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Old 01-13-2025, 03:19 PM #1
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Where does ECU get system voltage?

2004 v8 225k

Does anyone have a wiring diagram, picture or description of where the ECU gets the system voltage from?

OBD2 shows about 12.4-8V consistently on Torque and get the little battery dash light for over 2 months.

I have a cigarette lighter USB charger that constant reads 13.4-6v from both center console and cargo outlets. Also if I touch a voltmeter to positive terminal on battery and ground on the engine it reads 13+ volts.

New battery in the past year, haven't had to jump it in the past year and not in the past several months since this issue has been happening. No issues starting, driving or any other electrical related symptoms or lights. So I'm assuming alternator is working and keeping battery charged.

Figure there is corrosion or a bad connection somewhere. I've checked / changed, cleaned any cables I can see in the engine compartment, but I know some go into the fuse / ecu box our elsewhere maybe that I'm not tracing.

Any idea where to look next? What would cause inside cigarette lighters to read 13+v. Battery to stay charged and run for several months, and multimeter to read 13+v when touching red and ground but still have OBD2 and ECU reading in the 12s?

Thanks,
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Old 01-13-2025, 03:34 PM #2
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Quote:
Originally Posted by drallan View Post
2004 v8 225k

Does anyone have a wiring diagram, picture or description of where the ECU gets the system voltage from?

OBD2 shows about 12.4-8V consistently on Torque and get the little battery dash light for over 2 months.

I have a cigarette lighter USB charger that constant reads 13.4-6v from both center console and cargo outlets. Also if I touch a voltmeter to positive terminal on battery and ground on the engine it reads 13+ volts.

New battery in the past year, haven't had to jump it in the past year and not in the past several months since this issue has been happening. No issues starting, driving or any other electrical related symptoms or lights. So I'm assuming alternator is working and keeping battery charged.

Figure there is corrosion or a bad connection somewhere. I've checked / changed, cleaned any cables I can see in the engine compartment, but I know some go into the fuse / ecu box our elsewhere maybe that I'm not tracing.

Any idea where to look next? What would cause inside cigarette lighters to read 13+v. Battery to stay charged and run for several months, and multimeter to read 13+v when touching red and ground but still have OBD2 and ECU reading in the 12s?

Thanks,
If your battery is good, the voltage measured at the battery with the engine running should be 14+ volts, if you are only getting 13+ volts with the engine running then the alternator is going bad, could be just bad brushes or a regulator going bad or both.
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Old 01-13-2025, 04:16 PM #3
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Electrical Diagrams for 4th gen 4Runner

Relay locations
Electric wiring routing
Power Source
Engine Control for 1GR–FE
Engine Control for 2UZ–FE
Attached Images
File Type: pdf em03m.pdf (219.0 KB, 9 views)
File Type: pdf em03m0up.pdf (1.75 MB, 5 views)
File Type: pdf em03m-power-source.pdf (66.8 KB, 10 views)
File Type: pdf em03m5-Engine Control for 1GR–FE.pdf (131.5 KB, 1 views)
File Type: pdf em03m4-Engine-Control-for-2UZ–FE.pdf (145.8 KB, 1 views)

Last edited by 88Center; 01-13-2025 at 08:54 PM.
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Old 01-13-2025, 06:26 PM #4
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Awesome.

Thank you!



Quote:
Originally Posted by 88Center View Post
Electrical Diagrams for 4th gen 4Runner

Relay locations
Electric wiring routing
Power Source
Engine Control for 1GR–FE
Engine Control for 2UZ–FE
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Old 01-13-2025, 06:33 PM #5
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Quote:
Originally Posted by AuSeeker View Post
If your battery is good, the voltage measured at the battery with the engine running should be 14+ volts, if you are only getting 13+ volts with the engine running then the alternator is going bad, could be just bad brushes or a regulator going bad or both.

Understand, it probably is time for a new alternator, but in the mid 13s it is still working good enough that it shouldn't be throwing the "Battery" light, and doesn't explain the discrepancy I see at the cigarette lighter and battery itself from the OBDII readout and apparently what the ECU sees (i think it triggers somewhere under 13 or close to 12)

I could also be wrong in my assumptions. Just trying to understand what is going on as it starts strong and runs fine, even in the cold weather and has been this way for a while. Don't have a garage and would prefer putting off an alternator until the spring, especially if I'm not confident that would actually fix my problem.

Concern is there is a corroded or dirty wire somewhere that is impacting the reading where the ECU or whatever drives the dash light and OBDII data for Torque and a new alternator won't solve the problem. I'll be in the same position, just with a new alternator.

Looks like someone else posted some good diagrams. I'm not a EE, but I have stayed at a Holiday Inn and have a multimeter. Next chance I get I can hopefully take a more educated look at more likely culprits.

But I'm open to suggestions on where to start. I know electrical gremlins can be a PITA to track down.
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Old 01-13-2025, 07:45 PM #6
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Quote:
Originally Posted by 88Center View Post
Electrical Diagrams for 4th gen 4Runner

Relay locations
Electric wiring routing
Power Source
Engine Control for 1GR–FE
Engine Control for 2UZ–FE
Would you check the last 2 documents for Engine Control, neither seems to be working correctly at least for me?

The first 3 were fine, thanks
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Old 01-13-2025, 08:54 PM #7
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Quote:
Originally Posted by AuSeeker View Post
Would you check the last 2 documents for Engine Control, neither seems to be working correctly at least for me?

The first 3 were fine, thanks
Reuploaded.

Maybe because files names had blank spaces?

Last edited by 88Center; 01-13-2025 at 09:00 PM.
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Old 01-13-2025, 09:04 PM #8
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Quote:
Originally Posted by 88Center View Post
Reuploaded.

Maybe because files names had blank spaces?
Still no luck on my end for those last 2.
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Old 01-13-2025, 09:14 PM #9
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let's try this
Attached Images
File Type: pdf em03m5-Engine Control-for-1GR–FE.pdf (131.5 KB, 1 views)
File Type: pdf em03m5-Engine-Control-for-2UZ–FE.pdf (145.8 KB, 0 views)
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Old 01-13-2025, 09:35 PM #10
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Quote:
Originally Posted by 88Center View Post
let's try this
Still no luck, thanks for trying again.

This what I get, so maybe it's just this website...but had no issue with the first 3.

"This page isn’t working
Toyota 4Runner Forum - Largest 4Runner Forum - Powered by vBulletin redirected you too many times.
Try deleting your cookies.
ERR_TOO_MANY_REDIRECTS"
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Old 01-13-2025, 09:53 PM #11
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Quote:
Originally Posted by drallan View Post
Understand, it probably is time for a new alternator, but in the mid 13s it is still working good enough that it shouldn't be throwing the "Battery" light, and doesn't explain the discrepancy I see at the cigarette lighter and battery itself from the OBDII readout and apparently what the ECU sees (i think it triggers somewhere under 13 or close to 12)

I could also be wrong in my assumptions. Just trying to understand what is going on as it starts strong and runs fine, even in the cold weather and has been this way for a while. Don't have a garage and would prefer putting off an alternator until the spring, especially if I'm not confident that would actually fix my problem.

Concern is there is a corroded or dirty wire somewhere that is impacting the reading where the ECU or whatever drives the dash light and OBDII data for Torque and a new alternator won't solve the problem. I'll be in the same position, just with a new alternator.

Looks like someone else posted some good diagrams. I'm not a EE, but I have stayed at a Holiday Inn and have a multimeter. Next chance I get I can hopefully take a more educated look at more likely culprits.

But I'm open to suggestions on where to start. I know electrical gremlins can be a PITA to track down.
Yeah, don't start throwing parts at it. I think trying to diagnose almost any electrical problem without a diagnostic tool will be difficult unless it's something obvious. I suggest watching a video or two on youtube of "Pine Hollow Auto Diagnostics". Listen to what Ivan says and watch what he does, he is an auto electical troubleshooting expert.
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Old 01-14-2025, 12:49 AM #12
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Can someone else try the two PDFs above to see if they are working?

Did you try a diff browser?
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Old 01-14-2025, 12:56 AM #13
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The battery light at the dash is triggered by the alternator wiring, take a look at your voltage when the car is idle, in Parked, if it’s dropping it will trigger the light.
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Old 01-14-2025, 12:07 PM #14
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Quote:
Originally Posted by Dave4RCR View Post
The battery light at the dash is triggered by the alternator wiring, take a look at your voltage when the car is idle, in Parked, if it’s dropping it will trigger the light.
Thanks,

I'm just trying to figure out where to take that reading from. All of my readings have been during idle, but I'm not sure where to take the reading.

I thought the big red positive cable at the battery was connected to the alternator, so I've been touching some exposed wire on the positive cable (not just battery post) and a good ground on the engine and getting mid-high 13v. While driving the light never goes off.

I know the alternator should be around low 14s, so maybe the mid 13s is low enough to trip the light and the alternator is just failing, but strong enough to keep the battery charged for now.

Just wish I could find out where the OBDII got the voltage I'm seeing on Torque and why it differs from what I see when I put the volt meter on the battery.

Does the OBDII reading measure a regulated 12v (TBH I never checked until this occurred so it could have always been down). What do others see when checking voltage on Torque?


Does anyone know what voltage trips the "battery" light on the dash? Is it below 14? I could be wrong in my assumptions.
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Old 01-15-2025, 11:41 PM #15
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Quote:
Originally Posted by drallan View Post
Thanks,

I know the alternator should be around low 14s, so maybe the mid 13s is low enough to trip the light and the alternator is just failing, but strong enough to keep the battery charged for now.
I am pretty sure that auto parts stores can test your charging system free. Why not ask?
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