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So I thought I'd look for some error codes by following the guide on miata.net (http://www.miata.net/garage/Engine_Diagnostics.html).

...But I get nothing. Turn the ignition key to ON and LED doesn't light up at all. Turned it around too, just in case I had it wired up the wrong way. Checked the voltage across FEN and B+ with a multimeter but it was 0. 0 V between GND and B+ too.

Am I doing something wrong? I vaguely remember reading something about the LED diagnostics not always working, but a google search didn't give any answers.

Any help is appreciated!
Martinez

Tried this?



When it says LED make sure you use an LED with a resistor, otherwise it will burn out pretty quickly.

Also be careful as B+ is a direct feed from the battery and has no current limiting, so it can source hundreds of amps if you short it to ground

Looks similar but slightly different technique

Reverse the led pins, they only work one way, yours is probably backwards.
"Flip" it.


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Caffeine wrote:Tried this?



When it says LED make sure you use an LED with a resistor, otherwise it will burn out pretty quickly.

Yeah, that's actually the circuit diagram I used, and with a 4.6 kOhm resistor in series with the LED to bring it within specifications.

Caffeine wrote:Also be careful as B+ is a direct feed from the battery and has no current limiting, so it can source hundreds of amps if you short it to ground

So I assume then that I should see 12 V between B+ and GND. But I get 0 V. Maybe something wrong with the diagnostics port wiring?

If B+ is 12 V, then what kind of voltages should I see at FEN? Must be more than 12 V if that LED is going to light up.

emily_mx5 wrote:Reverse the led pins, they only work one way, yours is probably backwards.

Yeah, that didn't work either. See original post

From both of your answers I do know that it *should* work! Thank you.

Martinez

Martinez wrote:

emily_mx5 wrote:Reverse the led pins, they only work one way, yours is probably backwards.

Yeah, that didn't work either. See original post

Did you just turn the LED around, or un-solder the resitor and re-solder it to the other pin? (or flip it over and re-solder to the same pin)

They only work one way around or something.



Have you checked all the fuses are ok? Not sure if there's a specific fuse for the port (or if it's fused at all)

If you don't have +12V at the B+ then you must have faulty wiring to the diagnostic port. Wiring diagrams are here. http://www.madracki.com/miata/wiring.html

Also, could you update your sig to show what model you have?

edit: here are the power connections to the unit for an NA8, copied from the POWER DISTRIBUTION circuit diagram. Appears to be 2 black/white wires, both from the engine fuse in the box beside the driver's foot. Not sure which goes to B+

hks_kansei wrote:Did you just turn the LED around, or un-solder the resitor and re-solder it to the other pin? (or flip it over and re-solder to the same pin)

Just turned the whole LED-resistor circuit around. Diodes (which LEDs are) are uni-directional, like a valve, so they only conduct in one direction (there are exceptions of course). With the resistor it doesn't matter which side of the LED it sits on or which way it's turned.

manga_blue wrote:If you don't have +12V at the B+ then you must have faulty wiring to the diagnostic port. Wiring diagrams are here. http://www.madracki.com/miata/wiring.html

Also, could you update your sig to show what model you have?

edit: here are the power connections to the unit for an NA8, copied from the POWER DISTRIBUTION circuit diagram. Appears to be 2 black/white wires, both from the engine fuse in the box beside the driver's foot. Not sure which goes to B+

Cheers! Looks like I've got some wiring debugging to do! Have updated my sig now. I've got a 1990 NA6 that has had the engine replaced with a 94-95 NA8 by the previous owner. There are a few loose connectors here and there which will probably be the topic of another post

Anyways, it's suffering from that seemingly common 'engine sometimes just dies while I'm driving' syndrome which is why I wanted to check for error codes.

Martinez wrote:Anyways, it's suffering from that seemingly common 'engine sometimes just dies while I'm driving' syndrome which is why I wanted to check for error codes.

It's not very likely that you'll get any diagnostic codes from that. You should note that the Main relay, which is depicted on the same circuit as the diagnostic 12V supply, supplies all the power to the ECU, injectors and coils.

Immobilisers are wiring cancer. If the car has ever had an immobiliser fitted then it might be a good idea to start by examining every connection made or unmade for that. Then search for any other cut and soldered wires, checking for dry joints.

manga_blue wrote:
Immobilisers are wiring cancer. If the car has ever had an immobiliser fitted then it might be a good idea to start by examining every connection made or unmade for that. Then search for any other cut and soldered wires, checking for dry joints.

Yup ... what manga said.

Pull the under dash panel off, center console and tombstone out as well as the glove box. Check every wire and connection. Some bodgy installers out there. I spent weeks on my NA's wiring. There were remnants of two alarms, an immobilizer, some sort of radar detector, two bunches of old head unit wiring and just bare wires that I never worked out what they did! Would not believe the amount of unnecessary wiring I pulled out!

Then I started under the bonnet ... !

But it all made a huge difference to how the car started and ran

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KevGoat wrote:Yup ... what manga said.

Great! ;) It's got an aftermarket immobiliser fitted which is a pain to use, but I didn't think it could be the cause of random engine shutdowns. Thanks, I'll remember that.

Haha, and like yourself I've also found some weird switches under the dash and loose connectors in the engine bay. But I haven't really made an effort to sort it all out.

Looks like I'm in for a long wire cleanup weekend! Will also have a look at the main relay as hinted by manga.

Martinez wrote:Looks like I'm in for a long wire cleanup weekend! Will also have a look at the main relay as hinted by manga.

The Main relay is known to fail but it's probably more likely that you have a wiring problem somewhere after the Engine fuse. First I'd try tracking things from there towards the Main relay and towards the diagnostic module.

So I've started doing some wiring clean up... Gah, it's going to take a while. Found a bunch of loose wires, unterminated connectors and aftermarket devices. Got rid of some of some Jaycar Air/Fuel display unit with spliced wires that were exposed. Taped up whatever exposed wire I could find. The diagnostics port still doesn't work, but I feel better

A bit of mucking around and I've now got 12 V at B+. However, the diagnostics LED still doesn't work. I hooked up the multimeter between FEN and B+ and noticed that there is a small voltage that slowly (~10 seconds) goes up to 400 mV and then cycles back to 0 V. Not sure what's going on there. It's not enough voltage to drive the LED.

On the brighter side of things, I think I've solved the problem with the engine cutting out: Loose AFM connector! For the hell of it I started the car and pulled the AFM connector out. Yep, engine dies while all electrics still work. Just the same symptoms as I had previously.