Update, I think maybe it's has something to do with the field terminals on the alternator.

I had the alternator bench tested and it passed.

With the car off, the battery voltage is 12.3v

With the car on, the system voltage is 11.98v and alternator positive terminal post is 11.98v.

So then talking with a friend he told me about the "exciter" wire that enables the alternator. Looking through the FSM I think it's refered to as K20.

EDITED: The following is inaccurate. At that moment I was still figuring out how the system worked. PLEASE IGNORE THE FOLLOWING.

Here where I'm getting head spun. What I expect is one field terminal is a "signal" wire and should read system voltage to tell the alternator to pump out more juice. The other wire is the "Exciter" to enable the alternator.

Using a multimeter how can I test these field terminals?

Any help would be greatly appreiccaited!

 

Had time to look through the FSM throughly and found this wiring diagram.

Post C has system voltage when the car is running. Post B does not, so either the PCM is bad or there is a wire issue between PCM and alternator. Hopefully the FSM manual is correct that pin 20 on the PCM is connected to post B. Tomorrow I will probe pin 20 and see it there is voltage.

 

Just wanted to update the final resolution and document what I learned about the charging system and diagnosis process for those with YJ charging system issues.

The following applies to late model YJ, mine is a 94', there seems to have been significant changes over the years, so earlier models might be different.

Basic Operation, reference image in previous post:

1. Alternator needs to be regulated to not over charge. The alternator turns off and on frequently to regulate charge. Many alternators have built in voltage regulator, in the YJ voltage regulation is controlled by the PCM.

2. The two field terminals on the back of the alternator are used to regulate the alternator. One terminal is connected to system voltage. The other terminal is ground switched by the PCM. This means when the PCM decides to enable the alternator it grounds this 2nd terminal completing the circuit and energizing the field coil inside the alternator that engages the alternator. The PCM uses PWM to control the field coil.

3. PWM allows the PCM to control "how much charge" the alternator produces. To run the alternator at 50% the PCM runs a PWM at 50% duty cycle, this means the k20 terminal on the alternator will read around 6v. When the PCM wants to run the alternator 100% duty cycle the voltage will be 0v, when the alternator isn't running the voltage will be 12v.


With the basic operation knowledge here are the steps I used to diagnose the system so I don't have to throw parts at it.

1. With a multimeter, measure voltage at the battery posts, with car running should be above 14. If less then 14 you have a charge issue

2. Move negative probe to engine block, should remain the same. If not you have a ground issue.

3. With a multimeter measure voltage at the alternator posts, with car running should match measurment at battery. If not you have a high resistence in the ground or positive wires.

a. Measure voltage at the postive battery and postive alternator. Should be less .1 volt, if not you have high resistence in the postive cabless.
b. Measure voltage at the negative battery and negative alternator. Should be less .1 volt, if not you have ground issues.

3. To eleminate the PCM as an issue, using a LED test light, confirm the PCM is grounding the K20 terminal. Connect the test light to battery postive and the light probe on K20 terminal of the alternator. If the PCM is full grounded the light should be bright, if the PCM is regulating voltage should glow less bright. If the light does not glow, then the PCM or you have a wiring issue between the alternator and PCM.

4. If you've gotten here and everything checks out, it's likely you alternator.

Even through my alternator bench tested fine, it was bad. I don't know if with a field coil issue the bench test passes, but even through the PCM was fully grounding the K20 terminal the alterantor would not engage.

Hope this help the next Jeep'er!

 

Hello hope your still out there. I'm having the same charge issue. Grounds good. 3v on battery post + to alternator post+ this I thought was ok because of your step. Grounds are good. I checked continuity on the exciter wire up to the firewall. I read somewhere that the exciter wire has its own ohm rating. I was hoping you could debunk this. Can you please give me the location pin of the exciter at the PCM which is the same as the ECU? I'm trying to check continuity from the firewall to the end of the harness. I changed the PCM. No change.

Thanks for any help you can give. I'm a novice at this electrical.

 

What's a PWM? Is it a device or a technique programmed into the PCM?

Good Luck, L.M.

 

Pulse Width Modulation -- it can be built into the alternator or PCM.

John D

 

I thought about the high resistance in the positive cable from the battery to the starter but the jeep starts but doesn't charge. The battery has 700CCA

 

It's not the alternator or battery. They're both new and been checked. Do you know which pin the exciter wire is going to the PCM?

 

Welcome to the Forum @Chimmy

Velorider562 was last on the forum in mid June, this year. He may, or may not, be back.
Take your alternator in to a parts store that'll check your alternator for free. Call the parts store first.

Good Luck, L.M.

 

What engine do you have? A 6 cylinder should have a Delco (GM) alternator. They have an internal regulator and aren't connected to the PCM. I don't know which alternator a 4 cylinder should have, but I suspect that a 4 banger of the same year, should have the same alternator. Perhaps a member with an '87-'90 with a 4 cyl. will chime in.

Good Luck, L.M.

 

 

as noted above 91-95 use a PCM to control alternator but you cannot just apply how it works to a pre 91

 

3v difference between battery post and alternator positive isn't good. Means you have some high resistance between the battery and alternator positive. Check for corrosion near the battery positive wire terminal clamp and the positive alternator connection. Alternator housing to negative battery and alternator positive to battery positive should be 0v or millivolts.

I imagine a 3v drop to the battery would prevent the the battery from fully charging. Because the battery is only getting 11ish volt when Jeep is running.

 

I've got a 4.2 6 cyl manual. Can you please hand draw how the wires go into the starter solenoid? If two fusible links both go on one post? I'm think the burned green fusible link are supposed to go to one solenoid post together? Can you see if that's correct?

 

My '87 is wired different than your '90. I rewired it to accommodate a '95 starter (with a new flywheel) but I rewired it by function rather than color.
It looks like that wire corroded off rather than burnt off. I'd simply put a new ring terminal and reconnect it to the starter relay.

Good Luck, L.M.