Alternator Over-voltage issue

Noticed something else whilst troubleshooting yesterday.

From a cold start, initially the voltage output of the alternator is 14.6-14.8, and regulates well when revving the engine, or adding electrical load.

Once warm having idled for 15mins in the driveway the alternator still outputs approx 14.8v. It’s higher than I would expect with a fully charged battery. When I rev the engine now however, the voltage fails to regulate and spikes to 15.5-16.5v.


If I turn on both the headlights and AC, the voltage initially drops into the 13v range, but then a few seconds later resettles at approx 14.8v. Revving the engine now, with the increased demand, the voltage is much better regulated, and will only raise to 14.9v or so.


After driving the car for 30 mins or so, this isn’t the case, and high revs regardless of electrical load from accessories causes a jump into the high 15v to mid 16v range.

It's not a V6 petrol alternator is it? They have a different pin configuration.

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Originally Posted by ScottAU

Hi Josh. I’ll be honest, I’m not an electrician, Please can you explain this?

My understanding is that the two control wires to the ECU work on a PWM/CANbus system, so measuring the voltage shouldn’t provide any clues? If I was to measure this, I imagine that i would have to measure across two ends of the wire, and not to an earth point?

i have checked for continuity from the connector at the alternator to the ECM connector and found no issues. When connected up correctly, I get no error codes, but if I break continuity on either the Control or Monitor wire, I get the alternator light and error codes. This makes me think the ecu is communicating correctly with the alternator.

Does my thinking sound right?

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Yeah they are pwm from what I've read.

You measure between the wire and earth.

If your multimeter has a pwm mode put it on. However if you just set it to normal volts you will get an idea of what's going on. Just need to look at alt control. Not monitor

Quote:

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Originally Posted by josh.huber

Yeah they are pwm from what I've read.

You measure between the wire and earth.

If your multimeter has a pwm mode put it on. However if you just set it to normal volts you will get an idea of what's going on. Just need to look at alt control. Not monitor

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awesome. I’ll try that. Do you know what a reasonable voltage reading would be?

Been reading this with interest, as I know nothing about D3s, so learning a lot.
On your last question, all I can say is my very non-smart Defender just sits on 14 volts continuously. Would that be a normal start up with a smart alternator?

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Originally Posted by ScottAU

awesome. I’ll try that. Do you know what a reasonable voltage reading would be?

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I think you will need an oscilloscope to check PWM

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Originally Posted by theelms66

I think you will need an oscilloscope to check PWM

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Absolutely. The voltmeter will give the average of the voltage it sees based on the pulses but this means very little in troubleshooting in all reality.

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Originally Posted by DiscoJeffster

Absolutely. The voltmeter will give the average of the voltage it sees based on the pulses but this means very little in troubleshooting in all reality.

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Yes and no. Different meters will give different readings on the same PWM signal, but if you get some baselines like "when it's making 14.5V it reads this, and when it's making 15.6v it reads that" you can get an idea as to what it's doing. Even just watching it vary can be illuminating.

The biggest issue is getting a handle on what your meter does and how it does it.

From what I've been able to ascertain the signals are :
- ECM to alternator, "wind the wick up a bit and give me xx volts please"
- Alternator to ECM, "this is how approximately much current I'm making (or to put it another way, my current load) at the voltage you've asked me to make".

I'd be more interested to see what the alternator does with the PWM input both open circuit and shorted to ground. Those would both be more likely than the ECM doing something funny.

Quote:

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Originally Posted by DiscoMick

On your last question, all I can say is my very non-smart Defender just sits on 14 volts continuously.

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If I break the connection on either the control wire or the monitor wire, the ‘smart regulator’ will revert to being a ‘dumb’ regulator and output a constant 14v. The only negative to this that I can tell is the battery light on the dash - so I’m very tempted to run in this configuration for the time being until I’ve found a proper solution.

Quote:

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Originally Posted by BradC

"I'd be more interested to see what the alternator does with the PWM input both open circuit and shorted to ground. Those would both be more likely than the ECM doing something funny.

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Thanks for your input.

By open circuit you mean not connected to anything? if so then the regulator behaves as expected (from this document) and outputs a steady 14v

I suspect the same will be the case when grounded, but I’ll have to double check.

EDIT:
The voltage through the control wire to ground is approx 6.8v. This rises to 7.6 as the alternator output rose from 13.8 - 14.8v (alternator output inferred from control module voltage measured with gap tool). There was no error lights on the dash, so suspect the regulator was behaving on usual instruction from ECM.