Hi all, I’m hoping someone has had similar experience and may be able to offer some advice…
When my battery went flat whilst driving, I tested alternator output and this proved to be not working so I fitted a new one. On testing this at the battery, got 14.8V so all good. On the journey home same day (25 miles) charging failed again. No output from alternator. Fitted another alternator and this too put out 14.8V at the battery, but once again, battery flat on the journey back home. At no point has the dash charge warning light given any indication of a problem.
This is getting very expensive and I am now wondering if I have a problem in the ECU which I know contains parts of the regulator (Mk 2.5).
Not an expert Jamie but have you had your battery checked? There could be an internal short circuit in your battery which could cause a trip in your alternator or elsewhere (ECU) to cut in. If there is no trip it could cause your alternator to burn out. I would check your battery before trying another alternator.
If the battery is being charged at 14.8 volts there is no problem apart from a slightly high voltage. I would be happier with around 14.5 - 14.6 volts.
That aside there is inconsistency with the charging as the battery would not go flat with that voltage charge.
My best guess is that your ecu has got damp and the regulation process has been interfered with.
Lift the carpet in the passenger footwell and check for corrosion and/or damp.  Â
Also check all the earth connections. Check where the battery earth connects to the body, the earth points in the engine bay, and, if the mk2 is the same as the mk1, the earth connection near the differential.
Good luck.
Hi Jamie,
my 1999 2-8 MK2 lifts the battery to 14-8.
My 1992 1-6 MK1 lifts the battery to 13-5.
My 2002 1-4 Fiesta lifts the battery to 13-5
I get these results from a little GISMO i Purchased on Ebay which plugs into the Cigar Lighter
With this little tool it gives Battery Voltage Ignition only on.
Battery Voltage with Engine running.
All three of them also give in car Temperature and two of them also have U.S.B port Max 2 Amp.
I paid ÂŁ5-00 each for them.
Ian.
Aftermarket alternators do not always suit mx5.I had issues few years back with new Mitsubishi brand alternator for mk2 mx5 1999ish .Customer had it fitted in Fast Fit type garage but intermittent charging/intermittent warning light on dash.I fitted used genuine alternator ,removed from same year approx and never a problem since.Thats few year ago.I still have the “new” aftermarket alternator here as the supplier would not take it back as he says “its listed as correct part” but I know better !!
The is several posts on here over past few years about aftermarket alternators not being compatible with ecu
Thanks for replies. I am fairly confident the battery is good and think the problem is somewhere in the charging system. New alternator puts out 14.8V (which I think is about right as battery a bit discharged by then)then it stops functioning during a 25 mile journey. Battery is on charge tonight - I will fit it tomorrow and test again, though I expect the alternator to no longer output - as happened with the last one.
I am thinking therefore that something is killing the expensive alternators shortly after installation (but can’t imagine how this could happen) or something else (perhaps ECU) ceases to function after short time. I will test this theory by disconnecting then reconnecting the same alternator to see if the 14.8V output is restored. I will also have a look at the ECU, but unless there is something physically amiss (eg. water ingress) I’m not going to find anything there.
I’m curious as to why the battery light isn’t functioning as it should and wonder if there is a clue there (it comes on with ignition).
Also, the drive belt was squeaking during the journey and has deposited some of itself on the alternator as though the pulleys were misaligned (but it didn’t do this on the last alternator). Nevertheless the alternator pulley does spin.
Hi Jamie,
just to let you know when i purchased these GISMO’S i confirmed the accuracy of them with my multimeter which has a thermocouple
and all are accurate.
Regarding the generater warning light it takes very little current to turn it off.
Regards Ian.
The first alternator was a recon from SAS Autoparts which cost me ÂŁ150 plus my old alternator. I have returned this for a refund, but they want to test it first which could take up to 12 weeks! Meanwhile, I bought the second one via ebay as second hand. Mk2.5 (regulator controlled from ECU) was specified in both cases
I am aware of generic causes for charging failure such as bad earths, dodgy battery, etc. but my car’s symptoms seem too specific for this.
I should be refitting the charged battery and testing again as outlined earlier but its 2C outside and pouring with rain so I’m holding out for a while!
The SAS Autoparts one may not be suitable but used 2 “original” one will be compatible .Possibly ecu fault then?? if everything else looks good.We have ecu,s if you do need one ,incl coded key and immobiliser unit.Just give me the number off the old ecu to be sure.Its 3 piece"married set" which have to fitted together.Not mix n match.Plug n play once all 3 parts are fitted from same car.I have few tips for fitting if you are interested ,PM me
Fitted charged battery, start her up. Unsurprisingly, 12V ish at battery @ 3000rpm so no alternator output.
Unplugged connector at alternator, disconnect battery. Reconnected, same test, but still no alternator output.
I am now in process of accessing ECU. Will have to figure out way to remove the two security bolts, but to be honest everything looks dry and rust free so far. Also, I am thinking that if the regulator in the ECU had failed, wouldn’t that result in OVERcharge? And why would a replacement alternator work when first fitted?
What could kill an alternator within 25 miles of fitting?
The only other avenue I have is the fact that the belt squeals constantly and has deposited some of its contents (powdery rubber) on the alternator - though it still spins the alternator. Could that have got into the workings sufficient to stop it working? I’m clutching at straws now.
A squeal means the alternator drive belt is slipping and wearing away, hence the deposit. I doubt the deposit is the problem, but the belt slipping is not right. I’ve an NC, but I’m guessing your car uses a vee belt. Vee belts must not run in the bottom of the pulleys, but wedge in higher up. I’m wondering if part of the problem is a worn belt is unable to transmit sufficient torque to the alternator. I’d certainly change the belt to be sure. This is assuming that as others have said that the alternator is correct for the car.
mk2.5 have Multi ribbed belts ,not v. V belt(early cars only up to 1993 approx). Tighten the belts until squealing stops basically.Check the ground/earth on the rear of the PPF frame (near the diff end )They corrode regularily
The squeal persists whatever the tension so I will have to investigate this further, ie. new belt, but the fact is it is spinning the alternator so problem can’t be there.
I have now removed the cover plate for the ECU and there is nothing apparently amiss there. The earth in the boot looks good - I haven’t checked the one underneath the car.
I keep coming back to the fact that two replacement alternators have worked on first fitting then failed after 10 minutes or so. So I am back to the question of what can destroy alternators in this way and how?
I suspect a replacement ECU would effectively render my car a write-off. Certainly I would need to be sure of it before throwing more money at it!
Sorry, but if it’s squealing and coating the alternator in rubber dust it is not turning the alternator correctly - it is slipping. It’s not the answer to just tighten it. Worn belts (of either type) don’t sit correctly in the pulleys and so don’t transmit the torque. Sort that problem first and eliminate one thing.
Jamie you do not state whether or not the warning light is working correctly also does the drive belt squeal when engine is on tickover.
If you have any doubts about the earth there is nothing to stop you adding another from the mounting bolts to any good earth in the engine compartment and will soon get warm if it is doing the job of a lost earth and the wire is thin enough.You must keep a constant eye on this
as it could be a bit risky.I am from a time when we had no test equipment and did allsorts of risky things.
Jamie you do not state whether or not the warning light is working correctly also does the drive belt squeal when engine is on tickover.
If you have any doubts about the earth there is nothing to stop you adding another from the mounting bolts to any good earth in the engine compartment and will soon get warm if it is doing the job of a lost earth and the wire is thin enough.You must keep a constant eye on this
as it could be a bit risky.I am from a time when we had no test equipment and did allsorts of risky things.