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This is one of those areas where "belief" and "facts" become intermingled.
The OE unit has been fine for people since 1977.
Most (to a massive extent) threads that say "Its low" is via the view of a 25-40yr old pod gauge where every one of 8-14 wire connection points is old, corroded, and add cumulative resistance to the path to the needle...so it is "low".
Or an alternator full of oil and crud, with a worn/weak VR...
I purchased my 928 when it was 7 years old with 39K miles. The alternator was weak then. It was replaced with a new one from Porsche under a service contract. Still low charging issues. I know for a fact it was replaced because I was half owner of the shop and I installed it myself. I also replaced an alternator in 1990 for a nearly new 1989 GT. The owner had it to the dealer several times and they replaced it once. He bought a new one and we installed it fixing the problem so there appears to be quality control issues with the alternators to begin with. The wiring in my console and dash are still a nice shiny clean connection. I replaced the battery to starter cable because I could measure a voltage drop across when the car was idling. It made the battery charging problem a little better but not fully resolved. I also tried several brands of voltage regulators with no change. Porsche adding a cooling hose was a patch as the alternators do get heat soaked and the performance does drop. Having the battery in the rear of the car doesn’t help. Having it right next to the muffler doesn’t help either. The overall design is inferior, and by adding a better alternators it offsets the other bad design elements.
I purchased my 928 when it was 7 years old with 39K miles. The alternator was weak then. It was replaced with a new one from Porsche under a service contract. Still low charging issues. I know for a fact it was replaced because I was half owner of the shop and I installed it myself. I also replaced an alternator in 1990 for a nearly new 1989 GT. The owner had it to the dealer several times and they replaced it once. He bought a new one and we installed it fixing the problem so there appears to be quality control issues with the alternators to begin with. The wiring in my console and dash are still a nice shiny clean connection. I replaced the battery to starter cable because I could measure a voltage drop across when the car was idling. It made the battery charging problem a little better but not fully resolved. I also tried several brands of voltage regulators with no change. Porsche adding a cooling hose was a patch as the alternators do get heat soaked and the performance does drop. Having the battery in the rear of the car doesn’t help. Having it right next to the muffler doesn’t help either. The overall design is inferior, and by adding a better alternators it offsets the other bad design elements.
I guess you're not a member of the "Porsche can't do wrong" religious cult ?
Be careful, you may get excommunicated from here or at least get blacklisted. The "believers" don't take kindly to us infidels.
Glad you piped in with your experience. My electrics aren't like new but in better condition than most and the alternator output was always marginal. I've lived with it for 25 yr.s, tried everything over the years to make it better, without noticeable improvement. The car ran OK all that time, essentials worked in-spite of the excessive voltage fluctuations whenever you turned on something drawing more than the absolute minimum current.
2 yr.s ago I installed a new Bergvill radio that I really like and unlike my old unit, it could not deal with the excessive voltage drops. It would shut down all the time. Put in a new battery, hoping for some improvement, but no such luck. This is when I decided I would go for a higher output alternator, come hell or high water.
A few causes for low battery voltage - It is often one or more of these items at the same time:
1) resistance in the exciter wire. The exciter wire from the 14 pin connector to the alternator is often corroded and that reduced the voltage provided to the alternator the excites the fields. Replacing the engine wiring harness is the solution there. We make them and so does Sean.
2) Bad engine ground. The alternator requires a good ground to work. It grounds through the engine to the frame via the ground strap on the right lower side of the motor. Roger sells them for I think $11. Replace, or at the very least, check yours.
3) Bad battery ground strap. Similar to #2 above, but check your ground strap from the battery post to the frame under the tool kit. Remove the terminals, clean them, and put them back.
4) Sulfated battery. Traditional wet-cell batteries do not store well, so 928's that see a lot of parked time are especially subject to battery sulfating as it sits. A sulfated battery will resist a charge and the voltage regulator "sees" that as a fully charged battery, so alternator output will be low. This is so common that many alternator manufacturers demand in their warranties that you must replace the battery at the same time as the alternator. I am a big fan of AGM batteries. They store well without aging at all. I have Optima Reds in all of my cars, and some of them are 10 years old, parked a LOT (like all winter) and they are just fine. I recommend them but do not sell them.
5) The alternator. The alternators in our cars were top-of-the-line units at that time. However, technology has marched on and modern alternators (especially Gen II systems) have more durable electronics, are smaller, and are less affected by heat. This is a good thing considering where our alternator is mounted so close to the exhaust manifold. Our alternator has 2 internal fans in it, and does not need to have the plastic cooling cap mounted on it like the OEM unit did. They also produce viable outputs even at idle speeds, where the OEM alternators struggled to do that. It is a wonderful upgrade to do to your 928.
I cannot speak to alternators that are nearly $1000 as mentioned above, but we have a very good alternator custom-made for us from a billet case so it bolts directly to the 928 without adapters. It is a Gen II unit, charges at idle, and is rated for 175 Amps so it's all you will ever need. Has a full 1 year warranty too. Our alternator is in stock, fits perfectly, and sells for $643.
A few causes for low battery voltage - It is often one or more of these items at the same time:
1) resistance in the exciter wire. The exciter wire from the 14 pin connector to the alternator is often corroded and that reduced the voltage provided to the alternator the excites the fields. Replacing the engine wiring harness is the solution there. We make them and so does Sean.
This problem limits the charging until enough excitation arrives, after which the alternator charges normally. If you see low voltage until the engine revs above 3000 after startup, go after this problem. There are other separate requirements for <'84 cars when a later alternator is fitted. The <'84 cars don't supply enough excitation current without some modifications in the instrument cluster.
2) Bad engine ground. The alternator requires a good ground to work. It grounds through the engine to the frame via the ground strap on the right lower side of the motor. Roger sells them for I think $11. Replace, or at the very least, check yours.
3) Bad battery ground strap. Similar to #2 above, but check your ground strap from the battery post to the frame under the tool kit. Remove the terminals, clean them, and put them back.
^^^ These things regardless of anything else. ^^^ Replace the two ground straps proactively, in spite of how good you think they look. Looks don't carry the current.
4) Sulfated battery. Traditional wet-cell batteries do not store well, so 928's that see a lot of parked time are especially subject to battery sulfating as it sits. A sulfated battery will resist a charge and the voltage regulator "sees" that as a fully charged battery, so alternator output will be low. This is so common that many alternator manufacturers demand in their warranties that you must replace the battery at the same time as the alternator. I am a big fan of AGM batteries. They store well without aging at all. I have Optima Reds in all of my cars, and some of them are 10 years old, parked a LOT (like all winter) and they are just fine. I recommend them but do not sell them.
Available battery technology is improving, but AGM batteries do age, just a little differently. Having fiberglass mesh sheets bonded between the plates helps limit the amount of sulfation that can short the plates, but does not prevent sulfation. The drop-out rate is the same, it's just the location and contact that changes. As the sulfate drops out of the electrolyte solution before it can replate with proper recharging, it's capacity that's functionally lost forever. The spiral-wound batteries like the Optimas are great, but have no serious advantage in that they still have plates and electrolyte. The shape allows more surface per total volume, but the chemistry is very familiar. I'm pretty sure there are no H6/G48 Optimas yet, so use in my battery S4 box will always be a space and mounting compromise.
Since our cars tend to have longer periods between uses, it's best to keep a maintainer attached whenever the car isn't being driven. Dipping any cell voltage below 1.7 or so risks rapid sulfation, same as about 10 volts in our 6-cell batteries. That includes dips below 10 on cranking with a cold battery. Maintaining capacity is critical so the voltage doesn't get drawn too low in normal use. The alternator is sized to recover from starting draw, and supply adequate current to run the car. If it has to supply significant additional current to help recover a weakened battery, the rest of the systems will suffer lower voltage.
5) The alternator. The alternators in our cars were top-of-the-line units at that time. However, technology has marched on and modern alternators (especially Gen II systems) have more durable electronics, are smaller, and are less affected by heat. This is a good thing considering where our alternator is mounted so close to the exhaust manifold. Our alternator has 2 internal fans in it, and does not need to have the plastic cooling cap mounted on it like the OEM unit did. They also produce viable outputs even at idle speeds, where the OEM alternators struggled to do that. It is a wonderful upgrade to do to your 928.
The new high-capacity alternators do a great job masking the symptoms of a tired electrical system. For many it will be the easiest solution, but to make them work for you, the underlying problems still need to be addressed. Replace the ground cabling at both ends of the car. Keep ALL the ground points clean and protected. Larger alternator capacity demands larger primary wiring to carry the current, demanding a new upgraded front-of-engine harness or an extra primary feed harness between the starter, alternator, ABS if you have it, and jump post. Find and cure your electrical leaks, those casual parasitic drains that slowly but surely drain the battery. Early car have virtually no parasitic drains. As we get into car electronics with memory to be maintained, the parasitic drain currents grow. LH and LHA, EZx controllers have engine management memory. That new stereo head unit has a clock and some station memory. The signal-sense circuitry in that accessory audio amplifier needs a tiny bit of constant power. This stuff adds up, and inexorably kills batteries. Add a battery disconnect switch and use it whenever the car is not being used for more than a few days. Get and religiously use a good maintainer. Keep a good battery in the car.
I like the battery disconnect switches - glad you mentioned them. They also function as a nice little layer of protection to make your 928 harder to steal.
Update on my 120 AMP alternator replacement adventure.
After chewing on it for a few days, I decided to take a closer look at the cooling cover interference. The more I looked at it, the more I believed, that I could modify it and make it work. Few days of messing with it and we are good to go. All that's left to do is find a 2" dia. breather hose to replace the original 2-3/8" dia. one. Few pics below.
Forgive me from pointing this out. I'm sure it's a trick of the camera in your fourth picture, but...
That wiring harness and the wires showing look like they were last serviced by a wild Madagascar feces flinger. Those large brownish crispy tubes used to be bright red primary conductors. The patched-together wiring to the pressure sender and the oil level switch are I'm sure atypical. Broken heat and oil sleeve too. I'm sure the rest looks brand new, and this is just a local condition meant to fool us. [/rant mode]
As shared earlier, we place a whole lot of faith in a new alternator curing some very basic wiring deterioration issues in our cars. Upgraded higher-output alternators make more heat in those wires and connections that now have higher resistance.
In my casual opinion, you'll gain back the lost capacity plus get to use any extra the new alternator has if you can replace the front-of-engine harness with a new one. Several of our vendors offer modern replacements with tinned corrosion resistant marine conductors, modern more oil- and heat-resistant insulation and heat sleeves, and will put bigger primary conductors in the harness to support the extra alternator capacity. Get with Roger at 928SRUS, Greg Brown of the massive alternator upgrade fame, 928 International, etc. You'll be a LOT happier, and a slew of sniggly electrical gremlins will exit the building once you can get enough current through those wires to bring the voltage up.
Sorry to keep jumping in Dr Bob BUT having just gone through all this I can share some photos.
OP, with a decrepit harness like that, inspect and consider replacing your positive battery cable too. The original cable’s insulation can crack and degrade over time - the insulation on mine was flaking off and I shudder to think what could happen with a short circuit. Same is true on the front-of-engine harness. It’s all unfused. Also inspect the engine ground strap, battery ground strap, and main engine compartment harness (through which all power flows from the front of engine harness to power the car when running) as well. I replaced all of these on my car - it’s time.
dr. bob, rjtw, - you're both right, you just don't go far enough.
Everything on my 40yr. old car could use replacement/rejuvenation. However, I am not going there just because it would be a good thing to do. Where would I stop ???
But you both have gone into the weeds. My update post was about the cooling cover mods, not the condition of my wiring or anything else.
My car is not a show car specimen, never was, never will be. If I wanted a new car look, inside, outside and underside - I would get a new car !!!
If I were forced to pull the engine, I would definitely replace/upgrade the wiring and many other things. In the mean time, I repair, replace what needs fixing and put-up with the blemishes ;-) The car served me well for 25 yr.s maintaining it this way and I sleep well. I hope your cars serve you as well as mine served me. Amen
according to the parts interchange the 928 alternator is the same as the later 944 alternator.
in the 944 side we have found that the 90s-2000s Nissan Quest/Mercury Villager minivans have a near-bolt-on alternator at about 125-130 amps stock that is widely available at most parts stores (since it was a Ford product). just need to grind some meat off the lugs to fit the alt bracket.
they have internal fans and lots of vent holes and are physically smaller/lighter than the 944/928 alt and survive happily without the cooling shroud even next to a million-degree turbocharger in the 951.
FWIW.
This looks like a great option. You can bet that these(if ford motorcraft part is used) have millions of miles of real world testing under its belt. Has anyone used on a 928 vs 944?
dr. bob, rjtw, - you're both right, you just don't go far enough.
Everything on my 40yr. old car could use replacement/rejuvenation. However, I am not going there just because it would be a good thing to do. Where would I stop ???
But you both have gone into the weeds. My update post was about the cooling cover mods, not the condition of my wiring or anything else.
My car is not a show car specimen, never was, never will be. If I wanted a new car look, inside, outside and underside - I would get a new car !!!
If I were forced to pull the engine, I would definitely replace/upgrade the wiring and many other things. In the mean time, I repair, replace what needs fixing and put-up with the blemishes ;-) The car served me well for 25 yr.s maintaining it this way and I sleep well. I hope your cars serve you as well as mine served me. Amen
I appreciate the idea that you can't replace everything. And that your discussion is about the fitting of the cooling cover to your wonder-what-that-used-to-fit replacement alternator. And that you are working on an early car that was fitted with a relatively lower-output alternator originally. The replacement options shared by others. including the Quest retrofit, do not apply to your car without some pulley changes and electrical refitting.
So how does one decide to replace or "upgrade" stuff like an alternator? It usually happens when electrical or mechanical performance drops off. Low voltage, poor cranking, weak or dead batteries, dim lights, all telltales of electrical weaknesses. The grinding noises from tired bearings usually warn us of impending failure, if we happen to hear them. Otherwise the vibrations from bad bearings kill the rotor windings, and you start to see the electrical symptoms.
Meanwhile, those same electrical symptoms start to appear as the wiring deteriorates. Replacing or upgrading the alternator without taking care of the wiring issues means you are still just part way to taking care of those electrical symptoms. A bigger/badder/higher-voltage alternator may help overcome voltage drop in failing wiring. So long as it works for you and the car, awesome!
As far as replacing that harness in front, no need to pull the engine. Next time you do a timing belt service, you'll have the front half of the replacement work done in the process. The connections you see at the alternator and oil switches, and the two at the starter, plus two clamps, are all that's left.
02-16-2020, 04:13 PM
