A while back I had a pretty solid running rig, the 1988 with the VG30i. Only issue was a slight leak from the valve cover gasket. I replaced the valve cover gasket, a fairly easy job, and afterwards the truck would run like it was running on 3 cylinders and would die without throttle input. The next day it wouldn't start at all, and I now have no spark. (STILL CRANKS HEALTHY AS EVER) The ECU will sometimes give (CODE 21) (loss of ignition signal) and sometimes gives (CODE 55) (no issue code)

What I've done:
[*]Tested every electrical component of the ignition system with a multimeter based on the FSM
[*]Checked for signal on the G/Y and G/B wire, checked good all the way back to the ECU
[*]Checked continuity of every wire in the ignition system going back from the component to the ECU
[*]Checked for spark at plug, wire, distributor, and coil, got none.
[*]Tested the capacitor/resistor in-line with the transistor, they tested fine.
[*]Cut away the entire harness wrap to visually inspect wires going to and from the ignition components.

What I've replaced:
[*]Battery
[*]Plug wires
[*]Distributor
[*]Coil
[*]Coil transistor
[*]Starter (unrelated but worth mentioning it is NEW)
[*]Alternator (also unrelated but worth mentioning)

What I haven't replaced:
[*]Ignition switch
[*]2.2k Condenser
[*]The resistor right next to the 2.2k condenser, not sure what it's called.

What I know:
[*]Coil gets battery voltage when [ON], drops to ~10.5ish while cranking, I've read this is normal.
[*]Distributor gets battery voltage
[*]Transistor gets battery voltage
[*]All grounds that I could even think to check are good, in the cab and out.
[*]Wiring is visually damage-free all the way to the firewall, and as far as I can see under the carpet and up into the dash.

ONLY ELECTRICAL PROBLEM I COULD FIND:
FSM directed me to check for battery voltage between ECU terminal 3 and ground while the ignition is on. I checked and got no voltage. This white wire with red dots on it shows on a diagram that it goes into something called JOINT CONNECTOR C and then to the 2.2k condenser that I've already tested as being fine. I can't seem to figure out where that connector C is though...
I tested this with the ECU unplugged and forgot to ever try testing it with the ECU plugged in as I figured it wouldn't change the outcome, but I can test it plugged in tomorrow. There is a chance the circuit is fine and I've just tested it incorrectly.

My guess is that there is an issue with this joint connector C, as this wire disappears under the dash and doesn't seem to come out in the engine bay.
I have also heard that the ignition switch itself could be the issue, but that doesn't make sense to me considering the engine cranks perfectly fine, and everything on the coil side is getting 12+v
I'm also considering the issue may lie with the ECU, but these are all just guesses and I'm trying to avoid the continuation of hopelessly replacing parts until something works.


Thanks to everyone

According to the WD, that white wire is feedback to the ECM telling it the coil has fired. The 2.2K component is a dropping resistor and not a condenser, it's there to reduce the high-voltage "kick" on the coil primary to a level a zener or clamping diode inside the ECM can handle. With the ignition on you should see 12V on both sides of the resistor, white and blue. If there's voltage on blue but not white, the resistor is open, the white wire is shorted to ground, or the ECM input has failed internally (shorted to ground). If there's no voltage on blue or white, either there's a bad connection to blue at the distributor, the coil primary is open, the power transistor is melted short, or the white wire at the distributor (transistor trigger) is shorted to power. The latter would probably mean a blown ECM.

VStar650CL wrote: ↑

Tue Jan 05, 2021 7:01 pm

According to the WD, that white wire is feedback to the ECM telling it the coil has fired. The 2.2K component is a dropping resistor and not a condenser, it's there to reduce the high-voltage "kick" on the coil primary to a level a zener or clamping diode inside the ECM can handle. With the ignition on you should see 12V on both sides of the resistor, white and blue. If there's voltage on blue but not white, the resistor is open, the white wire is shorted to ground, or the ECM input has failed internally (shorted to ground). If there's no voltage on blue or white, either there's a bad connection to blue at the distributor, the coil primary is open, the power transistor is melted short, or the white wire at the distributor (transistor trigger) is shorted to power. The latter would probably mean a blown ECM.

If I'm getting 12 volts on the blue side and 10 volts on the white side would that indicate an issue with the resistor?

No, the resistor is there to drop voltage, that's its job. The inductive "kick" from a coil primary is typically over 150V and can top 350V in HEI systems. The resistance reduces that, allowing the ECU to monitor it without blowing up its inputs.

PS - It might indicate a problem with the ECU input, depending whether it has an internal dividing resistor or just a diode clamp. That isn't the sort of info Nissan puts into the ESM. I suspect that would be a normal reading with the ECU connected, but that's a best-guess and not a certainty. The only way I can think to find out would be to measure it on a known-healthy system.

VStar650CL wrote: ↑

Fri Jan 29, 2021 1:44 pm

PS - It might indicate a problem with the ECU input, depending whether it has an internal dividing resistor or just a diode clamp. That isn't the sort of info Nissan puts into the ESM. I suspect that would be a normal reading with the ECU connected, but that's a best-guess and not a certainty. The only way I can think to find out would be to measure it on a known-healthy system.

Would you happen to know which terminals on the ECU connect to the power transistor? I'd like to test the continuity of those wires before I spend money on an ECU. I'm not really sure what else to test, but I'd like to try and get my hands on a donor ECU just to see if that's the issue before I go ahead and buy one.
It's starting to look like I have no wiring issues whatsoever, but those are the last wires I haven't tested for continuity, just because I cant seem to figure out where they lead to.

The Motor WD shows the white coil trigger wire connecting to pin 5 on the ECCS, still white. The white feedback wire from the resistor should connect to pin 3 on the ECCS, also still white. Note that your engine should also have a CAS (crank angle sensor), I believe on the front of the RH cylinder head. If that's flatlined or out of sync, you'll also get a no-spark condition.

VStar650CL wrote: ↑

Fri Jan 29, 2021 3:17 pm

The Motor WD shows the white coil trigger wire connecting to pin 5 on the ECCS, still white. The white feedback wire from the resistor should connect to pin 3 on the ECCS, also still white. Note that your engine should also have a CAS (crank angle sensor), I believe on the front of the RH cylinder head. If that's flatlined or out of sync, you'll also get a no-spark condition.

If i'm not mistaken, the CAS is within the distributor on these trucks, I've tested that and am getting pulse signals where I'm supposed to. I checked continuity on those wires and they checked out fine, I guess I should look into getting an ECU..