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  #1  
Old 11-07-2016, 02:57 PM
twmattox twmattox is offline
350 Buick
 
Join Date: Feb 24, 2003
Location: Indiana
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Keeps blowing a fuse

1988 Jeep Grand Wagonneer. Everything was great until last night...nothing changed or different. Turned on the headlights and POP...blew the 20 amp Park Taillight fuse. Figured maybe it was just old...put a new one in, turned on the headlights and POP.

Electrical is my weakest discipline. Where do I begin? What do I do??
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'88 Grand Wagoneer (SJ) / 360 / TF727 / NP229 / DANA 44 (2.73)
'05 Wrangler Unlimited (LJ) / 4.0L / NSG 370 / NV231 / DANA 30-44 (3.73)
'15 Wrangler Unlimited (JKU) / 3.6L / 42 RLE / NV 241 / DANA 30-44 (3.73)
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  #2  
Old 11-07-2016, 03:56 PM
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serehill serehill is offline
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Well

I'd pull out all the bulbs & then put in another fuse. if it survives it's a bulb problem if it dies it's a wiring problem.
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  #3  
Old 11-07-2016, 04:06 PM
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tgreese tgreese is offline
 
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Location: Medford MA USA
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Before I spent a bunch more fuses on trying random things, I would pull the bulbs and check continuity between ground and the pins (not the ground or shell) of the bulb sockets. For the tail lights, the shell (outer part) of the bulb is at ground potential, but the inner contacts should be floating (connected to nothing but air) with the switch off. Two of the three headlight plug contacts should be floating.

If you find that the bulb contacts are connected to ground, or even low resistance, you have a short somewhere. Start unplugging the harnesses step by step and try to localize the short. First, pull the plugs to the headlights and tail lights and see if the short is still there. If not, the dangling part of the plug going back to the car should still have a short. Disconnect the plug to the headlight switch and high beam switch and see if you still have a short in the lights harness or in one of the switches. Keep dividing up the harness until you localize the short in something that does not come apart.
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Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk
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  #4  
Old 11-08-2016, 09:17 AM
twmattox twmattox is offline
350 Buick
 
Join Date: Feb 24, 2003
Location: Indiana
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Thanks guys...electrical is really my area of greatest challenge on these old rigs. I have a small box of 20 amp fuses so I figured I would play for a few minutes last night (only used 2)...

I noticed that I had about 2 seconds after turning the switch on before the fuse blew. So, I had someone stand at each corner of the rig with me in the driver seat. I turned the switch on and had them tell me which lights turned on...

All front side marker and front lights all lit up. All dash lights light up. Both rear side marker lights light up. However, the tail lights AND license plate light did NOT light up.

Now, I know this did not test the switch, the chime module, nor a few of the other lights (HVAC control light, cigarette lighter light, radio light etc). But, this seems to tell me that the issue is in the rear. Does this sound right? About a year ago I removed the rear housings, cleaned them, replaced the bulbs (using dielectric grease), brand new license plate light and housing, and re-wired the trailer lights...I would have thought there would not be issues there...

I figure I will take the rear harness apart tonight and see if the wiring tones to ground. If not, I will start working on bulbs. Am I missing something???
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'83 Scrambler (CJ-8) / 258 / T-5 / D-300 / DANA 30-AMC20 (3.31)
'88 Grand Wagoneer (SJ) / 360 / TF727 / NP229 / DANA 44 (2.73)
'05 Wrangler Unlimited (LJ) / 4.0L / NSG 370 / NV231 / DANA 30-44 (3.73)
'15 Wrangler Unlimited (JKU) / 3.6L / 42 RLE / NV 241 / DANA 30-44 (3.73)
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  #5  
Old 11-08-2016, 10:49 AM
tgreese's Avatar
tgreese tgreese is offline
 
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Location: Medford MA USA
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Not looking at the '88 diagram, but typically the whole rear harness unplugs from the under-hood harness a few feet from the firewall bulkhead. Unplug the rear harness and see if that isolates the problem. Also, does it blow with just the park/running lights on, or do you need the head lights on too?
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Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk
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  #6  
Old 11-08-2016, 03:16 PM
twmattox twmattox is offline
350 Buick
 
Join Date: Feb 24, 2003
Location: Indiana
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I know there is a harness plug near the bumper. That was where I was going to unplug it. I will look for one closer to the firewall.

Will pop a fuse if just the parking lights are on or if both parking lights and headlights are on.
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'83 Scrambler (CJ-8) / 258 / T-5 / D-300 / DANA 30-AMC20 (3.31)
'88 Grand Wagoneer (SJ) / 360 / TF727 / NP229 / DANA 44 (2.73)
'05 Wrangler Unlimited (LJ) / 4.0L / NSG 370 / NV231 / DANA 30-44 (3.73)
'15 Wrangler Unlimited (JKU) / 3.6L / 42 RLE / NV 241 / DANA 30-44 (3.73)
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  #7  
Old 11-08-2016, 04:02 PM
tgreese's Avatar
tgreese tgreese is offline
 
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Location: Medford MA USA
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Quote:
Originally Posted by twmattox
I know there is a harness plug near the bumper. That was where I was going to unplug it. I will look for one closer to the firewall.

Will pop a fuse if just the parking lights are on or if both parking lights and headlights are on.

Ok, I would not mess with the headlights yet. Check for that connector to the rear harness, unplug it, and see what happens.
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Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination ATs, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
ECO Green: '15 FCA Jeep Cherokee KL Trailhawk
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  #8  
Old 01-03-2017, 03:20 PM
twmattox twmattox is offline
350 Buick
 
Join Date: Feb 24, 2003
Location: Indiana
Posts: 1,282
Finally got time free to spend on this issue.

Disconnected the rear wiring in the engine bay... Still an open short somewhere.

Removed all bulbs in the front (side markers, headlamps, front markers), and disconnected the marker lights. Still an open short.

Removed the interior stuff (radio, HVAC controls, speedo, etc). No melted wires or anything I could notice grounding out. Removed the headlamp switch and dimmer switch. I would like to test these but can't find any instructions in my Factory Service Manual...

Also, with the switches disconnected, front bulbs disconnected, rear harness disconnected, (however, with the dash bulbs still installed) I tried something. I put a multimeter (set to tone) connection on the negative terminal of the battery and the other end on the supply side of one of the front marker lamps. I get tone... Is this because there is a short in the front end or is it because the dash lights are still installed???

I plan on disconnecting the bulkhead connector and trying it again. I am a tad frustrated because all my wiring (with some small exceptions) looks pristine. All the sheathing is still in place, all well mounted, I can't see anything that looks chaffed or anything.
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'83 Scrambler (CJ-8) / 258 / T-5 / D-300 / DANA 30-AMC20 (3.31)
'88 Grand Wagoneer (SJ) / 360 / TF727 / NP229 / DANA 44 (2.73)
'05 Wrangler Unlimited (LJ) / 4.0L / NSG 370 / NV231 / DANA 30-44 (3.73)
'15 Wrangler Unlimited (JKU) / 3.6L / 42 RLE / NV 241 / DANA 30-44 (3.73)
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  #9  
Old 01-03-2017, 04:58 PM
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babywag babywag is offline
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I believe the foglamps are also tied into that circuit.
Might want to unplug those, or pull the fog lamp relay.
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88 GW, 67 J3000, 07 Magnum SRT8
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  #10  
Old 01-03-2017, 07:43 PM
twmattox twmattox is offline
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Join Date: Feb 24, 2003
Location: Indiana
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Great point. I will pull the relay and unplug the fog lamps just in case...

Have I said before just how much I HATE electrical?
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'83 Scrambler (CJ-8) / 258 / T-5 / D-300 / DANA 30-AMC20 (3.31)
'88 Grand Wagoneer (SJ) / 360 / TF727 / NP229 / DANA 44 (2.73)
'05 Wrangler Unlimited (LJ) / 4.0L / NSG 370 / NV231 / DANA 30-44 (3.73)
'15 Wrangler Unlimited (JKU) / 3.6L / 42 RLE / NV 241 / DANA 30-44 (3.73)
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  #11  
Old 01-04-2017, 07:41 AM
rocklaurence rocklaurence is offline
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Quote:
Originally Posted by twmattox
I know there is a harness plug near the bumper. That was where I was going to unplug it. I will look for one closer to the firewall.

Will pop a fuse if just the parking lights are on or if both parking lights and headlights are on.

This is the first step--does it blow with running lights on or only when the head lights are turned on? Most of these failures happen because of damage [burnt or crushed wires]. What was the last thing you worked on?
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  #12  
Old 01-04-2017, 01:03 PM
twmattox twmattox is offline
350 Buick
 
Join Date: Feb 24, 2003
Location: Indiana
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Quote:
Originally Posted by rocklaurence
This is the first step--does it blow with running lights on or only when the head lights are turned on? Most of these failures happen because of damage [burnt or crushed wires]. What was the last thing you worked on?

Last thing that was done was to rotate and balance the tires and have the shop adjust the front transmission band. The fuse blew when I picked the rig up from the shop. Can't imagine either of those crushing anything...

As said above, the fuse blows with running lights on. I have disconnected the rear lights at the bumper (still blew), disconnected the rear lights at the bulkhead (still blew), removed front side marker lights and disconnected the front lights (still blew). Removed the radio, HVAC controls, speedometer cluster, dimmer and headlight switch...

None of the wiring anywhere looks crushed. All of the wire looms are perfectly in tact and still connected to the "factory" locations; with some small exceptions...
1)Over a year ago I wired in an adapter harness for the radio
2)5-6 years ago I replaced the fuse holder for the cruise control modul
3)6-8 years ago the engine bay wiring along the passenger side was re-routed and re-loomed some of it seemed brittle from running near the exhaust manifold.
Again, can't think of anything changed from the last time I drove it at night (this summer).

I really think it is the headlight switch. But, I don't just want to throw $$$ at it. Any ideas on how to test the switch?
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'83 Scrambler (CJ-8) / 258 / T-5 / D-300 / DANA 30-AMC20 (3.31)
'88 Grand Wagoneer (SJ) / 360 / TF727 / NP229 / DANA 44 (2.73)
'05 Wrangler Unlimited (LJ) / 4.0L / NSG 370 / NV231 / DANA 30-44 (3.73)
'15 Wrangler Unlimited (JKU) / 3.6L / 42 RLE / NV 241 / DANA 30-44 (3.73)
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  #13  
Old 01-04-2017, 01:54 PM
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babywag babywag is offline
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Borrow one of the breaker(s) in the fuse box so you can test more...

Put the breaker in the fuse slot that keeps blowing.
Reinstall the headlight switch.

Remove the center bolt on firewall plug, the bulkhead connector is actually 2 connectors.

Disconnect the bulkhead connector that has the front light(s) wiring @ bulkhead (right/driver's side). Reconnect the other main bulkhead connector so you have power for testing.

With that bulkhead connector removed, measure for power @ the light(s) wiring connectors where the bulkhead normally plugs in.
If you can maintain power and the breaker doesn't trip, the short is somewhere in the wiring running from the bulkhead to the lights.
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  #14  
Old 01-04-2017, 02:09 PM
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will e will e is offline
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I like the breaker idea.

Unconventional but has worked for me:

Park in a garage. Turn off all of the lights. Let your eyes adjust. Turn on the parking lights (with all the bulbs out) and watch for a spark. if you don't see it coming from under the dash get a helper and watch under the hood. Then under the truck and such.

It's worth a shot.


I would also take a look at the trailer light wiring.

So to make sure I understand: the brake lights don't blow the fuse, emergency flashers don't blow it, and turn signals don't blow it. Only when you go to parking lights?
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Last edited by will e : 01-04-2017 at 08:48 PM.
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  #15  
Old 01-05-2017, 02:49 PM
twmattox twmattox is offline
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Location: Indiana
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Quote:
Originally Posted by will e
I would also take a look at the trailer light wiring.

So to make sure I understand: the brake lights don't blow the fuse, emergency flashers don't blow it, and turn signals don't blow it. Only when you go to parking lights?

Correct, the brakes and flasher don't blow the fuses. I disconnected the rear lighting harness at the bulkhead. The trailer harness (along with the whole rear set of wires) is no longer connected at all.
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'83 Scrambler (CJ-8) / 258 / T-5 / D-300 / DANA 30-AMC20 (3.31)
'88 Grand Wagoneer (SJ) / 360 / TF727 / NP229 / DANA 44 (2.73)
'05 Wrangler Unlimited (LJ) / 4.0L / NSG 370 / NV231 / DANA 30-44 (3.73)
'15 Wrangler Unlimited (JKU) / 3.6L / 42 RLE / NV 241 / DANA 30-44 (3.73)
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  #16  
Old 01-05-2017, 02:53 PM
twmattox twmattox is offline
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Quote:
Originally Posted by babywag
Borrow one of the breaker(s) in the fuse box so you can test more...

Put the breaker in the fuse slot that keeps blowing.
Reinstall the headlight switch.

Remove the center bolt on firewall plug, the bulkhead connector is actually 2 connectors.

Disconnect the bulkhead connector that has the front light(s) wiring @ bulkhead (right/driver's side). Reconnect the other main bulkhead connector so you have power for testing.

With that bulkhead connector removed, measure for power @ the light(s) wiring connectors where the bulkhead normally plugs in.
If you can maintain power and the breaker doesn't trip, the short is somewhere in the wiring running from the bulkhead to the lights.

Excellent idea. I will make notes and try this when I get more time to work on it. Unfortunately, I don't have a garage I can fit the rig in and it is VERY cold this weekend. I don't want to break any brittle plastic.
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'83 Scrambler (CJ-8) / 258 / T-5 / D-300 / DANA 30-AMC20 (3.31)
'88 Grand Wagoneer (SJ) / 360 / TF727 / NP229 / DANA 44 (2.73)
'05 Wrangler Unlimited (LJ) / 4.0L / NSG 370 / NV231 / DANA 30-44 (3.73)
'15 Wrangler Unlimited (JKU) / 3.6L / 42 RLE / NV 241 / DANA 30-44 (3.73)
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  #17  
Old 01-10-2017, 07:15 PM
JDD JDD is offline
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Fuse

Just a shot in the dark, my '90 Wagoneer kept blowing a fuse every time my wife put the sun visor down. There is wiring that runs up to the lights next to the mirrors. Mine were disconnected and exposed, I guess when the visor was moved, the wiring touched the metal roof? I took the visor off, wrapped electrical tape over the exposed end, and haven't had a fuse blow since. Just something else to try, good luck, wiring problems are difficult to track down.
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  #18  
Old 01-12-2017, 09:53 AM
twmattox twmattox is offline
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Quote:
Originally Posted by JDD
Just a shot in the dark, my '90 Wagoneer kept blowing a fuse every time my wife put the sun visor down. There is wiring that runs up to the lights next to the mirrors. Mine were disconnected and exposed, I guess when the visor was moved, the wiring touched the metal roof? I took the visor off, wrapped electrical tape over the exposed end, and haven't had a fuse blow since. Just something else to try, good luck, wiring problems are difficult to track down.

Wait...the visor is on the same fuse as the parking lights??? I disconnected my vanity light and have it sitting above the headliner waiting on me to get my visors recovered. Worth a shot...

The schematic I have been following places the vanity lamps on the overhead light circuit that ties into the 20amp dome fuse. Though, if I follow it correctly, those are somewhat connected via the dimmer switch. Have I said how much I hate electrical?!?!
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'83 Scrambler (CJ-8) / 258 / T-5 / D-300 / DANA 30-AMC20 (3.31)
'88 Grand Wagoneer (SJ) / 360 / TF727 / NP229 / DANA 44 (2.73)
'05 Wrangler Unlimited (LJ) / 4.0L / NSG 370 / NV231 / DANA 30-44 (3.73)
'15 Wrangler Unlimited (JKU) / 3.6L / 42 RLE / NV 241 / DANA 30-44 (3.73)
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  #19  
Old 01-12-2017, 10:56 AM
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babywag babywag is offline
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The visor vanity light wiring is hot all time. If the problem was in that portion of circuit the fuse would likely blow immediately, not only when you turn on lights.

From your descriptions it appears the problem is somewhere between the headlight switch and the lights.
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  #20  
Old 01-12-2017, 10:44 PM
Joe Guilbeau Joe Guilbeau is offline
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Join Date: Apr 17, 2002
Location: San Antonio, Texas
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Trouble Shoot Shorts at Fuse Block

I have used a 12Vdc bulb socket with pigtails (wires) and connected it to the fuse location. The bulb should light, when you find the problem the bulb will no longer be lit.

Might somewhere in the harness, unless it is a pinched voltage carrying wire shorting to electrical ground.

Think back to what you were doing just before the short revealed itself.
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