issue on non-tilt column trucks.
That is where I would look first.

[wibilly]

i did disconect the cruis under the hood, same deal. after breakfast i'll disconect all but ignition switch from colume and try that.thanks so mucc for your help. how does the speedometer tie into this? this thing was in duluth when they had the flood this summer not sure if it was in the water or not

[wibilly]

no luck on the cruise switch either. i didat onepoint notice that the dash light dimmer also controaled volume on the warning buzzer

[ChevyTech]

Unplugging the cruise unit under the hood does not get the cruise switch out of the picture.

A power circuit through the fuse, then to the cruise switch, then to the brake switches/clutch switch, then to the cruise module under the hood.

You can unplug the cruise switch on the left side of the column in the area of the bracket that holds the column to the dash.

In your first post it looked like you were saying it still blows the fuse with the cluster removed, so the speedometer must not have been the cause.

[ChevyTech]

Here is a not very good photo of the cruise switch connector.

[wibilly]

at this point the cruise switch is disconected,i have also taken the bulk head conector apart and the litle thing by the computer what's it called fuel manigment somthing and still a dead short

[ChevyTech]

Does it have a defogger on the rear glass?

Unplug the key / headlight warning buzzer.

Unplug anything plugged in to the convenience center by the park brake pedal that is in the row labeled WHT

If this does not do it then it may have the wire harness rubbed through its insulation somewhere.

[wibilly]

thank you chevy tec i'll do that. what is and where can i find the hot fuel modual?

[ChevyTech]

The fuel module will override the PCM two second timer and the fuel pump will run for 20 seconds and then “off” if the vehicle is not started. This circuit corrects a hot restart vapor lock during high ambient temperatures.

All “G” vans with a 5.7L engine had the hot fuel handling module also just called a fuel module sometimes.
Other vehicle also have the module including ¾ ton pickup and heavy duty emissions pickup and suburbans, but it was also used ½ pickups that do not fit into the heavy duty category but I have not been able to figure out what is the exact criteria of when it gets included. It can easily be added to any TBI truck.

The hot fuel handling module is next to the ECM, usually taped to the harness. It is small like a pack of cigarettes, with three wires to it.
1993 Wire colors Black with white tracer, pink with black tracer and gray
1990 wire colors = there was a Tan with white tracer instead of a gray wire
I don't remember where the change happen between these years.