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Battery light flashing, erratic idle

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Hi guys!

Driving home from MOT , battery light started flashing. At first from time to time, but 3-4 days along the line it always shows up a few miles after startup. Maybe it has to do with heat because light never flushes immediately after start (cool engine). The usual trigger is when I get on bumpy roads. If light doesn't flash while driving, it almost always flashes when I come to a stop in the parking lot. I normally let the car idle for 30 seconds or so before I shut down the engine and this is when it happens. Lately, idle is also erratic when this happens.

What I checked:
Both belts spin when idling. I've recently changed them actually. I can't see them when I rev, but I doubt they have to do with lights flashing when idling.

Battery voltage: 14.18 - 14.4V at idle and under load (lights) about the same. So the alternator charges!!
Voltage between starter and battery earth = Voltage between alternator and battery earth = battery voltage.

Blue and Green Cable going from the alternator to ECU pins show less than 1.2 ohm resistance (between ECU and harness connecting to the alternator). I checked what was visible and they don't seem broken.

If I unplug the blue/green cable harness, idle goes back to normal, no more ups and downs.

I'm not exactly sure how or what to measure in regards to green/blue cable but:  

Voltage coming from green cable at the harness is always fluctuating at idle. Sometimes 11.2 Volts, sometimes 10V, sometimes 13V, but never over 13.8V. So this is a clue.

Blue cable has a low voltage. 0.2V or so. Manual says blue cable is responsible for transmitting a PMW signal to ECU to calculate electrical load on the alternator. So no idea how to test that.

If green cable is responsible for telling the alternator the voltage of the battery, then possibly this indicates the issue, since the voltage is always much lower than 14.2V and fluctuating.

So how do I know if the ECU sends the correct battery voltage to the alternator? And how do I know the ECU receives the correct battery voltage info.

I don't have extended knowledge about diodes and power distribution, but I traced the smallest cable on top of the positive terminal (most likely the one responsible for informing ECU about battery voltage). This cable goes into a diode+3 fuses attached to the outside ECU box. It's easy to spot as it's the only blob hanging around from the box. There are 4 terminals going into the diode. 2 of them show exactly the same voltage as the battery (12.6V engine off). the other 2 show 1-2 voltage less, but are not identical (one is less than the other). Also the red/white cable that apparently is supposed to tell the ECU the battery voltage shows less than the actual battery voltage (something similar to those 2 above).

If this means anything to anybody, or need more info, please let me know.

Thank you so much!


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I once had something like this on a Ford Sierra and it turned out to be a bad joint on the connector into the alternator.  However in your case It sounds more like a component in the alternator voltage regulator is playing up as it gets warm   I would either remove the alternator and get it tested or see if you can find a matching unit from a scrapyard or ebay and see if that makes a difference.  

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i once had this issue on a 3vz-fe v6 engine id swapped into a mr2.


i pulled the alternator out (not easy in a mr2 with a engine much bigger than the designed one) and sent it off, it tested as no fault. i told them to change the brushes anyway, when they took it apart the voltage regulator disintintigrated. replaced it and all was well :)

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