Electrical Gurus Needed- Not enough power to ecu

[VETTE_50_TH]

Ok. Ill start from the beginning. We took out the old harness and replaced it with a harness we removed all the unnecessary plugs from. We removed them directly and fully de-pinned the ecu plugs. We also rewired the fuel pump and relocated the battery to the back.

Now, we go to start the car because we finally finished the engine bay and we dont get any fuel. We find that I didnt rewire the ground correctly. Fix that. Nothing. We find that the car isnt sparking. We found a ground we missed and still nothing. We think it might be the mpi/fuel pump relays. Nada. All checks out.

Then we find that the ecu is only getting 2.7V at pin 12 and 25, but the rest of the car is getting the 12V from the battery, including the backup power for the ecu, fuel pump, etc.

This is also where it gets confusing. The car starts when we jump the relay and the ecu finally kicks on. We are jumping pin 38 which is the MPI relay control. We know this because we get the CEL like we are suppose to. Then we can start the car. Next, we try to connect evoscan to the car, but it says less than 5V, cannot connect. That data pin is pin 62.

Finally, we realize that all these pins we are having trouble with go directly to the chassis harness under the dash. What is up there that could be causing this.

The way the owner before i had fixed this is by using another relay between the two mpi and fuel relays. Thats how he was able to jump the car. Im guessing the car has a bad ground up there or something because the ecu is getting some power. Just not enough.

Please give me any thoughts or recommendations.

 

[1995gsx20g]

iirc on a 2g there's a major ground point behind the radio.

 

[VETTE_50_TH]

Yep, both are taken care of. It seems to be the chassis harness. The reasoning for this, is the blue plug that connects to the engine harness is where all of the problem wires go to. So im not sure if it is that or not. But we will find out.

 

[snowborder714]

I was just going to ask if you removed the blue plug that sits near the black plug (fuel pump test connector) on the firewall. If you remove the plug, you must connect the two wires that run to it or your car won't start.

 

[rstchris]

Look into the ecu power feed wire, there are lots of unfused power feeds that can do strange things when sensors internally short or when voltage drop occurs. Look for anything that could be loading off of the power feed to the ecu. Also try popping open the relays in question and look for carbon on the contacts and clean them.

 

[VETTE_50_TH]

I was just going to ask if you removed the blue plug that sits near the black plug (fuel pump test connector) on the firewall. If you remove the plug, you must connect the two wires that run to it or your car won't start.

I know what blue plug your talking about, thats still there. Im talking about the main hanress plus that goes to the chassis harness under the dash.

Look into the ecu power feed wire, there are lots of unfused power feeds that can do strange things when sensors internally short or when voltage drop occurs. Look for anything that could be loading off of the power feed to the ecu. Also try popping open the relays in question and look for carbon on the contacts and clean them.

Even if they test ok?

 

[slowboy101]

this is just a thought but I could be wrong but when you relocate the battery to the trunk you have to get a better alt. from a saturn ive heard of this problem similar to yours and he swaped out his alt. and it solved the problem

 

[VETTE_50_TH]

Nope. This is before the car turns on im not getting the right voltage. And i do have a new alt in it. A galant 90A

 

[slowboy101]

cool I thought I might just throw that out there

 

[DIRQUIN]

On the mfi relay are you getting 12v supplied to it? The relay gets its feed from a fusible link. #4. 20amp. Check to make sure its getting 12v. Check for any corrotion on the terminals too.

 

[VETTE_50_TH]

As in, before the relay, is 12V going to it. I believe we checked that we are. Im not positive. Ill check tomorrow. Also, the relays are all good, and look brand new.

 

[DIRQUIN]

If there is 12v to the relay and its not putting out 12 to the ecm then its bad. Pin 38 is the ground control of the mfi relay from the ecm. Is it turning the relay on? Also try disconecting all solinoids and the fiav. All these use the same power supply from the mfi relay. Try and pull out the mfi relay, jump the 3 & 1 pins and check the supply at the ecm.

 

[VETTE_50_TH]

The ecu works. We can bypass the mfi and the car starts. So..we know it works.

 

[92AWDHX40]

Hey james did you figure it out?

 

[VETTE_50_TH]

Hey james did you figure it out?

Nope. Not yet Vic.

 

[92AWDHX40]

Sounds like maybe a short somewhere, perhaps a loose wire, or bad ground?. I would still keep checking everything you could. I'm still working on my electrical issues. Good luck man.

 

[DIRQUIN]

Take some leads and put them in under the relay on the terminals. check if there is a ground coming from the ecm. It turns the relay on to provide power to it. Have you sub'ed in a known good relay in its place?

 

[VETTE_50_TH]

We tested all of them. I have 4. And we supplied power to it and tested to see that it had continuity. And it did.

 

[DIRQUIN]

How about the ground from ECM.

 

[VETTE_50_TH]

No i dont think so. I dont think we checked. I dont know where the ground is?? We checked all the power connections.

 

[DIRQUIN]

There are 4 pins on the relay. #1 is the 12v to the ecm pins 25 & 12. #2 is the ground feed from the ecm to the relay. #3 is the 12v supply for the relay switch to #1. #4 is the 12v supply to the relay to operate the switch. We know we have 12v to the relay on #3 & 4. You are getting a small amount to the ecm. You say the relay has continuity, But is it letting enough get through the contacts. The relay can have continuity but still not operate properly. Or the wire going from the relay to the ecm has corrotion build up, or is half way broken only letting a small bit through. Check the wire from relay to ecm. Do a continuity and ohm test. Also, is the ecm getting a good ground. Check pins 26 & 13 for a good ground to the body.

 

[VETTE_50_TH]

Yes, we redid the ground on the chassis. Here is the other problem though, is that the data cable for the obd2 port to reflash the ecu is not getting 5V. Thats why i suspect something in the harness on the dash.

 

[DIRQUIN]

Fuse 11 in the J/B in the engine compartment is the power supply for scan tool power. Is it blown?

 

[VETTE_50_TH]

Ill check all of this when i go over today. Thanks for the hlpe btw.

Actually. I know its getting some power because the light on the OPENPORT 2.0 cable comes on.

 

[DIRQUIN]

You may have a bad connection in the J/B.