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G4 Storm + Miata VVT = Flowback voltage from solenoids? (IAC + VVT)


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Hello there, I have a miata '01(nb2 vvt) that I bought used and I am trying to sort some things out.

The previous owner told that there is a ground issue with the ECU that needs to be shorted but he didnt have time to get on that.

So I am trying to figure out what the hell is going on. 

its not a plug in ECU, with a custom wiring loom.

I checked the harness wire by wire and there is no shortage or anything weird going on and ecu grounds ok.

The weird thing is that what he thought as a ground problem, after all is that when the idle control valve or the vvt are operating, the ECU reads elevated voltage that progressively goes 17+v(and fluctuating) depending on their duty cycle. As I understand there is a flowback voltage from the solenoids(?) but I cannot figure out why.

If I turn them off, everything is back fine and the ECU reads steady voltage.

Are there any suggestions on this?

I am afraid that even if I start making the entire engine loom from the beginning I ll face the same issue

 

 

edit: attached you will find the schematic of the wiring.

wiring_diag.jpg

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ok.. I think I got it.

All this time I was focused on the grounds and the power to the accessories but never tried to move the power of the ECU.

As you saw in the schematic, the same line that powers the ECU(from ignition switch of the key) its also powering the "hot" side of the relay coils for the accessories.

As last try to figure this out, I got the ECU power connected directly to the battery I got a very little flowback from the solenoids.

Before when engaging both solenoids from 12.3v(engine off) to ~15v instantly(when engine running add ~+2v)

now from 12.3, when engaging the solenoids it fluctuated between 12.31-12.32. Thats some progress but yet it has a bit of flowback that I think it may be acceptable(?)

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The problem is the diode you have drawn in the main ECU power supply - you need to remove that.  The flyback from the solenoids needs to be able to go back to the battery though that wire, having the diode there will prevent that.  With nowhere for the flyback to go it will just "pump up" the ECU voltage.

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First of all, thanks for your reply.

On the issue now, I didnt know that the diode could cause that issue, but I did try that(to remove the diode) and it turns out that they put it there because without it, after turning the ignition key off, because of flow back(I assume) it wouldn't disengage the ignition relay(which also powers sensors/solenoids etc) and the car shut down because of fuel starvation( and to be honest I never checked if the volt issue was fixed with the removal of the diode).

So from what I understand, either way I have to get a different source of power to either the hot side of the coils of the relays(I use the same coil feed for the ignition/fan/hs fan relays) or the ecu.

In general the wiring on the car is a bit sketchy(i.e they have bypassed the main engine relay) but I want to try to avoid doing the whole wiring for now. 

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The back feed can only be caused by one of the accesories that is connected to an aux output being "hot fed" (live all the time, not ignition switched).  Remove the diode, find which aux is causing the backfeed and move it to an ignition switched feed.  The easiest way is often to pull say 2 aux output pins out of the main connector at a time until the problem goes away, then you can narrow down which one it is.  Ignition outputs and injector outputs on the Storm cant backfeed so it is only the aux outputs you need to investigate.  

Correct main power wiring is below, and I have also attached some extra reading about backfeeding from the help file. 

QOYqpOt.png

2020-10-29_14-09-22.png

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Thanks for the tips.

 

As far as I checked I dont have any of the accessories that are hot fed.

The only aux that I see connected(and managed by the link) are

VVT solenoid (12v feed from ignition coil)

ISC Solenoid (12V feed from egnition coil)

Ignition Relay coil(As I told you before this relay feed 12v to coils, injectors, solenoid, and 12v sensors)

Fuel Relay Coil

Engine Fan Relay coil

Engine Fan HS Relay coil

Non of those are hot wired

 

When I ll have more time I ll try to check again

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As I say, unplug a couple of aux pins at a time until you find your culprit.  They come out easy - no tools needed.

You want at least Injectors and ignition coils to remain connected to the same source as the ecu so it can apply correct deadtimes and dwell times. 

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So, I changed the wiring based on the diagram and with your inputs(i didnt try the aux I went full send :p)  and everything is working fine as I can tell.

 

So its time to tune this thing.

 

Thanks again for your help and your time

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