521 constantly blowing flasher, reverse and meter fuse

[d.p]

I got a 521 that the PO told me that fuse would get hot and stop working so he would just reseat it which worked.  Now it blows constantly and I have no idea why but it seems to be blowing more now that my truck is lowered.  Nothing I can tell that causes it, just look down and see the gas and temp gauges not working.    Could a loose battery cable cause this?  My negative won't get completely tight so it has some play in it.  

 

Anyone have any idea what could cause it?  Left most fuse on the fuse box:

 

 

[datzenmike]

Simple enough. If the fuse blows then it's drawing too much current through it. Probably a wire somewhere is pinched and touching something grounded. Trial and error.... unplug the rear light harness under the passenger seat and see if this stops the problem. If it does the short is probably in that harness going to the rear tail lamps.

 

 

Something that should be done is a through cleaning of the fuse box so you don't have to keep resetting the fuses. Mark the wires or take a picture then remove it. Wash thoroughly and use a scrub brush or as I like to do an old toothbrush. Now submerge in a bath mixture of lemon juice and salt overnight. Alternately you can use just vinegar. Scrub, rinse and dry in the morning.

[d.p]

Simple enough. If the fuse blows then it's drawing too much current through it. Probably a wire somewhere is pinched and touching something grounded. Trial and error.... unplug the rear light harness under the passenger seat and see if this stops the problem. If it does the short is probably in that harness going to the rear tail lamps.

 

 

Something that should be done is a through cleaning of the fuse box so you don't have to keep resetting the fuses. Mark the wires or take a picture then remove it. Wash thoroughly and use a scrub brush or as I like to do an old toothbrush. Now submerge in a bath mixture of lemon juice and salt overnight. Alternately you can use just vinegar. Scrub, rinse and dry in the morning.

I took a look at the under the pass seat and didn't see a harness.  You sure a 521 has that harness?  Its blown 3 times this morning alone so it seems to be getting worse.  I already cleaned the fuse box via your method, that is just an old pic before I did it.  

[datzenmike]

The harness is connected to the pass side frame and goes back to the rear lights. I had a 521 back in the '70s and '80s but I think it exits the cab under the passenger seat. Maybe wayno knows.

[DanielC]

On a 521. the wiring harness is under the truck.  There are two connectors in the engine compartment, on the fire wall, close to the starter.  the harness goes down the right frame rail, but has some branches that go across to the fuel tank, and toward the transmission.  It is fairly close to the exhaust pipe, and muffler.  At the end of the right frame rail, it crosses over to the left tail light.

You can unplug the tail lights, and the rear harness where it connects to the engine room harness, by the starter.

[d.p]

Ok will try that but again if I do that I got no brake lights and hard for me test it because it only blows when I am driving the fucker.  

[flatcat19]

You can test all the wires for continuity without driving.

 

 

No continuity, open circuit/short to ground. Find the open, repair.

 

Does the brake fuse pop when you turn another function on?

[d.p]

Ok, I am fuck all at testing continuity. 

 

Can you give me a crash course?  I have pen light and multi-meter but have only tested household stuff like my water heater elements and other easy shit.   Also how do I get the cluster off?  I want to have a look behind it but not sure how to remove it. 

[DanielC]

If this were my truck, I would get it up off the ground with some blocks of wood under the tires, or jack stands, or some other SAFE way of raising it about 4 to 6 inches.  Before you go under the truck, try to push it off whatever is holding it up.

With a good bright work light, inspect every inch of the rear wire harness, going toward the rear of the truck.  Pay close attention if the harness goes between the frame and the cab of the truck.  Pay close attention close to the harness when it is close to the exhaust system.  At the rear of the frame, the harness for the tail lights goes through a hole in the frame.  Check the wires there.  If a previous owner has added some connections for a trailer light plug check that.  In fact, if there is added trailer light wiring, check that first.

[d.p]

Ill do that right now.  

 

There are a two cut wires on the passenger side frame going towards the front, a black and a red wire.  Red wire comes out in the engine bay but is cut there as well.  No idea where the other end of the black one is.    

 

Guess it has a trailer light wiring, didnt even know it did.  Anyways traced all the wires from the back to the front along the rail and dont see anything exposed.  Exhaust mount, where it goes through the frame its all wrapped nice and tight.  

 

[banzai510(hainz)]

MY left fuse keeps blowing also. 72 521

 

don't know if this helps you but I had my tail lights fill up with water. Never popped a fuse, I drilled a drain hole under the plasic housing

 

Im going to go home and ck mine also. On #1 as you stated there is 2 wires. Pull one wire off then run the truck. if it don't pop its not that wire. then try the other wire and if it pops then write the color code of wire and look on diagram wht circuit it feeds. then remove bulbs or systems that wire feeds.  I will try this also when I get back home . I just seem to panick as I just popped a good Volt reg thinking it was something simple.  But corression can cause a high amp draw also. so clean connectors

Good luck to the both of us

[d.p]

MY left fuse keeps blowing also. 72 521

 

don't know if this helps you but I had my tail lights fill up with water. Never popped a fuse, I drilled a drain hole under the plasic housing

 

Yeah no rain here at all but I did wash it last weekend, not sure whatever was wet would still be today.  All lights are dry though.   

 

There were two wires, one to emissions (blue/yellow) and the other to the flasher/meter/reverse lights (green).  I pulled the emission one off and it didn't blow for a while, then I lowered it, washed it and it started blowing like crazy.   So it has to the be green wire or something along its path.  

 

Before I lowered/washed it I didn't drive it at all so no idea if it helped when I pulled that emission wire off.   I checked the wiring diagram for the green wire coming off #1 and its combination meter, flashers and reverse lights.   It goes through the combination meter, turn signal switch, tank gauge unit, hazard switch, flasher unit, reverse lamp switch and then to the reverse lights.  Fucking needle in a hay stack at this point. 

 

[banzai510(hainz)]

don't know if that drawing is exactly correct as I got 2 wires off the 1st fuse(10A)

 

 

just remember ck your voltage. As my solid state volt reg popped and now overvolts.

I will have to try to do this when I get home. I don't have a computer at home .  I was hoping Datzenmike will chime in on this while I was on here also.

 

 

usually its a corrssion issue. 

but you say reverse light is on that fuse. wires not hooked up by trans touching ground?????

 

 

my next step if I get that far is see if I pop a fuse with the instrument panel not connected but this might not work as it might feed the alternator. to charge.

[d.p]

don't know if that drawing is exactly correct as I got 2 wires off the 1st fuse(10A)

 

 

just remember ck your voltage. As my solid state volt reg popped and now overvolts.

I will have to try to do this when I get home. I don't have a computer at home .  I was hoping Datzenmike will chime in on this while I was on here also.

 

 

usually its a corrssion issue. 

but you say reverse light is on that fuse. wires not hooked up by trans touching ground?????

 

 

my next step if I get that far is see if I pop a fuse with the instrument panel not connected but this might not work as it might feed the alternator. to charge.

 

Yeah its not exact and I have two wires off #1 as well but blue/yellow doesn't do anything for me and DanielC seemed to think it was the emissions.  

 

I do know when #1 fuse blows 3 things don't work, gas/temp/turn signals and haven't had a chance to check the reverse lights but I assume they don't work either.   And how you not have a computer at home?  

[DanielC]

I believe the hazard, and turn signals are two separate fuses.   The hazard will work if the key is on or off, the turn signal only works with key on.  However, turn signal electricity goes through the hazard switch, but is switched off when the hazard is on. 

 

Brake lights will work, key off.  Since brake lights are separate from turn signals on the rear of a 521, the brake light circuit is just a fuse, feeding a wire that goes to a brake light switch, to the brake lights.

 

The reverse lights are wired similarly, although the fuse for them also supplies other stuff.  But one side of the reverse light switch on the transmission is hot when the key is on, and when the transmission is put in reverse, the reverse lights get power.

 

Since you have trailer light wiring installed, look for a multiplexer that combines the function of turn and brake lights.  A 521 has separate lights for each function, a lot of trailers do not.  The multiplexer could be bad, and causing problems.  The multiplexer is a small device, with wires going into it, and coming out of it, probably the size of a pack of cigarettes, or smaller.

 

There are copper traces (circuits) on the combo meter.  Some of these are supplied power when the key is on.  If there is a loose wire, or even a piece of metal that contacts the hot traces on the printed circuit board, that is a possibility of a short circuit.

 

I have made threads on rebuilding tail lights, and instrument panel testing, but they are old.  Try searching "instrument panel" and "tail light rebuilding" using my name to limit the search options.  I may not have the exact spelling on the two topics.

[d.p]

I believe the hazard, and turn signals are two separate fuses.   The hazard will work if the key is on or off, the turn signal only works with key on.  However, turn signal electricity goes through the hazard switch, but is switched off when the hazard is on. 

 

Yes and yes.  Harzards work regardless if they key is on and turn signals only work with the key on.  

[d.p]

UPDATE:  Related to throttle somehow but not sure how?  

 

Start truck, put in In neutral, floor it and the fuse blows.  Did that twice and it blew both times, if I just let it idle or give it a little gas it doesn't blow.   Would that be alternator or fuel pump?  

[DanielC]

Part of the electric emission wiring on the truck uses switches on the carb, and on the throttle pedal to control if the distributor uses the advances, or retarded set of point in the distributor.

Look for a wire shorting under the air cleaner base.  There should be two switches, on the carb, that are activated by a cam on the throttle shaft.

 

The colored version of the 521 wiring diagram does not show the USA emissions wiring.

[d.p]

There are two wires but no short that I can tell.   One wires was missing the rubber sheath (first pic) so I wrapped it in electrical tape to no avail.  Two switches under the cleaner, one square one and one cylindrical one.   Tried disconnecting the wires to the square switch to no avail, fuse still blew.  Not sure which is which.  

 

 

 

 

[datzenmike]

Well the alternator puts out less at low speeds and when revved up can 'push' more current. This may seen a bit simple but if you are running a 10 amp fuse when it should be 15 it might do this. It safely draws 9 amps but when revved up draws 11 and blows.

 

Do you have the correct fuse size?

 

 

About the harness under the pass seat... it's been a long while and I assumed the 521 was like the later 620.

[d.p]

Pretty sure its a 10AMP which is what I have in there so unless the paper is wrong on the inside of the fuse box then I don't know.

 

At a loss to what is causing it but can reproduce like clock work.

[datzenmike]

Yup I see it now. That fuse powers the turn signals through the 4 way emergency switch when (and only when) it is turned off, and the dash gauges.

 

Disconnect the emergency flasher switch. Try turning the 4 way switch on and off several times too.

 

Pull the 10 pin connector out of the back of the dash gauges.

 

 

It would seem it has to be one of these two causing the problem.

 

 

I don't see where the reverse lights are run off of this fuse.

[wayno]

The 521 rear wiring is on the fuse box side of the engine compartment, it goes down the fire wall till it gets to the frame then follows the frame back to the taillights, there are 2 4 wire cream colored plugs near the engine compartment light.

I would imagine that when the fuse box was moved to the interior, that is when the rear wiring went under the passenger seat thru the hole in the floor.

[datzenmike]

Yes.

[d.p]

You think the 4 way flasher or back of the gauge cluster is what is causing it to blow under heavy load?  Disconnect the flashers then try to turn them on/off a handful of times?   Or do both separately?  

 

Blew 10+ fuses today troubleshooting this shit, fucking frustrating as hell.