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dustinkhen

Voltage to ECU, 3SGTE MR2 ST205

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Hello everyone, so i might be overlooking this but what's suppose to be the normal voltage to ecu for PIN 71 +14V??

My tuner told me that my the g4+ plugin was not seeing a good amount of voltage so he had made a jumper wire from the alternator positive post straight into the (B) Pin 71 thus ECU getting 14V, and had continued tuning my car like that. I eventually had to take that jumper wire off bc it would leave the ecu activated and drain the battery even after I would turn off the car bc it's straight powered from the battery.

Right now I only see ~12.8V to the ecu.

With car on i have 14V at the battery posts and alternator positive post to chassis ground.

Battery is in front of the car, positive cable runs through the whole length of the car into the trunk where the fuse box is located.

In picture provided,
     it's 13V at the 15A EFI fuse
White/Red Wire to (Batt) Pin 61: 12.8V
Black/Yellow to (+B) Pin 71: 12.8V

Here's a log that I've recorded, ECU voltage varies bc of loads like lights, signals, fan, etc..
Can someone give me some advice?

EngineControl.png

Enginecontrol2.png

Log 2019-12-15 4;37;23 pm.llg

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you want the ECU to be reading the same voltage your injectors and ignition system are receiving.  In older toyota's like that I've found tarnished/hot relay and fuse contacts can cause a voltage drop, along with corroded ground points from battery to chassis and engine to chassis etc.  Trace the circuits back one by one through their junction points in the chassis and you'll slowly identify the source of the drop

 

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On the sw20 mr2 chassis, ive had issues with the main power wire corroding where it meets the fuse/relay box in the back left corner of engine compartment.  I would start there.  this is where the power the the ecu is taken from.  Basically you just need to follow the power from the battery back to the ecu, checking voltage levels along the way.  I have a feeling you will find significant voltage drop at that last box.

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Checked my st205 voltage today. 13.2v ish ign on at the ecu. Cars not running but usually shows in the forteens running (cars not running at the mo trigger issues just cranks n trys).

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17 hours ago, Brad Burnett said:

On the sw20 mr2 chassis, ive had issues with the main power wire corroding where it meets the fuse/relay box in the back left corner of engine compartment.  I would start there.  this is where the power the the ecu is taken from.  Basically you just need to follow the power from the battery back to the ecu, checking voltage levels along the way.  I have a feeling you will find significant voltage drop at that last box.

I have my suspicions on that as well. 
main power from battery goes like this..

battery > am2 40A fuse > engine bay fuse box > 15A efi fuse . *provided picture shows the rest of the diagram*

I’ll check that circuit if I have a chance this weekend. 
 

14 hours ago, Chris Simms said:

Checked my st205 voltage today. 13.2v ish ign on at the ecu. Cars not running but usually shows in the forteens running (cars not running at the mo trigger issues just cranks n trys).

Thanks for checking your st205, so I am really having issues then. *facepalms*

i wasn’t having any problems at all with the factory ecu and all that. But I guess this is the headache I’ll get for wanting to go a bit faster lol

 

22 hours ago, JMP said:

you want the ECU to be reading the same voltage your injectors and ignition system are receiving.  In older toyota's like that I've found tarnished/hot relay and fuse contacts can cause a voltage drop, along with corroded ground points from battery to chassis and engine to chassis etc.  Trace the circuits back one by one through their junction points in the chassis and you'll slowly identify the source of the drop

 

I have back up Circuit Opening, EFI, and Ignition Relay just in case it was just old aging relays. However, the issue still persists.

What I have not done yet, it’s to check for corroded ground points from battery to chassis / engine ect., I was thinking this could be my problem.. but who knows

I’ll be checking this hopefully this weekend!

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the simple version of how to test this is what you are doing now - check voltage at a few points along the circuit and see when you see a significant drop. Remember to always use the same ground point because low voltage can also be caused by bad grounds.

The more accurate way (that rules out ground side issues too) is to load the circuit up (remove the ECU, connect a head light bulb (so around 4-5amp load, which is safe on that circuit) between the ground and 12v ign wires, then turn on the key. Now set your meter to voltage and measure *along* the wires - eg first measurement will be battery positive to the screw down point in the main fuse box where that battery/alternator line comes in. On a healthy wire you will see something like 0.1-0.2v (or the same negative amount if you have the meter backwards, but it doesnt matter). If you see 1v or so on your multimeter it means you lost 1v between those test points and should go looking for a bad contact. Then repeat for each individual wire/pair of available test points. ie you could test both in & out sides of the EFI relay if you lift up the fuse box and pop the bottom off. Also test the grounds using the same process - ive worked on an sw20 before that had ECU voltage issues when cranking because the ground straps to the engine had poor contacts.

You dont need the car running for this as the drop will be nearly the same at 12v vs 14v. 

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7 hours ago, dustinkhen said:

I have my suspicions on that as well. 
main power from battery goes like this..

battery > am2 40A fuse > engine bay fuse box > 15A efi fuse . *provided picture shows the rest of the diagram*

I’ll check that circuit if I have a chance this weekend. 
 

 

Seriously though, i have had a couple sw20s come through my hands that had the large power wire to the engine bay fuse box literally corroded through.

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Welcome back to my own thread.

So I've already tested and tried a new power circuit that I've built to power the ecu and i still get the same results.

battery > AM2 7.5 fuse > fuse box - EFI 15A fuse (Measured 13.5) >
Split 2 way:
     1) BATT pin on ecu / Pin 60 on Link Pnp (Measured ~13.5V)
     2)EFI Relay >
          Split 2 way:
               1) Circuit opening relay to power fuel pump + giving ECU its power source (Measured ~12.7V - ~12.9V average)
               2) Into separate connector to power up EBC, TVIS, IACV/ISC Valve, then lastly ending at the +B pin diagnostic port.

Every power source in the car with engine on:
Battery ~14V
Alternator ~14V

Tried out spare EFI & Circuit opening relay with same results.

Bypass fuel pump relay (Fuel pump circuit powered by Circuit Opening Relay):
     Measured about ~12.7V to ecu
 

Built brand new power source circuit and still the same results.
I'm at a lost here everyone.
Ground side of things everything is fine.

Edit: I've also added 2 Ground cables / wires.
1) Passenger side: Head to body
2) Driver side: Head to body

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You need to be very methodical about how you check for this.

1) Circuits have to be loaded - this can be either by having the engine running, or by connecting a test load to the circuit being tested. Engine running is simpler to set up but may be harder to get test leads to exposed wires.

Create a spreadsheet and then with the help of your wiring digram and/or tracing some wires yourself, write down a list (similar to that in your last post) of EVERY fuse, relay, connector, etc along the circuit in question.

Now with the engine running (or you test load connected) probe the circuit before & after each "thing" and write down the observed voltage. At some point you will see it drop, and then you will know it has to be the relay/wire/etc between the "good" voltage and the low voltage.

 

eg from your post above, i'd like to know the voltage as seen at both sides of the 15A EFI fuse, then the entry into the main relay, then the output, then the +b & +b1 pins on the ECU (ie the end of the wire coming out of the main relay).

Another note, the 7.5a AM2 fuse only feeds the ignition key, which in turn feeds the control side of the main & fuel relays. It does not feed the load side of those relays. That load side feed comes from the 40A am2 fuse. So following the logic above, i'd test either side of the 40A fuse, then then 15A EFI fuse, and both sides of the ignition main relay, etc. Best place to check voltage with things running is probably the bottom of the fuse box as you can get to the underside of the relay connectors.

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