3.8 volts brown wire at alternator ???

[1929fordtudor]

First post, as I have been gleaning from all the knowledge on this forum. My first c10 (69) as well so I am really benefiting from this forum. I made the swap, per the schematics found here, to a three wire 94 amp alternator. The voltage on the brown wire at the alternator, which I have read should be 12 volts key on, is only reading 3.8 volts key on. Any ideas why, and is this harmful?

[VetteVet]

Hello and welcome to the forums. You will get the best help on the internet here. There are very knowledgeable people on all aspects of the chevy GMC trucks.

I will post the wiring diagrams on the brown wire you are talking about. You may have already seen them, but just in case you haven't and for the benefit of anyone reading the thread here they are.

The brown wire for the external regulator which you are extending to the alternator, starts at the key switch where it shares a connection with a 12 gauge brown wire on the accessory terminal. It begins as a 10 ohm resistance wire which the alternator needs to "turn it on". It runs to the inside of the cab to the firewall block and there it connects with a 16 gauge brown wire which runs from the engine side of the firewall block to the external regulator to the
no. 4 terminal. This is the wire that should have 12 volts on it with the key on and no volts with the key off.

If you are only getting 3.3 volts then you may have a high resistance in the key switch or in the firewall block.
Check the voltage on the wiper fuse or the heater fuse in the fuse panel for 12 volts with the key on. This is where the 12 gauge brown wire goes that shares the accessory terminal on the key switch with the resistance wire.
If it has less than 12 volts, then the key switch may be bad internally. You would notice the low voltage on the wipers, heater, and also the turn signals if that were the case.

That leaves the firewall block as the possible culprit for a poor connection.

Here is the key switch diagram with the two wires I pointed out with the black arrows.





Here is the fuse box and the path of the resistance wire going through the firewall block. Also sowing the 12 gauge brown wire from the key switch going to the fuse panel.

This one shows the complete path of the resistance wire with the light cluster and the gauge cluster. It goes to the external voltage regulator. It is the one just below the larger brown wire.
You would just have to extend the brown wire from the firewall to the no.1 terminal on the alternator, and
the red wire to the no. 2 terminal on the alternator to convert the internal regulator part of the wiring. Then you have to run the red wire on the back of the alternator to the main junction of the truck wiring to complete the conversion. Some guys use the white and blue wires in the regulator plug to extend the brown and red ones but I like to use the method I the last post that I redrew.







Here is how I did mine. See how simple it is and it eliminates the external voltage regulator .







Do you have the charging light or the battery gauge dash?

[1929fordtudor]

Thank you for all the information! I have a few things to check now. I do have the gauge cluster not the idiot light. Looking at the diagram brings up another interesting fact. The PO did quite a bit of "electrical" work and left a bit of a mess. In your last diagram (which is the diagram I used from a prior post of yours) you show a black and a black/white wire with a fuse. These two wires have been cut and just hanging out of the harness. The ammeter gauge does not move and I'm wondering if this could be the reason why. Fuel gauge acts all screwy also even with new sending unit.

[VetteVet]

Originally Posted by 1929fordtudor

Thank you for all the information! I have a few things to check now. I do have the gauge cluster not the idiot light. Looking at the diagram brings up another interesting fact. The PO did quite a bit of "electrical" work and left a bit of a mess. In your last diagram (which is the diagram I used from a prior post of yours) you show a black and a black/white wire with a fuse. These two wires have been cut and just hanging out of the harness. The ammeter gauge does not move and I'm wondering if this could be the reason why. Fuel gauge acts all screwy also even with new sending unit.

I always shudder when I read that a PO has been cutting and hacking wires cause it really makes the fix on a problem difficult. That's why I almost always post how the original wiring came from the factory. It gives a base line for correcting the mess that gets made by chopping and cutting.

The two wires you asked about are indeed the wires for the ammeter. They run to the cluster plug to pins 1 and 12 and are both positive. The solid black wire is from the battery positive at the end of the fusible link if you have one.
The other is the black/white stripe one at the main junction where the alternator output wire goes. They sense the voltage differential between the battery and the alternator. They have a small 4 amp fuse in each wire to protect the ammeter gauge from high voltage in case the battery or alternator should surge.

Check the fuel tank sensing wire for good connection from the sender to the fuse panel FUEL terminal and from there to the gauge cluster plug.

Also check the tank for a good ground with the sending unit and the tank to cab ground. There's more to check but let's not get too far ahead of ourselves.

[1929fordtudor]

Ok, working on the truck today. So at the fuse block I have about 4 volts. At the switch I have 12 volts brown/white, brown. I trace both to the fuse block and At the fuse block on the brown/white wire I have about 4.5 volts. It appears the voltage drop is because of the brown/white wire. The larger gauge solid brown runs to the back of the fuse block, 12 volts, whereas the brown/white is the one that goes through the firewall, 4 volts. So the voltage drop makes sense but not sure is correct for my 12si 94amp alternator conversion. I'm a little confused.

[1929fordtudor]

I also replaced the ignition switch as I was suspect of it. I do have 12 volts on the wiper circuit. It just appears the voltage drop is because of the brown/white wire. They do not join at the back of the fuse block where it comes through the firewall like the diagram shows. They split at the ignition switch and never join up again. Brown goes to the fuse block, brown/white goes through the fire wall.

[VetteVet]

Originally Posted by 1929fordtudor

I also replaced the ignition switch as I was suspect of it. I do have 12 volts on the wiper circuit. It just appears the voltage drop is because of the brown/white wire. They do not join at the back of the fuse block where it comes through the firewall like the diagram shows. They split at the ignition switch and never join up again. Brown goes to the fuse block, brown/white goes through the fire wall.

Look at the middle diagram in post 2 and you can see the two wires at the key switch. the brown wire is 12 gauge and goes to the back of the fuse panel for the wipers , heater fan , and turn signals. You should have 12 volts on it on any of those fuses.

The tan/white stripe wire is the alternator exciter wire that goes to the alternator through the external voltage regulator and if it is converted to an internally regulated alternator then the wire will go to the no.1 terminal on the alternator. If you go to a CS alternator, it will go to the L terminal and you'll need to add a 85 ohm 5 watt resistor inline with the wire. you have to have 12 volts on the wire with the key on and zero with the key off.

When it goes through the firewall it becomes a 16 gauge brown wire. I can't see anywhere you could have a voltage drop unless there is a bad connection in the firewall block. Are you running it straight to the 12SI alternator to terminal 1 or are you going to the old external regulator?

[1929fordtudor]

I do have 12 volts on all those fuses. When I test the brown/white at the fuse block it is at 4 volts before I even get to the other side of the firewall. That brown/white wire is only a single strand which I assume is a resistance wire. That resistance wire runs from the ignition switch to the fuse block. At the switch it is 12 volts then 4 by the time you get to the fuse panel. I am running the brown wire when it comes through the firewall straight to the alternator. Why is there such a voltage drop from one end of the brown/white to the other (switch to fuse panel)?

[VetteVet]

I think you may have the two wires mixed up. The resistance wire (24 gauge B/W stripe goes to the firewall block and the 12 gauge brown wire goes to the back of the fuse block. Check the first two diagrams in post 2 and follow the wires with the black arrows. The two wires start at the key switch accessory terminal on the key switch, and go to the fuse panel (12 gauge brown), and the inside of the firewall block (brown with white stripe 24 gauge resistance wire). Check the third diagram for the original routing of the resistance wire and the wire for the charging light which the gauge dashes didn't have.

[1929fordtudor]

At the switch the brown and brown/white have 12 volts. I trace both wires to the fuse block. At the back of the fuse block the 12 gauge brown wire has 12 volts. I trace the brown/white to the firewall. At the firewall on the inside of the truck, before the connection, I only have 4 volts. When I trace the brown/white wire from the ignition switch, testing every several inches along, the voltage drops from 12 to 4 as I move closer to the firewall. It doesn't make much sense to me unless that brown/white wire being a resistance wire is causing the voltage drop and the distance from the ignition switch to firewall through that resistance wire is causing the drop. Do you have any suggestions to remedy this? I really do appreciate your help.

[VetteVet]

Are you checking the voltage drop with the brown wire disconnected at the alternator or where ever the end of the wire is. If it is plugged into the alternator, or any current flow ,then you will get a voltage drop anywhere along the wire.

The problem is you're getting way too much voltage drop at the end of the resistance wire.
12 - 3.8 means an 8.2 drop. Or the current draw is way more than it should be.

Measure the resistance on the B/W wire from the key switch with the alternator plug out and see what you have. I would expect 10 ohms which is what the factory rating is.
The charging light in the dash on the light clusters is rated at 10 ohms so the factory matched that with the resistance wire.

[1929fordtudor]

You nailed it! 10.9 ohms from switch to alternator unplugged. Yes I have been testing the voltage with the plug installed on the alternator. Ok, now we are getting somewhere! I have 12 volts on the brown wire at the alternator with the connector unplugged. So from the switch to the alternator I have 12.67 volts. So what could be causing the voltage drop when I plug the connector into the alternator? So let me guess, this is normal with the plug connected and I've been chasing rabbits?

[1929fordtudor]

On to the fuel gauge now. Gauge tests good and new sending unit by PO. There is no ground to the sending unit and there is no tab or pigtail on the sending unit. Where or how do I connect a ground to the sender if there is no pigtail or tab to connect to?

[1929fordtudor]

Ok, I attached a ground to the sending unit. There was a very wide tab that I trimmed down to accept a connector. Ran the ground to the frame rail. With key off the gauge is around 10 o'clock. With the key on it moves to just barely below empty. There is approximately 1/2 to 2\3 tank of fuel. Gauge checks out perfectly fine. Could it possibly be the PO purchased and installed the wrong sending unit?

[VetteVet]

Originally Posted by 1929fordtudor

You nailed it! 10.9 ohms from switch to alternator unplugged. Yes I have been testing the voltage with the plug installed on the alternator. Ok, now we are getting somewhere! I have 12 volts on the brown wire at the alternator with the connector unplugged. So from the switch to the alternator I have 12.67 volts. So what could be causing the voltage drop when I plug the connector into the alternator? So let me guess, this is normal with the plug connected and I've been chasing rabbits?

According to Ohm's Law a 10 ohm resistance on a 12 volt circuit would result in a current of 1.2 amps I = V/R or 12/10 equals 1.2
Since you are reading an 8 volt drop with a 10 ohm resistance You must have more resistance or more current flow. I would suspect more current flow which probably means a bad diode in the alternator. Are you positive that the brown wire is connected to the no. 1 and not the no. 2 terminal on the alternator?
Check the diodes by using the meter leads between the terminals and ground and then reversing the leads. You should get continuity one direction and not the other. Include the large output terminal in the test.

Originally Posted by 1929fordtudor

On to the fuel gauge now. Gauge tests good and new sending unit by PO. There is no ground to the sending unit and there is no tab or pigtail on the sending unit. Where or how do I connect a ground to the sender if there is no pigtail or tab to connect to?

The fuel sender is configured to ground to the fuel tank which should be grounded to the cab or frame. Then the sender terminal is connected to the fuel terminal in the fuse panel. If you ground the terminal on the sender you will get the empty reading you're seeing on the gauge. Un ground the terminal on the sender and connect it to the fuse panel instead of to ground and read the gauge. Your Ohm range is 0 to 90 Ohms. Take a reading between the terminal on the sender and the sender body (flange) looking for 45 ohms which would be close to 1/2 tank of gas.

Originally Posted by 1929fordtudor

Ok, I attached a ground to the sending unit. There was a very wide tab that I trimmed down to accept a connector. Ran the ground to the frame rail. With key off the gauge is around 10 o'clock. With the key on it moves to just barely below empty. There is approximately 1/2 to 2\3 tank of fuel. Gauge checks out perfectly fine. Could it possibly be the PO purchased and installed the wrong sending unit?

See Above check.

[1929fordtudor]

I will check the alternator tomorrow. Maybe I got a bad alternator. It is a rebuilt unit from the local auto parts store.
I have the tan wire from the post on the the sender connected to the fuse panel. I connected the ground wire to the tab on the sender flange. Get the same reading with the ground connected or disconnected. It doesn't seem to make a difference. With the key off the gauge needle sits at about 9 o'clock. With the key on it will move up to just below empty. This is with 1/2 tank of fuel.

[VetteVet]

The empty reading is an indicator that the sender wire is grounded. What do you get if you pull it off the FUEL terminal on the fuse panel? You should read between two and three o'clock. Reconnect the tan wire to the fuse panel and see if your read empty. Then disconnect it from the sender and look for the 2/3 o'clock reading. If it still reads empty then the sending unit tan wire is grounded. They like to chafe under the rocker sill panel and ground out.
If you show the 2/3 o'clock then the sender is bad or the float is stuck. Pull the sender out and check the float arm for free movement and then connect the ohm meter to the tan wire terminal on the sender and the sender flange. Move the arm up and down and watch the meter for travel between 0 and 90 ohms.

[1929fordtudor]

Finally got around to checking a few things. The wire checked out to the fuel tank. I pulled the brand new sending unit that the PO installed and the brass float had several cracks in it and was filled with fuel, therefore resulting in a continuous empty reading. Tested the sending unit and it is good. Ordered a new brass float today.
The alternator is another issue. Made sure all connections were correct. I believe after testing, that it must have a bad diode. It is a reman so chances are good I got a faulty one I suppose. Now to see if they will swap it out.
Thank you so much for your help. On to the next! Looking at disc brake conversion kits/dropped spindles, lowering springs and a 200-4r transmission.
Already got the VA unit installed, a must here in San Antonio!

[pilotXC]

[/QUOTE]Here is how I did mine. See how simple it is and it eliminates the external voltage regulator .




Attachment 1653986


Do you have the charging light or the battery gauge dash?[/QUOTE]

Apologize if this is a dumb question, but is this the correct way to wire the CS 130 alternator with an LS engine and harness (factory 72 body harness)? Mine has the L and F pins to the PCM and the screw on 8g wire to the battery and that is all. I've been asking around and get a variety of answers, just trying to find out if it's possible to keep my ammeter working with the swap and make sure I'm going to still have everything functioning properly. If the ammeter doesn't work, that is ok, but would like to have all the factory gauges function if I can.

[VetteVet]

The diagram you posted is my rendition of the way to wire an SI style alternator. The CS alternator can be wired independently of the computer as long as it doesn't require inputs from the alternator via the I and F pins. I have read that the I pin is an input for the tachometer and the F pin may be the voltage control for the alternator. I will see if I can verify that.

The CS alternator for the 350, sans computer is wired as below.






The pins may be marked different such as P L I S or what you have but the S and L wires are the ones to use. Looking at the diagram, You will want to wire the exciter wire from the key switch, through the firewall, to the L terminal, and insert a resistor in series as shown. The S terminal should be connected to the main power junction in the harness along with the alternator, power output wire. Certain years of the CS do not require this S voltage sensor wire, I think 94 and later. The resistor ohms value is variable. 85 ohms 5 watts is what Painless Wiring recommends, but 300 ohms .5 watts seems to work.

We recommend also that the power output wire be upgraded to at least a 10 gauge or better yet an 8 gauge to handle the increased power output of the alternator if you need additional amperage.

Do not run the power output wire directly to the battery positive terminal or you will by pass the ammeter wiring shunt and it will not work. I have covered this many times in the past.
Here is a link to one of the threads.

http://67-72chevytrucks.com/vboard/s...d.php?t=417872


page two starts the explanation on the battery gauge.

[pilotXC]

Thanks VetteVet for the reply and details. I used your diagram as it's very easy to read and helpful. I used your info a few years back when I first got my truck and had some other wiring issues and it helped tremendously, so refer back when needed... If you could verify the I and F pins that would be great. Mine has L and F pins, but seems like L and I are the same, depending on the alternator year?

I did an average to poor job trying to illustrate how the wiring would be done if I connected everything up with how I got my harness back from the person that modified it for my truck. I don't know if I need to include the brown wire with the 85 ohm 5 amp resistor somehow and if I should have the power wire from the alternator going to the terminal block on the driver side with a fusible link or something? I hope this makes sense and helps with my questions. Thanks again!!

[VetteVet]

Here is my rendition of a diagram that would best use the output of the CS alternator.
It shows the non use of the PCM to energize the alternator. Unless you have a
check engine light and other sensors I don't think you would have to worry about a malfunction in the computer or any other PCM functions. A factory tach would need an input for the PCM but it would probably work with a tach harness just as well.

If you wanted to use the PCM to activate the alternator, I would definitely use the resistor in the L wire terminal.

The two black wires with 4 amp fuses are needed to operate the battery gauge and the red shunt wire is needed to measure the voltage potential between the battery and the alternator junctions in order to show charge or discharge of the system. They are 18 gauge if I recall correctly.

The 16 gauge fusible link allows these connections to get voltage power from the battery and also protects the shunt circuit from shorts.

There has to be a junction inside the cab firewall to power the fuse panel, the key switch, the headlight switch, and the horn relay. The power wire for this normally comes from the soldered engine bay junction as shown in the diagrams in the earlier posts.

[VetteVet]

This is how I would wire the alternator without the main junction like the factory did.

[VetteVet]

Check out this video.



https://youtu.be/c4AV971yRvc

[pilotXC]

Thank you as always, this is great. Do I need the busbar to starter 8 gauge wire if I went that route? The only reason I had it on the diagram I did because that was the only route to the battery from the alternator without jumping to the terminal block in your second diagram. If I used your diagram, the busbar would route to the battery from the alternator correct?

Would I need to fuse the 8R wire to the terminal block on the fender well? I assume so, just want your input on what amp fuse I should use. Also, you think it's alright to splice the brown ignition wire into the L wire going to the PCM? I'm going to call the person that altered my harness and ask him on this as well...