Carl Fausett

You're not the only one to try that - even Porsche kept changing the alternator pulleys down over the years as they experimented with solutions to the "I don't charge at idle" complaint.

Note that as the pulley size went smaller, the belt width went larger. 4-rib to 5-rib to 6-rib. The smaller the diameter of the pulley, the smaller the belt/pulley contact patch and the less tractive force available. It seems they widened the belt to follow the smaller pulleys in an effort to reduce belt slip.

Bill Ball

Isn't there something more basic going on? I've never had the "no charge at idle" problem. I'm still running my original alternator. It developed bearing whine about 100K miles ago, so I took it apart, replaced the bearings and one plastic bearing spacer, turned the slip ring on a lathe to eliminate a wear groove and replaced the brush/regulator assembly. Still going strong, even at idle, at 278K miles. Car idles at 650-675 per LH/EZK control. Running aftermarket sub and main audio amps. At night with lights and everything going, it might drop to 12V at idle, but otherwise runs close to 14. Why can't others resolve the low idle output with a stock alternator?

j.kenzie@sbcglobal.net

Hi Bill,
In my case, with my MS Supercharger, I have added 2 extra cooling fans for the radiator, and one for the intercooler, along with its water circulation pump. In fact the stock alternator did pretty much OK, but remember when it's running at 12.0 volts, it is not really charging the battery. I notice this most in the summer, at night, in stop and go traffic with the a/c on.
I think my dash board gauge is way off; when it reads 10.5 volts, it is usually 12.2 or so at the jump post. That still is not the greatest.
Thanks,
Dave

Carl Fausett

Bill, I think there are a lot of variables to the "my car doesn't charge at idle" conversation. Year of the vehicle defines the alternator that's in it and the pulley size (hence, how fast it turns at idle). The idle speed itself varies throughout the years also.

We find that the condition of the battery and the wiring harness are a large factor - a lot of "low output" problems get corrected when they replace the engine wiring harness.

Then there is the voltage indicator in the dash itself. Known to be an "indicator" and not a gauge - it is not calibrated. Sometimes a customer who thinks they are not charging at idle can be shown with a proper voltmeter that in fact they are.

In your case, I have no doubt that the level of care and maintenance that your car receives is also a plus - not all 928's are so lucky.

Carl Fausett

Dave - I have always felt that a little while spent at 12v at a traffic light at night with all your gear turned on is not a big deal and not worth chasing. It's one of the things the battery is for in the system - to provide amperage when the alternator cannot. I just dont feel that the system must always be charging no matter what - maybe I am alone in this - but if it charges the moment the light turns green and you drive away, I call that "normal". (I'll probably get some s**t about this, lol)

This is not to say that in daytime, with far less drain on the system, and it can't keep a charge at idle - that I wouldn't chase that. I'm just talking here about headlights on, AC on, stereo on, radiator fans on, and idling. Given the amp draw at that moment, the fact that your alternator can maintain 12v means it IS charging even then. Just that the load is close to matching the output.

It's a judgement thing. And again - check the voltage at the battery posts with a voltmeter - don't trust the indicator in the dash pod. You may find even at "12v" on the indicator you are actually charging even in your worst conditions. You can do this at night on the driveway. Turn everything on, and see what the voltmeter says at idle.

dr bob

Observations & opinion: Charging


I think perhaps the biggest "issue" that we enthusiasts face is that the voltmeter reads higher at idle when the car is still cold than it does when the engine is warmed up. I do advocate watching the gauges of course, but like the oil pressure gauge, it's -normal- to see the lower readings when the alternator is heat-soaked.

Add in wiring deterioration over time, and the voltmeter readings can sink below 12V even though the battery is being charged. Look carefully at the harness that connects the starter and alternator to the jump post, and see if it includes cracked insulation and wire corrosion at the jump post end. Most I've seen have this problem. While it's tempting to put a few wraps of red electrical tape or shrink sleeve around the end of the cable there, you are only hiding damage that's already started, and will continue under the new covering. Remember also that that is just one end of a cable that routes across the front of the engine, then tucks behind the alternator near the exhaust by the motor mounts, runs back over the crossmember under the exhaust, passes back under the exhaust on its way to the starter. That section you see by the jump post probably sees the easiest conditions of any part of the harness, except for the flexing when the engine rocks on the mounts. The rest is out of sight, out of mind, but easily causes a lot of the low-charging symptoms we experience. We treat the charging voltage symptom as an alternator problem, when in fact the alternator is doing its best trying to push current through effectively smaller and smaller wires.

----

While testing to find the cause of my apparent battery problems a few years ago, I spent a bit of time with long test leads routed from key points in the system to voltmeters sitting in the cabin. This ultimately lead to replacing the battery-to-starter cable (did nothing really) and the ground strap, since they appeared to have the biggest voltage drops.

The engine ground cable is on my list for PM replacement, and Ill probably get on Carl or Sean's wait-list for a new front of engine harness at some point too. Or maybe, now that I have a winter 928 hibernation season, I'll fab a new harness with fine-strand tinned marine-grade cable and eliminate these symptoms.

WallyP

Bob Webster, my local auto electrical guru/rebuilder, said that he could get several different regulators for our stock Bosch alternators, with one variable being different heat/voltage curves. Some models are designed to prevent overcharging a heated battery that lives in the engine compartment, so the lower output at higher temp is built in.

RFJ

Hello,If anyone is thinking of a fan and hose blowing on an alternator I would just use a 4 inch marine blower off ebay with the hose they have for that very cheap long lasting and can be wiried to be on all the time or just with the headlights or whatever , just saying

Alan

You might find your car would behave rather differently in Phoenix at 120F (or even 100F).

Environment makes a big difference to the deration curve - not only directly - via higher ambient temperatures, but also because in the summer in AZ the AC is just always on - so that means the cooling fans are always on...

Alan

Carl Fausett

Thats an excellent point.

dr bob

This website has been shared previopusly here for folks interested in some creative regulator solutions. I threatened to buy one of the adjustable versions but never pulled the trigger. I think Alan was in on the discussion too, thinking about the external regulator option. I'm sure some searching here will turn up the original thread if you are interested. In the meanwhile: http://www.davebarton.com/AdjustableVoltage.html

j.kenzie@sbcglobal.net

Hi Dr. Bob,
I ordered one of these Dave Barton adjustable voltage regulators for my 90 amp Bosch rebuilt OEM type alternator, but it would not fit. They allowed a return, and were nice about it, but no real progress there.
Dave

ajricker

I bought one of the remote adjustable regulators. It does take a little filing to get it to fit. I placed the regulator in the fender just under the headlight.
Charging is better but you do still get some drop off when the alternator gets heat soaked.

Alan

I have the external (remote) adjustable regulator too (in the front fender) - it helps, but it helps most if you don't set the voltage too high. Its tempting to increase over stock voltage level but that is a mistake - it makes whatever deration loss you do get just start sooner. As long as you can get less droop at temperature you can set the nominal voltage a bit lower - this delays the point at which it starts to droop.

The loss of performance at low rotational speed is still an important issue and is partly driven by the nature of the regulator circuit design. At idle the field current needs to be at its highest to generate well - but the simple regulator design we have (for size reasons) has a rather limited range. A more substantial regulator that isn't subject to self-powered limitations could certainly do better. Regulators like this are usually very expensive ($300-$400) and bulky and are typically designed primarily for boat battery charging (always external). Many will allow remote battery voltage sensing and battery thermal monitoring - both of which would be good for our configuration.

But things become a real project real soon...

Alan

j.kenzie@sbcglobal.net

Hi guys,
I thought I'd give a followup. I bought a 3" marine bilge fan on ebay for $25 and some 3" flexible ducting. I wired the fan to come on with ignition on. I ran the fan near the front grille duct opening, and led the duct to the rear of the alternator. It makes a big difference. I get very little heat soak of the alternator. The fan is a little noisy, but otherwise, the alternator seems to be much happier.
Thanks for looking,
Dave