Holy smokes man,thats insane.Im going to add to my fuse knowledge on these cars this is critical,if in doubt,change it,clean with de oxit without fail and fill withDS-ES Electric Switch Lubricant - Lubriplate

 

Glad to hear you got it solved. I this is a good example of how the power of understanding and using a circuit diagram to methodically test and isolate the fault can be effective vs. trying to do it without a diagram.

Once you found that you weren't getting the 12V to the Main Relay's ignition switch line, if you knew exactly what fuse that was (BTW, usually looking for the 7.5 label on the fuse, and reading the fuse box label, "IGN" as labeled on the diagram), you could have tested the fuse, replaced it, swapped it with another 7.5A one in the box, tested voltage on both sides of it. And you'd have confirmed that the ignition switch was good and the fuse was bad.

I know doing things like swapping in ECU's might seem quick and easy if you have easy access to them, but that can bring in a whole batch of new troubles. And I know it takes some time and patience to understand a circuit diagram, even one as straightforward as this one.

And while we're hatin' on weird fuse problems ...
As I mentioned in one post above, on my '91, completely separate from my fuel pump issues, I had a time when the 7.5A ST (starter) fuse in that driver's footwell fusebox caused intermittent no-starts. For a few weeks in this situation, I would bypass that by using a paperclip to hotwire the starter relay. Then it failed completely and I knew I had a chance to do a more methodical debugging (very tough to do this with intermittent problems since you know that every single measurement result is a random variable) eventually traced it to the 7.5A ST fuse. Glad I did that before swapping out the Neutral Safety Switch, which I was about to do. I really don't know how a fuse could be causing intermittent failures like that, but it did. No problems in the few years since then, so that fuse was definitely the one and only problem in that case.

So I guess, related to your case ... It is usally said that a blown fuse is just an indicator of another problem, and that just replacing the fuse makes no sense since it will just blow again. Well, like most things, that's not always true of course. These cars are getting pretty old. So hopefully in your case, the fuse truly was the only problem, as it was for my starting issues.

 

Hello you all, I have a sc400 and Im having the exact same issue thats talked about in this thread the car is cranking with no fuel or spark so I did a bench test on the main relay for continuity across the 1-o-clcock to 5-o-clock pin and no resistance

Then I jumped pin the 1-o-clock pin to 5-o-clock (pins 2-4) to close 11-o-clock to 7-o-clock pins and I got a resistance of 1.68 not the best but my fuel pump turns on and pumps when I do this and I get power to B So should have a failed relay?… but I swapped my main EFI relay with a working one for my sc300 and nothing happens (no fuel or spark just cranks)

But I did some more digging around and I found that my IG 7.5 fuse was blew then I changed it and it blew again, So I believe thats where my issue is located not actually my Main EFI relay.

Also sometimes my gauge cluster will completely off randomely I'm not sure this has anything to do with the issue I'm having . If anyone could help this would be greatly appreciated.

 

Also I suspect the Ecm to be going bad, I opened it up and noticed 1 capacitor slightly leaking but I want to know for sure if it’s the Ecm, wiring or ground issue so I can properly diagnose the issue

 

The important first two steps are to replace all those QAS capacitors and to check the down stream of the fuse IG 7.5A. Is it grounded or the resistance too small?

Those QAS capacitors fail unconditionally when aged. This is a big known issue and you can't escape from it.

 

Ok great and thanks for the help I will definitely look into getting the capacitors repaired, but as far as checking the downstream of the fuse IG 7.5 A how do I go about checking if it is grouded or resistance to small

Also would you know the diagram of this fuse 7.5 A (which is located in the foot panel inside the vehicle) so I can figure where downstream would be located?

 

Remember this is a LS400 Forum, most of members don't have the wiring diagram for a SC400. You'd better ask it at the SC400 Forum. Oldskewel's post #8 and the link there may help you some but it's for a 91 LS400.

To check the downstream, use an ohm meter or a DVM to measure the resistance between the line and the chassis. If it's too small, remove units connected to the line one by one and distinguish what is causing the problem.

According to the diagram for a 91 LS400, the fuse IGN 7.5A supplies +B 12V to theEFI Main Relay, the Circuit Opening Relay and theECU. To remove these one by one is the way to distinguish in case a SC400 is designed the same way with a 91 LS400.