6mil928

Ok guys I've searched the forums and found numerous threads related to alternators and battery charging. I've also contacted firemed about his Chevy alternator mod. Here's my question:

I just changed out my alternator a couple of months ago and now when I come to a stop my alternator drops off and I get a low voltage warning on my radar detector. I also hear the volume on my stereo drop when I have it on. I've confirmed that at rest my battery has 12.3 volts. When the car is running it has 14.45. When the rpms drop below 1K say at a stop light the volts drop to around 12 or below. From what I've read the Porsche alternators use an igniter wire to excite the alternator and they self ignite above 2K rpms. I haven't tested my igniter wire(Blue wire) at the 14pin plug yet but it appears to be working since it is charging below 2k rpm but when it gets below 1K it stops. I also do not see the belt slipping. IIRC I installed a paris rohn alternator. Any insight you guys can provide would be much appreciated. Jason

Dennis Wilson

The alternators that self excite above 2K stay charging at all rpms or at least mine does. It's possible the alternator has an internal problem, like a bad diode which aren't replaced on normal rebuilds.

Dennis

Alan

I'd suspect an alternator/regulator problem.

Classic symptom of failed exciter circuit is that the red light stays on at start-up. It may go out when revs get to ~2K+ but not always - depends on the magnetization of the alternator rotor - new ones may not self excite at any rpm.

If yours starts up OK - seems its just not able to sustain the required curent at low rpms - this is a common regulator wearout symptom or in some instances may indicate a blown output diode which will loose you one of your 3 generated phases causing similar symptoms... (and possibly excess radio noise too)

Take it in to an alternator shop for load testing.

The older alternators like the P-R may struggle if you have big audio/lighting etc..

Alan

6mil928

Thanks guys. I'm going to make sure I have power to the blue wire tomorrow and if I do I will probably do the GM alternator mod. Alan should the blue wire have costant power or is it only at start up? Once the alternator has the initial excite charge it should stay chrging the whole time correct? If that is true then mine must be defective sense it drops and comes back up. Also my red light is on before starting and goes off after the car is started. Even when the voltage drops at idle the light doesn't come back on. Jason

Dennis Wilson

Jason,

It should have constant voltage but don't expect 12 volts.

Dennis

Alan

The exciter circuit stays on. The alternator starts driving this circuit high itself when it starts generating - from there its part of the regulation circuit. The light is in that circuit and it goes out when both sides are close to the same voltage one side driven by ignition the other by the alternator. With the alternator connected to the circuit the voltage will be much lower than 12v until the car is running.

Soince you charge light comes on before starting and goes off after starting the circuit sounds fine - sounds like the regulator/diodes

Alan

6mil928

Alan that's kinda what I was thinking too after reading so much on this subject in the forums. Looks like GM here I come. The Gm alternators charge at low rpms right? Not sure if you know the answer to that one. Charging problems seem to follow the 928's by what I've read. Thanks guys, Jason

Alan

A good alternator of any make should work OK even at idle - bigger than stock loads at idle don't help of course .... GM alternators are designed for GM vehicles - some mounting & wiring adaptation will be needed (connectors). Some GM alternators have some very nice features for extra controls we don't have (voltage adjustment, generation shutdown etc).

In general the problem with 928's (for almost any alternator) is that the stock pulley ratios spin the alternator slower than on most other cars. Its tough to get much smaller of an alternator pulley and anyway this leads to more likelihood of belt slip. Geting a much larger crank pulley is also problematic - lots of work to install.

Before replacing the alternator make sure the belt is not slipping - the relative loading per revolution is highest at idle - so this is where its most likely to slip... maybe a cheap solution - check this first...

Also note that many "upgraded" alternators only generate higher current output at high rpm - they may actually generate lower current output at (especially our) idle rpms than their non-upgraded versions... not good for most users...

Alan

Dennis Wilson

Our 928's are also getting a little long of the tooth and unclean electrical connections can cause increased loads on the charging system. Wally P's electrical maintenance checklist is a must and adherence should be a requirement for every 928 Rennlist user. Actually, it is a very good guide and if used routinely, would alleviate many of the problems we are seeing. IIRC it is in the new user thread but should actually have a thread of its own.

Dennis

Landseer

My GM alternator is charging at idle using the pulley that was installed on it. Yours is a little older, you might need to switch to a V-belt pully. Engine speed makes no apparent difference to the dash guage reading. Simple to try one. No car mods needed. If you don't like it, you can probably return for a refund, depending on where you purchase. Just a thought.

6mil928

Thanks guys. This weekend I'm doing some work on my 14 pin connector and making everything right. I'm going to check everything over at that time before I change out the alternator for a second time. Landseer do you remember if the pully was the same size as the original one? Jason

Landseer

Not sure why you have electrical problems. My ParisRhone was fine, then went to 'partial mode', then seemed to completely fail. Shop analyzed and said 2 out of 3 of the voltage regulator diodes were fried. Wanted $100 to fix.

Answering your question, pulley on new delco was just a little bit smaller, and the case had a little different design that interfered when swung, so I had to change down 1 or 2 belt lengths to get the belt tight without having to swing the alternator so far that it interfered with other stuff, I think power steering. ( I bought each of the next few smaller belt sizes and fit them til one fit. Took 5 minutes to make my choice. Returned the others. )

Mine was the ribbed type pulley because thats what the 88 camaro had, I think yours needs the V-type? Other threads here speak about that, IIRC. The parts places didn't speak "delco" they spoke car make/model. I feel certain there exists a make/model that has a V-belt pulley already installed, but not sure how you find that out.

Firemed has older 928's running the Delco including a v-belt early Euro. 928Fixer from Chesapeak VA has a thread here somewhere that talks about this, as did twofast928 --- which is where the idea came from for me.

If you can get the front of car into the air a foot or so, and buy or borrow a Delco, its pretty easy to hold it up and mess with the configuration of the belt and the pivot bot (need washer shims) and determine first if the drive pulley and alt pulley align well enough. The key for my install was fore / aft positioning of the alternator pulley so that the belt tracks nearly parallel to the other belts, plus, doesn't hit anything. I positioned the whole alternator with washers on the pivot bolt, other people here changed pulleys and/or shimmed the pulley on the alt shaft. Mine fit without that.

Can't quite visualize whether their pulley changes were for belt type, pulley diameter or fore/aft adjustment.

If your alt doesn't spin fast enough, which I doubt would occur, then you can pretty easily take it apart again. Somebody here suggested the shaft diameter of the original alt and the new delco would share the shaft size. Not sure.

Only real funky part is soldering-on the little pigtail harness onto the exciter wire, but use a crimp first to make sure it works and if you are happy, solder it, laying on your side on the ground.

PM me a phone # if you want to discuss more and I can call you. Remember, I've done one for an 84 US, but am not an expert. PM Firemed and 928Fixer, one of them also might help.

https://rennlist.com/forums/928-forum/245494-delco-alternator-conversion.html


Most importantly, above is a link to where I snatched the idea. First couple posts in the thread are the important ones.

Alan

Chris how long have you been running this config.? The biggest challenge for alternators is very hot weather - especially very hot at night. The car loading at idle is highest - headlamps and cooling fans (on electric fan - esp dual - models) with AC & blower contributing extra loading.

Compounding this at high temperatures the regulator works less well and aging affects this to be progressively worse too.

So applicability may depend on ambient conditions where you live... Phoenix is about the worst case for this...!

Alan

Landseer

Allen,

Thats a great point. The conversion is recent. I myself do not have any data under those conditions noted.

I have sketched a couple of cooling duct set-ups, and have a brand new 928 cooling cover to work with. Probably will add this feature to help.

As it is 105 Amp vs 90 Amp, am hoping it will at least be equivalent performance, though, and should be better. Its working great so far.

6mil928

I PM'd firemed a couple days ago and he is going to send me some info on it actually today now that he is off shift. Jason