Thread: Alternator Over-voltage issue

Originally Posted by ScottAU

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.

Awesome. Now you have a baseline to work with. See what it does when it's misbehaving.

Hi,
Ur disco is basically telling u that the battery is knackered, hence the 16v.
I suggest u sack ur auto sparky and get a competent one. A very simple thing to remember regarding alternators, the lower the voltage the higher the output amps, the higher the voltage the lower the output amps. This is ecm controlled in ur disco and the ecm is asking for minimum amps and protecting the vehicle from a knackered battery when the over voltage is sensed.
A new battery will immediately be able to correctly load the alternator and will cure the higher than 14.8v readings. A load check on the old batteries will show the same result is knackered battery.

Agreed that dead battery's do cause issues. But the BCM should be backing the alternator off before it shuts down the car.
He said he's already load tested the battery twice. I'm not sure on a D3 but the D4 can definately monitor current and voltage.

Does it start well everytime? Does it give low battery warnings? What's the battery voltage after it's been sitting for a while and the car is asleep? Can you put the multimeter on and see what the voltage is during cranking aswell?

Originally Posted by josh.huber

Does it start well everytime? Does it give low battery warnings? What's the battery voltage after it's been sitting for a while and the car is asleep? Can you put the multimeter on and see what the voltage is during cranking aswell?

Starts just like it has for the last 3 years without problem. No battery warnings or battery related error codes (other than when high voltage shuts down ECM, in which case I get multiple associated error codes, but only the transmission reports high voltage). Voltage holds well, when I returned to the car this morning it was 12.9v. Haven’t noticed any major drop off during cranking, but I’ll check that again tomorrow.

Description and Operation.pdf
Looking at the DCT codes in the attached manual it looks like there is several error codes which can be thrown up by the alternator and charging system. It would be interesting to know which sensors are responsible for determining the above faults, and their parameters.
P1146, P1244, P163, P2504

Originally Posted by ScottAU

Starts just like it has for the last 3 years without problem. No battery warnings or battery related error codes (other than when high voltage shuts down ECM, in which case I get multiple associated error codes, but only the transmission reports high voltage). Voltage holds well, when I returned to the car this morning it was 12.9v. Haven’t noticed any major drop off during cranking, but I’ll check that again tomorrow.

Description and Operation.pdf
Looking at the DCT codes in the attached manual it looks like there is several error codes which can be thrown up by the alternator and charging system. It would be interesting to know which sensors are responsible for determining the above faults, and their parameters.
P1146, P1244, P163, P2504

The ECM itself would be monitoring those. No external sensors.

Originally Posted by Simo74

Hi,
Ur disco is basically telling u that the battery is knackered, hence the 16v.
I suggest u sack ur auto sparky and get a competent one. A very simple thing to remember regarding alternators, the lower the voltage the higher the output amps, the higher the voltage the lower the output amps. This is ecm controlled in ur disco and the ecm is asking for minimum amps and protecting the vehicle from a knackered battery when the over voltage is sensed.
A new battery will immediately be able to correctly load the alternator and will cure the higher than 14.8v readings. A load check on the old batteries will show the same result is knackered battery.

Put a new battery in today (Varta g14) and it didn't make any difference sorry.

Originally Posted by ScottAU

Put a new battery in today (Varta g14) and it didn't make any difference sorry.

Have you had a chance to look at the PWM control line while the voltage is through the roof?

In cases like this I tend to run a small multi-core cable through the firewall and sit a meter/logger/cro (as required) on the passenger seat so I can actually catch it in the act.
It's usually pretty easy as when I'm chasing a fault my wife will refuse to go near the vehicle, so the passenger seat makes a great diagnostic bay.

Seeing as you have a GAP. In the section with extended info (don't have mine handy, but it's not the first "tested" items), do you have an element for requested or commanded alternator voltage?
I have that on the TDV6, and while it bears only a vague resemblance to the actual voltage, the battery voltage does indeed go up and down as the reported figure goes up and down. Just while it's saying the ECU is requesting 15.1V I actually see mid 14's and down from there.

edit : found it
IMG_1267.jpg

So what ended up being the issue