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Old 06-16-2015, 09:49 AM   #1
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Default I Messed Up- ST SIG HELP

Hey guys, I did something stupid. I started my car with the ST SIG 10A fuse still in. I have a DIYPNP V1.5. Which transistor, and possibly anything else, did I most likely fry? I think it is the one in spot Q17 but I'm not entirely sure.

Side note, why does my car only idle when the fuse is in place and not when I take it out? The IAT sensor does go into pins 1 and 6 of the AFR pigtail, yeah?

Cheers!
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Old 06-16-2015, 11:42 AM   #2
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If this is a '90-'93 Miata, probably the TIP125 in the high side driver section.
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Old 06-16-2015, 11:48 AM   #3
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Oh, right. I have a '90 with the stock 1.6. I'll check that out in a minute. Is it likely that anything else would be messed up? And what about the IAT sensor pins with the AFR pigtail?
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Old 06-16-2015, 11:59 AM   #4
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it was working because the AFM controls the fuel pump, since you probably blew the transistor on the MS and it sounds like you still have the AFM in place.

But it also depends if you bought your DIYPNP with that circuit populated/wired or not.
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Old 06-16-2015, 12:15 PM   #5
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I have removed the AFM. I installed the IAT sensor right by my TB and connected the wires to slots 1 and 6 on the factory AFM connector. Those are the right slots aren't they?
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Old 06-16-2015, 12:22 PM   #6
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I Messed Up- ST SIG HELP-80-image_0ba7999e85000a4afa49c510844ebb19f690e8d4.jpg  
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Old 06-16-2015, 12:23 PM   #7
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Quote:
Originally Posted by typicalsteve View Post
I have removed the AFM.
I'm going to take a wild guess here and say that until you replace the damaged transistor on the MS board, the car isn't going to run (with or without the ST SIGN fuse) after you remove the AFM.

With the transistor blown, the MS isn't able to control the fuel pump. With the ST SIGN fuse in, the starter circuit is powering the fuel pump during cranking, then the mechanical switch inside the AFM takes over once there's airflow.

You could work around this problem by placing a jumper wire between pins A and B of the AFM connector, or between pins F/P and GND of the diagnostic connector under the hood on the drivers side shock tower, which will cause the fuel pump to run all the time when the key is on. This creates a potentially unsafe situation wherein the fuel pump will continue to run in the event of a ruptured fuel line.


I am, admittedly, taking some liberty in translating your original post, presupposing that when you say "why does my car only idle when the fuse is in place and not when I take it out?" what you really mean is "why does my car only START when the fuse is in place..."[/i] With the AFM still installed, I'm guessing that you could have started the engine with the fuse in place, then removed the fuse and the engine would have kept running.
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Old 06-16-2015, 12:33 PM   #8
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Thank you for detailing that all out for me. I will replace the transistor as soon as I can tell which one exactly it is that I blew. Can you help me with that? They all look fine to me.

Also, my AFM is indeed removed and I have installed the IAT sensor by my TB.
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Old 06-16-2015, 12:35 PM   #9
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And my car was idling last night for a minute or so with the fuse in and the AFM removed. Here is my setup
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I Messed Up- ST SIG HELP-80-image_f4cb1ffd48c956f661d6d050538a6d1caee576fb.jpg  
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Old 06-16-2015, 01:20 PM   #10
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TypicalSteve responded to me via PM, which I post here so as to retain one single thread of conversation:

Quote:
Originally Posted by typicalsteve
Hey Joe,

I am bit confused about what you said earlier. Just to clarify, my AFM is removed and I have the IAT sensor installed. Last night, my car would not start or idle with the ST SIG fuse taken out. After I put it in, it would start and then it idled for about a minute. What can explain this? And I still cannot seem to find the transistor that is blown.
Two options occur to me:

1: The transistor isn't blown, however your ECU was clearly assembled at a shop located directly above an Indian burial ground, or

2: Are you sure about that full minute?

Reason I ask is that once the fuel system is primed and the engine running, it'll take a little while for the fuel pressure to drop and stall the engine while it's just sitting there idling. Not a full minute by any stretch of the imagination, but maybe 10-20 seconds of runtime after the fuel pump is shut off might be plausible before the engine sputtered to a halt.
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Old 06-16-2015, 01:25 PM   #11
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The car was most definitely on and idling for at least a minute. I then shut it off, though, and it was unable to maintain idle after that. But it did do it once.

Is there a sure fire way to tell if the transistor is blown or not? Shouldn't I be able to test the continuity with a voltmeter or something?
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Old 06-16-2015, 02:19 PM   #12
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When you key the car on, it should prime the fuel pump for 3 seconds. If it doesn't, that transistor is blown out, and you'll have to solder a new one in, ~$14 from DIY.

If you need the car to run, jumper FP and GND in the diagnostic box. The FP will be on when the key is on, which is annoying, but it'll work.
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Old 06-16-2015, 02:34 PM   #13
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Okay, that's pretty straight forward. Thanks.

Is there any way to tell if the FP is priming other than just listening to it?
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Old 06-16-2015, 02:44 PM   #14
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Quote:
Originally Posted by typicalsteve View Post
Is there any way to tell if the FP is priming other than just listening to it?
That's pretty much it.

The NB having a returnless fuel system, you can't hear fuel moving under the hood as with the earlier cars. Easiest way is to open the fuel filler and stick your ear against it while someone turns the key on.
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Old 06-16-2015, 02:55 PM   #15
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Quote:
Originally Posted by typicalsteve View Post
its under the green board.
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Old 06-16-2015, 02:58 PM   #16
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Alright, cool. I will probably order that resistor today then.

Thanks
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Old 06-16-2015, 03:13 PM   #17
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transistor.

honestly, I'd wire 4O to FP and then jump the wire in my AFM connecter when you install your AIT sensor and call it a day.
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Old 06-16-2015, 04:04 PM   #18
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I meant transistor... And Okay I will do that and post a pic to verify that it's all correct.

Thanks
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Old 06-16-2015, 04:09 PM   #19
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FP to 4O puts the fuel pump output to the AFM connector. you then have to jump that pin in the AFM connector to the fuel pump relay itself.
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Old 06-16-2015, 04:13 PM   #20
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Would that method be better than just grounding the FP in the diagnostics box?
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