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Old 04-03-2019, 12:40 PM   #1
snowball
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Alternator wire routing

Hi all,

I have a 90 burb R1500 350, vin k, 2wd.

Ok. the idiot light for the alternator keeps flashing. Looks like I have a broken wire.

What's the best way to fix this?

What's the routing for the wires (2 on the back of the alternator) . I see it go from the alternator, over the valve cover to the back of the engine.

Any way to hack the wiring, at least temporarily?
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Old 04-04-2019, 03:33 PM   #2
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Re: Alternator wire routing

This probably is related to your other thread about the voltmeter and may be the cause for the 9 volt reading you are seeing. Take voltage readings as I suggested in that thread and compare them. If you're seeing fluctuating readings compared with the flashing charge light, I would say that you are having charging problems and they are most likely caused by the wiring to the alternator.
You should have a CS style alternator and it has two wires in the plug into the alternator.
One will go to the S terminal and one will go to the L terminal. The S terminal wire should be large gauge than the L terminal wire. It is the sensing voltage wire and it should be connected to a battery junction wire with the alternator charging wire.
The L terminal wire is a smaller gauge wire and will come from the key on, ignition circuit. If it has poor connections or broken strands it will cause the charge light to flash.
The alternator may also be the cause of the light flashing if it has a bad voltage regulator or diode inside.
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Old 04-04-2019, 03:45 PM   #3
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Re: Alternator wire routing

Thanks Vettevet.

The Voltmenter problem has been going on since I bought the truck in 03. It's not registering even though I can read voltage when I put pull the VM and put my DMM probes right on the terminals. Therminals show power. 12+ off 14.5 when running. VM shows 9.

Lets put that on the back burner.

So if I wanted to run new wires for the L and the S, how are they routed and what are they connected to?

IS there any situation where L or S would have continuity with ground?
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Old 04-04-2019, 11:27 PM   #4
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Re: Alternator wire routing

The L wire runs from the dash cluster charging light through the firewall to the alternator plug. If you reroute it to a key on 12 volt hot source the alternator should charge but the light will stay on all the time unless you take it out of the circuit or take the bulb out of the holder. Keep in mind that the light is a resistance for the alternator exciter circuit and it also helps protect the regulator from over voltage. You might need to wire in a resistor in series with the new L wire to avoid this. After a certain year I think it was 94, the resistor was included inside the alternator. I will try to find my diagram for the 90 year and see if it shows the path of the charging light L wire.

Both of the wires are 12 volt positive and do not run directly to any ground.
If you have not determined that the L wire is bad, you should take the alternator and have it tested to be sure it does not have a bad regulator inside.
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Old 04-05-2019, 01:22 PM   #5
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Re: Alternator wire routing

I believe the alternators "L" terminal begins at the key switch. When on ACC, it provides B+ to the dash light. That bulb is a 194 peanut bulb, and when lite, it's resistance drop to about 460 ohms. This make is a current limiting device. To apply a full 12 volts dc to the "L" terminal, will blow out the regulator. The CS alternator used a "Delta" style stator or called "Y" connect. The single connection is ground when the alternator is off or not working. But let's say, it's just off. It provide ground to the dash light and with B+ coming from the key switch, the build is on. Once started, the alternator's "Y" terminal becomes hot. Now you have B+ from the alternator to the dash bulb & B+ coming from the key, then bulb goes out. The normal reason for the dash bulb to come on, either bright or dim, is usually because the Diode Trio inside the alternator has a bad diode in it. It sole job is to feed the regulator with voltage so it can use the "Sense" input to control the field magnetic strength by controlling the ground side at 400 hz. The stronger the field, the higher the output.
Diagnostics. Unplug the alternators connector and have a fused jumper wire and digital multi meter handy on volts DC. With the key-on / engine off, a check with the DMM will show B+ or battery available. So good.

The use of a 460 ohm 1/2 watt resistor used in series from the key switch will work the same way the dash bulb does now. On passenger cars, for years, they wired a 460 ohm resistor in parallel with the bulb. Bulb goes out, reduced current at B+ still allows the alternator to work.

With key-on / engine-off, if you have B+, then install a fused jumper wire to the lights wire. This will make the bulb come on steady. Good. You have tested the dash light circuit.

I believe you said the bulb was blinking, even thought I have never seen that, but if it passes the above test, the problem is in the alternator. Easy fix with front / rear bearing, new brushes and a diode trio.

The resistor inside the alternator that is removed during repairs, is a wire-wound resistive wire, around a ceramic core, but it's job is to sense temperature. When hotter outside, charge voltage is reduced. When cold, it increase the charge rate. Therefore, the readings during seasons will show between 13.2 - 14.2 volts DC.

Their should be no reason to suspect the voltage regulator, but is you want to test it. prepare the DMM, set to Volts DC, place on the battery. Look on the back case of the alternator for a case hold shaped like the letter "D". This window allows a pocket flat blade screwdriver to hit the voltage regulators ground tab, while grounding the screwdriver on the case to the alternator. This will full-field the alternator and with the engine rev'd up, the voltage at the battery will increase along with a whining alternator. It is at this point where a digital readout amp clamp will show within 10% of the alternators output. If rated at 80 amps, you'l commonly get 70 amps.

Rebuilding the alternator is something many do not do anymore, because boxes of alternators are found everywhere. They test each component (In Theory), clean everything, put new brushes & bearings, test it and call it a day. This is called "Spray & Pray"
Good luck.
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Old 04-16-2019, 05:35 PM   #6
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Re: Alternator wire routing

Thanks Lead foot. trying to figure some of your instructions. here.

Quote:
Originally Posted by LH Lead-Foot View Post

With key-on / engine-off, if you have B+, then install a fused jumper wire to the lights wire. This will make the bulb come on steady. Good. You have tested the dash light circuit.

The use of a 460 ohm 1/2 watt resistor used in series from the key switch will work the same way the dash bulb does now. On passenger cars, for years, they wired a 460 ohm resistor in parallel with the bulb. Bulb goes out, reduced current at B+ still allows the alternator to work.
I think you're saying to plug the resistor between the L and S connection on the wiring harness side for the alternator plug. Correct?

Quote:
Originally Posted by LH Lead-Foot View Post
I believe you said the bulb was blinking, even thought I have never seen that, but if it passes the above test, the problem is in the alternator. Easy fix with front / rear bearing, new brushes and a diode trio.
What I mean by blinking, is sometimes it's on. Sometimes it's off. If I tug on the wiring that connects to the alternator plug, I can get the light to go off and stay off, and get 14.4 V. but over time and with virbrations, the light comes back on, and voltage drops back down to B+


I pulled the alternator and brought it to my local parts shop. They tested it and it passed. I explained the situation, and since it had a lifetime warranty, they replaced it anyhow. Still have the same problem.
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Old 04-17-2019, 04:13 PM   #7
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Re: Alternator wire routing

Quote:
Originally Posted by snowball View Post
Thanks Lead foot. trying to figure some of your instructions. here.

I think you're saying to plug the resistor between the L and S connection on the wiring harness side for the alternator plug. Correct?

(Sorry I screwed up the quote thinngy / My Bad)
LEAD FOOT - The only charging system wiring I saved to memory was from the GM paper service manual. The IP has a battery light that should come on during "Bulb Check" operation with the key in ACC or ON, with all lights and some go off after time of the system check was complete.
(ABS, SIR, things you may not have)

On your system, it is a very simple system for wiring, but 2 types are used. I don't have the paper manual but most likely you have a BRN wire from they key, to the dash bulb, a simple 194 peanut bulb, but then passes thru the firewall connector and goes to the regulators terminal "L" for lights. This is a turn on signal getting voltage from the key on. During crank, the circuit is dead to reduce the weight of pulling the "ON" Alternator. Key spring to ON, alternator works, the "Y" connector stator produces voltage, now the dash bulb has voltage on both sides and have no path to ground, go out.

The 2nd design, is my explanation of the 460 ohm resistor is an additional path for voltage to get to the alternator if the bulb burns out. If there was a 460 Ohm resistor added, it's behind the IP on the printed circuit board. This a common a cars, but not trucks for some reason. (Note; The 194 peanut bulb when turned on, measures at about 460 ohms using voltage drop & DMM / Plus, resistors are not made is all sizes of resistance)

What I mean by blinking, is sometimes it's on. Sometimes it's off. If I tug on the wiring that connects to the alternator plug, I can get the light to go off and stay off, and get 14.4 V. but over time and with virbrations, the light comes back on, and voltage drops back down to B+

LEAD FOOT - To find the "Root Cause" of this problem, I would be following the wire and the larger harness it disappears into, visually inspect for any type of damage from rub-thru, burnt spot, previous repair and aftermarket stuff was installed, like theft deterrent system, then removed. (Sorry, I forgot the year of your vehicle)
But if it has a carb, the factory used a spice to run the dash bulb, but also the electric choke. This is done with a relay placed on top of the engine, but energizes the coil side of the relay. Doing so this way, the alternator signal tells the choke relay to turn on, allowing fused power to heat the choke only when the engine is running, but not using the BRN alternator wire alone to do this, but using a relay.
Using ohms law, 12 volts used in a relays coil with 90 ohms, only takes 0.133 amps. That's nothing. If you have a carb, it will have an electric choke. Now, where the relay is located, is something I can't tell you because I have no paper manual.
NOTE: I understand pulling on the alt. connector, but this action can spread the terminals inside the plastic connector. Now, these are easy to tighten up. For tools, a large paper clip, ground down pocket screwdriver to make it flatter. My favorite is old style wipe blades that have two spring steel runners that can be removed easy. Make a finger loop on one end, cut off at 4", going the sides to make it thinner. In the front of the connector, there should be a little square pocket on top of the larger hole you can see the terminal. Push the wire inward and insert the tool into the small hole, wiggle and it pushed down the lock tang. Pull the wire back and it should come out if the locking tab was pushed down. Try again until it works. When out, use the same tool but push up that large flat tang that make the contact. Carefully bend the lock tang on top, back up under 1/8th". When out, check for pin fit, it should slide on with a firm feeling and the same when removing. This is a very common procedure in the shop.
But, still have to check the BRN wire down to find a short to ground. When it shorts to ground, the bulb comes on. Are there any aftermarket things installed under the hood / now removed leaving witness marks. They often use a test light and find that power or ground they need, but poke into wire, they leave a hole, this does create a ground but an area for corrosion. Cover with a small dab of silicone. Ford has a "Wiggle Test" in their electrical diagnostic section. With the key off, there is no power or ground to the bulb with the connector removed from the alt. So, with a test light to B+, or DMM set to OMHs with buzzer on, it will allow you to move the harness with a little of any damage or loss of the root cause which is touching ground.
Yes, this un-wanted ground short, it turns the light on, but it can't be good on the regulator.

I pulled the alternator and brought it to my local parts shop. They tested it and it passed. I explained the situation, and since it had a lifetime warranty, they replaced it anyhow. Still have the same problem.
LEAD FOOT - It's good you had it tested, plus they gave you another. For the bulb to come on means the alternator side of the circuit to the bulb, has to short to ground. Having a loose terminal at the alternator will just create an open, no path to ground, bulb goes off. So that is correct.

If the bulb continues to go off, then on, with the new alt. you have to follow that wire thru the harness where it goes into the large firewall connector. If I was standing there, I would disconnect it, look for aftermarket stuff, see it you have a choke heater relay (Might be in firewall, while wires run over the hot intake where the EGR port is located). With the alt. end off, the firewall connector off, the BRN wire should be open from ground. This where I would wiggle the harness in place, every 2" or so, and if you have a buddy, they can tell you if there is a change. Some digital multi meters include a buzzer with the OHMS feature. So it will buzz when it detects ground, plus it has a clip.
You will find it.
Best of luck as I gave you a lot to absorb. Diagnosing electrical for some is like pulling teeth or would volunteer for a root canal operation.
PM if a clear explanation is needed. I will try anything to help.
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Old 05-02-2019, 11:12 AM   #8
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Re: Alternator wire routing

Quote:
Originally Posted by LH Lead-Foot View Post
you have to follow that wire thru the harness where it goes into the large firewall connector. If I was standing there, I would disconnect it, look for aftermarket stuff, see it you have a choke heater relay (Might be in firewall, while wires run over the hot intake where the EGR port is located). .
Thanks Lead Foot.

Just to confirm, key on, engine off, I'm not getting any voltage to the connector on any pin. If I giggle the wiring behind the engine block I sometimes get voltage.

This is on a 90 square body with TBI.

Where is the large firwall connectector? I see the wire go from the alternator, to over the valve cover and into the wiring behind the engine block. and then I loose it. I know there is a hole back there that connects to the ecm and another connector to the bulk head. What direction should I be looking? Should I be looking under the engine for that wire?

Thanks for all your help. Everyone. Thank you.
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Old 05-02-2019, 01:14 PM   #9
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Re: Alternator wire routing

Welcome to the shop / dealer tech world! Yeh.

The wire (Brown I think) at the alternator, key-on, engine-off, should have close to the same as battery voltage with a loss of voltage used to power the dash light. Wiggling the wire at the back, over the valve cover is your main clue. Although you're getting voltage, you can do the reverse and supply it with a ground jumper. Doing so, key-on, engine-off, the dash light should come on and stay on as the ground jumper is completing the circuit from the key switch, thru the dash light, to ground.
Now, if the light only comes on when wiggling the wire in the same spot or general area is the same results. So un-wrap the harness if needed, but find the BRN wire and follow it. It will most likely disappear into the firewall connector otherwise the "Black Hole of death", Just kidding! Since everything for an 1990 square body for service is on paper, having the "Factory GM Manual" will be helpful...but do not buy a Haynes or the other one, only GM will isolate the charging circuit, show harness views and show all of the connectors by number. By the 90's, GM was really good about info in the Electrical Service Manual.
But we don't have one. So to be helpful, I am 6'3" but on trucks, year ago, I would place a moving blanket or fender covers across the front fender without hit the radiator, and place a 1 X 12 about 3 -4 foot long, then crawl on top of it.
But the bulkhead connector should have a 7/16" bolt in the middle on the engine side, unscrew it, the carefully wiggle the connector, perhaps with some WD40 on it, but should come apart (Unless someone on this forum know different / these wonderful older trucks where not in my work bay often).

Remember, we are looking for the same brown wire, that goes to the alternator and failing the wiggle test over the valve cover pulls both ways, but most likely to the connector. The grease used from the factory on the connector side will be black most likely. But, we are looking for aftermarket wires, burn't-melted wires, cut-abraded wires or just a plan old loose connector at the bulkhead /firewall. You are on the right track. Remember, I believe their are grounds on the back of the drivers' side cylinder head, so a pinched wire, cover with PVC insulation maybe cut or grounded as well. Grounded will keep the light on so thats' not it. At this point, it is you, a good flashlight, WD40 to clean these 29 year old wires to find the brown one. Follow it both directions as pulling on it causes an intermittent should be visible.
Now, when I talk about "Pin Fit", is the male terminal fitting into the female terminal. Once you can hold it and see it, using another factory terminal you insert it and pull it out to feel if it has tension. How much? The same as 3 or 5 other terminals just like it. Each terminal has a tiny tab to use a small tool to flatten the tang, while holding the wire into the main plastic connector, will allow for the terminal to be removed. In the 90's, it should a 1/4" wide terminal with the tang in the middle on the top. So the shape of the connector with have a little window to flatten the tang for removal. Don't worry, they can usually be re-shaped, tang bent back up for re-locking, but in the center of the terminal will be the tension tab. From the wire side, a small tool can raise this tab to increase tension if it does not pass the "Pin Fit" test. The wire can have 18 copper wire threads, but if it has been brutally abused by someone else, it can have broken copper wires which causes resistance. You are looking for an open in this wire.
If both ends are out and visible, you can use you DMM set to resistance (Some have a Beeper option), but attach it to both ends, then pull apart with both hands every two or three inches of so. I have seen good insulation, but open wire touching inside, but open when pulling on it. Watch you meter or listen for the beeper to change or quit. There it is.
Having an intermittent is the worse problem to fix, but many vehicle are repaired using a new wire properly run from one end to the other.

You may choose to start a new "Thread" and ask for 1990, GMT400, alternator schematic as a member maybe able to scan a page from their book. Working on wires and the connectors are not too bad on the 1990 Trucks, but take some time to learn how to remove terminal from connectors.

I giving you a lot of things to do, so see what you can accomplish, because pulling on the harness over the valve cover and changing the reading is a good start.
Note; Plastic split wire loom cover can be purchased at home improvement centers and is usually cheaper that auto stores, but temperature rating, exposure are either good or crappy. If you have electrical tape covering the harness, if you have a fabric store near buy, go there and ask for a "SEAM RIPPER". These are cheap and the front is shaped like the letter "U". One tip has a plastic ball (It can come off), but the center is sharp. It is designed to place over thread sewn into fabric, they cut it and does the same on electrical tape as well. It will save time.
Best of luck. (See attachment)
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Old 05-02-2019, 02:50 PM   #10
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Re: Alternator wire routing

Leadfoot.

You sir are a saint!

Thank you. I'll have to re-read this several times but THANK YOU!
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Old 05-03-2019, 12:40 PM   #11
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Re: Alternator wire routing

Start a new thread in "Electrical" for "1990 Alt. wiring diagram needed" to see if a forum user has a factory book that show the alternator control circuit for your 1990 C2500(?) Engine size and firewall connector.

I hate to have you do more than you need to find your problem. That is every service tech goal is by visual, wiggle, testing with ground or power, then dis-robe some of the harness to put hands on the "BRN" wire and find the problem.

The lessons learned will last you a life time. I am watching the thread and don't be shy about some photos, but just make sure you have good light. The area behind the brake booster is dark and hard to get into, but this is traditionally where the bulkhead / firewall connector is located.

You can unbolt the 4 nuts at the back of the brake booster and slide it forward leaving master, lines everything connected. Now, you will have to remove the under dash clip on the brake pedals linkage. Rather than a cotter pin, they probably have a clip that is a folded type. The back side is "U" shaped and fits onto the foot pedal. The other half is a long clip that has a tab that clicks into place over the knob end of the brake pedals pin. Use a light that does not get hot under the dash, as the incandescent bulbs burn your face or arms....Don't ask me how I know.

Once upon a time, there where no LED or florescent type lamp for techs, but then house phone where I grew up was wall mounted oak wood.
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Old 05-03-2019, 01:44 PM   #12
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Re: Alternator wire routing

Are you referring to this diagram?

http://www.mediafire.com/file/odh0ta...ing_Manual.pdf

On page 328 it shows brown going to ESC Spark module to page 337 where brown connects to C309 pin 486.

Does that sound right? its definitely in the area of the wiring wiggle.
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Old 05-03-2019, 02:33 PM   #13
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Re: Alternator wire routing

I am so sorry but was unable to open or make it work. I am a "PC" guy and last failure, my son sent me a MAC AIR and have no idea how to use it.

Now, the (ESC) electronic spark control generates a PWM for the VCM to change spark timing depending on the signal for the knock sensor. If you have changes when pulling on the harness in this area, maybe un-related just happen to go together.
The color brown is used in more that one circuit, but most ESC modules I've seen are on the passengers side of engine.
I would feel better with the firewall connector removed, the correct wire located and then test with OHM's or supply fused jumper wire to B+ and attach a light on the other end, then go back and do the wiggle test.
Any open will cause the dash light to go off, while a temp short to ground will cause the light to come on. The voltage from the "Y" connect stator is ground when off, but B+ when working. We have to find which cavity the alternator light control wire (BRN) goes thru and test it from there.
Although, many aftermarket things are added under the dash.

From fuse B+ to key switch, on position B+ to IPC dash light, thru bulb to alternator. It does not get any simpler than that. Pull the dash and trace the wire both directions and put eye balls on it. Check and count the cavity where the wire passes thru the firewall connector. It can have 40+ terminals with some having two wire crimped onto the terminal. Make a drawing of it, then remember it is backwards on the engine side. Almost every GM uses the area around the brake booster, toward the fender to place the firewall connector and sometimes a gasket to keep out water where the two meet.

The nice thing about phones with cameras or point & shoots, is if you can get you hand in there, click the button, you got sometime to look at. The rub is that fact that many wires come out there, but having a schematic will indicate which cavity it is in.
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Old 05-03-2019, 02:59 PM   #14
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Re: Alternator wire routing

Try this link
http://download1350.mediafire.com/zr...ing_Manual.pdf

Yeah, I hate it when people mess with my computers.

Looking at the diagram a second time. it says they're using brown to connect to ground on the generator bracket.
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