Alternator with aftermarket ECU mk2.5

Technical Area Electrics & ICE
2

I have a turbocharged NBFL. It runs an Emerald ECU and I bought this car already running the turbo and ECU. It also has a mk1 60amp alternator which I always assumed the guy who did the install work had swapped in so he could rob the clean, low mileage original 80amp alternator.

With the recent Spring Rally looming I thought I’d remove this crusty eyesore from my engine bay, sourced a clean 80amp mk2.5 alternator, gave it a bit of extra tarting up and swapped it in ready to wow the judges.

I returned home from the Spring Rally on a towtruck due to a flat battery, having had to dive into halfords for an emergency fresh battery in on the way down .

I was thinking either my replacement alternator was faulty, or I had somehow damaged it, but upon a bit of research it appears that mk2 MX5s have alternators regulated by the ECU, whereas I THINK older ones had inbuilt regulators in the alternator and as my 5 no longer has its ECU, the mk1 alternator was put in to overcome this. Application of a multimeter confirms my 80amp alternator is not charging my battery.

Can anyone conform this logic? How do other users of aftermarket ECUs regulate their alternators?

Unless I know Im barking up the right tree, I’m unsure how to proceed.

 

Ben  

 

3

I think you are correct - the emerald ecu does not regulate the alternator so an alternator with a builtin regulator would be needed.

All MK2/MK2.5 cars have ecu regulated alternator. All MK1/Eunos 1989 -1998 have builtin regulators.

Given your situation the best alternator I can suggest to fit is the OE one off the MK1 1.8 that has the correct pulley for your ribbed belt.

It’s a Mitsubishi A2T80391.

If the one fitted was a 60 amp, probably another mitsubishi from a MK1 1.6. Would have had to have a pulley swap.

The problem is that the MK2 seems to get more water on the alternator, hence why they corrode up badly.      

4

We have late mk1 1.8 alternators that have the Micro V belt(multi-ribbed) and built in regulator.PM me if interested.Robert

 

PS the pulleys wont swop.I remember trying this years back to no avail.Shaft thickness and length are different(

5

Thanks all, Ill probably pop the old one back in and see how that performs, then look at getting a more aesthetically pleasing replacement should that prove to work.

 

Ben

6

I’ve done it with the mitsubishis Rob and rigged up a main bolt to test several on my MK1 1.6

By the way OP, these are 65 amp as opposed to the usual 60 amp for the 1.6. Not really sure what difference this would make but MK2 was fitted with a 70 amp alternator and MK2.5 with an 80 amp.  

 

7

Having put my scabby old mk1 alternator back on, I put a voltmeter to it and found it to read 15.5v while the engine is running. Is this normal or too high? I thought it should be in the 14s ideally.

 

Ben

8

Too High Ben - voltage should be about 14.5 - 14.7. Anything more than 15 will cook the battery over time. Normal reason for such a high reading would be a faulty regulator in the alternator.

This is starting to sound a bit odd. Do you have access to a MK1/Eunos 1.8 car you can test that old alternator on?

I guessed that the Emerald does not regulate the alternator but given the voltage from that alternator would check with them to be sure.

http://www.emeraldm3d.com/   

 

9

I presumed the additional amps were maybe for heated seats etc.65/70 amp would be fine

10

Internet searching reveals a number of references to the fact that the Emerald ECU doesn’t regulate the mk2 alternator. 

I’m gonna borrow my dad’s more sensitive multimeter, but I think the plan is still to source a good mk1 1.8 alternator.

Thanks for your help

Ben