Where does ECU get system voltage?

D

drallan

New member
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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drallan said:
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,
Click to expand...

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.
 
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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Last edited:
AuSeeker said:
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.
Click to expand...


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.
 
88Center said:
Electrical Diagrams for 4th gen 4Runner

Relay locations
Electric wiring routing
Power Source
Engine Control for 1GR–FE
Engine Control for 2UZ–FE
Click to expand...

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
 
drallan said:
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.
Click to expand...

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.
 
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.
 
Dave4RCR said:
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.
Click to expand...

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.
 
drallan said:
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.
Click to expand...

I am pretty sure that auto parts stores can test your charging system free. Why not ask?
 
drallan said:
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.
Click to expand...

I read in a post that you can get the volt measure from the alternator by connecting the multimeter to the alternador post. It’s easily accessed you just need to remove the plastic cap.

Now. If you follow the Alt-S fuse in the engine bay, there are 3 wires that connects to a plug just below the alternator post. I believe the PIN4 is the one that goes to the computer.

See attached
 

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Thank you, that worked!
Had some super cold weather hear in Maryland and I don't have a garage so it has been slow going. Good news is runner has been working, got a different OBD reader (non-Bluetooth) and it is showing same voltage as I'm reading with the multi-meter, Mid-13s+. On top of that, tested my other Bluetooth adapter in a different vehicle and it was constantly showing a volt lower than expected, so for some reason that adapter is off. It was an inexpensive Amazon find and probably ran its course. So, good news is, I'm now showing consistent readings.

But, I'm still not sure why I'm getting the battery light as I feel 13.6+ should be plenty. Understand it might be a little low, but alternator isn't that old, and battery is less than a year old. I've cleaned every connection and ground I can find. Have disconnected, cleaned and reconnected the alternator plug and cable, as well as checked all the fuses. Only one I can't really get to is the starter ground for obvious reasons. Did even take it to the carwash and gave engine bay a decent light spray. I do have some minor valve cover leaks, but nothing has changed. Next step when I get a chance is to change out the battery terminals, they are snug, but pretty much maxed out and deformed.

Maybe when it warms up one of these days I'll do the big 3 upgrade (if I ever get this solved). Any write ups on that? I've found some for the 3rd Gen and other Gens, but not the 4th.

For those who have changed out battery cable terminals, what did you use / how did you do it? Not much slack, especially on the negative end. Afraid to cut / remove too much or even pry off old terminal and not have enough slack to attach new terminal end. Any recommended brands to go with others have used? A lot of the local auto part store ones don't look any better than what I have on there now.

Is there any way to just shim the battery post itself a bit to make the connection tighter? Just don't want to be mixing metals. WOuld be nice to isolate the problem before I start cutting cables and potentially make it worse.
 
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