Alternator or PCM

[NCJL]

‘21 JTRD. So the dash battery light comes on. Pull up the off-road page. See 12….volts while running. Verified on the IDash as well. Check all battery connections. Looks good. I’m thinking alternator. Start driving home, 30 miles. We were in the mountains 4x4ing. Loosing voltage as we drive. I can verify the electronics work till 11.0v not 10.9v. This happened 6 miles from home. Pulled battery hitched a ride to part store. New battery just to get home. 11.9v reading, made it home. During the battery swap, parts store charged battery verifying the original battery was good. It was dead when I dropped off.

How does a person verify the alternator is bad or the PCM is not telling the alternator to charge?

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[rharr]

You pull the alternator and put it in a test rig and spin it?
Or you figure out which wire is the trigger wire to activate the alt and introduce a outside voltage to trigger the alt while running to read voltage at the battery with a multimeter?

You still running a Aux battery? Has that been tested?

 

[NCJL]

No Aux battery. Deleted that about a year ago. No problems till this.
When the battery light came on I disconnected all accessories from the electrical system, 2nd battery, winch, ARB compressor, switch pod, oil cooler fan, power steering cooler…etc. Trying to save “start” battery life.

I do have a second battery wired in parallel with the main battery. This battery is also still good. Although having this setup with the issue described has me wondering if I could have overloaded the alternator or maybe the PCM got smart and shut it off due to charging more battery capacity than factory.

Not using winch or any other hi amperage draw other than the ARB compressor when issue occurred. I leave the compressor on full time. It only runs for about a minute every 8 hours unless I’m airing up tires. I didn’t air up tires this day, saving battery life to get home.

The ARB compressor did “chirp” telling me the voltage was low. The brushless is a smart compressor. I think the chirp was before the dash battery light.

Everything is properly fused. No fuses blew. Was thinking maybe something shorted out to start the issue. No evidence of a short.

 

[NCJL]

You pull the alternator and put it in a test rig and spin it?
Or you figure out which wire is the trigger wire to activate the alt and introduce a outside voltage to trigger the alt while running to read voltage at the battery with a multimeter?

You still running a Aux battery? Has that been tested?

No aux. see below.
Yea, I was hoping to verify before removing the alt. Trying to figure out the trigger wire. Stop!

I just went and pulled the control wiring connector off the alternator. It only has one control wire. Must be the trigger. So verifying alt or PCM should be easy. Let ya know.

 

[rharr]

pull and check your z fuse for continuity between legs. You may need to disconnect your battery first to pull the z-fuse not sure if current will spike when battery is connected.

 

[NCJL]

pull and check your z fuse for continuity between legs. You may need to disconnect your battery first to pull the z-fuse not sure if current will spike when battery is connected.

Not feeling smart. By z-fuse you mean the fuse labeled “z” in the fuse box? What fuse is this for?

It will be tomorrow before I get to this.
Thanks

 

[DylanM]

See below for checking the alternator for function. If it passes the function test, check to see what fault codes are present.

 

[Lost1wing]

Fuse array is the group of terminals on the side of the fuse box. N7 is the alternator output on the 3.7l gas version. Should be the same on the 3.0l.

Start the engine and place your meter on N7 and ground.

 

[Lost1wing]

See above post, diagnostic strategies for testing.

 

[rharr]

Z-fuse is this guy... fuses for your fuses.......

 

[NCJL]

Reconnect battery, started right up with some under voltage relays clicking, leveled out. Voltage at battery before, after connection the same 12.5v . After engine start, 11.4v. Dash battery light comes on. Yes, I need to get a battery charger for this process.
Check IDash for codes, none.
Check across all N terminals. N7 is 1v lower than anything else. Others matched the voltage at battery.
Start to remove control wire from alternator. The engine idle RPMs change. A little playing around with the control wire connection I determine the Alt has an internal short. Seemed like I was engaging and disengaging the Alt with the movement. Couldn’t get it to stay engaged. Noticed what seemed to be excessive movement on the Alt side of the connection.
Checked voltage on the control wire. Constant 8.5v. Everything else still at 11.4v. Connector movement to verify no short, did not change voltage.
Not sure of the control parameters. Thinking the PCM sending out voltage on the control wire less than 13ish would tell the Alt to be on.
Thought about having the wife hold the leads on the N buss while I played with the Alt connection. Decided I saw enough, Alternator.

 

[J Sierra]

Disconnect both grounds
Measure from plus battery to alternator should be near 0 ohms
This is across N7 fuse

 

[Lost1wing]

You need to get access to that control wire at the alternator. Remove the connector, inspect and reinstall. It would be nice to check your voltage at n7 when you move that wire around and the rpms change. You may want to follow that same harness looking for critter damage.

 

[DylanM]

Checked voltage on the control wire. Constant 8.5v. Everything else still at 11.4v. Connector movement to verify no short, did not change voltage.
Not sure of the control parameters. Thinking the PCM sending out voltage on the control wire less than 13ish would tell the Alt to be on.

The wire between the PCM and alternator is a LIN bus communication wire, not an old school field wire. Reading the voltage from that wire doesn't tell you anything useful with a smart charging system.

To determine if the smart alternator is bad you need to check three things:
- Check for battery voltage present at the heavy charge wire lug at the alternator with engine off. If battery voltage isn't present, check for a bad N7 fuse or wire break.
- Check that the alternator case has good ground continuity to the engine.
- Start the engine then disconnect the communication wire from the alternator. With the communication wire disconnected, the alternator output at the charge wire lug should default to 13.5-14V at idle. If the output doesn't default to that, then the alternator is bad.

 

[smlobx]

Following because I don’t have a clue what you guys are talking about but hope to learn!

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