77 brake light fuse blowing - - WEIRD symptoms !!!

[TheHardWay]

I can take 10, 15 mile trips without the fuse blowing, but 25, 30 miles and POP. After repairing a PO trailer splice, replacing the brake light switch and replacing two wires I found crushed between the rear body mount and frame (wait til ya see the photos ), I'm guessing it's heat related. And it's either engine/exhaust or electrical heat that's overloading the wiring or a switch and blowing the fuses.
Since the constant power connects from the brake light switch to the hazard switch, today I pulled the hazard switch. Looks okay from the outside:

This looks like the original switch, and I do NOT want to break it open, so am hoping some knowledgeable Mudder can steer me in the right direction as to what else might cause the fuse to blow.

IMMA STUMPED

 

[charliemeyer007]

If you gently pry those tabs back ( just enough) you can open the switch and clean the trash out and polish the contacts, a dab of dielectric grease wiped on with a q-tip - then close it back up. If the tabs break off then wrap the switch with small zip ties and cover it with heat shrink.
There are spring in there - have good lighting and a clean area. A deep, big box will help contain flying parts. Take a pic when you first open it up carefully.
Go to a junk yard and pull some switches to practice taking them apart and putting them back together - better to learn on stuff that isn't on your rig.

 

[JohnnyC]

If you’re not confident in opening up the switch … you can take the easy clean way and spray some electrical contact cleaner inside the switch …. Just load it up and work the switch … after you’re confident it’s dry … reinstall and try

I use crc brand for stuff like that

I would if I were you take a spare appart and see… it’s not hard and I think there are some threads here taking them appart

 

[pb4ugo]

Miles traveled or time is not going to blow a fuse. I had a similar issue, my brakelight fuse started blowing, seemingly for no reason. I had previously rewired the turns and brake circuits and were working, so i knew the wiring was all good. I needed to see what else was in the circuit. I had to look at the wiring diagram, then think of what I might have done to make it blow. I found the horn is connected to the brake light circuit and the hazzard switch was incorporated in the circuit to make both rear lights activate. I knew i could hear the horn relay working but I hadn't installed the horns yet. I decided to disconnect the horn relay plug and my problem went away. I think I would inadvertently hit the horn button while driving and blow the fuse. Afterwards, I would periodically pull the hazzard switch to confirm the fuse wasn't blown.

 

[Living in the Past]

Do you have a stock six cylinder or a V8 with exhaust close to wiring harness. Next do you know exactly when the fuse blows? If so is the fuse warm or hot to the touch? If the fuse is warm the problem is close by. I know on the earlier fuse blocks the connects would come loose and general heat. The pieces the hold the fuse can be the problem. These are tempted steel. Heating up multiple times it looses that and because a resistor in the circuit. Guess the same thing could happen inside the hazard switch. Unless it something that is shorting directly to ground it will probably feel hot when the fuse blows.

 

[TheHardWay]

Thanks guys, for responding with some good advice!

@charliemeyer007 - thanks for the detailed method for opening up and cleaning the switch. I've taken apart a slew of electrical devices over the years, so I'm not concerned about this, but really didn't want to cut off the clear shrink wrap thinking it couldn't be reinstalled like it was.

@JohnnyC - Hey, Buddy, glad to have you back in Mudville! If I chicken out on cutting away the clear wrap, I'll spray the hell out of the switch and work it 50 times... then do it again.

@pb4ugo - I like your story. Will review my schematic again to make sure I don't have any other switches on the brake light circuit.

@Living in the Past - stock original 6 cylinder. I never know when the fuse blows, only that when I get home after the long drives, it's blown. I've thought about looking at the fuse panel backside for anything amiss, but keep forgetting to JUST DO IT.

I'm gonna get busy this morning. Keeps those great suggestions coming!

 

[Living in the Past]

I would pull a couple other fuses and compare the spring in the clips to the circuit that keeps blowing fuses. If that doesn't have the same spring feel that could be the problem. Also remember when ever you pull a fuse to squeeze the clips together before installing a new fuse. Loose connection acts like a resistor and generates heat. Resistor will increase the load. Every time it happens it increases the resistor.

 

[TheHardWay]

Timely comment! I just finished pulling the fuse box and cleaned it all up (pics later), even shot each female plug with Deoxit. Will give those fuse clips a squeeze before reassembly.

 

[45Dougal]

Before you pull the hazard switch apart, id take a look at the hazard switch connector. They get wet underneath the leaky vent and inevitably corrode. Worth cleaning up even if it's not the cause.

Do you have the correct stop light bulbs fitted?
Do you have Amber turn signals or is it wired to flash the stop lights?
If the latter, maybe you've got a bad diode - check if your front turn signals come on with the brake light.

Horn or horn relay is another good culprit - check where your harness passes through the bulkhead for chaffing, and around the horn/ relay for stray wiring

 

[TheHardWay]

Luckily, my 77 doesn't have the vent on top of the cowl, so the hazard harness connector is in great condition. I am still going to shoot the two connectors with Deoxit before reinstalling.

I have the amber turn signals.

Funny you should mention the horn relay. The horn works like it should, since I replaced the relay last year, but I was just looking at my schematic and noticed the 20 amp fuse feeds the horn relay in addition to the brake lights and hazard - possible culprit?

 

[pb4ugo]

Luckily, my 77 doesn't have the vent on top of the cowl, so the hazard harness connector is in great condition. I am still going to shoot the two connectors with Deoxit before reinstalling.

I have the amber turn signals.

Funny you should mention the horn relay. The horn works like it should, since I replaced the relay last year, but I was just looking at my schematic and noticed the 20 amp fuse feeds the horn relay in addition to the brake lights and hazard - possible culprit?

As I mentioned in my previous post, the horn circuit was in my case. It's easy enuff to disconnect. The plug was old and brittle, so I taped it up too.

 

[pb4ugo]

I also found when the fuse blew my flashers didn't work, that's why I'd pull them to confirm the brake lights were still working.

 

[TheHardWay]

Spent most of the day cleaning and inspecting the fuse block. Seems to be in good shape:

Cleaned up pretty well too

Also cleaned all fuse holder springs with a .22 cleaning brush. Made sure to give the brake light fuse prongs a squeeze before inserting the fuse.

It occurred to me - the clear shrink cover on the hazard switch was necessary for 76 and older models to protect switch from leaky cowl vent? If so then I'll cut it off tomorrow with no regrets, and dig into the switch.

STAY TUNED

 

[45Dougal]

Looks pretty clean, nice.
Any luck tracing the horn wiring?

I'm sceptical that the hazard switch is the culprit. Generally I avoid non-reversible disassembly until everything else is rulled out. Just disconnect it for a while and jumper the turn signals?

 

[B y r o n]

Spent most of the day cleaning and inspecting the fuse block. Seems to be in good shape:

View attachment 3643051

Cleaned up pretty well too

View attachment 3643053


Also cleaned all fuse holder springs with a .22 cleaning brush. Made sure to give the brake light fuse prongs a squeeze before inserting the fuse.

It occurred to me - the clear shrink cover on the hazard switch was necessary for 76 and older models to protect switch from leaky cowl vent? If so then I'll cut it off tomorrow with no regrets, and dig into the switch.

STAY TUNED

Looks like you are missing the insulator that goes between the fuse block and the body. Any chance the fuse block isn't grounding out against the body? If it does that would blow fuses.

 

[TheHardWay]

Guess it couldn't hurt to cut a thin sheet of plastic to replace the crumbly ass old insulator, ya?

 

[charliemeyer007]

Good plan to make it yourself - it would be $55 plus shipping at $OR

 

[TheHardWay]

I JUST SAVED $55 (plus shipping):

Big thanks to @byron

Whoops, this picture should be the third of the five.

 

[TheHardWay]

Opened up the hazard switch today:

Great condition - the minor contacts corrosion was the worst of it:

Wiped off the old grease, a little brass brushing followed by fine sanding, dielectric lubricant and BABAM:

Reassembly was pretty easy - I was even able to slip the clear cover back on:

I'm glad I did it, but don't ever want to do it again. Ran out of time today, but as soon as I get the radio reinstalled I'm taking a long drive, find out if the brake light fuse blows again.

WISH ME LUCK

 

[TheHardWay]

LUCK, OR SKILL?

Yesterday I reinstalled the Retro radio and buttoned everything down for today's test drive, and thanks to @45Dougal and @pb4ugo, inspected the horn relay re-e-a-al close:

The connector is pretty beat up and the terminals are grungy and a little corroded. The 3 wire sub harness is bent and has a split in the harness tape right where it rests on the fender. I decided that's a project for next week, removed the relay, wrapped some harness around the bent section of harness so it wouldn't touch and possibly short and blow the fuse, and off we went!

Put 25 miles on the rig, pulling out the hazard switch every 5 miles after I counted 14 (the longest previous drive that didn't blow the fuse since the previous blow out), and THERE WAS NO BLOWN FUSE - WOO HOO (too many disappointments from previous fixes to get excited now).

I'm theorizing that the horn relay is the culprit, because the horns work fine when the fuse is between blowouts, and that tracking down the suspected harness short and fixing that plus cleaning the connectors will fix the problem ONCE AND FOR ALL.

But for now all I can say with certainty is so far so good.

To be continued...