ECU Ground and Chassis Ground, car wont start.
 

Hey guys,

I new to owning RX7s and I have a 87 turbo with an AEM EMS I bought from another member, the car left me stranded the other day, everything just cut off going down highway. I basically found out that my ecu is not getting power. With further diagnostics if found that the chassis ground wire for the EMS got hot and have a ecu grounding issue. I had to solder a jumper in the AEM EMS to another wire not being used. The way it was wired was to this brown/blue wire which I don't think is a ground wire? I'm trying to find a good stock wiring diagram and stuff I found is some what complicated to understand. I basically need to know where a factory ecu ground wire is so I can ground it appropriately and hope it works. I have to get a new volt meter today, but everything turns on, cranks, but no fuel or spark and I can't connect to EMS so I know the ecu is not getting power. I hoping the EMS isn't fried, upon inspection it seems ok, only that pin was toast. Here are some pictures.

Thanks guys, I'm hoping I can get it running again and the ecu isn't fried. If so, I'm going to to take it to Ben at rotary evolution for a haltech and a tune.

Case

Have you thought about checking the 15 amp Engine fuse?

Yes, fuses are good.

i can't really tell what you're pointing at in the 2nd and 3rd pictures. something to note though is: for the factory harness solid black is almost always a ground. black/white and black/yellow are accessory power wires to turn on the ignition/injectors and emissions solenoids.

There is supposed to be a Black/White wire which powers the ECU. You can check this wire w/key to on to see if it is powered or not. Next to the lead coil is a Green check connector w/four wires. One of these wires is B/W and it's the same wire which powers the ECU. Check this wire w/key to on.

Well, my friend who's pretty knowledgable on wiring etc came over and we just got done diagnosing everything. We tested all the ground and power leads, which led us to one of the three grounds not grounding the EMS. The EMS is theoretically shot due to a faulty ground wire, so the EMS basically overloaded and ultimately fried that pin. We then took that ground and grounded it properly and hooked everything up, nothing... I'm going to talk to AEM and send it to them to see if its salvageable, if not, then it guess I'm going with the haltech.

strange that it would cook the circuit that powers up the ECU though, generally that doesn't carry much amperage. the ignition/injection ground though is the one that carries a bit more and the one i would guess that would be problematic if it had a bad connection.

What we were thinking is that since 1 of the 3 chassis grounds went bad that basically the ems overloaded and only had 2 grounds to use and that was the end result... This is our best guess..

grounds are usually divided for the system they have to control, some could quite possibly damage the main circuits on the board.

i'd just be concerned that there is a problem on the engine or anything else controlling the engine that may cook the new ECU/repaired unit as well. this is part of the problem with PnP units as they reuse the old and baked 25 year old original harness which could be grounding on the block somewhere.

even though it may appear to have been a faulty ground, grounds do not always follow ground but they can apply power and ground to other circuits through relays in the ECU. if the ECU could not manage to blow the fuse first it will blow the next weakest link which apparently was a semi poor ground wire which overloaded the board. problems like this are usually in the engine bay and of no fault to the controller.

normally i would just physically check the harness, sometimes the problems are easy to spot.

Ben,

Thanks for the info, PM sent