Electrical Issue with a '98 NB8 (V-Special)

dkbuchanan · Post by **dkbuchanan** » Mon Dec 08, 2014 6:25 pm

Hi All,

I'm new here, and have only had my mx-5 for just over a month, and on Saturday, it started doing some really weird shiz.

I took off from the lights and all of a sudden it just dropped 2k rpm out of nowhere, did more or less the same thing at the end of the road, and it did it again, but the revs bounced all over the place, drop 2k, back up, drop 4k, back up etc.

Drove it quietly to a mates place, and then took it down the road to see if we could figure out what was up, got halfway down the road and it just died, revs went straight to zero and wouldn't start after that.

Pushed it back to his place and tried to start it, long story short I got it to the garage (it was booked in for a warrant anyway) and left it there over the weekend.

I haven't a clue what is going on, any suggestions or has anyone had a similar experience?

So far the garage reckon that its not a sensor and could be either coil pack or ecu, but I'm not 100% sure as I don't really have much of an idea of what goes wrong with these engines (yet).

Any help or advice would be MUCH appreciated.

Kind Regards,
Daniel.

evel · Post by **evel** » Mon Dec 08, 2014 9:49 pm

Not trying to sound like a dick but did it have fuel in it? If so is the fuel very old, like has the car sat for a while and you just took her out for a spin? Could also be the fuel pump that has died. Did it only drop all its revs after a certain point when accelerating ?

dkbuchanan · Post by **dkbuchanan** » Mon Dec 08, 2014 10:26 pm

Not at all. It had a full tank, filled up on Friday night.

I've been driving at least a couple of times a week lately.

Somewhat, it kind of only did it above 3k, but not reliably, and only when you really gave it herbs.

I thought fuel pump, but it idles okay, and if the fuel pump was poked, I'd expect it to not idle at all.

WideOpenThrottle · Post by **WideOpenThrottle** » Tue Dec 09, 2014 8:24 am

Could be lack of fuel, could also be lack of spark, may be an intake track vacuum leak. Could even possibly a timing issue {CAS or belt]. No doubt you will post what is was as we have an NB too and I'd be curious to know!

Hope it turns out simple and inexpensive.

dkbuchanan · Post by **dkbuchanan** » Tue Dec 09, 2014 11:55 am

Yeah, it could be a lot of things, fingers crossed I will hear more from the garage today.

Last I heard they were trying to source a replacement coil pack to rule that out. They have tested all the sensors apparently and everything looks normal even when it plays up, so not sure what is going on there.

I'll definitely post once I know what is actually going on.

SLYDIT · Post by **SLYDIT** » Wed Dec 10, 2014 9:32 pm

The most common problem with nbs is the cas +crank sensor on the front. if i was doing parts swapping. id start with those

Ian · Post by **Ian** » Wed Dec 10, 2014 9:53 pm

The NB1 coil packs are notorious for failing...which is part of the reason for COP conversion popularity.

dkbuchanan · Post by **dkbuchanan** » Wed Dec 10, 2014 10:24 pm

Thanks for the suggestions.

At this point, the garage have replace the ECU and reckon that the CAS and Crank sensor are fine, as the waveforms are coming up normal.

I'm not convinced its a coil pack problem, as the symptoms seem nothing like those normally found with the NB1 coil pack faults.

So far it will reliably cut ignition above 3.5k rpm and will bounce around a bit of you give it some herbs, but mostly when the engine is under load, i.e. not when free-revving.

I'm kind of at my wits end here, and am probably going to tell the garage to call it quits as the bill is already mounting and its getting rather expensive. Though I might get them to check the grounds first.

They seem to think that the ingiter circuit is in the ECU, but all the documentation I and marcellarius can find says that its actually part of the Coil Packs and the ECU just signals it to fire on a different circuit.

Any suggestions?

evel · Post by **evel** » Wed Dec 10, 2014 10:45 pm

Would of though the coils would have their own ignitor and would just be earthed through the ECU like you said.

Cut your losses and get a Link?

Teamjelly · Post by **Teamjelly** » Wed Dec 10, 2014 11:35 pm

Try some new leads and some new resistive plugs (NKG BKR6e)
The leads could be failing, putting the spark energy into the block and ground plane which will cause all sorts of havoc with the stock ECU.

Or could be a the NB Cam sensor.
They are prone to failure - I had one fail, and it sent the spark timing so advanced it broke all four rods instantly and blew holes in the sump/block

The garage should have scoped the ignition output, and see what's happening or at least check all the sparks with a timing light. - not just replace parts until it hopefully is fixed and you end up with a huge bill for what normally is a simple issue.

The NB coilpacks have inbuilt igniters - and are normally pretty reliable

dkbuchanan · Post by **dkbuchanan** » Thu Dec 11, 2014 1:14 pm

Thanks for the suggestions.

The plugs have been replaced, and were pretty shot anyway.

They have scoped the ignition output and seen it doing weird stuff when it was playing up. However I disagree with their diagnosis that the circuit between the coils and the ECU is shot, I would expect if the coils were having issues with the igniter circuit that the transistor in the coil packs would have packed a sad, not the other end. I have asked them to double check the signals and swap the old ECU back in to see if it actually made any difference whatsoever given it is still doing the same thing with the ignition cut when it plays up.

As of this morning I have asked them to go back and check the grounds, cam sensor and crank sensor (take them out if necessary).

SLYDIT · Post by **SLYDIT** » Thu Dec 11, 2014 8:22 pm

90% of the time when a mechanic says the ECU is faulty .....it's not. and you need to find another mechanic! i dont think ive ever heard of an MX5 ecu fail in NZ.

replace plugs
replace leads
reterminate ALL earths ECU earth Coil earth block and chassis earths.

borrow a CAS and try that.
borrow some coils and try those.

if that doesnt sort it then you need to stop and get an mx5 specialist to look at it at the very least.

SLYDIT · Post by **SLYDIT** » Thu Dec 11, 2014 8:27 pm

not sure if yours has the single wire crank sensor down the front bottom of the engine but the later ones that have it often fail where the loom is held to the block by a P clip causing erratic issues. the wire breaks inside the sheath at the p clip... also the sensor needs to be gapped properly to the wheel

dkbuchanan · Post by **dkbuchanan** » Thu Dec 11, 2014 9:32 pm

Thanks for the suggestions SLYDIT.

I'll definitely have a work through those, the plugs have been replaced and were pretty shagged.

I was wondering if it was the earth on the coil packs as they earth directly to the block, and as the problem seems to be heat/motion related (i.e. idles fine, and doesnt seem to do it when cold, but when hot and revving is like clockwork).

They are going to continue with checking the grounds etc on Monday (short day tomorrow for work function), but if that comes up empty I may consider taking it to AutoWay, although they have called Don and apparently he had not seen anything like it and had no suggestions other than coil packs. That being said they will likely go through the same procedures again, so I'm stuck between a bit of a rock and a hard place lol.

Anyway, I'll keep y'all updated as to where I get to and what happens, but I don't expect to get anywhere before the end of Jan, although I'll probably try to get it back over Christmas/New Year and have a try of a few things on my two weeks off.

dkbuchanan · Post by **dkbuchanan** » Tue Dec 16, 2014 2:52 pm

Right, well I went to see the garage yesterday after work.

They finally agree with me that it is not the ECU, and more than likely not the coil packs.

They have checked and cleaned all the grounds, and currently have the crank sensor out, as it was giving weird signals, they are going to swap it with another one today to check that.

@SLYDIT, the crank sensor is three wire (in this car), +, -, signal I believe. Is that what you meant by single wire?

dkbuchanan · Post by **dkbuchanan** » Tue Dec 16, 2014 6:44 pm

Another update.

I dropped by the garage this afternoon after work, and it seems that all of this was caused by a faulty crank sensor.

It wasn't signalling cleaning when it was on, the signal was jittery. That is now replaced and it seems to be running fine, one final check tomorrow and a warrant and I should have it back!

Moral of the story, pay close attention to the signals, not just that you ARE getting a signal.

Thanks heaps for all your suggestions and pointing me in the right direction.

dkbuchanan · Post by **dkbuchanan** » Wed Dec 17, 2014 6:08 pm

Got the car back!

All seems fine, we will see how it goes over the next week or so.

Thanks everyone for your suggestions and help, MUCH appreciated.

Teamjelly · Post by **Teamjelly** » Wed Dec 17, 2014 6:14 pm

Glad she's up and running again
do we want to know what the bill ended up with all the mechanics fuddling around in the dark?

dkbuchanan · Post by **dkbuchanan** » Wed Dec 17, 2014 6:17 pm

It wasn't actually as bad as I thought it would be, most likely because they didn't end up replacing the ECU or the Coil Packs. The total came to around $900 including replacing a few belts that needed doing (delaminating) and the warrant which oddly enough was what it originally went in for!

SLYDIT · Post by **SLYDIT** » Wed Dec 17, 2014 8:05 pm

dkbuchanan wrote: @SLYDIT, the crank sensor is three wire (in this car), +, -, signal I believe. Is that what you meant by single wire?

theres another down on the crank nose ...might only be on the vvti engines. your symptoms certainly screamed CAS once the basics were done.

dkbuchanan · Post by **dkbuchanan** » Wed Dec 17, 2014 9:42 pm

This was the sensor right down by the crank pulley which has notches on it which presumably signal TDC for each cylinder. Is that the one you meant?

Hmm yeah, it was quite interesting when they showed me the signal waveforms, the waveform from the old sensor, when signalling was really jittery, only small variations, but still quite a lot of jitter, whereas the replacement sensor was significantly less jittery when signalling.

Electrical Issue with a '98 NB8 (V-Special)

Electrical Issue with a '98 NB8 (V-Special)

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