ecu ground keeps blowing?

[97ebaygst]

i just fixed my ecu ground sensor wire and when i put it back in the car, the car was still running rich. Well I took the ecu back out and checked the ecu ground and sure enough it was blown again. My question is what sensors run too the sensor ground in the ecu. I think I have a sensor thats grounding put and frying the sensor ground in the ecu. I dont know which ones to check and i dont want to miss any either.

 

[steve]

You'll have to tell us what did you do just before this started happening?

99% of the time I find a blown sensor ground it was from the owner messing with the wiring or going cheap on a replacement O2 sensor and misconnecting the wires on a universal sensor to the plug.

 

[luv2rallye]

On a 2g (but not 1g) 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, and fuel tank diff pressure. As Steve points out a miswired O2 sensor can blow the ECU sensor ground foil. Definitely if the O2 heater ground is wired to it (only the o2 signal sensor ground can be wired there). The o2 heater ground must be wired to engine, chassis, or body ground since it has a large current. Another smaller possibility is if one of those sensors has shorted its signal ground to the +12V.

 

[97ebaygst]

any advice on what would be the best way to go around and check these sensors. I was just gonna follow the wires till i found a split in the rubber. would that be the best way?

any advice would be helpful

You'll have to tell us what did you do just before this started happening?

Well i actually bought the car like this. I know "why did you buy a car that already had problems?" I think it was a good deal and I wanted a car to tinker on as a project. I wont know if it was a good or bad deal until I get the car running in good shape.
The guy i bought it from said that he was driving it down the road, he punched it and it just all of a sudden started dumping fuel and billowing black smoke. He told me the reason he was selling it was cause he had something similar happen to one he owned before and it was a pain in the a** that he could never fix and ended up selling anyway.
So I'm kinda working a little blind, but i managed to track down the ground sensor wire being at fault when I had no clue what the problem was. I just gotta find out why it keeps blowing.

 

[steve]

That's the worst case scenario. You'll have to go through everything and maybe you'll get lucky and find it quickly or maybe not. It doesn't have to be any of the normal sensors causing it. A pinched or heat cracked harness could let the sensor ground come in contact with a power wire.

I guess I'd start by disconnected each sensor in list of those attached to the sensor ground and see which, if any, is causing the problem. You'll need to rig up your multimeter to measure the voltage on the sensor ground pin of the ECU harness connector and rig up a ground for the MPI relay pin to turn the power on with the ECU disconnected. I believe that's pin 38 you need to ground to turn on the relay. What's not emulated is the +5v power for the sensors going out from the ECU.

Usually when the trace vaporizes it's because it was shorted to B+. I don't think the ECU puts out enough current on the sensor power wire to melt the trace on the ground side but maybe it's possible.

If you don't find a sensor causing it or an obvious wiring error as your checking them you have to start inspecting the harness inch by inch.

I feel for you.

 

[97ebaygst]

That's the worst case scenario. You'll have to go through everything and maybe you'll get lucky and find it quickly or maybe not. It doesn't have to be any of the normal sensors causing it. A pinched or heat cracked harness could let the sensor ground come in contact with a power wire.

I guess I'd start by disconnected each sensor in list of those attached to the sensor ground and see which, if any, is causing the problem. You'll need to rig up your multimeter to measure the voltage on the sensor ground pin of the ECU harness connector and rig up a ground for the MPI relay pin to turn the power on with the ECU disconnected. I believe that's pin 38 you need to ground to turn on the relay. What's not emulated is the +5v power for the sensors going out from the ECU.

Usually when the trace vaporizes it's because it was shorted to B+. I don't think the ECU puts out enough current on the sensor power wire to melt the trace on the ground side but maybe it's possible.

If you don't find a sensor causing it or an obvious wiring error as your checking them you have to start inspecting the harness inch by inch.

I feel for you.

So am I setting my multimeter for voltage or ohms? Let me dumb it up to see if I understand how to test this. What your saying I have to do is connect one end of my multimeter to the 92 pin on my ecu harness and the other to the ground sensor wire on the pigtail of whatever sensor I'm testing? When I do that, exactly what am I looking for?
What does mpi stand for? There are so many abbreviations for things I cant really remember them all. And why would that relay have to be grounded?
sorry if these seem like dumb questions but I'm the kinda person that needs things laid out the first time before I learn.

 

[97ebaygst]

That's the worst case scenario. You'll have to go through everything and maybe you'll get lucky and find it quickly or maybe not. It doesn't have to be any of the normal sensors causing it. A pinched or heat cracked harness could let the sensor ground come in contact with a power wire.

I guess I'd start by disconnected each sensor in list of those attached to the sensor ground and see which, if any, is causing the problem. You'll need to rig up your multimeter to measure the voltage on the sensor ground pin of the ECU harness connector and rig up a ground for the MPI relay pin to turn the power on with the ECU disconnected. I believe that's pin 38 you need to ground to turn on the relay. What's not emulated is the +5v power for the sensors going out from the ECU.

Usually when the trace vaporizes it's because it was shorted to B+. I don't think the ECU puts out enough current on the sensor power wire to melt the trace on the ground side but maybe it's possible.

If you don't find a sensor causing it or an obvious wiring error as your checking them you have to start inspecting the harness inch by inch.

I feel for you.

can any one elaborate on the process that he is talking about here? like maybe a step by step?

 

[#40Fan]

Take it to a mechanic.

 

[97ebaygst]

Take it to a mechanic.

it's a project car for a reason. If ## aren't gonna help please don't post

this is my rear o2 sensor. the wires on the right were intertwined and the other two were wrapped up separate

 

[steve]

Well if it was hooked up wrong it could do it. For now make sure the wires still on the car are insulated from each other and repair the sensor ground.

Kinds of sounds like I said in the second post.

 

[97ebaygst]

i just had an idea on how to check for my problem and I wanted to run it by someone. What if i fixed the ecu ground senor and disconnected the sensors in question and connected them one by one until the wire on the ecu ground blows again. That should help me narrow down the problem right? I know it sounds like a newbie way of doing it and I'm blowing my ecu ground but I can't figure out any way of doing it. so does this sound like a good idea or what? I want your opinions weather this is a good idea or not.

 

[luv2rallye]

Ah, the old destructive testing technique . That's like removing one thing at a time on a running engine to see which will cause permanent damage . Before getting that drastic there's a couple better non-destructive things you can try.

The most common cause of a blown ECU sensor ground is wiring an O2 sensor heater wire to the sensor ground wire instead of to the chassis (body or engine) ground wire. The high current melts the ECU's sensor ground foil. For O2 wires identification: http://www.dsmtuners.com/forums/articles-electrical-wiring/244376-oxygen-sensor-wiring.html. So check this first before going on to harder tests.

If that's not the problem then check for shorts to +12V and ground .
First you need to check that the harness wire going into ECU pin 92 is not grounded anywhere else which it should not be (for 2g ECU pinout: 2G Turbo ECU Pinouts). If grounded (or low resistance) elsewhere, that indicates an improper sensor connection (or shorted sensor) which may cause the ECU's pin 92 to carry a high current which should not be there. Disconnect battery negative cable and then unplug the ECU. Now with meter set to resistance put the minus (or common) lead to ground (bare metal on body or frame) and the positive lead to the harness side connector's pin 92. You should read infinite. Now leaving the one lead on ECU pin 92, place the other lead on the battery + cable terminal (leaving + connected to the battery is ok since battery negative is disconnected) to check for a short to +12V. You should read infinite (or a very high value).

Now lets check for voltage on pin 92. With battery still disconnected, reconnect the ECU (remember to connect the ECU ground casing wire or bolt ECU to body/frame). Reconnect battery. With the sensor ground still blown inside the ECU (so ECU pin 92 no longer is connected to ground inside the ECU) and everything being connected: Set the meter to read volts and connect the minus lead to ground. Now connect the positive lead to ECU pin 92 either literally, if you have the ECU case off, OR if the case is still on, to it through the wire side of the harness connector where the wire connects to that connector pin (insert probe there). If you still can't get at it, you can pierce the wire's insulation with a sharp probe tip (or safety pin) to touch the wire. Now turn the key to ON but not Start (to turn on the MPI relay). If you read +12V that is the problem and you should disconnect each of those sensors (starting with rear O2) one at a time until the 12V goes away to find the culpret.

 

[steve]

Well, was it the rear O2 sensor?

 

[97ebaygst]

Sorry it took so long for me to get back but its been pretty busy here. ok so i checked the front o2 sensor and the last guy did a hack job and used duct tape to connected the wires. I fixed it using blue wire crimp connectors and wrapped electrical tape around it. I then proceeded to do the test that luv2rallye spelled out for me(thank u so much for that by the way, process was very easy). When I checked the resistance between pin 92 and the frame I got infinite until I switch the multimeter to 2000k and the reading was 998. That reading was made grounding it where the ecu mounts but when I went into the engine bay and grounded it I didn't read anything. I know u said that there shouldn't be any resistance but I didn't know if that small bit of resistance is ok.
When I did voltage check that read 5 volts. Is that normal? I would think so because its the 5 volt sensor ground. Well I'm hoping it was the front o2 sensor and not a grounding issue because I really don't want to go thru the whole harness. But for right now I'm gonna see if I can find whats causing the small bit of resistance since I don't know if that's a problem (hopefully not).

 

[luv2rallye]

998k ohms is ok. It's just going (or leaking) through one of the sensors to ground which should read very high resistance (if you unplugged every sensor that's connected to pin 92 you would read a true infinite). With auto engine circuitry any resistance reading that high is usually considered infinite since there's always a very small leakage through some sensor/device/capacitor.

Yes, 5 volts is what I'd expect you to see. I suspect the front O2 improper heater ground was your problem.

 

[97ebaygst]

998k ohms is ok. It's just going (or leaking) through one of the sensors to ground which should read very high resistance (if you unplugged every sensor that's connected to pin 92 you would read a true infinite). With auto engine circuitry any resistance reading that high is usually considered infinite since there's always a very small leakage through some sensor/device/capacitor.

Yes, 5 volts is what I'd expect you to see. I suspect the front O2 improper heater ground was your problem.

sweet now just gotta get the ecu repaired and hope that it doesn't blow again. wish me luck

 

[97ebaygst]

awesome i fixed the ecu with some very crude wiring and the car started right up and idled a little ruff but I think that is due to the plugs being dumped in gas . so I'm gonna change those out when I get a chance and hopefully all is well. took it out for a spin and it would cut off on me when I came to a stop but it would idle low if I gave it a little gas. But i guess I can say that this thread has officially been solved. Thanks everyone for they're help.

 

[luv2rallye]

You're welcome. Hopefully others will gain from this thread too. Improper O2 heater wiring is the #1 reason for ECU sensor ground foil blowing.

 

[Afroclipse]

This was very helpful thank you so much luv2rallye

 

[Tanro]

Do the 98-99 H8 Ecus have the same style sensor ground? I am trying to narrow down a problem and its either a Bad Sensor ground, or a Bad Mass airflow sensor.

 

[steve]

The 98/99 ECUs use the same pinout at the earlier ECUs so yes they have the same style sensor ground.

Where they blow up when you feed them 12v on the sensor ground is different but it shouldn't be difficult to check if you've damaged yours, open the ECU and follow the circuit trace from pin 92 on the ECU. Measure the resistance from it to ECU ground to see if it's open. I believe it's D22 in this picture. It looks like it might have a buried trace (inside the board) since I don't see anything at the end of the trace on the bottom.

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