- My model of MX-5 is: NBFL, ‘03, 1.8
- I’m based near: Bicester
- I’m looking for technical help or recommendations on: Alternator
Hi all. Noticed a couple of weeks ago my cigarette lighter was only putting out 11.7V. Got home and stuck a meter on the battery, sure enough it was sat at about 12.0V.
Car was happy to start and even pootled around for the next couple of days with no gremlins but it was bugging me. Checked again, battery was at 11.8V. With car on driveway, even when at 4K RPM, voltages recorded at battery and at the large alternator stud both measure battery voltage - no charging!
I’ve taken the battery out and charged it, it charged up happily.
I’ve replaced the Earth strap from block to PPF, and the PPF up to chassis cable too.
Checked the belt and it is in good condition and as far as I can tell it is sufficiently tight. When running there is no sign of slipping at the pulley.
At this point I’m thinking the battery is bad, the alternator is bad, or the ECU is not sending the excite signal.
I’ve checked the voltages at the excite connector with the ign off and on, but not car running as I wasn’t sure if it would cause further problems or not. Both pins read at 0.03V relative to engine block as ground, with ignition off and on. But the battery light on the dash lights up with key on, then goes off as it should when the car is running - I’ve not seen it come on at all when running the car.
Help please! Had to take my ratty Bora to japfest today and now I’m sad it’s sat on the driveway, especially having just had new wheels and a short shifter fitted.
Thanks! Will
Will,
If, as you say, the battery charges up well out of the car, but not in the car, I would suspect the alternator is in trouble, rather than the battery.
Can you monitor the voltage at the cigarette lighter socket while driving? That should give you more information. When charging mine reads 13-14v.
David
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Hi David,
I can’t currently. I noticed it as I have a USB plug that displays the voltage when ignition comes on but then turns off the display. I’ve been running the car on the driveway with a multimeter on the battery terminals, in neutral, changing the throttle by hand at the throttle body.
I have looked at buying a Bluetooth OBD for live feeds but would rather fix the problem now than spend another £20-30 and have to buy an alternator etc anyway.
Thanks!
I suspect alternator but take a voltage reading between the battery connection on the back of the alternator and a good known chassis ground with the engine running. Even if you have to support the battery to keep it running. Expect around 14v off the back of a good alternator. Even if you switch off the support briefly, if the alternator is faulty the voltage will drop suddenly.
If you determine that alternator is actually ok, then carry out a resistance check with vehcile switched off between alternator case (its ground) and another known good ground. Expect around 0.03ohms. Touch probes first to determine lead resistance so you know what your actual reading. If that’s all good, then you know it is the connection between the b+ on the alternator and the battery, however that is routed.
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This all makes sense. With car running the back of the alternator just reads at battery voltage.
Will have a check of resistances tomorrow, thank you!
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No worries. If you are not getting the charging voltage of around around 14v out the back of the alternator when running then it is probably at fault. So would need new or a tested used one 
Just a side note to check, just to be safe is all puleys. I work on Land rovers and I’ve only had it once but the outside of a crank pulley had sheared off so belt wasnt rotating correctly. Still turning but not under correct tension and rpm, gave the same symptoms
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Ah checking the pulleys is a great shout. Thank you! Will update here tomorrow evening once I’ve checked over the bits and bobs but have resigned myself to needing a new alt I think.
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I had a similar issue with my Wifes Mk2 California.
The charging regulation is controlled by the PCM (ECU).
I checked the alternator and it was giving out the right output, I checked all of the wiring for breaks etc.
After studying the wiring diagram I finally arrived at the PCM and checked the pin numbers and traced the wires back to the pins.
The PCM sits upside down in the passengers footwell and can be subject to water accumulating there if the hood drains have been blocked.
I removed the PCM to find that there was corrosion on the pin which controls the charge rate.
It might be worth checking the PCM for corrosion.
I ended up changing the PCM, but don’t forget they are coded to the immobiliser system.