ECU not getting power, HELP!

[MunkyX69]

So my ECU isn't turning on. I check the original MPI relay and found that between pins 10 and 8 there was no continuity. Bench tested it and it wasn't flipping the switch. I have since replaced the MPI relay. Now the ECU isn't getting power. New MPI relay bench tested fine.

With the new MPI relay pluged in, I'm not getting power to pin 4 or 5 at the relay. If I've read correctly, the ECU switches this part of the relay. I'm getting power to pin 10 and power out of pin 8. Were else does ECU get power from? When I turn the key to key on engine start, the fuel pump kicks on and the engine cranks. I also get power to pin 56 and 63 on the ecu with the key on.

I did notice that I don't have power at pin 103 @ the ECU, but if i'm not mistaken this wouldn't keep the ecu from powering. This is for keeping the FT correct?

Can someone help with this? I'm stumped...

 

[steve]

With the new MPI relay pluged in, I'm not getting power to pin 4 or 5 at the relay. If I've read correctly, the ECU switches this part of the relay. I'm getting power to pin 10 and power out of pin 8. Were else does ECU get power from? When I turn the key to key on engine start, the fuel pump kicks on and the engine cranks. I also get power to pin 56 and 63 on the ecu with the key on.

The ECU has to pull MPI Relay pin 8 to ground to turn on the relay and switch the power entering on pin 10 to 4 and 5 so it gets to the ECU pins 102 and 107.

The ECU needs the backup power on 103 in order power the circuit that activates the MPI relay.

 

[MunkyX69]

So I got constant power again to pin 103. ECU still isn't coming on. I jumped pin 8 on the MPI relay and forced power to pins 102 and 107. Again ECU isn't turning on. What else could this be? Pins 63,56,and 66 have to do with the MPI relay also, thies should always have power when key on?

EDIT* Pins 66 and 63 have .02v when MPI relay is jumped on pin 8, key on. Pin 56 has power key on with MPI relay jumped. ECU still does not turn on.

 

[steve]

If you got 12v at 102 and 107 and the ECU isn't powering up it's time to pull the ECU and have it checked out.

Pin 56 is an output from the ECU used to activate the fuel pump side of the MPI relay.
Pin 63 is an output from the ECU used to activate the power side of the MPI relay.
Pin 66 is connected to pin 63.

The way these work is you have a coil of wire in the relay with one side connected to power and the other side is connected to the ECU. When the ECU isn't trying to activate the relay the ECU side is for all purposes disconnected so you see the same voltage on both ends of the coil but when the ECU want to activate the relay it switches the connection to ground using a transistor, that causes current to flow, and generates a magnet field pulling the contacts together inside the relay.

 

[MunkyX69]

Already put in 2 known good ecu's and tested the original in another car. ECUs all check out and start the other car... So I'm at a loss on this.

 

[MunkyX69]

OK! This was a pain but it wasn't the ECU or the actuall wires either.

Basically I pulled the ECU out to the passenger side to make things easier. I then grounded pin 63 from the ecu and finally got the computer to power on and got the check engine light on the dash to light. I started the car and let it run for a while. Before I turned off the car I decided to wiggle the wires going to the computer. I litterally touched the wires and the car shut down.

Thought this was a bit odd. So I turned the key on again to see if the check engine light came back on and there was nothing. Oh great... I then moved around the wires again and tried the key again, and I got the engine light again. I figured out there was a loose connection to the ECU. I then depined all the wires to the ECU connectors and bent up all the little tabs in the female ends and put everything back together. Replugged everything back in and the car started and ran. I wiggled the wires again and no more loss of connection, the car stayed running.

Just thought that I'd share this if anyone else has this issue.

 

[steve]

Good work diagnosing the problem

 

[4ggg63]

Great post!!! I'm having the same issues. Gonna give this a try and hopefully save another DSM! Thank you!