Electrical issue

[Vova]

I have a weird little issue. When I fire up the truck it runs great. As soon as I shut it off it blows the main fuse off the battery.
I replace the fuse and it fires up until I shut it down and it blows the fuse.
What would cause this?

[franken]

Any idea what kind of truck this is?

[VetteVet]

Originally Posted by franken

Any idea what kind of truck this is?

I guess it's this one.







http://67-72chevytrucks.com/vboard/s...d.php?t=537635

[VetteVet]

Do you have an external voltage regulator and alternator?

[ptc]

Blowing a fuse means you have an overload or a short within that circuit. Being the "MAIN" fuse means it could lead to many circuits within the ignition wiring circuit.

You have to get a copy of your trucks wiring diagram and begin tracing the circuits with an ohm meter to see where a short exists with key in ON and OFF positions. Thats the correct way....

If you want to find it really fast? - just wrap the fuse in FOIL and try it again and wherever the short is it will start BURNING..... NOT A RECOMMENDED WAY OF TROUBLESHOOTING!

[Vova]

Originally Posted by ptc

Blowing a fuse means you have an overload or a short within that circuit. Being the "MAIN" fuse means it could lead to many circuits within the ignition wiring circuit.

You have to get a copy of your trucks wiring diagram and begin tracing the circuits with an ohm meter to see where a short exists with key in ON and OFF positions. Thats the correct way....

If you want to find it really fast? - just wrap the fuse in FOIL and try it again and wherever the short is it will start BURNING..... NOT A RECOMMENDED WAY OF TROUBLESHOOTING!

Why would it only blow the fuse when I turn the truck off?

[VetteVet]

Originally Posted by Vova

Alternator.

Is this the one that you got for free that someone had put the battery in backwards and fried the harness? Then you replaced the front harness?

Originally Posted by Vova

Why would it only blow the fuse when I turn the truck off?

Did you just start having this problem?
Is the fuse you're talking about, the fusible link that runs from the positive battery post to the right fender junction.
Have you replaced the stock harness with a stock harness or is it a modified one?
Is your alternator an internal regulated one or does it have a separate regulator on the core support?
I assume it is internal since you answered my previous question by saying "alternator."
Is it a one-wire or three wire SI series?

just to save time here, unplug the alternator and take the large wire off the back and tape it off, cause its 12 volts hot. The truck won't charge but it'll run off the battery long enough to try this test.

Now replace the fusible link and start the truck, let it run a minute or two then shut it off and see if it blows the fuse.

I have a suspicion that one of the diodes in the alternator is blown and shorting to ground.

[Vova]

On a suggestion I took out the ignition switch from the dash and it stopped blowing the fuse. Does that mean there is a short in the switch?

[Vova]

Originally Posted by VetteVet

Is this the one that you got for free that someone had put the battery in backwards and fried the harness? Then you replaced the front harness?




Did you just start having this problem?
Is the fuse you're talking about, the fusible link that runs from the positive battery post to the right fender junction.
Have you replaced the stock harness with a stock harness or is it a modified one?
Is your alternator an internal regulated one or does it have a separate regulator on the core support?
I assume it is internal since you answered my previous question by saying "alternator."
Is it a one-wire or three wire SI series?

just to save time here, unplug the alternator and take the large wire off the back and tape it off, cause its 12 volts hot. The truck won't charge but it'll run off the battery long enough to try this test.

Now replace the fusible link and start the truck, let it run a minute or two then shut it off and see if it blows the fuse.

I have a suspicion that one of the diodes in the alternator is blown and shorting to ground.

Missed your post.
Yes that's the truck.
It has a new alternator.
I have a fuse between the positive post and the fender junction.
It's a stock harness and ran fine for a long time before I started having this issue.
I'll try your test and see if it blows the fuse.

[Vova]

Originally Posted by VetteVet

Do you have an external voltage regulator and alternator?

Alternator.

[starterman99]

When you say main fuse what exactly are you refering too? Is it a fuse or fusible link and what rating is it and where is it located ? Picture might help.

[Vova]

Originally Posted by starterman99

When you say main fuse what exactly are you refering too? Is it a fuse or fusible link and what rating is it and where is it located ? Picture might help.

It's a fuse off the battery.

[VetteVet]

Did you try the test I told you to do in post 8 ?

[starterman99]

That should have a fusible link not a fuse holder and atc fuse in that location. You probably don't have a system problem but a momentary overload of that fuse when switch gets turned off.

[Vova]

Originally Posted by starterman99

That should have a fusible link not a fuse holder and atc fuse in that location. You probably don't have a system problem but a momentary overload of that fuse when switch gets turned off.

So according to this diagram http://www.madelectrical.com/electri...evymain1.shtml I should have a 14 gauge fusible link instead of a fuse.
You might be on to something...

[VetteVet]

Or you can try a 30 amp maxi fuse.

http://www.littelfuse.com/products/f...fuses/299.aspx

[starterman99]

Originally Posted by VetteVet

Or you can try a 30 amp maxi fuse.

http://www.littelfuse.com/products/f...fuses/299.aspx

You would not want a 30 amp maxi fuse either. If you have a low battery condition and the alternator is charging higher than 30 amps it will blow the fuse. Because it charging the engine will continue to run but voltage spike could be very damaging to other electrical components such as stereo or electronic ignition module. The best solution is the fusible link.

[VetteVet]

Originally Posted by starterman99

You would not want a 30 amp maxi fuse either. If you have a low battery condition and the alternator is charging higher than 30 amps it will blow the fuse. Because it charging the engine will continue to run but voltage spike could be very damaging to other electrical components such as stereo or electronic ignition module. The best solution is the fusible link.

Maxi Fuse or fusible link?

This subject has been debated ad nauseam on the internet and the result is, use what you want. the maxi fuse is a slow blow fuse with the same characteristics as the fusible link. They are however, much easier to change if you're out on the road.

A battery would not normally pull 30 amps when charging even if it was dead. , unless it was shorted internally.

The battery charging wire on our trucks which has the fusible link installed is not designed to provide full current load to all the trucks accessories. That's the job of the alternator, That is if it's wired correctly.

The fusible link and the maxi fuse are designed to protect the wire from a direct short and both will blow if that happens. Any circuits that would be powered by the alternator alone that are current sensitive should be protected by their own fuses.
JMHO YMMV

[Vova]

Good info boys. I'll try the Maxi fuse first because it's an easy fix. If it still blows I'll get the fusable link.

[mac808]

Originally Posted by Vova

Good info boys. I'll try the Maxi fuse first because it's an easy fix. If it still blows I'll get the fusable link.

Wow. This is a oldie but goldie!

Having the exact same issue as well. This is the only thread on this exact phenomenon I could find anywhere. Any update on if the Maxi fuse fix worked...?

Btw, just a quick shout out to VetteVet, who is all over the electrical troubleshooting threads that I was searching through before finding this one. You're a wealth of knowledge and valuable information. Just wanted to say 'thank you'!