j.kenzie@sbcglobal.net

Hi guys,
I have had several alternators on my 1984 928S over the years. It came with an Audi alternator. I used one of Carl's high output alternators, which needed a rebuild after 5 years. I tried a OEM style Bosch, which failed after one year. I am back to Carl's aftermarket item.
Unlike the OEM, the aftermarket units suffer from heat exposure due to their position near the exhaust, and behind the radiator. They work great until they get hot, then their output drops to maybe half strength.
I want to see if anyone has ideas for heat control. I can see schemes divided into insulation, and external cooling types. There are some nice radiant heat shields available, and some kind of ducting from the front brake duct opening seem like good places to start. Does anyone have any experience along these lines? I have even considered a copper coil containing coolant, wrapped around the alternator.
Thanks in advance,
Dave

WallyP

Is there a reason that you don't use the standard cooling cowl and duct?

Ducman82

On my GM unit, I had adapted the stock cooling housing. But I needed up leaving it off. It seemed to not make much of a difference

j.kenzie@sbcglobal.net

Hi Wally,
The OEM cooling duct system only works for the OEM alternator. I am not using that right now. My project is to minimize heat gain to the open fan style alternators.
Thanks,
Dave

mike77

Out of interest apart from low voltage have you ever noticed any other symptoms related to this?

My 1990 s4 experiences issues such as voltage dips, low idle and stalling. The symptoms are worst (ie stalling) when the car has been for a run then parked for a few minutes and is heat soaked. I've had the feeling low voltage is causing it and wondered if a hot alternator would cause it.

Carl Fausett

I wonder if a fella couldn't fashion a little heat shield between the exhaust manifold and the back of the alternator. I think that would help a lot. Any metal would work, but stainless steel would be best because it has very low thermal conductivity.

Here is the little OEM heat shield between the exhaust and the gas tank. It is sheet metal wrapped with an aluminum cover. I'm thinking something like this. Just an idea. I don't have a late-model in the shop right now, but I will look into making one the next time I do.

PS: heat will not only shorten the life of your alternator (newer alternators are more durable than old style ones in this regard, but they are not immune) but heat also reduces the output of an alternator.

j.kenzie@sbcglobal.net

Thanks,Carl. I was thinking along those same lines. I also ordered an alternator cooling kit called QuickCool, which consists of an in-line blower, sensors and ducting. Maybe together they will improve things a bit.
Mike, yes I have noticed poor idling and stumbling when the alternator is hot due to low voltage. It is a viscous cycle when you are using electric radiator fans. The alternator gets hotter, its output drops and the fans cool less, so everything gets hotter. Worse case is stop and go traffic in 100 degree weather with a/c on.
I am hoping a cooler alternator will run the radiator fans faster and cool the car better.
Thanks,
Dave

Carl Fausett

I think that's good logic and it should be true. It will certainly extend the life of the alternator if nothing else.

Another factor in alternator life is demand. Two identical alternators, both 200 AMP for example. One goes into a car where it will be called on to supply 100 amps all the time. The other goes into a car where it will be called on to supply 180 amps. The 180 amp application will have a shorter life. It's harder on the diodes to run at or near max capacity.

Carl Fausett

I keep a "development" list of new product ideas I want to consider.

This alternator heat shield concept just hit the list - next time I get an S4 or my GT in here I'll have at it and see what I can come up with. I will look at a stock 16v application too.

mike77

Does sound like what's happening to me. When the cars cold the voltage will be at 14 and then drop to about 12.5. If I park the car up for a short time and then start it back up I'll have about 11 volts, and all the issues are worse. Alternator is caked in layers of atf from an old ps leak. I'm thinking a clean and a new cooling duct might cool things a little.

Does the age of the alternator make it more susceptible to heat issues? Maybe I need mine checked out.

Carl Fausett

Back in the day, we knew that each alternator had 3 diodes in it, each responsible for a 0.8v rise. 2.4 volts for the three together, raising a 12v input to a 14.4v output. So, if an alternator was running 13.6v at max it likely had 1 failed diode, if running 12.8 it had two... etc. This presumes the voltage regulator and battery are good - which is a big assumption.

With modern alternators, this may no longer be true. But it was still a good yardstick through 1995 anyway.

The 928 alternator comes out pretty easy, and a good Auto Parts store will likely have a test bench and they can spin it up for you and tell you what your output is. Most do not charge for the test. But have it tested if you have any doubts or concerns. It's cheaper than head gaskets because you over-heated your motor.

Alan

I think honestly the best thing for our mount location would be to find an alternator body that will fit that has dual internal fans (front and rear) so dual feed with center case exhaust (various delco models have this config). Use a shroud for the rear (some have them - but may need to be clocked) - feed the rear shroud as the stock version is. Dual direction air flow will cool better

Then switch to use external rectifiers and external regulators (nearby - likely in the fender away from engine heat) - but these would still need to be fan cooled.

Most of the heat generated within the alternator is from the rectifiers - this affects the nearby regulator that then reduces the output voltage. Now the alternator does also get hot from ambient convection & radiation heat too - but removing most of the self-generated heat helps this tremendously. Moving the regulator away from the high heat areas will help keep the output voltage up.

Note that at ~80A output the rectifiers are internally dissipating about 150W + the self heating from bearing & slip ring friction, and rotor & stator coil dissipation. You can see why the alternator temperature spikes when the airflow drops at idle - high ambient heat and often glowing manifolds right above it.

The downside is that converting an alternator that way is really a lot of work. A heat shield either way would be a good helper too if it can fit in there.

Alan

j.kenzie@sbcglobal.net

Interesting, Alan. Does someone sell such an alternator? If so who?
Thanks,
Dave

Alan

Er No. Alternators have only 2 rectifier diodes in series with the output at any given time. But there are always at least 3 pairs of diodes (a pair for each phase) but all stock 928 alternators have doubled pairs per phase (in parallel) so 12 primary rectifier diodes. At any given time only 2 parallel pairs are directly contributing to the max B+ output voltage. If you lose pairs you get a weird waveform that has lots of noise and a lower RMS output - but the same peak output. Most meters will read lower - but its not at all as clear as Carl implies. What you use to measure it will affect the readings you get, and you can only really see what is going on with an oscilloscope.

There are also always at least 3 more smaller rectifier diodes (1 per phase) for the regulator circuit (on simple alternators like ours this is directly related to the 61 exciter circuit) NB These don't feed B+ at all. So all of you have 15 rectifier diodes in your 928's stock alternator.

These 3 smaller diodes feed the regulator and allow it to control the output voltage by varying the rotor field current, this circuit uses the one side of the primary rectifiers for its return path.

The battery must be used to create the initial rotor field current before the alternator itself has started generating. But the battery 12V doesn't get stepped up by the diodes in any way - they can't do that. Diodes used like this always step voltage down. The battery here is used as a temporary initiator - it is not an input to the alternator for any other purpose. All voltage & current to the rectifier diodes comes from the stator coil windings.

The rotating magnetic field of the rotor is what induces the voltage in the stator coils. This voltage must be higher than the B+output voltage since the diodes step it down.
These rectifier diodes create a full wave 3 phase bridge that creates the DC output from this - the diodes always drop voltage and therefore also dissipate a lot of power at high current (Power=Voltage x Current). The stator coil output voltage differences (max delta across all phases) will be ~a few volts above the B+ output terminal due to the 2x series diodes (these typically have a rather high forward voltage drop).

But remember that this is a 3 phase rotating machine - while the B+ output looks mostly like DC - the input is rectified from 3 constantly changing phases at 120 degrees phase angle difference. Each phase taken alone is an AC sine wave. Alternators really haven't changed all that much in the last 30 years, certainly not the ones we can use.

Alan

Alan

Well the alternator part is just a typical alternator (that would fit) with bits removed (they are usually quite modular). So remove the regulator and fit a brush only pack with pigtails and remove the rectifier plates and wire the 3 phase wire outputs to an external rectifier pack. (phases are connected in a delta or star(Y) config - usually star for a 928.)

You'd then need to source an external regulator, a full wave 3-phase high current rectifier pack, a 3 phase half wave rectifier pack and some kind of heatsink with a fan cooling solution, then a means to mount it all and interconnect the components robustly. Not a very easy job or one for the feint of heart. There are a few details beyond that but that is mostly it.

One day I may do it - I already did build some parts for this kind of idea. Phoenix is of course a very challenging place for alternators in the summertime...

Some people have created pseudo external rectifiers - designed to be used in parallel to the internal rectifier - Quicktifier is one. I'm not sure I see the benefits as being sufficient unless you can move everything out of the hot zone. You can easily find remote regulators for bosch alternators (and bosch brush & pigtail kits), like this http://www.davebarton.com/AdjustableVoltage.html


Alan