Hi all. New to 5’s and the forum. Owned the car for just 3 weeks and have a small niggle which I’d like to sort out. I’ve had the battery warning light come on a couple of times. This happens within 10 meters of starting to drive.
From the begining. Car starts fine, battery light goes off as it should. Pull away, and within 10 meters the light is on again. Very bright. The battery is new, only 4 weeks old. Voltage reads 12.5v without engine running. Alternator seems fine, I have 15v across the battery with the engine running. Inside the car appears bone dry, although I haven’t been under the carpets to check there. There has been a lot of rain recently, and the car is parked on a drive with the front wheels about a foot above the back wheels. If I drive the car for a short while then switch off and start up again, the problem seems to have gone away.
Anyone come across this sort of behaviour before? Any ideas as to the cause?
The most obvious thing to do is to check the electrical connections to the alternator (try unplugging and replugging) and the tightness and condition of the belt. If it was the latter you’d probably hear some noise and it would be worse in damp weather.
Could be a faulty alternator, I’ve know NB ones to fail before. Higher revs tend to make the light come on. It’ll store a fault code, something like “Generator no generate electricity”, I don’t know the number. Do a search on this forum.
If you are getting 15 volts across the battery when the engine is running, the alternator would seem to be charging the battery. I would say that 15 volts is a little on the high side - perhaps the battery is being overcharged.
Have you done a diagnostic check for error codes? - there is an error code for an overcharged battery as well as most other possible issues here.
Unlike the MK1 which has a builtin regulator within the alternator body, the MK2 regulator is on the ECU. When I had problems with my battery not charging at all, it was water damage to the ECU that had trashed the regulator. Your voltage symptoms are totally different but perhaps a damaged regulator could also cause overcharging.
The alternator needs testing. What is the voltage at 2500 r.p.m. with the headlights, heater blower full speed, Heated rear window, rear foglight on? This is testing under load. the voltage of 15v is fine by the way, It charges at a slightly higher rate due to the AGM battery. Confused me when I first bought my car!
It finally stopped raining this morning, so I was able to have another look around. The car was serviced just 2 weeks ago, had a new cambelt and waterpump fitted. The belt on the alterantor seems fine, no strange noises. Today I had no issues with the battery light at all. But did test the alternator under stress. Switched eveything on and took a voltage reading. So, with engine off I had 13.5v across the battery. Engine running was 15.5v. This lowered slightly to about 15.15v with everything running. Didn’t make much difference if the engine was ticking over or up at 2500 revs. But slightly higher voltage when reving. (This is a very inexpensive multimeter, so not sure how accurate it is.)
I’m not sure I understood what Geoff Peace meant when saying ‘It charges at a slightly higher rate due to the AGM battery.’ But I hope 15.5 volts isn’t cooking the new battery.
I haven’t yet unplugged the alternator to look at the state of the connector because it seems stuck in place, and I’d like to leave that until the battery light comes on again. Then I can do these tests again to see if there’s any difference and then maybe unplug, wd40, and replug the connection.
Error codes? I’m clueless here. It’s a 1998 MK2, so doesn’t appear to have a port to plug in a code reader near your right knee. I have seen somewhere mention of an led code reader, but have no idea how that might work. Though it might be handy to find out.
Next time the battery light comes on I’ll whip out the multimeter and take a voltage reading. Until then I’m stumped. Interestingly this problem may go back a few years. I noticed when looking at previous MOT advisories on the internet, that in 2010 the MOT advised about the battery light being on. Seems a long time to leave a problem. But the previous owner only covered 2000 miles in that time, and only ever took in out of the garage on fine days. It hasn’t seen rain since 2007, but has made up for that in the past few weeks. The previous owner has also told me this was the only time the battery light came on, and he had kept an eye out for it since. But it never reappeared.
Thanks for the help so far. If you have any more thoughts I’d like to hear them, and I’ll update this when I have something to report.
Your readings look to be OK. The normal voltage output from an alternator is 13.6 - 14.4 volts. This is for the more usual ‘wet battery’ The MX5 has a Panasonic battery which is an AGM type and requires a slightly higher charging rate, up to 15 volts. So, no, it will not cook the battery. Smart battery chargers such as CTec have a setting to charge at this rate for the AGM battery. I hope this explains the situation a little more clearly. 15v by the way is the quoted figure from the Mazda manual.
Well it could easily be a bad connection. If the battery never actually goes flat or boils it may just be a false alarm. Two things to do, next time the light comes on switch on the lights and screen heater etc and see if they affect the charge light. See if the lights dim and brighten with the revs etc and if the charge light goes out or still stays on. At any time though I’d check and clean/replug all likely connections and earths. They would definitely include the alternator plug and the ecu plug under the carpet
Yes also worth checking for ecu error code as others have said. No OBD plug but you can do it with a led if you search the web or may be worth getting this from MX5parts.
Ok, so, this morning the battery light came on pretty much as soon as I pulled away. I got the multimeter out and checked the voltage. Still just over 15v. Tried switching all the electrics on and this made no difference apart from a very slight voltage drop (less than a quater volt). The battery light doesn’t fade at all, it’s full on and stays that way until I turn the ignition off and restart. After the restart, no problems at all. I’m pretty sure now there isn’t a problem with the battery or charging. But where to find the bad connection or where water may be getting in is a mystery.
I tried lifting the carpet on the passenger side. Almost destroyed the door step cover getting it off, but that’s another story. It all looks as though it came out of the factory yesterday under there. All bone dry, and I’m loathe to disturb the ECU until I’ve tried all other possiblities. I’ve given the alternator connector a good spray with WD40 now, so we’ll see how it goes. It appears to be the initial movement of the car after standing overnight which triggers the light. Could be a water droplet getting where it shouldn’t? But where that might be is beyond me at the moment. Could be a dodgy earth? But being unfamiliar with the car, I’m not sure where to start looking. Any more suggestions would be greatly appreciated, and my thanks to eveyone who has responded so far.
PS. I have an LED error code reader and accompanying book from Autolink on the way, ordered from ebay last night. This may throw some light on the problem, or just confirm what we already know, but at least I’ll get a little more familiar with the 5.
Have you tried starting the car, letting it idle and then wiggling all the electrical connectors one at a time and see if the light comes on.
On a Ford Sierra I had some years ago it kept on cutting out as I started off. It done it several times over a period of months. I changed the coil because the breakdown man said he’d had a lot of this but it still kept doing it. One day I run the engine and wiggled every connector. I wiggled the one coming from the ECU and the engine stopped! I then pulled the connector right out and the contacts inside were green - water had obviously got in. Cleaned them all up with white spirit and sprayed them all with Maplin Contact Cleaner (NOT WD40), I never had another failure after that.
If you do try that: BE CAREFUL WHERE YOU PUT YOUR FINGERS WHEN THE ENGINE IS RUNNING
One thing I would do is clean and remake the earth connections. The one from the battery negative terminal, (check this connecttion first) connects to the boot interior, then continues under the car to the power plant frame, just in front of the diff, drivers side. This is an exposed position and has been known to give trouble. The other is at the rear of the engine block passenger side. Poor earth connections can cause all kinds of problems.
It’s been almost a week now since I had the battery warning light on. It all now appears fine. I’ve had a quick look at the earths, as suggested, but they all seem fine at the moment. I don’t want to disturb them unnecessarily. I’ll bear them in mind for any future problems. It would appear the problem was caused by a dodgy or rather dirty connection on the alternator plug. A quick clean and I’ve had no problem since. Thanks to all those who contributed.