Member Since: 25 Dec 2019
Location: Gold Coast
Posts: 5
Alternator Over-voltage issues
TL:DR
After approx 30 mins of idle or 1 hour driving time, the voltage output from the alternator rises to above 16v, and causes the electronics to shut down. Have replaced the alternator twice. Battery has been checked.
The Long Story
Greeting from Australia. I'm having trouble with the alternator in my Discovery 3, 4.4 V8 petrol. I've already found lots of useful information on other threads (and loads of related documents and wiring diagrams) which has got me this far, but nothing that specifically addresses the issue I'm having.
I drove through some very deep mud in Chichester State Forest which packed into the alternator on my disco 3. I didnt receive any battery or warning lights until about 40 mins after the incident, at which point my crank battery was pretty much dead. Being a fair distance from anything i had to bridge across to the second battery to provided enough power to maintain the engine (but not really any of the electronics), and with a couple of jump starts was able to make it out of the forest in 1 wheel drive and get towed to a mechanic.
A new OEM alternator was fitted and I was able to drive 6 hours home to the Gold Coast without any issue.
Over the next few weeks the car wasn't used at all. Then i used it for a few short journeys and started to have issues when the engine Revs exceeded about 3000 - at this point the alternator output would jump instantly from approx 14.5 to 16+ volts causing all of the electronics in the car turn off until the voltage dropped to an appropriate level.
Thinking the issue was related to the alternator or regulator, i took the car to my usual mechanic, and had a genuine Denso Alternator fitted. The mechanic noted that the harness connector (C0053) was badly damaged and would need to be replaced, and that a poor connection could have been the cause of the voltage issues - but they could not get hold of a replacement part. I collected the car with the new alternator (also new belts and tensioners), but with the damaged harness connector still attached.
I managed a few short drives without any noticeable issues, but on a longer drive to Brisbane, the voltage again spiked and caused the electronic shutdown. At this point, and knowing about the damaged harness connector, I located the part locally and replaced it at the roadside. A 10 minute drive later, same issue, and a flat bed to get the car home.
Whilst waiting for the tow from Brisbane i hooked up the multimeter a couple of times and started the engine. Slightly different behaviour - Initially the voltage output would be around 13.5v, and very stable. Gradually over the next 10 mins, it would slowly increase, 13.6, 13.7, 13.8...etc, and as it increases, the speed of the rise accelerated, 14.5, 14.7, 15..... until i shut the engine off to prevent damage. When restarting the engine within 20 seconds the voltage resumes where it left off. If leaving the engine off longer the voltage will begin lower, but quickly rise. The longer the car is left, the lower the initial voltage, and the longer till failure.
It has now been home for 10 days or so (there's no garages open over Christmas), and I've tried a number of things:
- Check and cleaned the earth point under wheel arch beside the alternator
- Checked and cleaned earth points at fuse box
- Checked and cleaned ambient air temperature sensor
- Double checked and cleaned B+ terminal on alternator
- Double checked and re-wired harness connector
- Checked F20 5w fuse in fuse box (see below)
- Removed Dual battery setup to eliminate this as a cause
- Had the crank battery load tested twice and checked for bad cells.
The 5w Alternator fuse in the engine bay fuse box is not blown. When measuring the voltage accross the fuse, it is consistent with the alternator output. However, removing the 5w fuse does not cause any warning light on the dash as suggested below. The engine can be started without issue with this fuse removed
.
Running the engine with either the ALT CON, or ALT MON connections disconnected causes the battery light on the dash to illuminate, and the Generator Field Circuit error code to trigger. The alternator and voltage regulator in this situation function as advertised, with the alternator outputting a consistent and stable 14 volts.
Given the above information i would like to assume that the alternator itself is not the issue, and until everything else has been investigated, i would like to assume that the ECM is also not the issue. Knowing that the ECM dictates the voltage set level for the alternator based on various information it is receiving (temperatures, load, idle speed etc...), I anticipate that one of these data streams is feeding the ECM bad information, and given that the issue does not present itself until the engine has run for a short period of time, i suspect it is either a battery voltage sensor, or a temperature sensor.
However, i can't find any information on which data streams specifically the ECU is using to base the voltage set level of the alternator. Any help on this (or other suggestions) would be greatly appreciated.
30th Dec 2019 3:27 am
gstuart
Member Since: 21 Oct 2016
Location: kent
Posts: 13725
Hi
I have seen a few threads where an alternator can take a battery out
assume the alternator replaced was a Denso and the ECM has been checked for any water ingress
But others here with more electrical knowledge will be able to advise better than myself
Last edited by gstuart on 30th Dec 2019 8:12 am. Edited 1 time in total
30th Dec 2019 7:44 am
gstuart
Member Since: 21 Oct 2016
Location: kent
Posts: 13725
Member Since: 25 Dec 2019
Location: Gold Coast
Posts: 5
Thanks so much, I hadn’t seen that last document in full!
Interestingly, there is the following paragraph in the generator section:
Quote:
The ECM monitors the load on the electrical system via PWM signal and adjusts the generator output to match the required load. The ECM also monitors the battery temperature to determine the generator regulator set point. This characteristic is necessary to protect the battery; at low temperatures battery charge acceptance is very poor so the voltage needs to be high to maximise any rechargeability, but at high temperatures the charge voltage must be restricted to prevent excessive gassing of the battery with consequent water loss.
I don’t suppose anyone knows how the temperature of the battery is monitored. Or whether it is inferred from other data.
Also, the ECM housing did not get wet or dirty so I don’t believe that it is damaged from direct contact from water.
30th Dec 2019 8:24 am
gstuart
Member Since: 21 Oct 2016
Location: kent
Posts: 13725
Glad it was useful
Here’s also a link for the full workshop manual which hopefully will give what ur looking for
Think I saw something about battery temp, will see if I can find it
Member Since: 21 Oct 2016
Location: kent
Posts: 13725
Is this what ur looking for
Click image to enlarge
Click image to enlarge
30th Dec 2019 8:38 am
Globetrotter448
Member Since: 21 Mar 2017
Location: Londonderry NSW
Posts: 1788
It may be a capacitor charging up, like the one on top of the gearbox. If the vehicle was deep enough to fill alternator then the capacitor would be under water. Just a thought as you say it creeps up.
30th Dec 2019 10:16 am
Pete K
Member Since: 15 Jan 2016
Location: GL
Posts: 10404
You may want to measure voltage drop in cables when engine running.
Put one end of dvm on one end of cable and other lead on other end.
Ideally it should be 0v
But you will get a little. Check thick earths and supply
Unlikely to find anything I guess
Also check the ecu main earth and supply points in case these are used for voltage monitoring.
Member Since: 25 Dec 2019
Location: Gold Coast
Posts: 5
Globetrotter448 wrote:
It may be a capacitor charging up, like the one on top of the gearbox. If the vehicle was deep enough to fill alternator then the capacitor would be under water. Just a thought as you say it creeps up.
Thanks for the suggestion. The connector to the radio interference suppressor was pretty dusty inside, but sadly, disconnecting the capacitor didn’t help
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.
1st Jan 2020 3:12 am
gstuart
Member Since: 21 Oct 2016
Location: kent
Posts: 13725
Hi
Happy 2020
Clutching at straws here, but as it looks like the charging system also uses the air temp and engine coolant temp sensor could one of these be faulty ????
Or would it help disabling some sections by removing fuse to see if it’s causing the issues , to which I assume the disco was fine before going through the mud
Also see a test for the rectifier , start engine, put multimeter onto AC volts , put a test lead onto each battery posts and should be zero or under 0.5Vac
Just an idea and know it can be very frustrating when trying to pin down an issue and hope u get to the bottom of it
Ps, hope ur also safe and well with the horrible fires
1st Jan 2020 10:36 am
Pete K
Member Since: 15 Jan 2016
Location: GL
Posts: 10404
I would get another alternator from somewhere else.
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