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Hi Guys

After a bit of research I decided to purchase an NB, which I did on the weekend, a '98 year model, with 160K on the clock. I have no idea what the real service history is, as the book has one service stamp from 10 years ago, so I will assume it hasn't been serviced well. What It did have was 2 receipts for new batteries in the last year, so I immediately thought there was going to be a problem with the charging system and I was right. A look at the battery compartment showed signs of acid corrosion, a clear sign of overcharging. A multi-meter showed 14.7 volts charge which is too high, and it didn't move under load or revs.

A check on the forum found the service manual and the pin outs to extract the diagnostic codes(thanks guys). To my surprise It flashed up code P1631 -"Generator output voltage signal no electricity" which confirmed there was a problem. I thought it would just be the internal regulator, which after some research found it doesn't have one and it is PCM controlled. My initial thought is that it probably isn't the PCM as it knows it isn't getting a signal, so it is probably a wiring problem or alternator problem, so I would start there.

I removed the P/D connector from the alternator and it seemed fine. I went to remove the main cable from the alternator and it snapped off the connection from the shunt! So i pulled the bloody thing out, and by the look of it, it was going to need replacing anyway, as the bearings were noisy and it was covered in carbon, no doubt from worn brushes.

I am not replacing it with the same model alternator that came out of it, because it is a terrible design. It is a refurbed OEX RXA305, so I suspect this might be a common problem? I ordered a Bosch Equivalent for $198 but I am thinking of sending it back when I get it and buying an internally regulated model, just in case there turns out to be a problem with the PCM. Does anyone know an internally regulated model alternator that fits? Obviously I would need to make a work around for the charge light that would no doubt stay on, I think would be simple enough though.

Thx

Col

the NA MX5 alternators fit, and are internally regulated. However you will need to change the front pulley on the alt to suit the multi-rib NB belts.

Otherwise, I believe Bosch sell external regulators which may be the simplest.


In saying all that, you'll need to do some research to make sure the factory ECU will run fine with an internally regulated alternator, as from memory the ECU uses the alternator input voltage etc to determine electrical load and compensate the idle for things like thermo fans, aircon, blower, etc.


I'd perosnally also check the ECU looks ok, obvious things like corroded contacts on the plug etc, and if you're game, open the case and check for any other damage.
By virtue of it's location under the passenger side carpet, if there's a roof leak etc the ECU will get damp.

I may be wrong but I'm not aware of any huge history of faults in the alternators. So I would be inclined to take out the ECU first, remove its covers and check it for any visible signs of damage, shorts or water ingress. If it all seemed OK then I'd just go ahead and put in a new/refurbished OEM NB equivalent alternator.

What do you think, Paul?

Put a fiver on it being 1ngress re' roadster life, re' pcm placement.
Check pcm plug first for corrosion, pcm internally next for general exposure, or damage to diodes on regulator circuit.
Plug corrosion at either alt' or pcm could alter load signal and blow reg circuit.

BTDT, it was the alternator (on the GM - NB8A). Just get the right alternator, and take it from there.

It is possible to replace it with an internally (or externally) regulated alternator, but you need to kludge the wiring to make it work as factory (probably the warning light you identify).

I know people say 'it is the ecu', but a search on here shows that it is usually the alternator that is the problem, and the solution. Just ask DTH ... who just may have a surplus NB8A alternator too http://mx5cartalk.com/forum/viewtopic.php?f=55&t=74645&hilit=alternator&start=15#p911690

Thanks for the replies guys.

I had actually checked the ECU and the plugs expecting them to be corroded, but it looked brand new and the foot well looks like it is has never had a drop of water in it, so I suspect it will be in the wiring somewhere - hopefully. I will throw the new alternator on when it comes and see if that gets rid of the diagnostic code, if not I will have to look into it a bit deeper.

Col

manga_blue wrote:I may be wrong but I'm not aware of any huge history of faults in the alternators. ... What do you think, Paul?

Agree Phil not a huge history of faulty alternators on here, but DTH's NB8A comes to mind, although he was initially advised by auto elec's computer guy that ECU/PCM was faulty, however turned out to be alt. Good that OP has already checked ECU/PCM.

Well,

The new alternator did not solve the problem, it is now not charging at all, still with the same trouble code. I checked the continuity of the wiring all the way to the PCM and that is fine, and there is no short to ground, so all good there too. If the trouble code is correct and the PCM isn't getting a signal, there should be no reading from the (P)terminal. From what I understand, there should normally be between 3 volts and 8 volts AC not DC at this terminal. I am not sure whether that is supposed to be read closed circuit with the P&D plug connected or open circuit, does anyone know? I did try both and got zero volts on the multimeter, so it is either my error or a faulty replacement alternator. If that side of the alternator is working I should be getting up to 2 volts DC to fully field the alternator as the battery is low. I am thinking about apply 1.5V through a 1.5V AA battery to terminal D and seeing what happens, what do you guys think?

Thx
Col

Turns out the Alternator I was supplied was an L & S alternator not P & D. You couldn't tell by looking at it as it looked exactly the same, so I swapped it out and it is all good now.

Well, that's good to hear. I was just about to send you wiring diagrams and wish you luck.

manga_blue wrote:Well, that's good to hear. I was just about to send you wiring diagrams and wish you luck.

Thanks mate. At least I have learnt everything there is to know about field control alternators!

Good result! What tipped you off that it was the wrong one?

greenMachine wrote:Good result! What tipped you off that it was the wrong one?

I found an OEM alternator for sale on Ebay and a seller had commented in bold that that it was a P&D alternator not L&S. From that I realised both types looked exactly the same, including the connector and I just had a feeling it was not the right one.

Col

hi guys, so I get a better understanding of this thread and for the simple people like me what is "P&D alternator not L&S"

GR124 wrote:hi guys, so I get a better understanding of this thread and for the simple people like me what is "P&D alternator not L&S"

A P&D alternator has a 'P' Terminal and a 'D' terminal connector, as well as B+. The 'P' terminal is the phase output voltage to the vehicle PCM. The 'D' terminal is the driver input voltage from the PCM. The output voltage of the alternator to B+ is controlled by the PCM increasing or decreasing voltage to the 'D' terminal (field control unit) based on idle speed and air intake temperature.

L & S alternators look exactly the same including the plug but it has a full regulator inside. 'L' terminal goes to one side of the dash battery light and 'S' is the sense terminal straight from battery. The output to B+ is controlled by the internal regulator by reading battery voltage at 'S' and comparing it to the output voltage.

Col