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SC300 will not start, no CEL!

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Old 12-14-13, 10:57 PM   #1
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Exclamation SC300 will not start, no CEL!

Ok so i have a problem, my 1992 manual SC300 died last week while i was driving to work and would not start back up, just turned over as if it did not have spark. I noticed while waiting on a tow, that the CEL would not illuminate while the key was at the ON position, which form experience form my old SC400 and research on here pointed towards a bad ECU. So my used ECU came in today and i just went to put it in, same thing, no CEL and no start. any other ideas of what else could be the problem? any diag tips?

Thanks in advance!

EDIT: the EFI relay seems to be good (haven't jumped it jet, will try tomorrow), EFI fuse is good, IGN fuse is good, and all other fuses are good.
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Last edited by Tam4511; 12-14-13 at 11:38 PM..
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Old 12-15-13, 02:47 AM   #2
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Have you inspected your rotor on your distributor?
They could be gone and you might need a new cap and rotor for the distributor. They usually go out around 150k miles and one they're bad you'll get no spark.

Also people will tell you to do the 12v fuel pump mod. It's possible your fuel pump ecu has gone bad, the 12v fuel pump mod bypasses the fuel pump ecu.

I don't know what would cause the CEL not to illuminate with the accessories on, it could be the bulb for it went bad on your dash. Normally when the ECU goes bad the CEL is on all the time as the ECU simply provides a ground for the CEL light, but it's possible (but unlikely) that somewhere along the ground path there was a break or cut in the wire making the ECU unable to make a ground to finish the circuit for the CEL light.

Anyways, I'd inspect your cap and rotor first. On the inside of the cap, all of the contacts should be metal and not calcified, corroded, or burned. The rotor is also supposed to be a nice solid piece of metal, if it's calcified or burned/charred it could mean that it's time to replace it.
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Old 12-15-13, 06:17 AM   #3
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If the cel is not illuminated, that means the ecu is either not getting power/ground or is bad. I'd bet the ecu is bad. Every bad one I've seen the cel either won't trigger at all, or if it does, when you check codes, it won't flash, but just stays lit. Try checkin codes, see what that does. Since you've gone through the dog and pony show of checking fuses, its time to pop open the ecu. That's where my money is. This sorta thing in m experience shows itself when cold to hot/hot to cold temp changes in the weather come, as in first extreme cold/heat wave of the season.
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Old 12-15-13, 04:28 PM   #4
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If the cel is not illuminated, that means the ecu is either not getting power/ground or is bad. I'd bet the ecu is bad. Every bad one I've seen the cel either won't trigger at all, or if it does, when you check codes, it won't flash, but just stays lit. Try checkin codes, see what that does. Since you've gone through the dog and pony show of checking fuses, its time to pop open the ecu. That's where my money is. This sorta thing in m experience shows itself when cold to hot/hot to cold temp changes in the weather come, as in first extreme cold/heat wave of the season.
I cracked my old ecu open and didn't see any leaky capacitors, and like I said I just got a different used one In the mail and plugged it in and same results. Does any one know where the main ecu ground is?
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Old 12-15-13, 04:59 PM   #5
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Same problems have been occurring to my 92' SC400

Took it to the local shop and they said it was the Ignition Switch, so I would highly recommend you check that out because I experienced the same symptoms.
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Old 12-15-13, 05:28 PM   #6
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Same problems have been occurring to my 92' SC400

Took it to the local shop and they said it was the Ignition Switch, so I would highly recommend you check that out because I experienced the same symptoms.
well it will spin over, so the ignition switch seems to be doing its job, it just wont start, which i believe is because the is a problem with the ecu circuit, as like i said, the CEL will not light up at ON.

i had called an ecu re-builder and he said the power goes in to the ecu through the EFI relay and comes back out to go straight to the check engine light. he said if there is no check engine light then either the ecu is bad or there is a problem with the circuit. I just hope the ECU i got in the mail isn't bad.

does anyone know where the ECU ground is?

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Old 12-20-13, 08:10 PM   #7
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Ok, so update:

Iv'e been sick for the last weekend so i havent been able to mess with it until now.
i jumped terminals FP and B+ which bypasses the fuel pump ecu, and it still wouldn't start. so then i took out the cluster to check if the CEL bulb was burnt out, i swapped it with my air bag light, and put it all back together.
once i got it together i turned the key and every thing but the CEL lit up, just like before. So I turned it off and on again and boom, the CEL comes on, and it starts. runs a little funny but it has a new ecu in it. I turned it off, back to on, CEL was on again, and it started it again, i let it run but then it died.
turned the key to on again and nothing, and now I'm back to square one with no idea what i did or what the problem is. i really need some insight here.
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Old 12-22-13, 02:22 PM   #8
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Did some pinout tests today. I am getting good grounds to the ecu, but no power through the IGSW wire. I tested the ignition switch and its goo though. Does anyone have experience with this?
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Old 12-23-13, 12:36 AM   #9
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Did you pull the cel codes?
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Old 12-23-13, 07:53 AM   #10
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Did you pull the cel codes?
there are no CEL codes. The CEL does not illuminate at all, and the ecu is not getting power.
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Old 12-28-13, 03:37 PM   #11
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Messed with it a little more today, using a test light, i traced the IGSW wire all the way back to the ignition switch and its not even getting power at the switch, so maybe it is the switch? it seems like the only thing i can think of.
i ordered on and should be here early next week.
anyone know of any thing im missing??
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Old 01-03-14, 03:09 PM   #12
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finally fixed it, it did end up being the ignition switch, but this morning the "new" used ecu i had bought took a dump. so i had to go home and get my old ecu. Runs perfect now.
I have Alldatapro at work ann it help immnsley, I will upload the pages i used later tonight. if anyone one had a similar problem and needs some input, feel free to PM me!

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Old 01-03-14, 10:15 PM   #13
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Glad you got it resolved
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Old 01-06-14, 10:04 AM   #14
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I am having very similar issues. I took it to my local mechanic as stated above, and he changed the EFI Relay, and the switch. car started fine and they just told me to start the car daily and let it run for a while to make sure this problem doesn't return.

I am now having issues where my CEL sometimes doesn't illuminate and it wont start. It has only happened once since I got it fixed at my local garage, but then 30 minutes of sitting there the light turned back on and fired up. If the CEL is lit when the key is in the ACC position, it starts fine.

I plan to buy a new ECU and hope it fixes it.

Also, I am experiencing weird idle RPMs, it sometimes idles between 1250-1500 while im driving. I also experience bucking and it seems a little louder than normal. But stopped it idles like 700-900, is this normal?

What should I go about doing? Is buying a new ECU my best bet?
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