Alternator Over-voltage issue

[ScottAU]

TL/DRAfter 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
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 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.PNG image.jpg


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 described below, with the alternator outputting a consistent and stable 14 volts.Open Regulator_for_Ford_6G_PCM_Controlled_Alternators_WAI_Transpo.jpg


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.

[theelms66]

I would check external voltage sense terminal whilst engine running . When its behaving and when its playing up. As a poor connection or low voltage at this point would increase charge rate . I THINK.

[Graeme]

Is a battery monitor fitted to the positive or negative terminal of the battery?

[ScottAU]

Is a battery monitor fitted to the positive or negative terminal of the battery?

I usually have the Traxide system installed. However, at the moment i have the second battery removed (and both terminals isolated), and dual battery isolator disconnected from the positive terminal of the main battery (also isolated with insulation tape) for the purpose of troubleshooting.

Other than this, i do not have anything third party connected to the main battery.

[DiscoJeffster]

When I removed my alternator I had a lot of trouble getting the rear wire connector unplugged which I understand is the wire that drives the required alternator voltage. It wouldn’t surprise me if this plug is damaged though the behaviour is odd.

[ScottAU]

When I removed my alternator I had a lot of trouble getting the rear wire connector unplugged which I understand is the wire that drives the required alternator voltage. It wouldn’t surprise me if this plug is damaged though the behaviour is odd.

I have replaced the connector with a new one, so I don’t believe this to be the issue. The old one was completely destroyed (below).52F28C95-A35D-4B30-95C2-5A8354D3EFE9.jpg

[Graeme]

Sorry, I was referring to original fitment although I suspect that no battery monitoring device was fitted to D3s.

[josh.huber]

The body control module should control the voltage from the alternator.
Do you have a scan tool? Gap or similar. Does the voltage on the scan tool match that of the multimeter on the battery.
If you had a bad connection to the module that controls battery voltage this would spike your battery voltage.
Also if you have a scan tool. Reset your battery. If not the alternator might be trying to "boost" the battery back to life. Mine did this. Was over 15 volts on a down hill run. Way to high for a good battery. Are you sure you got the wiring right? I commentated on a post a long time ago where the owner replaced that same plug and wired it wrong. Was easy to do. The schematic he followed wasn't 1, 2 , 3 was 1 , 3 , 2 and caught him out. Ill find it

[Graeme]

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.

Are you saying that you measured the voltage across the fuse pins or between the fuse and earth? If you detected voltage across the fuse pins then the fuse is blown.

[josh.huber]

Alternator Wiring Harness troubles
Alternator Wiring Harness troubles