I am really happy to have stumbled across this thread! While the experts are all still here I want to quickly share how I broke my RX7.

I recently fixed some waterleak on my 1990 Turbo 2 and forgot to tie the cable loom to the wheelhouse after the fix. Mid Testdrive i lost power and any throttle input beyond 10% would just make the car bog. Limping back home i found that I had completely melted the cable loom next to the Turbo. Of course it wouldn't start. I have gone ahead and repaired all broken cables by crimping in new sections. Still did not start. Checked all Fuses (with an Ohmmeter), non were blown. Tried to check for error codes but the check engine light does not light up at all when i turn on ignition.

My suspicion is that i fried the ECU. Currently I am in the process of removing the kickpanel. I will thoroughly go through the suggestions in this thread to be sure before I get a new/used ECU. I do have access to a stock S4 Turbo 2 ECU but am not sure if this part fits on my S5. Maybe someone can tell me if it is sufficient for checking if the ECU is the source of my problem.

Like I said I am very grateful this thread is still fresh and would love to hear if the ECU should be my main suspect!

 

A trick I just learned for diagnosing where a short circuit is, is use a low wattage light bulb instead of a fuse! It acts as a current limiter, saving you from having to do so much guesswork replacing fuses over and over till you found the short. And if you've got access to one, a thermal camera can help as well once you've got the light bulb hooked up. Glad you got your car running!



Start with the basics: Check the ECU is getting power, An S4 TII ECU won't work for your S5, I'm pretty sure - there are a bunch of changes to the cars between series.

Check all your repairs - that the wires have continuity and aren't shorting to ground or to another wire somewhere else. Especially if any of them melted somewhere other than right at the turbo caused by the short the turbo made. Check the wires that you didn't touch, in case you missed one that was damaged, or a connection between two wires was missed.

Then verify - is the fuel pump turning on? Is it getting spark? Are the injectors squirting? If so... chances are it's just flooded, and if not, you know what to check next, or point more fingers at your ECU as being the problem.

It's been a very long time since I've been inside one of these ECU's, but IIRC, they aren't potted - so if you open it up and carefully look it over, and something looks or smells burnt, it's almost certainly your problem.

 

What Terr said.

I reread: You already tried to pull codes, but check-engine isn't coming on. That's ECU. Either cooked, or no power.

Try to test the power. There are a couple of power connections, the unswitched one is pin 1A, which is in the upper row, rightmost pin. Per above, you can stick a safety pin or something else pointy in there, you don't have to pull the connector.

If you have no power, double check the room fuse. The same one which gave me so much trouble. Internal fuse block, top center. 7.5 or 10A.

It's a bit odd. Your description of what happened on your test drive sounds like limp mode. I had that happen to me a few years ago when I lost the metering oil pump. Limp mode is controlled by the ECU. But it sounds like once you got it home, it won't start or do anything. That sounds like ECU fried, or no power. It's hard for both to be true.

 

Thanks for the suggestions. When fixing the broken cables, I made sure to check continuity on each one before I shrunk hoses around the crimps. There is one big grey cable with a blue stripe that is also a bit toasty but I was afraid to touch it as it looks different and seems to have multiple strands of wire inside. I will try to find out what this cable connects to from the wiring diagrams.

I can't open up all of the engine loom just to check if something else is fried but I will try to understand the wiring diagrams to a point where I can check continuity between the pins inside plugs to be sure I won't fry any replacement parts I get.

That being said I did go ahead and pulled the ECU. I found a burnt spot around some resistors so that part definitely needs to be replaced. Luckily I found a N380 ECU nearby so I just need to make sure I fixed all the shorts before I put the new ECU inside the car.

Thinking about it I am still surprised I made it home with the car when it broke down on me. Is it possible that the ECU was already dead at that time? I am not sure how the limp mode alters the behaviour and can only explain what I felt. I will try to give a bit more insight into what had happened in case someone is interested. From one second to another the car would terribly bog beyond 10% throttle input. Of couse it would not build boost aswell. I could still rev high though. With the clutch depressed it was also possible to give it almost full throttle and the motor would rev up just fine. Only without load though. I also had to constantly give it throttle to prevent it from dying at stops. At home when it was still running I checked what it would do without my constant gas pedal input and it did actually idle at around 100-200rpm. Pretty rough but it was still kinda alive. Also no unusual smoke coming out the back. Like explained in my previous post after shutting it off it would not start again.

 

I am by no means an expert, so please someone correct me. I don't believe it would continue to run if the ECU was dead. That suggests to me that it was still at least partially working, and failed completely after you got it home. As to why that would happen, I don't think we know enough yet.

You first job, IMHO, is to try to figure out what fried your ECU, so you don't fry the replacement. I suggest you carefully check the voltages at the ECU connectors. There's a post in here someplace which references what the correct voltages should be, but I can't find it now. I'll search some more, or maybe somebody else has it bookmarked. It's hard to see why any short or open in the harness could put overvoltage to the ECU, but never say never.

Re the behaviour of the car when it started to go wrong: That really does sound to me like limp mode. When that happened to me, it was because the MOP had failed (actually, the feedback sensor had failed, the pump itself was fine) and the ECU took that to mean it had to limit power. I could start the car, and drive a little, but if I stepped into it much, it would all of a sudden light the check engine light, and it would bog at any significant power. I could drive it, but slowly, and could barely accelerate. It was not RPM dependent. Like you, I could rev the car up substantially with no load on it, because that didn't take much throttle.

I don't know all the causes of the ECU deciding to go to limp mode. Maybe somebody here has that. It will be different for the turbo than my normally aspirated engine anyway. But you know what damage happened to the harness. Correlating that with what the ECU does might tell you about exactly why it was unhappy.

[Later] Maybe this is what I was thinking of. See https://www.rx7club.com/2nd-generati...m_content=post This is for an S5, which may or may not be what you have, and it didn't look like this was for a turbo. But it should still be close.

 

I found the pinout for the S5 Turbo (at least for US and Canada models) in the S5 wiring diagram of the FSM. As luck would have it I also stumbled across a physical german copy that should lign up better with my N380 Euro ECU. I will crawl back into the passenger footwell next weekend and make sure my repairs are good.

Within the ECU I can pinpoint the burns to the area of the electrical components numbered T809, T807 and resistors 842, 843 and 841. Maybe there is a way to tell which cables run current to these parts. If someone knows I would be very happy to hear.

As a side note I never got the check engine light nor any other warning light while I was limping home.

 

Ok, I think I don't know what I'm talking about. If you didn't get a check engine light, the ECU hadn't gone into limp mode. I'm still sure you need the ECU to keep the engine running; if nothing else, it controls the fuel pump. So that sounds like your ECU was ok. Some happened to it since then.

So I guess you keep checking things. Ensure that there's power to it, and check other voltages if you can. Once you confirm it's safe to plug in the replacement, you can check other stuff.

When you try to start it now, what happens? When I was debugging my problem as detailed in the earlier part of this thread, when I had the room pulled, which meant the ECU was not powered, and I turned the key, I would still get all the annunciator lights (except check engine), and the engine turned over just fine. Is that anything like what you're getting?