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1G 1990 No spark. Resistance on ground circuit...

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Ssm90gsx

Probationary Member
17
1
Jan 6, 2015
canton, Ohio
Trying to continue my diagnosis of this no spark issue I’m having. I blew my motor last fall, and finally rebuilt it and swapped it in.

This issue started out as a no spark on 1/4. Swapped out my coil for a spare and still no 1/4 spark. I checked the wiring at the pigtail of the coil and found one wire exposed and barely attached. I re-pinned a new connector and installed it. Still no 1/4 spark. I then took a break from the car, and came back to find no spark at all, and randomly I will get like one or two pulses from the 2/3 coil.

So today I found a broken wire at the ECU connector for the 1/4 coils, I removed the pin, replaced it with a wire and pin from a spare harness and made sure resistance was less than .1ohm between the ECU connector and the PTU plug. While I had the connectors exposed, I went ahead and checked continuity between the transistor plug and ECU plug. Everything checked out fine.

I’m getting proper voltages at the PTU connector from the ignition switched 12 volt, have a pulse coming from the ECU to the PTU on both the 1/4 and 2/3 wire. The one thing I did notice though, is there is continuity between any ground point, and the 1/4 + 2/3 wire at the PTU plug. I’m at the point where I think my PTU is fried, but with all the other things I’ve found wrong, I believe there to be something deeper. I’ve swapped out a known good 90 ECU with the one I have now.

Any insight would be amazing !
 
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CAS in good working order?
 
CAS in good working order?
Yes. As far as I know it is good.

When I turn it by hand all of the injectors pulse like they should. I’m pretty sure I’m getting a pulse At the two wires (yellow 1/4 and yellow/red stripe 2/3) from the ECU. I tested my PTU according to both the factory manual, and the light bulb method and it registered good. I have my wiring harness out of the car as well as the ECU. I checked resistance on both of those circuits and came up with less than .2-.4 ohms. On the wires from the PTU to the coil pack wiring, had less than .2 ohms. None of the wires were shorted to ground or power.

I’m really stumped here
 
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I believe so, yes. Years ago on my last spyder, I did a 6 bolt swap. Would get random no start conditions that always pointed to misfire. Even had misfire codes. Always had a code for injector 2 malfunctions as well. When I finally found out my CAS had a random failure, and replaced it, all my problems went away.
 
Welp after more diag, I still have come up with no spark. Anybody have any suggestions? CAS and PTU tested good. Coil pack is brand new. Swapped the ECU, and went thru the wiring harness with a full pin out. What am I missing here
 
So upon having weird issues with my car (no spark on 1/4 and weird injector firing issues) I decided to backtrack and check out some basic things.

The battery was relocated half a decade ago, and the engine bay was painted last winter. This brought the idea of a grounding issue. I verified all the ground locations were adequate as voltage drop tests showed me. Now here is where things got interesting. I started measuring resistance between the negative terminal, and any given point with the key off, and had no resistance in any spots. I repeated the same test with the key on (ECMLink powered on) and there is a ton of resistance at the same points tested before with the key off.

So anytime the MPI relay is activated, there is resistance on the entire ground circuit. About two months back, I had an MPI relay short out internally to ground, and it blew my MPI fuse a good 4-5 times before I realized what the issue was. I haven’t been able to trace out any issues with the MPI circuit, but I also haven’t had any luck with a complete diagram for a 90.

I had the car running a couple weeks back, attempting to get it dialed in on speed density with the new setup. I noticed while it was running, the tach on the dash was not reading correctly at all, but ECMLink logged the RPM right. Also the car wanted to fall on its face when giving it big throttle variations, and no matter what adjustments were made to the throttletipin, the problem stayed the same. So after a busy week, I’ve come back to fire the car up to send out a log to someone more knowledgeable than I am, but the car wouldn’t start at all.

With little diag, I realize my injectors are not firing while cranking the car. I don’t really know what to do at this point.
 
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Its not unheard of to measure weird resistance when you have voltage flowing through a circuit, the ground included. The battery should be grounded to the body and the engine block, anything ECU in or out should be grounded to the ECU grounds and not the body or engine.
 
I apologize for the super late response..

So problem is still intermittent, but I’m onto something. I swapped in an unmolested 90 harness after swapping out every broken chipped connector. After install problem still persisted, no injector pulse. Key on I had 12v to each pin on the injector connectors. I removed the CAS and spun it by hand and I hear all of my injectors clicking. Installed the CAS again and while cranking I don’t have a pulse at all, but 12v remains at the connector while cranking.

I figured again I had grounding issues. I relocated the battery back into the engine bay, again no injector pulse, but now the car acted as if it got one pulse on one injector. I installed 2 grounds directly from the battery to both of 2 grounding points in the engine harness. This time the car started up, ran for about 15 seconds, I gave it some throttle as I was curious if it would break up, and it climbed in rpm naturally as it should.

Got the car buttoned up, I went to start it and same thing, car doesn’t want to fire, injectors aren’t pulsing. So I’m convinced that I have a grounding issue somewhere, that or I have a small amount of voltage present on the ground circuit. As stated I had a faulty MPI relay shorting to ground and popped my MPI fuse 4-5 times.

I’m unsure of where to even begin looking for where it could possibly be shorted to ground, or if that’s even my issue. Such a strange problem, has had me stumped for years.

Ground your negative terminal externally (while car is stationary of course) and see if that changes how it runs.

How do I ground the battery externally?
 
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