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Electric problems need help a.s.a.p

C

Colttan

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Bells, Texas
OK IN MY BLAZER my truck when running will idle fine for a few secs and then instantly jumps to 18.3volts and then pops all of my fusible links replace links and alt. and did it again 7 times what the hell is making it freak out and charge so hi, the only thing i can think of is the junction block on the firewall above the booster cause its the first fusible links that blows........Please help
 
Dead short somewhere,maybe the positive cable is melted to the exhaust manifold or the two thick red wires going from the starter to the fusible links is shorted against the tube those wires are housed in,or is shorted somewhere behind the engine where its hard to see.....they do go to the junction block near the brake booster,then back into the cab via the fuse box..I would assume it has to be a heavy gauge wire thats shorting out,otherwise you'd see smoke and see burnt wires by now,if they were smaller gauge...possible a bad ground might make the altenator spike up and put out maximum voltage too,but a short is more likely...
 
I'm going to go with a bad wire/connection somewhere in the charging circuit. As I recall, if the alternator "thinks" the load on it is great (ie broken wire) it will continue to increase voltage output. One of the reasons you never disconnect the alternator with the engine running.
 
Yep, heavy short somewhere.
Don't worry too much about the high voltage right now, its a symptom, not the cause. Unless you had a big overvoltage crowbar circuit in the system somewhere, which you almost certainly don't, the high voltage will not blow the fusable links.

Check the main battery cable all the way to the starter, and also the main ground cable.
Where is the main battery ground hooked up?
If its hooked to the frame, run a heavy wire from the frame to the engine.
Also, make sure the frame of the alternator is grounded well, with a ground that has good connection to the negative post of the battery.
If you are not sure, run a heavy wire from the frame of the alt to the main ground wire from the battery.

Its possible that the firewall connector is flaky. The overvoltage sounds like a ground problem somewhere.
 
Ok so I checked my main bat. Pos. Lead and its perfect and on the ground side I have main neg lead to frame and then to the alt bracket and a ground strap on the bellhousing and back of the head both to the firewall. And then I traced the 10gauge wires all the way from the starter to the fuse block and there is nothing wrong with any of it. The only two thing I can think of is either alt. Or the junction block on the firewall above the booster because that is the first fusible link that blows....this sucks what do I need to do guys
 
Well if the junction block doesn't have continuity to ground, it's not a problem. Since it's plastic, not sure how it could be grounded. :)

Did you run a continuity check on the wiring?
 
No I havnt but when I unplug the alt it won't do it could it be the plug on the alt?
 
Well the alternator is the only component in the system that can change the voltage over the batteries rating (12V) so that's not surprising.

It could be the plug, IMO need to start checking continuity to make sure all the plugs and wires in the system are good. Visual inspection is only one part of diagnosis.
 
Colttan said:
No I havnt but when I unplug the alt it won't do it could it be the plug on the alt?
Click to expand...

I didn't notice what year it is, so I just went with the schematic on a late 80's.
The fuse links go from the battery, to the firewall connector, to the alt.

I had been discounting the alt, since you said you replaced it. But, it or the connections to it would be a great place to cause all your problems.

If you unplug the alt, and it does not blow the links, then the problem should be from the firewall connector to the alt.
Including the plugs and the alternator its self.

Its unlikely, but possible that the main wire on the alt could be the problem, or an internal problem.

That firewall connector could short out with the right circumstances, but the alt and its wires and plug are more likely.
 
Perhaps the altenator has grounded the brush inside,by melting that plastic insulator bushing...the one you remove when making one into a weldenator ,that makes it go full tilt and put out maximum amps and voltage..I assume this happens with every altenator you have tried??..that kinds shoots this theory down..:doah:...I have seen those junction blocks melt and ground out to the firewall before,but usually after a battery cable shorted to the exhaust..
 
One way is to dissconnect the wire at each end,and use a battery and test lamp to see if current passes thru it and lights up the bulb...................................................................................................If your checking a "hot" wire thats supposed to have 12V,with any switch that controls that curcuit "on",you should get 12V at the wire ,with the test light connected to the wire on one end,and the other side of the bulb grounded....however,if the wire has shorted to ground,it can still test "good" as far as delivering 12V--usually the wire heats up and melts,or pops a fuse or fusible link though, if the wire gets shorted to ground..
 
Other way is to get a multimeter that has the "buzzer" sound for continuity checking. At one end of the wire you put one probe to the wire, then to ground. If the wire is supposed to be a ground, the buzzer will sound. If the wire is NOT supposed to be a ground, and the buzzer sounds, it's shorted somewhere. If the wire checks out ok that way, then you move on to clipping one probe onto one end of the wire, and the other probe to the other end, and making sure the buzzer sounds. If it doesn't, the wire is broken internally.

Broken wires internally though are pretty rare, without either massive corrosion which is usually evident, or severe trauma to the wire, which is also normally evident. I always look very good at the connectors, and make sure the wire ends/terminals are locked into it good. Sometimes if the locks are bad when you plug a connector in, and the lock is bad, it will push the wire out just enough to not make contact, or make a poor contact.

Did this just suddenly crop up out of the blue, or was some re-wiring done to the truck first?
 
See that's the thing truck drives ****ing awesome and the all the sudden boom 18.3voltsand nothing has changed electricly.
 
ok guys well thanks to everybody that put any input on my problem but i fixed the problem. My super worn out ignition cylinder grounded out and blew the fuisble link by the booster and all my fusible links looked questionable so i replaced them with inline fuses because i couldnt at the time get ahold of any fusible links and the inline fuses couldnt handle the load so it keep blowin em which caused open loop to the alt. and made it charge at max. today i made a new wiring harness for the whole system with fusible links and replaced the ignition cylinder and everything is perfect so just to let you guys know the solution encase it ever happens to you or anybody you know....thanks again everybody.
 
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