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2G 2g ECU sensor ground pin 92 5v?

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Wilsonf4i

Wilsonf4i

Probationary Member
22
3
May 29, 2017
Ocala, Florida
Is pin 92 Sensor ground harness side of the 97 EPROM ECU supposed to get 5v with key on just depinned from the ECU?

I blew the ground in the ECU and had it repaired. I'm doing some troubleshooting to figure out what might've blown my sensor ground inside the ECU. I depinned 92 and I have it connected to my multimeter. with key on it's getting 5v unless I unplug the entire 71 - 92 plug & then it drops down to 0v.

I just need to know is it a 5v reference or is it supposed to be OL (infinit) or grounded .01 ohms if I put my other lead to a good known ground? I read 5v isn't enough to blow sensor ground?

I have multimeter on 92 harness side and I'm pulled these pires with everything connected to the ECU one at a time to see if the 5v goes away. This is the troubleshooting process I should be following?

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On a 2g ECU pin 92 (note: this pin is incorrectly labeled Ignition switch on some 2g ECU pinout diagrams) goes to the signal grounds of the following engine sensors: manifold diff pressure, engine coolant temp, front O2, rear O2, TPS, volume air flow (MAF), and on the 98 & 99 fuel tank diff pressure. So if one of these sensors (often an aftermarket O2 sensor harness) is wired wrong you could get +5v on that line when the ECU is unplugged.

Also: http://www.dsmlink.com/wiki/blownsensorground
 
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I ran out of time last night but, today I'm going to work on it some more. I was unplugging the wires on the same ecu harness connector for those sensors you mentioned above and checking to see if the 5v dropped. is this the best way to troubleshoot? I referred to that dsmlink wiki to discover this issue! and I've been reading your posts replies in the past they've been very helpful! Manifold Diff Pressure is fuel tank #73 I believe?
 
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Remember that if there isn't any current flowing there can't be any voltage drop so what you read from a floating pin is whatever potential voltage is connected to the other side. In this case the other side is the +5v sensor power, so seeing +5 is a reasonable voltage to see when the pin is lifted and the ECU is on.

A better test here might be to measure the current to see if there is a potential issue. 5v to ground as a short will heat up and melt circuit traces almost as well as 12v does. All depends on how much current is available at the source.
 
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steve said:
Remember that if there isn't any current flowing there can't be any voltage drop so what you read from a floating pin is whatever potential voltage is connected to the other side. In this case the other side is the +5v sensor power, so seeing +5 is a reasonable voltage to see when the pin is lifted and the ECU is on.

A better test here might be to measure the current to see if there is a potential issue. 5v to ground as a short will heat up and melt circuit traces almost as well as 12v does. All depends on how much current is available at the source.
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Thank you for taking the time to respond to me here Steve & i'm grateful to read your other messages on this subject in other posts I've read. So I guess I'm still confused about this. It should read 5V? You want me to measure how much current is going through that wire do you have an example how? I couldn't figure out how to verify the ecu ground is ok. it olms out on both sides of the solder job I paid to have done but, I have symptoms of the car's ecu signal ground being burn out again without any obvious visual notes on the ecu this time.
 
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ECU pin 92 should go to chassis ground only inside the ECU unless that circuit trace inside the ECU got melted because of too much current flowing (which it sounds you had). So it's supposed to be 0V but if the trace is melted (open circuit) then as Steve says, pin 92 will float to whatever voltage is at the other end of that wire (a sensor) when ECU is plugged in. You can test if it still goes to ground by unplugging all the ECU connectors and doing a continuity test of ECU pin 92 to it's chassis case. If infinite resistance that circuit trace (or if now replaced with a wire) is most likely melted again.

If melted, you must figure out why before powering on the ECU, or it will just melt again. Usually it is because someone wired a sensor wrong (often one of the O2 sensors, because the after market connector wires aren't wired properly - you can't trust the colors - after market uses any color they want for all of them - and/or someone wired the plug wrong themselves believing the colors were correct to match the harness which often they are not). This might then put a +5v (or +12v) source wire onto that pin 92 wire which would then be a direct short to ground on that trace and melt it.

I suggest if the ECU trace is still melted, leave it that way temporarily to find the +5V source (on pin 92 harness wire) by plugging the ECU back in, turning key to ON, verifying you have +5V on pin 92, and then start unplugging all the sensors I mentioned one at a time until that +5V goes away. The last sensor is the problem so examine it's wiring carefully.
 
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There are more automotive multimeter videos on youtube than you can shake a stick at but some are painful to watch and you might not find one that looks like your meter.

Here's how I measure current by first moving the meters probes to the Amperage terminals, then set the control to measure Amps (start high so you don't damage the meter) and lastly you disconnect the two point that you want to check the current at and put one probe on one and the other probe on the other so the meter becomes part of the circuit. So in this case you have unpinned the wire to pin 92, the sensor ground. One probe to pin 92 on the ECU and other other to the loose wire. You shouldn't read anything more than a couple hundred milliamps.

Like Gary I usually see people burning the sensor ground by miswiring the O2 sensor and connecting the +12v for the heater to the sensor ground but the MAF and CAS also have sensor ground and battery voltage on the same connectors.
 
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steve said:
There are more automotive multimeter videos on youtube than you can shake a stick at but some are painful to watch and you might not find one that looks like your meter.

Here's how I measure current by first moving the meters probes to the Amperage terminals, then set the control to measure Amps (start high so you don't damage the meter) and lastly you disconnect the two point that you want to check the current at and put one probe on one and the other probe on the other so the meter becomes part of the circuit. So in this case you have unpinned the wire to pin 92, the sensor ground. One probe to pin 92 on the ECU and other other to the loose wire. You shouldn't read anything more than a couple hundred milliamps.

Like Gary I usually see people burning the sensor ground by miswiring the O2 sensor and connecting the +12v for the heater to the sensor ground but the MAF and CAS also have sensor ground and battery voltage on the same connectors.
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Thank you for the response today. I'm going to check that sensor thoroughly tonight. And Steve's message I'll be doing that next. Hoping to see 5v go away unplugging nearly everything or some interesting results from inspecting that o2 or my wideband wiring. I unplugged front o2 connection & nearly all the wires in the blue ecu harness connector and nothing changed though besides pin 82. Pin 82 would drop the 5v but, that's used to turn on the ecm really i'm uncertain because it's labeled 1g.
 
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Wilsonf4i said:
I unplugged front o2 connection & nearly all the wires in the blue ecu harness connector and nothing changed though besides pin 82. Pin 82 would drop the 5v but, that's used to turn on the ecm really i'm uncertain because it's labeled 1g.
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It's labeled IG but it should be labeled IG1 since it is the input to the ECU from the Ignition Switch IG1 terminal and should be getting battery voltage when the switch is in the RUN or START position. As you mention and I point out in https://www.dsmtuners.com/threads/2g-basic-ecu-mfi-mpi-relay-circuit-function.497188/ the ECU is waiting for this pin to see 12v before it turns on the MPI/MFI Relay by grounding (pulling low) ECU pin 38.

If you are seeing +5v on pin 82 there is something else going on.

There are two postscript files I use regularly from ECMTuning to remind me of what is what on the 2G ECU. They're not perfect especially the pinout list but between the two 99% is correct. The 2GA has differences around the MPI relay since it's a single box vs the two relays on the later cars.

 
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steve said:
It's labeled IG but it should be labeled IG1 since it is the input to the ECU from the Ignition Switch IG1 terminal and should be getting battery voltage when the switch is in the RUN or START position. As you mention and I point out in https://www.dsmtuners.com/threads/2g-basic-ecu-mfi-mpi-relay-circuit-function.497188/ the ECU is waiting for this pin to see 12v before it turns on the MPI/MFI Relay by grounding (pulling low) ECU pin 38.

If you are seeing +5v on pin 82 there is something else going on.

There are two postscript files I use regularly from ECMTuning to remind me of what is what on the 2G ECU. They're not perfect especially the pinout list but between the two 99% is correct. The 2GA has differences around the MPI relay since it's a single box vs the two relays on the later cars.

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My apologies if I confused you. It's 92 is the pin I'm getting 5v on when IGN is on. I will read these and refer to them from now on thank you for sharing them. So basically my understanding is I'm not getting this 5v when I unplug 82 because ECU isn't getting the 12v signal to turn MPI relay/relays. (the main power source for a bunch of these sensors one of which is ausing my 5v at 92 ground signal wire.
 
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Yes your understanding is basically (at least logically) correct.

[Technically the ECU getting +12V on 82 causes it to pull down (to 0V) pin 38 which turns on the MFI relay which provides +12V to many, many things including ECU main power and certain sensors (eg. O2, etc). Some other sensors (manifold diff pressure, TPS, volume air flow (MAF), and on the 98 & 99 fuel tank diff pressure) then get +5V from ECU pin 81.]

So something is causing pin 92 to have +5V which it should not have (should be 0V).

BTW, pin 92 MUST be the only place those sensors are connected to ground (inside the ECU) - that's why it's called sensor ground. They should not be connected to ground at any other place (eg. not at the sensor itself) or you can get electrical noise, voltage drops, and feedback causing sensor signal problems.
 
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The way I read your earlier posts was you were seeing +5v on the wire that would go to pin 92 when it was unpinned (disconnected) from the ECU but with the ECU turned on. Is that correct or am I getting confused?

If you are seeing +5v when the wire to the pin is connected then there is a problem with the sensor ground again.
 
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Update: I unplugged both sensors in top of driver side fuel tank. I unplugged both wideband (wired to rear) and front nox sensors. I wired everything I believe is to do with that ground sensor circuit on ecu. I unplugged pin 81 because this is what sends 5v to the sensors right? No change still about 5v.
Coolant temp sensor (big one), Throttle sensor, just coil pack and translator. What am I missing to unplug?

I just thought I didn't unplug the gm AIT I have wired. Could this be it? I don't use a maf so that entire plug is just exposed.

Steve: 5V with it disconnected and probed with my multimeter. I was not back probing to see if it was 5v. I depinned 92 and then plugged the connector back in & ecu turned on with key ignition your first statement correct. I'm supposed to be testing 5v with it connected back probing & not depinned?
 
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Wilsonf4i said:
I unplugged pin 81 because this is what sends 5v to the sensors right? No change still about 5v.

5V with it disconnected and probed with my multimeter. I was not back probing to see if it was 5v. I depinned 92 and then plugged the connector back in & ecu turned on with key ignition your first statement correct. I'm supposed to be testing 5v with it connected back probing & not depinned?
Click to expand...

So I did get the right idea, you are seeing 5v on the black wire that would go to pin 92 with it disconnected from the ECU. What's interesting now is that you seem to be still seeing 5v on that wire even after you disconnect the Green/Yellow wire from pin 81 which is the source of the 5v sensor power. That shouldn't happen.

I'm off to Austin for week so I'm going to be offline.
 
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steve said:
So I did get the right idea, you are seeing 5v on the black wire that would go to pin 92 with it disconnected from the ECU. What's interesting now is that you seem to be still seeing 5v on that wire even after you disconnect the Green/Yellow wire from pin 81 which is the source of the 5v sensor power. That shouldn't happen.

I'm off to Austin for week so I'm going to be offline.
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Correct exactly. I double checked this. the corner green/yellow wire next to 92. Maybe I need to send the ecu in again. It was repaired for a seperate issue because the iac went bent 1y ago.
 
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steve said:
If you are seeing seeing 5v on the Black wire for pin 92 even after you disconnect the Green/Yellow wire from pin 81 there is a problem external to the ECU. Only the ECU should be powering the sensor power line.
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Do you happen to know if there's a harness drawing or schematic for this sensor ground wire I can start tracing? I'd find in the service manual probably? I looked around 4-20 but man these drawings are a little confusing I may have to study them some more i'll be refering to page six on how to read the schematics.
 
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luv2rallye said:
Pin 92 sensor ground wiring is shown here (one on right): https://www.dsmtuners.com/threads/2...grams-tcu-diagrams-ect.528552/#post-153762041. Remember this is a schematic which shows the LOGICAL layout (what connects together but NOT in the physical order shown).
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THANK YOU for sharing this. I also selfishly neglected to share that this car has had an remtoe start installed from the original owner that I had got working for a period of time before. I wonder if this is faulty and I should rip it all out underneath the steeringwheel.
 
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Here's everything I unplugged still 5v at the signal wire (detached at the ECM 92 wire). is there any places in the harness I can split and test for 5v to determine what direction I need to troubleshoot where the 5v short to power is coming from? the "wideband rewire" is old I haven't changed in years. This is unplugged. I went underneath that car and checked old rear o2 wires are separated not touching and capped off. A thought process I just had that I am going to try tonight. I am going to disconnect the battery. then olms check 92 to each and every 5v wire to each sensor to see maybe if there's continuity. Previously, I was just unplugged each and then turning the key to see if the 5v went away. Is G302 a terminator I should check there does anyone know exactly where and what this looks like?

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G302 is just a ground point for the shielding on the O2 sensors wires. It doesn't connect to the pin 92 wire.

Are you sure you had ALL 7 sensors [front O2, rear O2, TPS, IAT (part of VAF or MAF), fuel tank diff, MAP (or MDP), ECT] all unplugged at the same time when you say you still measured +5v on the wire that connects them all to pin 92 (but it's not connected to ECU pin 92 for this test)? And the ECU is completely plugged in (but pin 92 wire disconnected somehow) and the ignition key to on?

If so, you must have something else still connected to that wire. Re-check any of your aftermarket stuff or rewiring (like the wideband connector or original O2 connector the wideband replaces, fuel tank diff unplugged, etc).
 
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luv2rallye said:
G302 is just a ground point for the shielding on the O2 sensors wires. It doesn't connect to the pin 92 wire.

Are you sure you had ALL 7 sensors [front O2, rear O2, TPS, IAT (part of VAF or MAF), fuel tank diff, MAP (or MDP), ECT] all unplugged at the same time when you say you still measured +5v on the wire that connects them all to pin 92 (but it's not connected to ECU pin 92 for this test)? And the ECU is completely plugged in (but pin 92 wire disconnected somehow) and the ignition key to on?

If so, you must have something else still connected to that wire. Re-check any of your aftermarket stuff or rewiring (like the wideband connector or original O2 connector the wideband replaces, fuel tank diff unplugged, etc).
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Bad TPS. I believe it was intermittent because I had to retest somehow I missed it. I remember unplugging it before I must of missed something.

Unplug TPS 5v goes away. My multimeter bounces between low Mil volts and .5v
 
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luv2rallye said:
ECU pin 92 should go to chassis ground only inside the ECU unless that circuit trace inside the ECU got melted because of too much current flowing (which it sounds you had). So it's supposed to be 0V but if the trace is melted (open circuit) then as Steve says, pin 92 will float to whatever voltage is at the other end of that wire (a sensor) when ECU is plugged in. You can test if it still goes to ground by unplugging all the ECU connectors and doing a continuity test of ECU pin 92 to it's chassis case. If infinite resistance that circuit trace (or if now replaced with a wire) is most likely melted again.

If melted, you must figure out why before powering on the ECU, or it will just melt again. Usually it is because someone wired a sensor wrong (often one of the O2 sensors, because the after market connector wires aren't wired properly - you can't trust the colors - after market uses any color they want for all of them - and/or someone wired the plug wrong themselves believing the colors were correct to match the harness which often they are not). This might then put a +5v (or +12v) source wire onto that pin 92 wire which would then be a direct short to ground on that trace and melt it.

I suggest if the ECU trace is still melted, leave it that way temporarily to find the +5V source (on pin 92 harness wire) by plugging the ECU back in, turning key to ON, verifying you have +5V on pin 92, and then start unplugging all the sensors I mentioned one at a time until that +5V goes away. The last sensor is the problem so examine it's wiring carefully.
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Just thought I'd update. Brand NEW TPS is still getting 5v only when TPS is plugged in.

Also, 5v at 92 ground signal pin when coolant thermostat is plugged in.

New things I’ve done and need to disclose:

Completely removed & unplugged rear o2 sensor connector/o2 sensor there was severed and exposed.

Verified that with ecu disconnected, no continuity between 2 wires at thermostat and tps signal ground (2, 4) and power wires.

Luv2rallye’s comment “You can test if it still goes to ground by unplugging all the ECU connectors and doing a continuity test of ECU pin 92 to it's chassis case. If infinite resistance that circuit trace (or if now replaced with a wire) is most likely melted again.”

-I tested the ground again on ecu it tests good just past the new solder but from the signal ground ecu 92 to the ECU case/fastening screws I get zero continuity. So for sure at least the ground is blown again somewhere else based on your comment? I’m unable to specifically verify the next place to further test the ground on the ecu no obvious new burns.

I still haven't checked the amperage going to 92 5v. I haven't plugged everything back in to see if it'll start since doing all of this and added 2 grounding screws near the ecu connectors that hold the ecu board down. Perhaps I should buy a new coolant temp sensor to start with? car won't start with a potentially bad one anyway?
Just to clarify TPS and or coolant temp sensors being plugged in will cause 5v at pin 92 harness connector (depinned). Goes away unplugged BOTH.
 
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