Question on alternator wiring

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Question on alternator wiring

Postby 77 Spyder » Sun Mar 25, 2007 1:51 pm

I am converting a standard gauge Harness to a Gauge type. (seeing I was an idiot thinking there were the same & junking my correct one)

Anyway, On the standard gauge harness, there is the small brown wire that goes from the Gen light bulb to the alternator. In The gauge cars, this light bulb is not needed. Can I cap that wire off? I thought that the small brown wire sends voltage through the lamp filament, & turns the alternator on by suppling resistive voltage to it?

I know when the engine is not running & the key is on, This wire acts like a ground through the alternator to illuminate the light on the dash. but it is needed to fire up the alternator when it sees 12V.

So, where would it get wired to in the gauge style cars seeing there is no Gen light ? Is there some sort of resistance wire used?

Any ideas?

Carl
Last edited by 77 Spyder on Sun Mar 25, 2007 7:13 pm, edited 1 time in total.
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Postby shytebucket » Sun Mar 25, 2007 5:35 pm

The brown wire is also used to energize the field coil when you start the car so you don't want to cap it. I would guess the brown wire would connect to the gage's (+) terminal and then to a switched 12V source.

I know some folks have wiring diagrams. Perhaps the forum admins should have them scanned to the FAQ or made a sticky topic for future reference.
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Postby 77 Spyder » Sun Mar 25, 2007 6:01 pm

I was thinking there was some sort of back up resistance in that brown wire, so I could cap it. If its only the light filiment that provides the 12V to the alt, then if that bulb burns out, the alternator would quit charging. Is that the way our cars are set up?

I also found that there is a A/C compressor cutout relay that is wired in parallel with the temperature light. looks like if the car run hot, it shuts down the A/C. I would imagine this will not work with gauges. It seems there is a little more than just swapping wires in the harness.

Also seems like the cars without the guages have no temp light, or oil pressure light. I suppose you loose oil pressure & its shuts off the fuel pump, but man talk about no clue whats going on with your engine. :?
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Postby shytebucket » Sun Mar 25, 2007 7:06 pm

Once the alternator starts charging it supplies its own energy internally.

Also seems like the cars without the guages have no temp light, or oil pressure light. I suppose you loose oil pressure & its shuts off the fuel pump, but man talk about no clue whats going on with your engine.


Not true about the cars w/o gages. Mine originally had an idiot light for gen, temp-press, add coolant, and brake. If you can't find the proper harness just do your own wiring. Even on a car as 'factory correct' as yours will be you can do a nice custom wiring job and no one will ever know.
Last edited by shytebucket on Sun Mar 25, 2007 7:16 pm, edited 1 time in total.
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Postby vega_man_larry » Sun Mar 25, 2007 7:11 pm

Another thing you could do is add a light. When I did a GT conversion this is what I did as the light gets your attention and then you will read the gauge. I also added extra dash lights. Hopefully you got the harness with the gauge dash. Just add a couple of extra lights and wire them into the existing circuit.

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Brown wire?

Postby stage169 » Sun Mar 25, 2007 8:58 pm

Carl, looking at the typical H - colored wiring schematic the brown wire off the altenator is terminal #25 (20 guage). On the other side of the harness connector (interior) there are two wires off of terminal #25 one being brown with a white stripe. This brn/wht wire goes to the ignition switch at the ignition #3 terminal. The other brown wire goes to the volt meter by terminal 39 on the dash printed circuit board connector.

I think you are talking about the harness in the dash? Maybe this will help. I'm guessing the harness in the engine compartment doesn't have a difference for just the light or guage car. Then again maybe we can get you a schematic so you don't have to go with what I'm saying :lol: Brian
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Postby Len » Mon Mar 26, 2007 2:04 am

When you turn the ignition on power is fed through the gen. light from ignition and grounds through the alt. (brown wire) When the alt is running fast enough power is back fed from alt to gen light, when this happens you have two 12v power feeds at the light. one from ignition and one from alt. So there is no way for the power to ground (from either side of the light). This creates a temporary incomplete circuit turning out the light as long as the alt is working propely. Power is also fed to the alt from ignition through the small red wire to energise the field in the alt wich allows the alt to charge and send 12v to the gen light (brown wire) and to the batt. (large red wire). as to where to hook the brown wire on the volt gage without the wiring diagram for the set up youre installing I couldnt say.
I did the same conversion a few years back on a 80 monza I rember that It took quite awhile and that I had to re configure the wiring connector at the printed circiut behind the dash I think a proper wiring diagram for both set ups ( gauges & non guages) would make this quite simpler I also rember having to run a totaly new wire from the distributor to tach.
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Postby spencerforhire » Mon Mar 26, 2007 8:12 am

Personally, I think you should keep the idiot light, even with a gauge. But if you want to do without you'll have to put a diode in the brown wire so the voltage won't feed back into the ignition and prevent you from being able to shut off the engine with the key. Another possibility is to switch to a 1-wire alternator which is "self exciting" and will simplify your wiring.
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Postby 77 Spyder » Mon Mar 26, 2007 8:28 am

Isn't it odd that There isn't a ciruit in the event the bulb burns out? As far as I can tell, There isn't. So if your Gen light burns out, your not charging anymore. I haven't cut the harness open to verify this, but it appears to be that way so far.

There are quite a few changes that need to be made, & I have allready found a couple things that are not correct with the wiring diagram that was posted a while back.

So apperantly, its ok to send 12V direct (through a diode) to the alternator? I was thinking the bulb acted as a resistor in that circuit.

I will work on it some more tonight. Sure wish I could find a complete harness. I would imagine one will pop up right after I get this sorted out :?
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Postby stage169 » Mon Mar 26, 2007 10:10 am

Isn't it odd that There isn't a ciruit in the event the bulb burns out? As far as I can tell, There isn't. So if your Gen light burns out, your not charging anymore. I haven't cut the harness open to verify this, but it appears to be that way so far.


I think the brn/wht wire from the ignition switch completes the circuit. The light does not matter. Brian
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Postby Sirshredalot » Mon Mar 26, 2007 2:14 pm

Actually...if the bulb does burn out ...the system will not charge.
Working at auto parts store so long has taught me that...even on alternator test charts there is a warning that states that if certain dash lights are burned out you will have an incomplete circuit and wont charge.

You are correct. Ive had many cutomers bring in good alternators claiming that they arent charging....this is often the culprit.

Not 100% sure about the H-body but I would imagine it is so....but alot of the guys on this board have alot more exparience than I do.
Just my .02

God bless
-Shred
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Postby stage169 » Mon Mar 26, 2007 8:23 pm

You are probably past this but the charging deal with a bad indicator bulb has been bugging me. I found something that supports what Len says.

The dashboard indicator light circuit also typically has an extra wire with a calibrated resistance in it. This wire is run in parallel to the indicator light and has about a 10ohm resistance. It's purpose is to allow slightly more current to flow to the alternator field current system at initial start-up to make sure the alternator begins producing power as soon as the engine starts. About 1 amp total current is flowing to the field current between the light and the resistance wire, with the resistance wire supplying about 3/4 of an amp. This extra resistance wire does not affect the functionality of the indicator light in any way.

I couldn't find it on the schematic and I have a hard time thinking that in the late 70's your charging system relies on a indicator bulb but stranger things happen. I'm still crushed that spark plugs are not one way traffic. Some newer ignitions split the engine ignition from the old fashion way of coil to ground and on the other side ground to coil. :( Keep us posted. Brian
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Postby 77 Spyder » Mon Mar 26, 2007 9:03 pm

Thanks for your efforts guys!

Here is where I am at, & I am pretty much done with the conversion, unless you guys see something that doesn't seem correct:

relocated the dash connector pinout according to the diagram that is posted on another thread.

removed two extra ilumination wires that are no longer needed.

removed one ground wire that was no longer needed.

added two 12V power wires

removed the brown Generator wire & replaced it with a resistor wire that extends from the fuse box, to the ignition switch. This is the brown/wht wire Brian mentioned above.
This wire was not present, & I removed it from the hacked up harness that I wrecked a while back. This Brown/wht wire has about 12 Ohms resistance from ignition switch to the fuse box. It was connected with the orange accessory wire on the ignition switch. There was no back up wiring that I could find for this circuit. on the standard harness.

Added a tach wire

Added a temperature sender wire

everything else seems to be working correctly. I was also able to pass all the extra wires through the fuse block, just like it should be. there were a few extra wires on the engine side harness, & I added a few on the interior side. most of this was pretty easy because the harness is spread out on the floor, not in the car.
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Postby 77 Spyder » Mon Mar 26, 2007 9:16 pm

One other thing, I am assuming the funtion of the warning lights changed from 77-80 the 80 didn't have the low coolant circuit, & had a different wiring setup for the engine/pressure light.

That added to the confusion initially. I was using the gauge bezel from the 80 Spyder, & it has a different grouping of warning lights.
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