37 replies to this topic

[Gary Moser]

plugs 1 and 4 fire together as does 2 and 3. only one coil fires once in the first couple of revolutions then nothing.

 

have pretty much eliminated the immobilizer circuit after spending a long time on the phone with a Mazda technician.

 

replaced several fuse that they recommended even though they tested good.

 

something is not telling the coils to fire

[davew]

Have you scanned for diagnostic codes. If you get a handfull of codes, record them, clear and try to start again. Then rescan.

 

I would guess you have one of 3 things wrong, cam sensor, crank sensor or air flow meter. When any of these do not sense movement of cam, crank or air it will shut off the system.

[Gary Moser]

shows no codes, cam and crank position sensor swapped for ones off of a running car

[FTodaro]

I have had to fix or repair many issues in our miata's over the years but last summer when i developed a miss in my VVT under a load, it was one of the most frustrating repairs to run down. Electrical issues are a bitch.

 

No need for me to go into the details of what it was but my point is that you have to start eliminating as many possibilities as you can.

 

It can be costly, but my first suggestion is that you have to be sure that the replacement parts you are swapping are in deed good. so i would either use new parts or if its used that you do one at a time and then you put that part back on the original car and confirm that it still runs, in other words verify that that used part works before and after you used it.

 

The last thing i replaced was the problem, the wiring harness.

 

You might just start there.

[Gary Moser]

there is a condensor mounted on the back of the head, anyone know what it does? I plan on replacing it with a new one.

 

been told to verify that the fuse box itself is supplying power to the fuses

 

will be cleaning all grounds that I can find

 

have finally been able to contact a Master Mazda mechanic willing to stop by and look at it

[Gary Moser]

had a Master Mazda Technician spend 6 hours diagnosing and scanning the car

 

two things stand out as being wrong, the egr valve is on and gets hot and the trigger voltage to the coils is always on and constant

 

tested the coils individually and they are good, for some reason the ecu is sending a constant triggering voltage to the coils

 

crank and camshaft position sensors are working

 

maf is good

[Gary Moser]

did scan for codes, none present except for egr

 

refernce voltages correct

 

grounds and power leads all good

[davew]

I have not gone out in the shop to verify this. But in general the ECU does not supply 12 volt source to any controlled device (EGR, Injector, O2 sensor). It will control the ground as a way of turning things on or off. The ECU does supply the 5 volt reference to sensors (TPS, ECT, etc.) 

[Gary Moser]

with the ecu disconnected and the ignition on, we were able to fire the coil by supplying 12 volts to the trigger wire

[davew]

Again, I have not studied a wire diagram.

 

Typically the coil is switched on and off by the ground side of the circuit. The ground side is controlled by the switch mechanism (points on an old car, ignition module on middle aged cars, and ecu on a new car). So if the ecu is unplugged, the ecu is not grounding that side of the circuit. If by touching the power side of the circuit, the spark fires, you by default have to have a always grounded situation in the ground circuit. Remember that a coil fires when the field is built up (power and ground are applied) and fires when the field collapses (either power or ground is removed). If both power and ground are constantly applied, the field never collapses and you never get spark. This is basic high school automotive theory 101.

 

I would look for a ground circuit that has a pinched wire someplace on that coil.

[Steve Scheifler]

SOLVED:

 

A few months late but for those who might run into similar problems I wanted to follow-up on this with what we found and how.  Believe me, this is the highly condensed version.

 

As mentioned already, multiple people tried their hand at troubleshooting this thing including a "30 year Mazda master tech".   Each repeated and reaffirmed the prior work and occasionally tried something new but to no avail. One thing that kept coming up was the immobilizer which I felt we had eliminated, but the way that the ECU responded to the procedure for reprogramming it was particularly odd and inexplicable so we couldn't rule it out conclusively.

 

I finally installed a Megasquirt PNP because it eliminates the immobilizer crap and has extensive logging capabilities.  With much cranking and sputtering the car would occasionally start but run like crap with lots of sync errors and VVT errors.  Monitoring the VVT position vs VVT maps it was clear that the VVT would get stuck in a position and the only way to break it loose again was to ramp the actuator duty cycle way up and then abruptly drop it back down. Then it would work until the next time the engine was stopped.  While it was working it ran fine even on the dyno.  This seemed to make sense because it explained both the sync VVT errors.  So, repair or replace the VVT mechanism and related pieces and we should be good.  Well, no.  I did find that the VVT actuator had been apart before and reassembled incorrectly so I cleaned it up and put it back together. The car still had bad sync errors. Opened it up again and compared the intake cam and cam position trigger lobes to a spare we had.  There was no obvious damage or conspicuous difference in orientation and the lobe ring was tight on the cam.  But the spare VVT mechanism was significantly different internally, and I think a better design (I'm guessing from a later year).  So I decided to swap that in, but because of the differences I also needed to swap the cam gear which of course meant pulling the belt again.  At that point I figured we may as well pull the intake cam for a much closer look.  Bingo!  When I placed the cam next to the spare in the identical position, the sensor pickup lobes were in a slightly different position.  And when I looked real closely I could see that on the one we took out it had not been pressed fully onto the back of the cam.  I'm told that the stock ECU won't tolerate moving that very much and still pass the sync test, which makes sense.  Add to that a VVT mechanism that may get stuck in the wrong position and maybe an old stretched belt, and things are too far out so the ECU just doesn't allow spark.  Unfortunate the ECU also did not provide any useful clues in the form of fault codes.  The Megasquirt doesn't care about any of that if you don't want it to so we could manipulate things on the fly while logging all the sensors and that helped us to focus attention in the right area.   Once we had it back together correctly it fired right up and ran perfectly though numerous dyno tests on both the Megasquirt and the stock ECU.

 

This car was built from new as an SSB for a prior owner of this site and has passed through several owners and shops since then, so there is no way of knowing when or by whom, but somebody decided to cheat a little on cam timing by moving the trigger lobes.  That might seem a little odd on a car that already has VVT but it's still designed to work within a range intended for the street with compromises not ideal for the track.  I seriously doubt it made a big difference but as usual with these cars it is many small things that add rather than one big conspicuous cheat.

 

So, recommendation to SM tech for next year, develop a simple way to check the trigger lobe orientation on the intakes cams.  Let me know if you need more information or help.

 

 

 

[chris haldeman]

Please explain to me how that ring changes cam timing?

[Steve Scheifler]

That's a very good question actually, since cam timing is a physical relationship between parts and not a matter of when the spark is fired relative to them.  So what this did was change where it thought the intake cam was in relation to the crank when the ignition is fired. Totally different.  My deep apologies, I obviously didn't think that through very well after finally getting the sucker running.  Odd though, why do it? It isn't the same as changing ignition timing since that, I assume, is off the crank pickup.

[Steve Scheifler]

OK, three edits into the reply I'll just start a new one.

 

I recall being a bit perplexed by this at the time and never really took time to suss it out or lock a clear explanation into my own head. So the ignition is triggered by the crank sensor. On a non-VVT the cam sensor would let the ECU know whether the cams are in sync with the crank and not fire if they are way too far off, but maybe that's about it. Right? (NA cars have only the one sensor). 

 

But the VVT can dynamically change intake cam timing based on a "map" similar to the ignition timing and fuel maps.  So in that system, if the ECU is tricked into thinking that the intake cam is in a different position than it really is, will that in turn change how much it tries to move the VVT actuator?  For example, with the Megasquirt I can overlay two round dial gauges, one for actual cam position as set by the VVT mechanism and one for the position it wants to achieve according to the map. If the cam is not where it "should" be at any given time the ECU can adjust VVT duty cycle up or down to reposition the cam (i.e. change cam timing).  If all that holds true with the stock ECU, and I think it pretty much has to, then indeed changing the orientation of the cam trigger will in fact cause a change to intake cam timing because the ECU will literally and physically move it to match the map.

 

Can I take back my apology, or at least amend it to "correct but incomplete"?

[chris haldeman]

i need to log a dyno pull with the stock ecu and re evaluate whether the ecu send a pulse width signal or just a single hole smash on off
Or you could for me :-) I am beyond busy

[Steve Scheifler]

For the VVT mechanism/actuator? That car is now out if commission with a dead gearbox so I won’t be able to put it on the dyno anytime soon. But based on the physical design of the system it is certainly intended to be truly variable based on whatever conditions, not just on/off (like the original V-tec just switching to an alternate set of cam lobes). The cool thing about a stand-alone ECU is that as with fuel and ingnition you can remap it, within the physically available range of course. On the NSX all I can do is move the changeover point.

[Bench Racer]

plugs 1 and 4 fire together as does 2 and 3. only one coil fires once in the first couple of revolutions then nothing.

 

have pretty much eliminated the immobilizer circuit after spending a long time on the phone with a Mazda technician.

 

replaced several fuse that they recommended even though they tested good.

 

something is not telling the coils to fire

Guys, I'm interestingly following this thread. Does this ^ first sentence fall inline with your current observations?

[Steve Scheifler]

Yes, but that’s making some basic assumptions, primarily that if while cranking the stock ECU determines there is a serious sync issue (cam vs crank) it just throws its hands up and supresses spark (and probably fuel).

BTW, after that post we did confirm that which of the pair of plugs off a given coil fired was primarily path of least resistance.

Do you see an inconsistency?