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An Independent Porsche Specialist providing Low Mileage High Specification Porsche for sale

Bosch Alternator

C

Cameltoe & Heel

Barcelona
Joined
26 Jan 2009
Messages
1,400
Guys, since rebuilding my car I've had the battery light on, not driven it properly yet other than briefly up and down the estate at Center Gravity.

There's no discernible increases in voltage across the battery terminals when running so really points to a dodgy alternator, I'd changed the Voltage regulator whilst the engine was out.

I've pulled the alternator to check the wiring which was correct, I then took alternator to local Auto Electrical garage who have since called back and told me the rectifier within the alternator is dead, so needs a new one. I've asked them to source the part and fit.

Anyone else had the rectifier die?, I'm guessing most places would just condemn the alternator and recommend a new one?.

Also wondering if the rectifier was dead before I acquired the car?, or it's gone bad due to being sat around not being used for last 3-4 years?.


Trev
 
I was going to suggest looking at the brown and blue wire, I was stupid enough to get them mixed up when i was assembling my alternator back on the engine a few weeks ago, I had brown to the insulated terminal and the blue to the chassis of the alternator, with the exact same symptoms. Its strange because the wires as they exit the sheath want to naturally go to those positions, not the way they should be.

Not much help to you, but maybe helpful to someone else who comes across this thread in the future.
 
yes Andy I'd spotted your post and I'd thought I'd check however it was all correct.

I did wonder as my wires naturally fell perfectly so there was no question in my mind when I installed it.

So had to pull the whole thing out and get it sorted, also Andy can you give me details of what paint and process you used to refresh your fan and shroud please, I may get them redone whist it's apart as I'm seeing some bubbling.

Cheers
Trev
 
Hi Trev,

The fan housing has been refinished in Cerokote burnt bronze, https://www.cerakote.com/finishes/H-148Q/burnt-bronze/ this is used for guns and is meant to be the dogs, we shall see! Its a ceramic paint and is applied on bare metal.

I wanted the fan itself painted the same finish as my wheels, so this was done in a wet paint. But could have been powder coat or Cerokote.

I have since found out that the magnesium is best treated with a surface treatment such as iridite NCP https://www.dorsetware.com/a-beginners-guide-to-iridite-ncp/ this seals the surface. Apparently the hydrogen in the magnesium slowly leaches out causing the paint surface to bubble, the iridite process seals the surface and stops this happening. So if I was doing it again I would go this extra step.

Hope thats is helpful.
 

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Regulator

I changed my regulator at approx 120 thousand miles, the carbon brushes just wear out, reducing the alternator output.
All that was needed was a check of the bearings and a good clean of the stator with a LEC (live electrical cleaner) brake cleaner will do.
 

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Or is this the part you have already changed?
 
Barry yes I've replaced the Voltage Reg, it's the rectifier that converts the AC signal from the alternator into a DC signal which it then supplies to the battery, this is the component which is dead within my alternator. It's made up of a bunch of diodes which only allow current to flow in one direction, generally it's these diodes that fail.

Trev
 
Trev only guessing, though if the auto electrician said your rectifier was duff then it should be a simple matter to replace it, there are four diodes in a full wave rectifier any one of which can fail and cause the rectification process issues... diodes are solid state devices thus generally very robust, but they do fail though rarely, The brush set or bearings are the more usual failings.. as you typed they act like one way valves relative to how they are connected to an AC circuit if you look at a diode in an electrical diagram it will appear like an arrow head in the circuit. to create a rectifier four are needed and connected in a box like pattern. Two arrows pointing to the positive DC pick up connection.... the other two pointing their tail to the negative pick up point.... so that is two "corners" of the box taken up with DC connections.... The remaining two "corners" of the box are the connection points for the AC output of the alternator and it matters not to which corner each of the connections is made as in AC the polarity of each of the output wires changes rapidly from + to - (alternates) as the alternator rotates...

The effect being that when the voltage in one of the two alternator output wires is HIGH, it passes through the arrows pointing towards DC positive connection. At exactly the same period of time the other alternator output wire is LOW, and it provides a DC negative connection via the arrows presenting their tail to the DC negative connection.... thus the DC output is maintained as each of the AC connections from the alternator is always opposite to the other AC connection in terms of their alternating voltage.... Simples...????

But then I expect you may already know this :?:
 
Yes I did know it very well as I have a degree in Electrical & Electronic Eng, but that was a looooong time ago :), finished my degree two years before my 993 was built.

Here's a basic diagram to support what you've described.

TPtxIgz.jpg


I've just picked up my fixed unit, this is the rectifier that's been replaced. There's actually 8 diodes on the Bosch Alternator. The chap did say it was giving out a weak signal @ circa 3 amp....

qlgDUzD.jpg


Trev
 
Trev... Thanks for taking the time to provide a heads up, great pics too.

HA..!!!! I should have known that there was a danger in the possibility I might be trying to teach my granny to suck eggs....Duh!

Like they say a picture is worth a thousand words...(-: Though the diagram does not define the DC output polarity from the rectifier, and while I appreciate that the + & - shown at the alternator refers only to an instantaneous snap-shot, I suspect there is room to add to the confusion there too..? But then I was ever the slow learner..

Congrats on your degree, I hope you were able to put it to good use, though looking at the work you carried out on your 993, you have great practical skills too..

I guess that Bosch used four physically small diodes on each of the positive and negative output heat sinks to make it simpler to fit them within the framework of the alternator as opposed to four larger diodes..?

Great that you solved the problem so simply and hopefully inexpensively too, compared to the cost of a new alternator..

An informative and interesting post... thanks. :thumb:
 
Good to get it sorted out Trev. Get it bolted back in ASAP.. the weather is excellent get the car out into the lakes.

Snap, another electrical engineer from Preston here. ⚡️
 
Fitted the alternator at weekend and fired her up, all is well and the battery light went out which was a relief :).

I hadn't heard of a rectifier problem on this site before so I'm guessing people just condemn the whole alternator and buy a new one?.

Trev
 
Porsche 993 alternator

Hi

Just spotted the post from Cameltoe & Heel re having the Rectifier replaced, I wonder if the part number is visible on the part or maybe on the invoice for the work

Thanks
John
 
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