Alternator Failure, Voltage Spike. Beyond Economical Repair?

[BUG4LIFE]

Wow, this thread has got really technical

Yes OP, it probably was my Readers Cars thread you saw. Was hoping I wouldn't have such an issue so soon after buying it, but it is what it is I guess! I think I'm going to go for the used option. The cost is much less than new and Xmas means I wouldn't get a refurb until the new year, so might as well roll the dice and hope the used item is up to the job [it'll be from the Jag specialists trusted supplier so I don't think I'm wrong putting some faith in them].

Sorry to jump on your thread. Hope the Focus is sorted soon.

[JohnWest]

Not at all, it’s a place of discussion so all contributions are welcome.

It’s a shame you’re having issues with it so soon after buying, I hope your resolution is more straightforward than mine!

[Penelope Stopit]

Max_Torque said:

I have explained how a typical alternator when failed hard short could indeed pull a typical car battery to close to zero volts

Have now noticed how you are under the impression that 1000 Amps will be flowing through a short circuit small button diode

On several occasions you have commented about the probability of a diode going short circuit being greater than one going open circuit and then continued with commenting about 1000 Amps flowing through that short circuit diode for a significant amount of time

I agree 100% with you that a diode when failing is more likely to go short circuit rather than open circuit

The problem lies in you not understanding what happens next after the diode goes short circuit

An alternator diode when failing goes momentary short circuit until that short circuit is burnt off it by the high current flowing through it

Having spent several years in a workshop environment involved in the reconditioning of alternators, starter motors and other automotive electrical components, alternator rectifiers were disconnected from the stators and tested. In the majority of cases which was a very high majority, faulty main diodes were showing as being open circuit, (field diodes were often found to be short circuit and as you no doubt know, this failing shows an illuminated ignition warning light in the instrument cluster when the ignition keys are removed from the ignition switch)

The reason why the main diodes were showing as open circuit was because they had more often than not gone short circuit and burnt out to open circuit

On the odd occasion when a main diode was found to be short circuit in a failed alternator, it was found that this was caused by incorrect connections being made at the battery and the error was noticed due to sparking when attempting to incorrectly connect. The sparking was enough to alert the person of the error made and the battery leads were not fitted and tightened to the battery terminals

Alternators with short circuit diodes being few and far between were caused by incorrect battery termination

Unless you have worked in a workshop for several years involved in the reconditioning of alternators you will be unaware of the above

I hope this helps explain things

[Arnie Cunningham]

Anyway. I think the 1.6 tdci engine in a 2010 focus is the DV6 or DV6C (2010 is roughly the crossover - DV6 is 16 valve, DV6C is the later 8-valve), also used in citroen, peugeot, volvo and mazda, as well as other fords, of that age.

Without wishing to get involved in the fruity debate about rectifiers and alternators, it's a very common engine so a new alternator, if that's what you decide has died, should be easy to get.

[JohnWest]

An update: car was recovered and was just sitting on the drive waiting for lockdown to be lifted (we’re in Wales) so I can go back, take photos and advertise as a spares or repair. My dad had decided it was best to cut his losses and get a new car.

He was speaking to a mechanic friend (not the original neighbour one) who started asking some questions about how it failed and the symptoms it’s now displaying.

Long story short, one new alternator, one new starter motor and the car is fixed! Everything works as it should, he’s made up.

Thanks to all the replies to this thread, thanks for the education on the finer workings of an alternator and the suggestions of possible fixes.

[Arnie Cunningham]

JohnWest said:

My dad had decided it was best to cut his losses and get a new car.

This is so often the case. And then, in "cutting losses", they often end up being replaced with something just slightly newer than then has a bunch of things that failed - because it was up for sale due to the previous owner also "cutting their losses", unless of course you are going with something very much newer.

Generally, unless something catastrophic or very expensive fails, I always repair - better the devil you know. Alternators and Starter motors I view pretty much as service items - albeit it I like to get at least a decade of service out of them.

[rambo19]

I drive a lorry, and my boss still drives an old merc atego.

A few weeks before christmas it started cutting out and then cut out completly and would not start.

Recovered to the garage we use, who, normally are very good, but they spent ages on it and could not get it started.
After much head scratching, the garage thought it could be the ECU.
They spoke to a firm that recon's ECU's, and they said to check the alternator output.
Alternator was kicking out 32v ! (24v system)

Turns out, the ECU will shut down if it detects a voltage that is to high, very clever and saves burning out the ECU.