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  #16 (permalink)  
Old 12-02-2010, 08:11 PM
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I would guess the brake lights are wired off of the wrong place and somehow backfeeding the acc terminal of the ignition switch. The brake light circuit is probably tied in in 2 places. The brake light feed is a direct feed from the fuse panel and should not come through the ignition switch at all. The brake lights should work with the key off so the feed for them is independent of the ignition switch. I would look for the remnants of a car starter or an aftermarket install of cruise control or something like that. trace all the wires that are attached to the BL switch and you will find the issue. If the directional switch was shorted it could cause this as it would feed the brake light voltage back through the directional circuit to the ignition.

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  #17 (permalink)  
Old 12-19-2010, 12:37 PM
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The problems' fixed, but this brings another question...

Well, it's been fixed, but not sure how...

I tore into the wiring, removing everything under the hood from all the wiring looms and laying all the wires out. There were a bunch of wires which had been cut and just stuffed back into the looms, and several which had been cut and spliced many times over. No idea what for, but they had.

So, I stripped out everything which didn't belong (including under the dash), replaced spliced wires with single solid wires, and hooked everything back up. There was still a dead short, but the problem with the brake lights was gone. Whatever, I'm not going to complain about good things, just because I don't understand how they happened!

Here's the question though! The large cable from the positive battery terminal runs straight to the terminal on the starter motor. But then, the way this truck WAS wired, a wire from that same terminal split, running both up to the junction block on the firewall, and to the RED side of the duel spade-type prong on the alternator. Not to the post connection on the back of the alternator, but to the red side of the spade plug. The post type connection ran straight to the junction block on the back of the firewall... so both wires were permanently hot.

Now, this truck really didn't match any wiring diagrams exactly, but some of the closer ones seemed to show that this wiring was correct. Several old hotrodders also said this was correct. However, every alternator shop I talked to said that the red spade plug was supposed to be run to a keyed power source, so that the alternator was only excited when the key was on. Honestly? I was confused...

The solution? For $15 this outstanding alternator shop I found installed a 1 wire kit which activated at 800rpm's instead of the normal 3500 or whatever. Dropped everything in, and it works great, the short is gone.

So the question is, was the diode in the alternator bad causing the short, or was it really wired incorrectly?

Or, is everyone else just as confused as I am?
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  #18 (permalink)  
Old 12-19-2010, 01:04 PM
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wiring

Quote:
Originally Posted by playswithfire
Well, it's been fixed, but not sure how...

I tore into the wiring, removing everything under the hood from all the wiring looms and laying all the wires out. There were a bunch of wires which had been cut and just stuffed back into the looms, and several which had been cut and spliced many times over. No idea what for, but they had.

So, I stripped out everything which didn't belong (including under the dash), replaced spliced wires with single solid wires, and hooked everything back up. There was still a dead short, but the problem with the brake lights was gone. Whatever, I'm not going to complain about good things, just because I don't understand how they happened!

Here's the question though! The large cable from the positive battery terminal runs straight to the terminal on the starter motor. But then, the way this truck WAS wired, a wire from that same terminal split, running both up to the junction block on the firewall, and to the RED side of the duel spade-type prong on the alternator. Not to the post connection on the back of the alternator, but to the red side of the spade plug. The post type connection ran straight to the junction block on the back of the firewall... so both wires were permanently hot.

Now, this truck really didn't match any wiring diagrams exactly, but some of the closer ones seemed to show that this wiring was correct. Several old hotrodders also said this was correct. However, every alternator shop I talked to said that the red spade plug was supposed to be run to a keyed power source, so that the alternator was only excited when the key was on. Honestly? I was confused...

The solution? For $15 this outstanding alternator shop I found installed a 1 wire kit which activated at 800rpm's instead of the normal 3500 or whatever. Dropped everything in, and it works great, the short is gone.

So the question is, was the diode in the alternator bad causing the short, or was it really wired incorrectly?

Or, is everyone else just as confused as I am?
The spade plug should have a wire running to the ING. SW I hook it to the ACC side then the other wire running to the stud on the ALT.looking at the back of the Alt. the left side to SW. the right one to stud. Both 14 wire. Off of the stud you run a 8 or 10 gage wire to the Batt terminal on the starter then you run the Pos. from the Batt.I would guess the diode was done for.


Bob
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  #19 (permalink)  
Old 12-26-2010, 10:49 AM
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I have a '75 GMC K25, so it should be the same wiring as your truck. It sounds like your truck was wired wrong.

It should be:
- Main battery cable runs to the starter post
- From that post there is a 10 gauge wires with fusible link. It runs up to the junction on the firewall and provides constant 12 volts power.
- If you have an ammeter gauge there will another wire connected to the starter terminal. This is for a shunt-style in-dash ammeter and it has nothing to do with the alternator wiring. In most trucks the ammeter no longer works.

- On the alternator
-- BAT post connected with 10 gauge wire to power junction on the firewall for constant 12 volts power.
-- Double spade connector (there should be numbers visible next to the plug)
-- Terminal 1 - White (aftermarket) or Brown (stock) wire runs back to the ignition switch, usually through a dash indicator light. My truck has gauges, so there is no indicator light. This is the wire that causes the alternator to turn on. If its constantly connected to power, it will drain your battery when the alternator is not running. In some cars that do not have an indicator light there is a diode or resistor to prevent power feedback from this terminal to the ignition.
-- Terminal 2 - Red (aftermarket) or white (stock color) wire runs back into the wire loom, and to the junction on the firewall. This is the alternator sense wire, and its designed to sense voltage drop and cause the alternator to charge more. It can be jumpered directly to the BAT post also, but they you are only sensing voltage drop at the alternator.

I think most of the confusion on your truck comes from the stock wiring colors vs. the aftermarket plugs. The two-wire OEM plug should have a brown wire to the ignition and a white wire to constant 12 volts, but most aftermarket wires use a plug with white (ignition) and red (12v) wires. Many installers match white-white and brown-red, not realizing they have wired the replacement plug backwards.

Bruce
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  #20 (permalink)  
Old 12-27-2010, 10:06 AM
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Just a note here; the brake switch has no power to it! It is a graounding switch and simply grounds the cicuit to make the brake light come on. So if your engine keeps running when the brakes are on you've got two issues. You have a power source that's not breaking to the ignition, and a bad ground that's being provided intermittently by the brake sw. when the pedal is pushed.
You need to check voltage at the coil when you turn the key off and see if it has 12v. to ground. Then figure out where that 12v. is coming from by pulling fuses until it goes away.
Second you need to ensure the engine is properly grounded to start. There should be a ground strap from engine to frame and engine to battery neg. side. Poor grounds can cause lots of false readings, and starting problems.
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  #21 (permalink)  
Old 12-27-2010, 10:33 AM
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Quote:
Originally Posted by 1971BB427
Just a note here; the brake switch has no power to it! It is a graounding switch and simply grounds the cicuit to make the brake light come on. So if your engine keeps running when the brakes are on you've got two issues. You have a power source that's not breaking to the ignition, and a bad ground that's being provided intermittently by the brake sw. when the pedal is pushed.
You need to check voltage at the coil when you turn the key off and see if it has 12v. to ground. Then figure out where that 12v. is coming from by pulling fuses until it goes away.
Second you need to ensure the engine is properly grounded to start. There should be a ground strap from engine to frame and engine to battery neg. side. Poor grounds can cause lots of false readings, and starting problems.
This is incorrect for this year truck. There is a direct feed from the fuse panel to on pin of the brake light swithch (orange wire) the white wire then comes off the switch and goes to teh directional switch and then is routed to the lights.

You theory may hold true on some of the newer vehicles but not on something this old.
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