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Fuse blowing - help needed

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bigredbird

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Hi all, got an electrical bug that needs squashing. I swapped out a non-rallye instrument cluster for a Rallye in my '72 Road Runner clone, fully restored IC and new wiring harness from YearOne. Everything worked great for the first 3 weeks after the job was done. But then the 25 amp MISC fuse started blowing, and taking out the fuel, oil pressure, voltage, and temperature gauges. At this point in the summer, a new fuse might last over 4-5 startups, or it might blow after just one trip around the block.

Since I don't want to "fix" this by carrying a lifetime supply of 25a fuses, I'm hoping someone can advise about a better solution. Maybe a separate inline fuse for these gauges, leading into the fuse panel?
 
Are you sure it’s the “Misc” fuse in question? That fuse provides unswitched 12v to the cigar lighter, park lights and the “batt” three-way power tap. The voltage limiter for the gauges should be connected to a switched power source protected by the fuse labeled “Acc”. The yellow three-way dash harness tap is the only other connection on that fuse circuit, disconnect anything plugged into that tap until you locate what is causing the short. BTW, those fuses are specified as 20 amps.
 
Thanks for the reply. Yep, definitely the MISC fuse that's the culprit - once that's swapped for a fresh one the gauges come right back to life. The shop that installed the new harness hadn't done one before and knows more about old Mustangs than old Mopars, but I sent along a color-coded wiring diagram and they said it was followed. What's most aggravating is that there's no clear pattern here, and the fuse might go a week or a day under identical usage.
 
Not the same year but I had my cig lighter touching ground causing a fuse to blow, bounced around while driving...
 
Well you see, what you got there is a "Homotron".
That would be a queer electron that goes around blowing fuses.
 
gaugecircuit.jpg
 
I don't have any words of wisdom re. the fuse that's blowing, but it sounds as if the installers did something not entirely right. There is a load on that circuit that shouldn't be there. It's not a short because it takes a while to blow.
That is a great wiring diagram! Any idea where I can get a diagram like that for my 73 RR?
 
try this....put a smaller rated fuse in there and see what happens. its very possible you got a rash of bad fuses. if it takes a while to blow , then its blowing under a load...( sounds like a **** movie ...huh ? )

you very well might have things on that circuit that dont belong on that circuit....overloading it and blowing...( huh huh,,,,i said blowing ) the fuse. if there was a direct short....you wouldnt get the fuse in the clips and it would blow!

find out whats SUPPOSED to be on that circuit and MAKE SURE nothing else is on there that isn't supposed to be there.
 
Miscfuse.jpg

Is your “new” harness fuse panel labeled as the original was? In the shot below, disregard the notation about the dash lamp for this thread
DSC07255r3.jpg
 
Not to hijack the thread, but what is that big solenoid looking thing plugged into the upper right-hand area of the fuse panel in the picture above? I've got one sitting in my glove box and didn't realize where it plugged into until now.
 
Nevermind.... I looked up at the diagram and saw that it was the seat belt buzzer. Good, I think I'll plug it in.
 
I would look to see if your cluster grounds are good. Random fuse opening might be bad ground or loose ground wire. Is the voltage limiter grounded properly?
 
Thanks all - I think you may have found the culprit. I got a replacement cig lighter a couple of weeks after the cluster went in and gauges had been working fine. The 'new' lighter didn't work but I left it in anyway. And if I'm remembering right, that's about the time the MISC fuse started breaking at random intervals. Lighter's out now, I'll put in a new fuse (20A, not 25 -- sorry about dual typos in original post) and see what happens when I get back from vacation in a week. Will let you know!

btw, 72RoadrunnerGTX, yes, the Year One fuse panel is marked exactly the same as in your pic. Thanks for the great diagram, too.
 
So far, so good - since the lighter didn't work and the car is a No Smoking zone anyway, I gutted all of the heating element and just left the handle, wrapped in electrical tape so there's zero contact going on inside the receptacle. Four days of start/stop later the gauges are still working fine and the MISC fuse is intact. Now if I could just track down the phantom drain that kills my battery at least twice a year.... :icon_confused:
 
bigredbird said:
So far, so good - since the lighter didn't work and the car is a No Smoking zone anyway, I gutted all of the heating element and just left the handle, wrapped in electrical tape so there's zero contact going on inside the receptacle. Four days of start/stop later the gauges are still working fine and the MISC fuse is intact. Now if I could just track down the phantom drain that kills my battery at least twice a year.... :icon_confused:
Click to expand...

Something still goofy there, the gauges shouldn’t be affected by that circuit. The “Misc” circuit is unswitched battery voltage, if the gauges are connected to it there would be a constant draw.
 
Well, I tried hooking up a multimeter to start tracking things down, especially the parasitic draw that's haunted this car for four years, but maybe because I've got a main battery disconnect hooked up to the negative battery cable, I'm not getting a DC amps reading of any kind. Same thing if I leave the negative hooked up and try jumping the positive instead. Do I need to bypass the disconnect (it's a Flaming River race unit, not a little Radio Shack toggle) in order to get a meter reading?

btw, gauges are still going strong, no more blown fuses since the lighter got taken out of the equation.
 
If you are using the multi-meter correctly set to measure current, the leads will need to be in series with one of the disconnected battery cables. Or in your case across the two battery switch terminals, with the switch set to off. Most quality meters will be restricted to max current flow by an internal fuse (around 10 amps) so be careful what car systems are turned on in this configuration. Too, should this internal meter fuse be already blown, it won’t register anything even if connected correctly. Most multi-meters require moving the test leads to separate terminals on the meter for the ammeter function.
 
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