Strange Electrical Issue at Fuse Block

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stoshzack

stoshzack

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Hi All -

I recently finished a bunch of mechanical work on my FJ40 1970 ('71 wiring diagram). I must have messed something up because now, when I go to start the vehicle, I constantly get a 20 amp fuse burning out. It is the fuse that goes to the yellow (oil press. ga, temp ga, and fuel gauge) and red-blue lines (back-up light/switch).

Reading past threads on the forum, I removed the fuse, attached an amp meter on the fuse block, and turned on the ignition switch. I got zero amps. I removed all the components on the lines, turned on the ignition switch, and still got zero amps. Thinking my meter was broken, I check the voltage across that fuse and got the normal 12 volts. I then put another fuse in, and it burned out right away.

Any thoughts as to what is going on? I figured with no amps, the fuse would not burn out. However, it continues to burn out right away.

Thanks for any help

@Coolerman
@Pin_Head
 
Green Bean

Green Bean

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Hi All -

I recently finished a bunch of mechanical work on my FJ40 1970 ('71 wiring diagram). I must have messed something up because now, when I go to start the vehicle, I constantly get a 20 amp fuse burning out. It is the fuse that goes to the yellow (oil press. ga, temp ga, and fuel gauge) and red-blue lines (back-up light/switch).

Reading past threads on the forum, I removed the fuse, attached an amp meter on the fuse block, and turned on the ignition switch. I got zero amps. I removed all the components on the lines, turned on the ignition switch, and still got zero amps. Thinking my meter was broken, I check the voltage across that fuse and got the normal 12 volts. I then put another fuse in, and it burned out right away.

Any thoughts as to what is going on? I figured with no amps, the fuse would not burn out. However, it continues to burn out right away.

Thanks for any help

@Coolerman
@Pin_Head
I'm probably wrong on this and someone may correct me, but I was under the impression that the instrument cluster gauges were on a 5 amp fuse

Fuse Block
 
Skreddy

Skreddy

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My 2/71, the instruments are on a 5 amp. Either way, I blew mine and it turned out in my excited rush, I didn’t get one of the wires on the post on the alternator and it was arcing. Maybe look there if you were near the alternator lately?
 
Pin_Head

Pin_Head

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Possibilities:
1. You are not measuring the current properly. Put the amp meter probes on opposite sides of the fuse holder.
2. You have an intermittent short.
 
Engineer8000

Engineer8000

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If it was a short, it may have blown your meter fuse before you noticed anything. My fluke 87 is only rated at 10A.
 
stoshzack

stoshzack

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If it was a short, it may have blown your meter fuse before you noticed anything. My fluke 87 is only rated at 10A.
Thanks @Engineer8000 - I think that is what happened. Ordered new fuses and an amp meter that will read up to 100 amps today.
 
73FJ40

73FJ40

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@stoshzack .

If it is a dead short blowing 20 amp fuses, hooking up a 100 amp ammeter may allow up to 100 amps through your problem circuit. If so, the short will be easily found, located at the end of the melted and charred wire harness. (for reference, the OEM starter draws about 100 amps on a warm day.)

I suggest you check the two circuits leading to the fuel sending unit and the backup light, continuity to ground, with your ohmmeter. Fuel is yellow/ red on the top of the tank, back-up from the light fixture with the transmission in reverse.

After those, the remaining sending units.
 
stoshzack

stoshzack

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Thanks @73FJ40 - I was thinking using the amp meter might be bad. Thanks for the advice. I’ll use your method to play it safe.
 

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