Emergency Alternator Question
 

The alternator cut out half way through my first lap at the Ring

I kept going and about 2 or 3 mins later it came back again, and has been working for last 20 miles or so

Symptoms were alternator light on, voltage gauge at min, tacho pegged at 5k

The car was fine first thing in the morning, and I switch on everything: lights, rear defrost, front defrost etc

But 20 mins later during the first lap the alternator fails

I did put the car through a car wash last night, but the jump start terminal does have the cover, and the multi-way plug was dry when I checked it just now

Belt tension is good, no sign of contamination, and no weird noises

Any suggestions as to the cause?

Any predictions about what might happen next?

Thanks

What year is your car?

Sounds like you need to clean you connections to you instrument cluster rather than a bad alternator.

First thing I would check is the voltage at the jump post. The voltage there is pretty much what is coming from the alternator and you should measure about 13.5 volts- if you measure 12 volts then you are running on the battery and the alternator has died [unlikely I suspect].

The tach signal is seemingly nothing to do with this so if that is wonky at the same time there is a voltage alarm and a voltage indication issue then as Anders says it may point to a fault in the dash panel connection tha tmay be more of a nuisance than a real world issue.

Done several more laps without issue so I think this was just water ingress

Smiffy has told me separately that he used to have a 928 where the alternator would cut out temporarily if he drove through a large puddle

My mistake was putting the car through the car wash and then driving just 100 yards before leaving it overnight

Water must have pooled somewhere and then caused a soaking once the hard cornering started

On the second red flag now, but the weather is great so will be enjoying the sun during the break

PS - all symptoms stopped as soon as the alternator kicked back in, including the tacho issue

However I'm not discounting the possibility that the dash connections are tarnised

Consider a look at the brushes when you get home. Perhaps the higher-than-normal engine RPM's and a little worn-bearing play pushed a worn brush back some. Plan on replacing bearings too while you have it open.

Any water in the passenger footwell, which may have entered through the blower box?

No sign of water in the cabin, but I admit I didn't look yesterday

However I now have a new symptom of something that has gone awry

When switching on the ignition, the 'bulb check' only lasts for half a second before the the dash changes to 'engine running' status, although it hasn't been started

This means I have flashing central warning lamp for low oil pressure

This has never happened before, and definitely coincides with the alternator failure yesterday

It is unlikely that I am going to go out of my way to try to fix this, but if it has a known cause then that would be very useful to know

check and clean the battery connections and the grounds for the engine,
remove the alternator and inspect the brushes,
they are probably close to being replaced.

I'm am sure that is good advice, I will add it to the list

Unfortunately the list has recently grown to include HVAC Actuators, Windscreen, Door Handle Hinge, and worst of all - Hard Fuel Lines

Is it worth replacing the bearings at the same time as the bushes?

I probably don't have the tools to press out the old bearings, so that might be beyond me anyway

Bearing taps out of the housing. You can heat the housing some if needed, but I've never needed.

"... I did put the car through a car wash last night, but the jump start terminal does have the cover, and the multi-way plug was dry when I checked it just now..."

So you did check and clean the 14 pin, the Alt signal wire runs through it, if dirty/bad connection Alt wont work ;)

Can anyone point me to a link for a brush set for the alternator?

The car is a 87 S4 as per the avatar, it is an early ROW model if that matters

Many thanks

You could try this link- they will need the alternator number if you have that handy. Could not see the Bosch part number from a quick glance in the WSM volume I have handy. Can take a look later if you are stuck.

https://www.jcrsupplies.co.uk/produc...TOR_BRUSH_SETS

The brushes come as party of the voltage regulator package. Easy to change without disassembling the alternator. Still worth disassembling to get a look at the slip rings that the brushes ride on, plus those bearings. You'll want an inpact gun to get the front pulley nut off so you can change the front bearing. It may be possible to replace the brushes and regulator assembly in situ, after removing the cooling cover.

Other diagnosis thoughts:
-- The alternator is self-exciting once the engine is running and alternator charging. If the car has started and run with normal voltage, the alternator -should- continue to charge unless something inside causes it to stop charging. If that happens, the excitation circuit would normally supply current from the external circuit, but only until there's charge reestablished. The only common element here is the brushes/regulator and their connection to the rotating armature via those slip rings.

-- Several have recommended that we avoid using some of the low-cost aftermarket replacement regulators and brush holders. With that in mind, shop for Bosch by name.

-- Resist the urge to install a regulator rated larger than 14 Volts. It won't improve charging voltage at lower RPM's, but will risk damage to electronics plus reduce the battery life as it boils out the electrolyte.

-- I found a Bosch factory-rebuilt alternator that needed only to have the rear case "reclocked" so the terminal would be in the right rotational places. I had a donated (thanks PNW members...) very worn out core to turn in, so it made some sense. The replacement looked like a brand new piece; almost "too new" for a while. The original one from the car turned out to be fine, so it received new bearings and brushes after a thorough cleaning. The slip rings were worn but serviceable, so that refurbished original was tagged and went into the working spares bin. The process avoided excess downtime, but also helped move my diagnostic effort along as I chased low charging voltage at idle.

I bought mine from Sahibs Auto electrical 3 Norbreck Parade, London NW10 7HR, UK - not far away from you!