Hello! We have a mystery overcharging issue affecting our 2008 Focus mk2. Every so often the battery light comes on, then a while later other lights on the dashboard (eg ABS), then the power steering goes. The issue doesn’t happen when idling or at very low revs, but at its worst only a touch on the accelerator makes the internal light get brighter and the power steering malfunction happen. The dashboard then blacks out and sometimes the car won’t start. The issue seems to ‘reset’ after being off a little while, sometimes then being fine again for 200+ miles.

An experienced mechanic has spent more than a month with it and had no luck, often because he just hasn’t been able to recreate the fault as it’s so intermittent. It came back passing the standard set of diagnostic tests. The garage fitted two (!) new alternators and that hasn’t solved the issue. 

Anyone experienced this before or got ideas? Here’s a video of the fault in action:
https://youtube.com/shorts/YXE3mi0h4mM?si=Oj81k71L87UY8zLM

Given the message on the dash about steering, id maybe hazard a guess that your mechanic should be looking at the electric power steering pump. Not 100 % sure but thats where id start.

What's the voltage across the battery terminals with the engine just at idle and when at 2000rpm ?

Last time we were able to catch the issue in time to get a voltmeter to it it was varying between 14v and 17v

1 minute ago, Tom Kite said:

Last time we were able to catch the issue in time to get a voltmeter to it it was varying between 14v and 17v

Thats not good !

4 minutes ago, Tom Kite said:

it was varying between 14v and 17v

Well it's unlikely to be a faulty alternator if it has already been replaced twice.

Has the car just started doing this recently or has it always done this since you've owned it ?

Fault has to be related to the charge control, but if this was a simple case of a broken wire or corroded connector then the alternator voltage would default to 13.95V as it is designed to do when control is lost.

PCM ?

Could this actually be an issue with the cluster connector solder joints cracking?

Well known and common issues as the mk2.5’s get older

The power steering malfunction first happened two months ago but we had a blown headlight a couples of weeks before that which I suspect was related.

The car has been in the family for around 10 years and the only prior electrical issue was a broken cigarette lighter about a year and a half ago.

May be worth checking all the earths are ok in the engine bay too

A high resistance or broken earth connection between the engine block and the main chassis earth point could lead to over voltage from the alternator.

Using a good digital multimeter, measure the resistance between the battery negative terminal and the metal case of the alternator (be sure to make a good connection). The resistance should be 0.5 Ohms or less.

Thanks @unofix and @troy45 those are good shouts, I will get my hands on a multimeter and test.

Would that make sense given how intermittent it is?

This morning added another chapter to the saga - battery completely flat after being left overnight. Nothing obvious left on. This happened before a couple of months ago after the first round of power steering issues. We got a jump start and then having been driving around with no issues all day since.

How old is the battery?

New battery went in with the new alternator a couple of months ago!

As mentioned above, have you had your instrument cluster repaired for the common cracked connector solder joint problem? (read about it and find my repair service here: https://www.ebay.co.uk/itm/314393033915). Yours is the type I repair by far the most. If bashing the dash above the cluster makes a difference when it's misbehaving then that would help confirm that the cluster is at least part of the problem. The power steering warning is a common symptom, as are various warning lights popping up unexpectedly. The sudden loss of power to the cluster could also be due to this. The cracked joints can cause havoc on the network communication lines that link computer modules and prevent them from going to sleep as they should which can thus cause battery drain, so this may explain that too. These problems can be intermittent as tiny movements due to temperature or flexing of the vehicle can bring the two sides of cracked joints in and out of contact. You haven't mentioned fault codes but a U1900 communications code is also a very common symptom.

I'm not sure how the charging voltage issue could be related, unless you're reading voltage from the cluster's LCD which can't be trusted when it's connector joints are cracked, so this may be an entirely separate issue.

1 hour ago, Tom Kite said:

New battery went in with the new alternator a couple of months ago!

.......and the resistance between battery negative and alternator case is ?????? 🤔

I won't be able to sleep until I know the answer 🤣

On 2/16/2025 at 8:21 PM, unofix said:

.......and the resistance between battery negative and alternator case is ?????? 🤔

I won't be able to sleep until I know the answer 🤣

0.1 Ohms! I know this because we just had to call the RAC out after a total breakdown. 😭 

RAC mechanic found all earths are fine. After a jump start car was working normally for a few miles but now the battery light is back on and battery not charging at all. The saga continues!

According to Sherlock Holmes, "When you have eliminated the impossible, whatever remains, however improbable, must be the truth."

In your case, the replacement alternator is either the wrong one for your car, or it is faulty. Is the alternator pulley the original or has that been replaced at the same time? 

Just because its supposed to be New, doesnt mean its right.

In the trials and tribulations of my alternator failures I ended up speaking to an auto electrician who was recommended to me. Massive help, the guy went through it with me on the phone without even coming out to see the car and charge me for it. 
 

Basically if the alternator is unplugged and not putting out the default 14V it’s goosed. That was my issue despite it being a month old Ford remanufactured part and tested by a ‘specialist’ who said it was ok but never tested it thoroughly enough to see if it still worked once it warmed up a bit

1 hour ago, troy45 said:

Basically if the alternator is unplugged and not putting out the default 14V it’s goosed.

Yes, that's what I said

On 2/15/2025 at 9:32 PM, unofix said:

Fault has to be related to the charge control, but if this was a simple case of a broken wire or corroded connector then the alternator voltage would default to 13.95V as it is designed to do when control is lost.