Charging Problem

[AlexJordan22]

To make a long story short my 84 starts and runs great, has sat since the mid-90s before I got it. Has a new battery but never showed above 13V while running (revving or not).

I checked voltage at the battery terminals, from the back of the alternator and negative battery terminal, and ammeter behind the cluster and they all showed around 12.5V while running.

I was confident I needed an alternator. Removed the old and had it tested; it failed. Bought another one and put it in but am still showing similar voltage.

Before I pull the new one to test is there anything else I should be looking at?

[tgreese]

How are you measuring the voltage? I would confirm the measurements with my multimeter at the battery. 12.5 volts is not charging - it should be something like 13.5 or 14.

An '84 uses the Delco SI (systems integrated) alternator, with the regulator inside the alternator. Not many external connections.

[sierrablue]

If I understand the system right, it'll put out more voltage the more the battery is dead. So if it's sat a long time, and the battery is low, it'll tend to be more in the 14-15 volt range (on the older ones it's 15), and if the battery's up, it likes the lower 13s. Maybe that's just on computer controlled stuff but I'm pretty sure yours is supposed to do that too. If your battery isn't actually getting any lower I would just keep an eye on it and probably keep some jumper cables with you (which you should probably do anyway...).

[tgreese]

No, it does not work like that. The electrochemical potential of the lead-acid cells is 12.6 volts. The battery has an internal resistance which creates a voltage by Ohm's law when charging; voltage equal current times resistance. These voltages add. The voltage regulator is just that; it acts to regulate the charging voltage of the battery. That regulated voltage should not exceed about 14 volts, even if the battery is completely flat.

The charging current goes up and down in response to the load and the condition of the battery. The feedback is by voltage, acting on the internal resistance of the battery. The voltage regulator maintains that voltage at a constant value.

[tgreese]

To the OP - an '84 has an ammeter as I recall. What does it show?

The alternator has three connections. The big red wire is the charging current, and goes up to the dash, feeds the dash and lights, through the ammeter, and back to the starter solenoid via a yellow wire. This is the connection to the battery.

There is also a plug on the alternator with two wires, labeled 1 and 2. #1 is the "bootstrap" wire connecting to the ignition switch. It provides a trickle of current at startup that starts the alternator charging. It can also be used to run an ALT indicator light, but not on our Jeeps. #2 is the voltage sense wire. This connects to the battery and provides the feedback described above.

If #1 is not connected, the alternator may not start charging. The alternator uses its own output current to make more output current, and this has to be started somehow. Even if #1 is disconnected, the alternator can still charge if you rev the engine fast enough so the residual magnetism in the alternator core iron starts this self-generation.

#1 connects to the ignition switch through a resistance wire. If this wire is replaced with ordinary wire, the engine will not shut off with the key switch. Instead the alternator will provide power to the ignition via the wire and the engine will keep running. The resistance wire is enough of a barrier that the alternator will start, but backfeeding to the switch cannot keep the engine running.

[sierrablue]

No, it does not work like that. The electrochemical potential of the lead-acid cells is 12.6 volts. The battery has an internal resistance which creates a voltage by Ohm's law when charging; voltage equal current times resistance. These voltages add. The voltage regulator is just that; it acts to regulate the charging voltage of the battery. That regulated voltage should not exceed about 14 volts, even if the battery is completely flat.

The charging current goes up and down in response to the load and the condition of the battery. The feedback is by voltage, acting on the internal resistance of the battery. The voltage regulator maintains that voltage at a constant value.

Let me put things this way--I KNOW for a fact that that's how it works on my dad's Mustang. That may not be how it applies here, but that IS how that one works. Probably a computer thing.

Additionally, as per the factory service manual, the early ones with the Motorola alternator are supposed to put out 13-14.5 volts at idle and like 14-15.5 at 2500 rpms. Even on my gauge under the dash, which has power coming off the ignition switch, not the regulator, it comes in at like 15 volts when I'm cruising and don't have anything but the radio on. That's just how it was designed and it still works, as a stock setup.

I can't speak for the later ones so I'll shut up now. But here's the info on the earlier stuff if that helps any.

[tgreese]

You're right that what you observe on a modern car may or may not be relevant to these cars. Computer control was not a thing in 1984, and many systems on that Mustang will not work without central computer control. Just like EFI, you can make complex responses with digital lookup tables that were only possible in 1984 with many layers of mechanical control. Witness all the stuff needed to make a carburetor emissions compliant, for example.

What bothers you about this, that the system in practice is not as exact as my description, or my description of how it works?

The voltage regulator is basically an analog computer with a single input and output. Whatever the manual says about its operating voltage is correct. If you look at the circuit, it's controlled by a few transistors connected to a zener diode voltage reference. The knee of an inexpensive zener is broad, and it gives varying pass voltages in reverse depending on the applied voltage. You've also got a long ways to go to get a response, with a third-party battery of inexact construction as the main scaling element. Seems likely there's a tradeoff between precise regulation and cost, and if precision were not needed, cost could be lowered.

[sierrablue]

You're right that what you observe on a modern car may or may not be relevant to these cars. Computer control was not a thing in 1984, and many systems on that Mustang will not work without central computer control. Just like EFI, you can make complex responses with digital lookup tables that were only possible in 1984 with many layers of mechanical control. Witness all the stuff needed to make a carburetor emissions compliant, for example.

What bothers you about this, that the system in practice is not as exact as my description, or my description of how it works?

The voltage regulator is basically an analog computer with a single input and output. Whatever the manual says about its operating voltage is correct. If you look at the circuit, it's controlled by a few transistors connected to a zener diode voltage reference. The knee of an inexpensive zener is broad, and it gives varying pass voltages in reverse depending on the applied voltage. You've also got a long ways to go to get a response, with a third-party battery of inexact construction as the main scaling element. Seems likely there's a tradeoff between precise regulation and cost, and if precision were not needed, cost could be lowered.

But you said in your previous post that it "should not exceed 14 volts." Either it's an imprecise system that's going to sit wherever, and as long as it's charging the battery w/o blowing it up, it's probably ok, or it needs to be around a certain number. You're claiming both sides of an either or and I don't understand. I agree with your second statement that it's imprecise but not your first that it HAS to be at or below 14 volts.

[tgreese]

Well, there you got me.

[will e]

I fought charging issues for a while on my 83 cherokee. I was planning on a CS140 upgrade but wanted to make sure the current system was working. Mine was a bit intermittent. Sometimes 14.2 volts, sometimes whatever the battery was at. Sometimes a bit in between.

While troubleshooting I finally discovered the 'exciter' wire, which was a single strand resistor wire in my jeep, had cracked and 'broken' due to age. I guess sometimes it wold complete the connection and other times it would not.

So, when I did the cs140 upgrade I purchased a connector with the resistor and bypassed the original resistor wire. My issues are all gone.

You can test yours by removing the plug and checking for voltage at the connector (not the alternator) when the ignition is turned to the 'on' position. The engine does not need to be running.

[AlexJordan22]

I figured out my issue here and I really appreciate all the help! It turned out to be a simple solution luckily.

My excitor wire (small brown wire going into the side of the alternator along with a small red wire) wasnt making good contact to the pin. Once I unplugged that and plugged it back into the alternator nice and tight I was charging at 14+ Volts. Amazing what a working alternator can do - my windows work better, my lights are brighter, etc. For anyone reading this who had a no charge condition and replaced the alternator and it still won't charge, double check that small pigtail on the side of the alternator and make sure its making a good connection.

[AlexJordan22]

I fought charging issues for a while on my 83 cherokee. I was planning on a CS140 upgrade but wanted to make sure the current system was working. Mine was a bit intermittent. Sometimes 14.2 volts, sometimes whatever the battery was at. Sometimes a bit in between.

While troubleshooting I finally discovered the 'exciter' wire, which was a single strand resistor wire in my jeep, had cracked and 'broken' due to age. I guess sometimes it wold complete the connection and other times it would not.

So, when I did the cs140 upgrade I purchased a connector with the resistor and bypassed the original resistor wire. My issues are all gone.

You can test yours by removing the plug and checking for voltage at the connector (not the alternator) when the ignition is turned to the 'on' position. The engine does not need to be running.

Awesome info, do you by chance have a link to the part you bought to remedy this?

[will e]

I fought charging issues for a while on my 83 cherokee. I was planning on a CS140 upgrade but wanted to make sure the current system was working. Mine was a bit intermittent. Sometimes 14.2 volts, sometimes whatever the battery was at. Sometimes a bit in between.

While troubleshooting I finally discovered the 'exciter' wire, which was a single strand resistor wire in my jeep, had cracked and 'broken' due to age. I guess sometimes it wold complete the connection and other times it would not.

So, when I did the cs140 upgrade I purchased a connector with the resistor and bypassed the original resistor wire. My issues are all gone.

You can test yours by removing the plug and checking for voltage at the connector (not the alternator) when the ignition is turned to the 'on' position. The engine does not need to be running.

Awesome info, do you by chance have a link to the part you bought to remedy this?

Sure:

https://www.amazon.com/ACDelco-PT2145-P ... 333&sr=1-1


ACDelco Professional PT2145 Alternator Conversion Jumper Harness

Because the original resistor wire was toast, I spliced the original connection to a full voltage circuit.

[tgreese]

You can fix the resistor wire if it's broken. It's a solid nichrome wire, like the heating element of a toaster. You cannot solder to it, but you can crimp to it - that's all the factory connection is. The solid wire is covered by a loose insulating sleeve, sometimes called spaghetti tubing. Push or trim it back and crimp to the solid wire.

Non-insulated connectors like this work well https://www.amazon.com/dp/B072V27QN4 ... join to the existing plain copper stranded wire and cover with adhesive lined heat shrink.

You can also use a diode pointing toward the alternator, connected with plain copper wire. All that's needed is something to block backfeeding into the ignition switch. An incandescent light bulb works too.

[J20Hunter1]

tgreese I am getting 15 volts on the Red wire that used to go to ingnition box/that I hooked Hei power to,But I only have13.5 volts @ idle on yellow wire feeding battery. I have had a problem with randon dying and I am thinking maybe getting modual in distributor hot. Any Idea why I have 15 volts at that wire Thanks for your help

[tgreese]

Year/model/equipment? It's important. Presuming it's a J20, so it's '74 or newer. Domestic J20s are V8s.

Yellow wire to me suggest you have an ammeter. You need to tell me the year and model.

Suggest you start a new thread with your question rather than hijacking this one. No need to address it to me only - better to have the whole forum participate.

[AlexJordan22]

My charging problem cropped up again. This time while it was running I verified my excitor wire is making good contact. If I unplug it it the battery shows ~11.5V, and plugged in it jumps to 13.6 and then immediately goes to 12.5 all while running. So I know the excitor wire is working because it goes from 11 volts to 13 right when I plug it in and then immediately down to 12.5. Alternator is new from Napa and appears to be working since I do see it go up to 13.6 momentarily.

It's almost like it wants to send 13.6 to the battery but maybe the regulator kicks it down to 12.5 for some reason. Not sure if it's the alternator or something else. Any ideas before I pull this alternator and have it tested?

[tgreese]

The regulator controls the voltage and has transistors and a zener voltage reference in it. The alternator is just a bunch of wire and steel plus some silicon diodes.

The regulator regulates in response to the sense wire; that's other wire on the plug with the exciter. Normally that's connected to the charge wire, which is always at battery voltage. The regulator feeds back current to the windings in the alternator to make current. The alternator makes enough current so the battery voltage is always the set point, like 13.5-14 volts. This compensates for the variable load, and for any drained battery potential from starting.

No "wants" or "appears" about it - don't imagine how it might work. A new alternator could be bad, and the regulator is part of it. Napa parts are usually pretty good.

[AlexJordan22]

The regulator controls the voltage and has transistors and a zener voltage reference in it. The alternator is just a bunch of wire and steel plus some silicon diodes.

The regulator regulates in response to the sense wire; that's other wire on the plug with the exciter. Normally that's connected to the charge wire, which is always at battery voltage. The regulator feeds back current to the windings in the alternator to make current. The alternator makes enough current so the battery voltage is always the set point, like 13.5-14 volts. This compensates for the variable load, and for any drained battery potential from starting.

No "wants" or "appears" about it - don't imagine how it might work. A new alternator could be bad, and the regulator is part of it. Napa parts are usually pretty good.

Alternator was installed about 4 months ago and has worked well up until a couple days ago. You're clearly really knowledgeable about the charging system, does this sound like an internal problem with the alternator? I'm assuming the alternator is internally regulated but could be wrong.

[tgreese]

Yes, the regulator is inside the alternator. These are "SI" alternators: Systems Integrated.

You need three connections - the battery, the sense wire and the exciter wire. If you have the alternator connected right, there is nothing much else you can do. While running, it should maintain the voltage across the battery at something close to 14 volts. This is the electrochemical potential of the lead-acid cells when fully charged of 12.6 volts, plus the voltage across the internal resistance of the battery due to the charging current. 12 volts is too cold (little or no charge current) and 16 volts is too hot (too much charge current, "boiling" the battery).

The TSM has good coverage of the charging system, both theory of operation and diagnosis/repair. Does not have to be the expensive '84 TSM; any of the '80s era books would be fine for this.

TSM.png