Electrical or ECU problem....

[Supra87T]

Here's the deal. I have an 87 Supra that would not start the other day. I have narrowed it down to the ecu only sending out approx 1 volt to efi main relay by using the tsrm. The wiring to the relay is fine. The ecu outputs about 1 volt at switched ignition terminal also. I got a another used ecu thinking that was the problem.... could it be that I have two bad ecu's or is my problem elsewhere?

 

[Nick M]

No, it isn't likely that you have two bad engine control modules. It isn't likely you have one bad module. It is possible, just not likely.

 

[jetjock]

Make sure you're referencing a good ground. Use the one right on the main relay. When troubleshooting always use the ground at the device giving a problem or at least confirm it first thing. Referencing to the battery or body is poor technique is most cases.

IGSW is an input that requires 12 volts so that could be your problem right there. Comes from IGN 2 on the ignition switch. IGSW is what the ECU uses to energize the main relay. It's a simple circuit so check the IGN fuse and see if you have + 12 there with the key on. If not work backwards towards the ignition switch itself. Or run a jumper from any + 12 source to IGSW and see if the main relay comes alive. If it does you'll know the fault lies in the switched + 12 IGN 2 line.

 

[Supra87T]

alright... i'll check that tonight and let you know. the tsrm is confusing to me... lol.

the ground seems to be good to me, it's reading 0 ohms. not quite sure if i'm checking it the right way though. i'm just checking it with a ground i know is good and seeing if it reads any ohms. the tsrm says to check E1 as the main ground, is that correct?

the ign fuse is in the main juction block in the engine bay, correct?

 

[Supra87T]

I have power at IGN 7.5 and fuse is fine. That means the problem has to be with the wire that goes from here to the ECU, correct?

 

[jetjock]

Sounds like it. Look at the schematic. ECU input terminal IGSW gets 12 volts from the ignition switch. When it sees that it should turn on the main relay output. The main relay coil gets it's ground from ground path A on the fender. It's also connected to ground path C on the other fender. (The car uses redundant grounds in nearly all circuits). Sounds like yours is good.

 

[Supra87T]

I ran a wire from IGN fuse to IGSW and the car runs fine now. I wonder why all of the sudden the wire went bad though....