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S Chassis Technical discussion related to the S Chassis such as the S12, S13, S14, and S15.


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Old 06-09-2022, 04:26 PM   #1
RaidenKing
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Mystery yellow wire with blue stripe

I've been working on an s14 with SR notchtop for some time now. I noticed after a new Tomei cam install that the idle would drop when the car warmed up. In testing, I've almost certainly ruled out leaks, air, and fuel issues. No vac leaks to be found, fuel pressure runs around 38 psi when connected and running, MAF is a new replacement confirmed to be working.


Notable moments of idle drop usually happen when trying to run the AC or when the fans kick on at about 185F. I have tried extra grounds, but this hasn't really helped aside from freeing up maybe .3 volts.

Seriously eyeing the alternator being the primary problem at this point, but I found this long yellow wire with a blue stripe (cut) coming out of the loom near the alt area. This was a car I bought from someone else as a project, so no idea if that is a now unused auto to manual wire, or something necessary for my power.

Any ideas on what the (Y/BL) wire could be for? The idle only comes down from 850 once the car has warmed up, and seems exacerbated when more accessories power on. If left as is, the car eventually stalls.

Any advice or info is appreciated. I've tried searching everywhere, but half the results wind up pointing me over to N/A Sr20 forums.
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Old 06-09-2022, 06:26 PM   #2
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Check and service your idle air control valve.

My S13 has similar symptoms.
...I have no IACV at all.

Fans, AC ETC draw a ton of power, especially when they first start. Seems like your IACV can't / isn't compensating.
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Old 06-09-2022, 11:21 PM   #3
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just a clarification

Y/B = Yellow wire / Black Stripe
Y/L = Yellow wire / Blue Stripe

the alternator has a small yellow wire that goes from the alternator to a 7.5 amp fuse in your engine bay fusebox and runs from there to a solid white wire tied up to your 100a fuse also in the engine bay in slot A. in the usdm fsm the yellow wire branches off and goes to the smj on the driver side

in the japanese fsm a W/R wire runs from the alternator to the super multiple junction on the driver side to your cluster.

there is no yellow/blue wire that goes to the charging system in this area.

The idle air control valve is only 2 wires, should be black for ground and B/Y (Black wire with a Yellow Stripe)

that wire should be running to fuse 32 in your engine bay fusebox and through your super multiple junction from the fusebox to your ecu female plug.

hopefully that information helps.

you can download the fsm with the sr20 notchtop in it here. : https://drive.google.com/file/d/11Vf...ew?usp=sharing

the page I'm referencing is EC-70 for the Idle Air control valve and EL-39 for the Charging System
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Old 06-10-2022, 09:50 AM   #4
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Quote:
Originally Posted by tuzzio View Post
Check and service your idle air control valve.

My S13 has similar symptoms.
...I have no IACV at all.

Fans, AC ETC draw a ton of power, especially when they first start. Seems like your IACV can't / isn't compensating.
Thank you, I went ahead and fully disassembled the IACV this morning and sprayed down with carb cleaner. Also took small bits of paper towels and Q-tips to get into those harder to reach carbon filled places. I had sprayed out the main body with carb cleaner before, but never really hit those important spots. Reassembled it with the springs in the right places and took a test drive.

It seems to have helped a little, as I drove for a couple hours today with about a 50/50 shot of my revs dipping between 500-850, or hanging out at 850-1k. In the times it dipped around the 500-850 zone, the car would eventually stall if not given a little throttle blip.

I noticed when placing it in neutral to coast up to a stoplight, the revs would mostly catch and hover at 1.5k - 1.2k before going back under 1k. After going under 1k, it was still that 50/50 shot.

Strangely, the idle was fine when the fans came on before I drove it. The problem seems to arise more when I have given it some revs or driven, as hitting low revs had manifested before I was able to get out of the neighborhood.

Although there is the other strange part where about half of the red lights I stopped at had a perfect idle. I don't know what could be making this so inconsistent? I checked for continuity on the plugs for both sides as well, and everything seems to be properly pinned/flowing.
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Old 06-10-2022, 10:03 AM   #5
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Quote:
Originally Posted by Om1kron View Post
just a clarification

Y/B = Yellow wire / Black Stripe
Y/L = Yellow wire / Blue Stripe

the alternator has a small yellow wire that goes from the alternator to a 7.5 amp fuse in your engine bay fusebox and runs from there to a solid white wire tied up to your 100a fuse also in the engine bay in slot A. in the usdm fsm the yellow wire branches off and goes to the smj on the driver side

in the japanese fsm a W/R wire runs from the alternator to the super multiple junction on the driver side to your cluster.

there is no yellow/blue wire that goes to the charging system in this area.

The idle air control valve is only 2 wires, should be black for ground and B/Y (Black wire with a Yellow Stripe)

that wire should be running to fuse 32 in your engine bay fusebox and through your super multiple junction from the fusebox to your ecu female plug.

hopefully that information helps.

you can download the fsm with the sr20 notchtop in it here. : https://drive.google.com/file/d/11Vf...ew?usp=sharing

the page I'm referencing is EC-70 for the Idle Air control valve and EL-39 for the Charging System
You're absolutely right with the mistake I made on labeling the wires. I should have said Y/L there. It hit me last night and I never corrected it.

Also thank you for the proper FSM. I've been scavenging little bits of other FSMs and I can't understand why they're so hard to find on a google search. Big help! That's the one I needed.

In line with this Y/L wire possibly having been from an auto-manual swap done on the car, I also noticed my original TPS had 2 plugs to connect into. My understanding is that the 2 plug TPS is specific to automatics. Everything I've found on searches regarding Y/L wires seem to apply to manual conversions so far. Chances are it is an unnecessary connection, though I do not like having this mysterious cut Y/L wire just kind of dangling around in the engine bay just behind the alternator.


In addition to the things I mentioned in the original post, I also timed the car to +15, and just completely disassembled the IACV to spray all parts down with carb cleaner and get as much excess carbon off as possible. TPS has also been adjusted as best I could for the .5 closed to 5 open. I had recently replaced intake manifold gaskets as well, but my understanding is that the idle would run high if a leak existed there.

The problem I'm having now is while the idle is improved, it is not consistent. When coming to a stop, I seem to have a 50/50 shot of the rpms dancing between 500-850, or 850-1k where its really happiest. Only seems to happen after I've driven it post-warm up to any extent. I have a P2M fan controller that sets the fans running at 185F, but it is also able to monitor voltage in the system.

I've noticed when the RPMS drop, so does the voltage. Generally I'm seeing between 12.0V-13.8V on the scale, then hitting 11v area when idle drops even lower. I do not know however, if the RPM drop is what's causing the voltage drop - or if it's the voltage drop causing the RPMs to go down. When I pulled into the driveway after a long cruise, the car idled down until it completely stalled out.

Adding to the mystery of all this, the car drives great in revs. No stutters or issues anytime I get going, and she pulls like the devil all the way to redline on these Tomei 258's and GT2871r. Just wish I could figure out this idle.

Edit: Adding one other thing of note. The cam gear I have in the car is from a redtop, so there is no longer VTC functionality. Could that be causing an issue for me where still having it plugged in could mess with the idle in any way? Would disconnecting the VTC plug cause any damage?

Last edited by RaidenKing; 06-10-2022 at 10:40 AM..
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Old 06-10-2022, 10:51 AM   #6
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I got an s13sr with a nissan quest alternator. I'd pop your ecu out and check for codes, if you have consult hooked up, leverage that if possible. I always feel upgrading to an aftermarket ecu and harness is always going to be the answer to a majority of my problems because I can see the faults in realtime instead of chasing demons.

Every now and again my car will randomly throw knock if it's cold and dump fuel fowling my plugs and 02 sensor until I heat the car up enough driving like an asshole to get rid of it all cause I'm too lazy to pull it off and clean the fuel and carbon. lol
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Old 06-11-2022, 07:21 AM   #7
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Quote:
Originally Posted by Om1kron View Post
I got an s13sr with a nissan quest alternator. I'd pop your ecu out and check for codes, if you have consult hooked up, leverage that if possible. I always feel upgrading to an aftermarket ecu and harness is always going to be the answer to a majority of my problems because I can see the faults in realtime instead of chasing demons.

Every now and again my car will randomly throw knock if it's cold and dump fuel fowling my plugs and 02 sensor until I heat the car up enough driving like an asshole to get rid of it all cause I'm too lazy to pull it off and clean the fuel and carbon. lol

I've been looking into getting a consult just to see if there are codes being thrown. As of now, the only light I have on in the cluster is airbag - typical 240 stuff. Should I be looking for a specific version of the consult? Or will the newest version I can find cover everything? The ECU I have right now is a Martin special from Enthalpy. Tuned to my current modifications and all that.

I keep inspecting and finding new issues. Saw a loose nut on an exhaust manifold stud. Tried tightening and found that the stud itself had loosened. Pulled it out to find the stud took some interior metal with it in the threads. I was able to tighten it back in with a new stud as best I can, but the next thing for me to figure out is how to get a new stud in there nice and tight now that the stud hole is bigger. Have a feeling I'll have to throw in a bigger bolt in place of the stud. Had this happen once already with a CAS bolt where I had to go a size bigger because the threads had dug metal out on the inside.

Oh glorious fun!
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Old 06-12-2022, 01:28 PM   #8
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Helicoil time.
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Old 06-14-2022, 10:35 AM   #9
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The Yellow/Blue wire from behind your alternator is the oil pressure switch that goes directly to the oil light in the cluster. Check and see if it's just not plugged in or what.

If you see a Yellow/Red wire in the same area, then that SR came from a S14 Kouki that was equipped with the triple DIN gauge pod. Y/R is for the oil pressure gauge.
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Old 06-14-2022, 10:50 AM   #10
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Few other ideas and things to check.

The S14/S15 SR throttle body has a wax-based idle control. Yes you read that correctly. You'll notice a cam with a few hash marks under the throttle body, as the engine warms up, the cam adjusts and drops the idle. This is mainly a cold start fix. The FSM has ways to test this out. So make sure your TPS was set to ~0.5V with a warm engine otherwise it'll drop to something like 0.25V.

Is the neutral switch from the trans connected to the ECU? Mine didn't want to idle correctly or trigger the VTC until I fixed the neutral wiring in mine.
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Old 06-15-2022, 05:58 AM   #11
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Quote:
Originally Posted by LoSt180 View Post
The Yellow/Blue wire from behind your alternator is the oil pressure switch that goes directly to the oil light in the cluster. Check and see if it's just not plugged in or what.

If you see a Yellow/Red wire in the same area, then that SR came from a S14 Kouki that was equipped with the triple DIN gauge pod. Y/R is for the oil pressure gauge.

Thank you for the extra input. I'm going to double check on everything you mentioned. That also gives me good reason to look further into the Y/L wire connection.

Took it out for another brief drive the other day and experienced less idle drop. Cold starts have been the best idles so far. Starts up and slowly steps down from about 1200-1500 rpm. I only experience idle issues after warm up. I have a backup P2M alternator to test out also, since the current one is old and beat.

I'm due to change oil as well since the new cams have had time to break in. Will see how the next few test runs go and come back with an update. Strangest part is just how inconsistent this has been, but further testing has confirmed idle will always drop below normal range when I turn on the AC.

I love Zilvia, you guys are awesome and thank you for helping me troubleshoot this thing.
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Old 06-24-2022, 05:29 PM   #12
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Alternator replaced, idle issue resolved. Not without drama of course, as I managed to hit the new alternator with a ratchet on accident when doing my oil afterwards. Sparks flew, and I nuked my 75A fuse. All well after fuse replaced.

Now when it dips in rpms, it recovers very quickly instead of stair stepping down to a stall.

I'm assuming the old alternator wasn't putting out enough to also power accessories, so the fans kicking on after warmup had enough draw to dip the motor for some reason. It would also explain why night drives had consistently bad idling (lights) before changing out the alt.

Also Lost180 - regarding the y/l wire, I think you were right on target with that. I changed the oil and noticed that I'm missing the oil pressure sensor. That cut wire must have gone to my now nonexistant sensor at one point of its life.

Seeing a max of about 185F coolant temps, and a lopey minor bounce off of 850 on idle. The cams are only 258's after all, but it was bizarre that this problem chose to show itself immediately after I upgraded cams and replaced intake manifold gaskets. Maybe the old alternator didn't appreciate getting drenched in coolant when I was doing manifold gaskets.

Thanks guys again, big time.
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