NB8A Alternator over charging

[DTH]

Hi guys, my '98 NB8A battery cooked last Monday. The auto electrician suggested the alternator had been over charging & when disassembled some of its parts had started to melt. He also suggested the on board computer might have something to do with the over charging. Any ideas will be appreciated, thanks.

[greenMachine]

The NB8A has the voltage controlled by the ECU (only MX5 to do so?). It is possible but highly unlikely that the ECU is the cause of the alternator problem - I have never heard of that happening, fwiw. It is much more likely to be a common or garden problem of the alternator failing due to reaching the end of its working life - it might be nearly 20 years old if it is the original!

Replacement of the dead original with the correct (ECU controlled) alternator would show whether the voltage is in spec, in which case you will know whether there is a problem elsewhere. If you need an NB8A ECU, I can probably find a couple in my shed.

Replacing it with an internally regulated alternator is also popular, and would sideline any ECU issues as far as the voltage is concerned.

[DTH]

Thanks for your informative response, greenMachine. According to the auto electrician the alternator is a cheap Chinese version so not the original. I'm waiting to hear his progress report today.
I'll bear in mind your offer of an ECU from your shed if needed, thank you.

[rascal]

I'd also vote for the alternator itself.
My NB8A is on its 3rd (two replacements) in 9 years of ownership, though mine cop a hard time from continual high rpm use

[DTH]

Hi again greenMachine,
The auto electrician has had a computer guy check it & he says the ECU is at fault or words to that effect.
I've put your suggestion re an internally regulated alternator which he says is fine but the battery warning light on the dash comes on and stays on. Is there some way around this?

Other than being able to do that I need a replacement. The numbers I've been given are BP5E 18 881A 279700-0121
The car is a '98 NB8A in the delightful Emerald Green :-)
Please advise if you can supply a serviceable replacement & cost.

Regards,
david

[93_Clubman]

Just out of curiosity, did you replace the cheap HIDs your NB8A came with when you bought it:
viewtopic.php?f=55&t=71388&p=888127#p888127

[DTH]

Well spotted 93_Clubman. Yes I did. A pair of 130+ globes fitted well and are an improvement (at least I now have a high beam) but not as much as I'd hoped. Although I've cleaned the lenses with Rain-X twice perhaps the plastic reduces the light somewhat. Thanks for your interest. Cheers

[greenMachine]

Hi again greenMachine,
The auto electrician has had a computer guy check it & he says the ECU is at fault or words to that effect.
I've put your suggestion re an internally regulated alternator which he says is fine but the battery warning light on the dash comes on and stays on. Is there some way around this?

Other than being able to do that I need a replacement. The numbers I've been given are BP5E 18 881A 279700-0121
The car is a '98 NB8A in the delightful Emerald Green :-)
Please advise if you can supply a serviceable replacement & cost.

Regards,
david

David, if it is a 98 it is Grace Green I suspect

Who is doing this work, anyone with knowledge/experience of MX5s? My memory tells me that there is a bypass or somesuch which fixes that, but it was a long time ago and I never knew all the details.

Do a search on here, and if nothing turns up try the big board (miatanet.com), both the garage section and the forum. But preferably, talk to an MX5 specialist up there. Or talk to any of the sponsors on here, perhaps starting with MX5plus which is in Brisbane.

Funny you mention it, I might have one, I will have a look and see. Watch your PMs, but if you can get something local it will be quicker.

[DTH]

Yes it is Grace Green per the logbook but i prefer Emerald Mica which is as it was advertised.
I've searched this forum but couldn't find anything. Just tried AutomotivePlus which I think is the new name for MX5plus with no real success. I'll try miata.net.com to see if they can help.
Thanks again

[93_Clubman]

auto electrician has had a computer guy check it & he says the ECU is at fault or words to that effect. ...need a replacement. ...numbers I've been given are BP5E 18 881A 279700-0121

NB8A ECUs appear for sale on here from time to time, but the search function on here last time I tried it (last week) was missing obvious hits, so if you haven't already it may be worth manually going back through the last year or so of MX5 parts for sale posts. Just checked ebay, but none currently on there, although there usually are. Suggest checking Gumtree if you already haven't. Usually they are advertised on here for $50-$100.

[DTH]

Thanks 93_Clubman, I'll check that. Another option I've been told about is to have the ECU repaired. A company called CoolDrive does it tho at what cost i don't know.

[93_Clubman]

Unless they get wet, which does happen being located in the passenger footwell due to blocked drains, the 16 bit NB ECUs, so far, have been quite reliable, so I'd simply buy a good known quantity used one. Yeah, ECUs can be fixed, & some like Toyota 4AGE 20 Valve Silvertop motor ECUs frequently have to be fixed due to leaking capacitors. I know the cost to get Toyota 4AGE 20 Valve Silvertop motor ECU capacitors replaced is $100-$200, & closer to the latter these days. That said, there has been at least one forumite that paid $1k to have his MX5 ECU fixed - yes he was ripped off & I'm not sure it even fixed the problem. So buy a good known quantity used one as they have been quite cheap & reliable so far.

[davekmoore]

Air con drain pipe gets kicked off at top of passenger footwell due to penny pinching by Mazda not cutting a clamp on it. ECU then gets wet.

[93_Clubman]

DTH, have PM'd you a few leads for a replacement NB8A ECU.

[bruce]

Cool drive are near me. They supply electrical auto parts, didn't notice they fixed ECUs?