Hello fellow mx5’ers today my alternator light flashed on then stayed on at tickover a slight rev and the light goes out, this morning my light stayed on permanently , I lifted bonnet and checked the alternatorconnections all seemed ok , but light still on, touched the large grey multiplug on the alternator and it made a “screech” as it started to charge and settled out ok. The light has not been on since.
I have been driving the car out and in of the garage for a few days but not using the car so I was expecting the battery to be slightly discharged I checked the charging rate and at tickover it is outputting 14.86 volts I expected this as the battery is not fully charged so the alternator will produce more voltage to compensate for the discharge I will put the battery on a charge overnight today and recheck charge rate as I go.
My question is : on some models of MX5 the engine ecu (computer) controls the regulator output does anyone have test info or experience of a good check in relation to this set up ?
I know that 13.5 volts to 14.5 volts is normal charge rate do you consider 14.86 volts a problem ?
Not quite sure what you mean by touching the grey plug and the alternator made a screech! Was the belt slipping? Is the alternator running quietly, no sound from the bearings? If all is well, the voltage of 14.68 is fine, anything up to 15v. it charges slightly higher than most cars due to the Panasonic battery. Run the car at around 1500rpm. with a voltmeter connected to the battery. Switch on the headlight main beam, heater blower, heated rear window, rear fog light. Watch the voltage, it will drop a little, but should still be charging with those loads. Check the earth connections, especially the one from the battery to the body,inside the boot then on to the power plant frame, drivers side, just forward of the diff. very exposed and liable to corrode. The alternator regulator is in the ECU but that is irrelevant, the charging test is the same.
The belts we’re not slipping the alternator was not charging at the time , but as I touched the grey connector it started to charge and the load on the alternator caused the belts to slip and the alternator to complain I am not unduly worried about the alternator it’s the ecu controlled theory that I would like more info on thanks for the input Geoff .
Wiring diagrams show the alternator incorporating the rectifier. The regulator is within the ECU. Apart from the main output cable there are two others shown, Grey/red to terminal 3M and grey to terminal 4T both on the ECU. Manual gives the test I indicated earlier plus a current test for output and then says if the voltages etc. are incorrect, dismantle and inspect the generator. Not a great deal of help really. If the regulator was faulty I would expect the voltage to rise too high and be erratic, if it stays below 15v. with various loads then the regulator is OK. Just a different system from most other cars.
As Geoff has confirmed, your alternator is PCM controlled. This system was introduced on the MX-5 from the introduction of the NB in 1998. The charge rate is controlled by duty cycle from the PCM. There is no more tests, other than those already mentioned that you can do yourself with out specialist tools. The outputs from the PCM can be monitored using data-logging equipment via the DIAGNOSTIC socket in teh engine bay, any stored fault codes can be read though the same and you can check the duty cycle output from the PCM with an oscilloscope.
Thats great I have an oscilloscope and will get a wiring diagram if the problem re-occurs I am going to Calke Abbey at the weekend there is a rumour of sunshine in the midlands !!! so it will get a good run and charge the battery as we go many thanks for all the input
As Robbie has indicated without dealer diagnostic and relevant data I do not think you will be able to test further. I am still perplexed by your phrase ‘I touched the grey connector and it started charging’. It would suggest to me the problem was more fundamental. Probably a check of the terminals and connector to the alternator for corrosion would be a good first step. There are two types of tests for alternators electrical ones for output voltage, current output, testing the rectifier for leaky diodes etc. which can cause all manner of problems. Secondly tests of a physical nature to ensure the bearings are not partially seized or the rotor shaft bent, brushgear, sliprings and internal wiring in good order etc. These are far more likely than anything wrong with the PCM, unless of course it has been subject to water damage. I hope this helps to put things a little more in perspective!
After my last post I did wonder what the proceedure would be if the regulator failed on a Mk. 2. A fault not unknown in alternators, so I have made some enquiries and my information is the system of the alternator being controlled by the ECU (PCM in the case of Mazda) was first used by Ford around twelve years ago, which I presume is why Mazda adopted it. Most of the regulator parts are, in fact, in the alternator but it is controlled by the ECU. The theory being that a battery will charge better when it is cold. When the engine is started, the ECU checks the coolant and air intake temperatures, if cold, it will boost the charging rate which then decreases as the temperature rises. It does not apparantly, compensate for high demand, this being done by, I can only assume, a normal regulator. This evidently is why the advice is not to jump start a car with this set up and a flat battery as the initial charge can be as high as 18v. in this condition. Such a high voltage can play merry hell with the ECU Such are the wise words I have heard today!
That makes sense as the load on the engine will vary at cold start and demand will vary ie heated rear window etc power steering load so the ecu would probably be in constant communication and adjust accordingly the fact that we cant test the regulator via the ECU is a bind but of the car it should test ok ? Drove 300 miles today into Derbyshire and back with no problems so all is well .
Bad new £60 pounds of fuel 305 miles OUCH!!! 33.5 mpg makes the Bmw look good Get 37mpg out of the BMW on a run 2.5 ltr straight six
thanks for the input Geoff .
33.5 mpg makes the Bmw look good Get 37mpg out of the BMW on a run 2.5 ltr straight six 