zachmlewis

Sorry about the duplicate post! I'm new to using forums!

Anyways, I have a slightly modified '86 gxl (headers, presilencer, cat back exhaust, 'cold air' intake, s5 intake manifold, diy modified 6 port actuator valves (like the atkins actuator valves), air pump/emissions, OMP, A/C delete, etc,) but for the most part it is still pretty stock.

Had it die on me out of the blue the other day while I was on the interstate. Before that I'd been driving it almost every day for almost two years with virtually no issues outside of just regular maintenance.

Well got it home and hooked up a timing light and sat in my car and cranked the engine. I get spark on all 4 of my plugs for about 2 or 3 seconds and then nothing. Tach stops bouncing as well (it runs of the trailing igniter right?)

Tested the resistance of my coils. Leading had no resistance so I replaced it with one I got along with a running s5 motor a friend gave me when he did an LS swap (He gave me a free running engine so I couldn't complain too much, but I know he will have to answer to the rotor gods when he dies). I decided to swap out my trailing coil too for safe measure.
Both test .8-.9 ohms and are getting a steady 12v from all three B/Y wires. I was also able to get 12v no matter where I touched my multimeter lead on the mounting brackets and bolts for the igniters so I know they are well grounded.

Checked the resistance of my crank angle sensor and it checked out fine. Then I took the CAS out of that s5 motor and plugged it in and hooked up a timing light to each of the plug wires. I got a steady spark from every plug when I turned the gear.

Only things I could think of beyond that are bad grounds, bad continuity on one of the other 3 wires going from the ecu to the trailing coil, or my ecu is fried.

If the coils are getting 12v that means the ecu is also getting power correct? The B/W wire on the green check connector also had 12v.

Is there a better way to check those other three wires besides checking for continuity? (I'd have to have a pretty long lead to reach from the ecu to the coil)... should they have any certain voltage when the key is on or cranking?

Also, I've checked all my main grounds and they seem to be fine. The motor was completely rebuilt by a good friend of mine who owned the car before me about 40,000 miles ago, and the wiring harness was pulled out with it of course and the engine bay was thoroughly cleaned, so even with an expectable amount of dirt build up from 40,000 miles the grounding points are all still very clean.
The only one I didn't check is the ecu ground mounted to the rotor housing since I'd have to pull the intake manifold off to really get to it. I tried to find it by looking down there with a flashlight but I just couldn't get the right angle to see it. I did take it to a shop once about 6 months ago(?) (Jeff's auto, really great rotary and miata mechanic, just before he moved his shop when he was still in Puyallup). I guess there is a chance he forgot to put that ground back on, but I can't imagine I'd get very far without the ecu being grounded?

Could an issue like this even be caused by a bad ground? I would imagine a bad ground would cause some really erratic and intermittent issues, and would have had some symptoms before just dying suddenly, but this is the exact same thing every single time I turn the key and it happened completely out of the blue.

Any opinions on this would be super appreciated. I'm pretty bewildered to be honest.

Rotary Alkymist

Have you removed and inspected the plugs? Strange that tach stopped working. Try wiggling X,U and M at ECU while cranking. Try wiggling F too. Tweezers are a great tool for this.

zachmlewis

Spark plugs were a little black from accidentally flooding it when I was trying to get it started on the side of the road (in my desperation I disobeyed one of the number one rules and pumped the gas pedal), but I put new ones in as soon as I got home. I'll try wiggling them later today when I've got someone here to turn the key for me. X goes to the tach right?

zachmlewis

Well, I know X, U, and M all go to the ignitor, what I should have asked is, X is the wire that sends the signal for the tach right?

Rotary Alkymist

Not sure, I know that 86-88 manual doesn't have as much ecu info as 89-91. I do think you have a wiring/ECU issue because the tach is involved. If the wiring hasn't been tampered with then your ECU should be fine...ECU is always last resort man. If ALL else fails, only then would I suspect the ECU.

Could you explain in point form everything that happens from the time you sit in your car, turn the key, and release the key? It would help form a diagnosis.

Edit: Just a thought.... have you checked all fuses? Specifically the Engine fuse(15amp). Have you grounded the main fuse box? Look at your bracket... is it rusty around the bolts that connect it to the strut tower? Rust will accumulate there because that's where the energy flows the most.

zachmlewis

- I turn the key to on. Everything is good. Dummy lights come on and then go off and i get all the normal beeps and what not.
- I start cranking the engine and all is good for the first ~2 seconds. Tach works as it should, and I get spark on all four plugs.
- ~2 seconds after turning the key, the tach stops and I stop getting any spark at all. Engine keeps cranking.

I've checked all my fuses for continuity and they are all good and there is no rust on the bracket. Didn't think to check the fuse box for ground though.
I haven't gotten a chance to try wiggling X, U, and M either as its been an almost constant downpour here the past couple of days and I don't have a garage at my current place so I've had to just leave a car cover on it. Soon as there's a break in the rain though I'm gonna go out and try it

zachmlewis

Well, I think I found my issue haha. Cant believe I didn't notice this until now. The top side of the harness (the side that was visible when I pulled off the metal plate to look at the ECU) looked perfectly fine. Pulled out the plug to inspect all the connections, flipped it over and saw this...





Luckily a friend has another s4 harness he said I could have. Now my question is: how hard is it t get this harness out and a new one in? Will I be able to unplug everything and pull it out through the firewall with the engine still in the car, or will I have to pull the engine?

Rotary Alkymist

Glad to hear that you found the problem.

Is that F-31 wire cluster? Are those wires melted? Damn ... I was gonna mention that I didn't think your last coil was dead because they're so robust but they do die. This would've helped destroy a coil. I'm surprised you don't have blown fuses but that's a good thing!

Front harness is harder to replace than the EM harness because it comes in from the driver's side and snakes over to the ECU.

I would unwrap that a bit and space those wires out to really see what you're dealing with. It looks worse than it is. I would do a good job of repairing and re-insulating those wires and call it a day. Harness replacement is a tedious job and you'll be playing with old connectors that can crack and break.

I'd look for clues that point to the reason this happened. Something at some point was crossed. Maybe rubbing from road vibration?

Edit: Harnesses are getting harder and harder to find. I wouldn't junk this one just yet. Your wires, in general, look good.

zachmlewis

I think they look chewed up, like a critter got in there (a mouse or squirrel?) Some of the insulation down there was pulled up too, almost like they had made a nest... luckily nothing came flying out to maul me when I pulled the carpet up! Hahah. Like I said, I don't have a garage right now, so I've been parking outside. Might explain why it happened so suddenly. For the majority of the two years I drove this car daily I was parking it inside, so less chance for a critter to get in there.

I'm not sure what cluster it is... the hanes wiring diagram I have is trash hahah. It's the rightmost cluster of wires plugged into the ecu.

I don't want to trash it, but I also figured it wouldn't be much easier to cut and splice a new wire onto every single connection there, especially with it down in the footwell with the carpet fighting against me (I really don't want to have to take a part the center console in order to pull the carpet back more.) It was hard enough just getting my phone down there to get a good picture.
I might just swap them out and do a good repair job on it when it is out of the car... unless it really is that much more work to get it out and put a new one in...

zachmlewis

So I went out today and unwrapped those wires to take a look at them. Didn't look too bad actually once I got it unwrapped. I wrapped all the ones that were chewed up in electrical tape just as a quick fix to see if it would fix it....
No dice. It's still doing the same thing, but this time all my warning lights stay on while I crank the engine, which makes me think it has something to do with the charging system?

zachmlewis

Never mind, warning lights should be on while cranking because the volts dip below 12v right?

Rotary Alkymist

They should dim somewhat but yeah they should be on.
There are wires that running from the alternator to F-31.

Look at all affected wires again. There may not be enough copper to supply the right voltage.
Cut and solder all wires that were exposed.
Be sure to use heat shrink.

How many wires were compromised?
It really isn't that much work when you think about it, at least to me. It seems like way less work to solder 3-4 wires than to take out the harness.
You could solder those wires in about an hour or two and then rip down the road again!
And what if you put it in to find that your ECU is fried? This is beginning to seem like a possibility. It would really suck to do all that work for nothing.

zachmlewis

True, true. It was about 5 or 6 wires. None of them seemed chewed up that bad to be missing any copper but you never know I guess. I didn't expect the electrical tape to be a permanent fix by any means, just figured it would have insulated them enough.
But yeah, you are right. I'll try to pick up a sautering iron tomorrow and sauter in new wires

Rotary Alkymist

I know it's tedious but get a trouble light in there or a phone and get your head way up in there lol. Look at them really good.
Those wires are tough... tougher than you or me. Don't be scared to really separate them one by one to inspect. Get the wire in question way out there and very visible. You'll find your problem guaranteed.
Just think of each wire as a separate job. Don't move on until you're satisfied with the wire at hand.