justinporsche

HI,
I have a 1988 Porsche 928s4 that will charge around 14 volts at idle with no loads. However, as soon as you start putting an electrical load of any kind on, (lighting, air conditioning, etc,) The charging voltage progressively drops down to less than 12 volts. Heres what i have done so far: Replaced battery. Replaced alternator twice with a bosch reman unit. Checked battery cable connections both ground and positive. Checked main engine ground. I checked the exciter wire circuit extensively, through the instrument cluster, central warning display module, and the Central electric board in front of the passenger footwell. I checked the blue exciter wire at the alternator and I have key on battery voltage. I have jumped the blue wire to the B+ terminal and the charging remains the same. The only thing i can find visually, is, at the central warning unit behind the dead pedal, the pin for the blue exciter wire is burned a little. However, if im understanding this correctly, once the alternator is excited, this is out of the equation. I have completely disconnect the central warning unit and the instrument cluster with the car running, and the charging voltage still stays the same whether it be low (with loads on) or normal (with no loads). Disconnecting these 2 units doesnt affect it.
I am checking the charging voltage using a meter directly on the battery posts. The needle in the cluster seems to be always about a volt and a half lower than what my meter reads. The red light at the bottom of the gauge does illuminate with key on, and goes off once the engine is started.
Any help appreciated.

FredR

What you have described in the first sentence sounds perfectly normal to me. The alternator at idle cannot match the demand so there is a draw on the battery and when that happens the voltage dips to 12 volts or whatever it takes to meet the demand. Wind the motor up a bit and by 2k rpms the alternator should be on top of things with the voltage at or close to 14 volts. Stick on all the lights, main beams etc, ac system, hi fi system and it might need 3k rpms to balance things up.

The display gauge on the cluster is not a fiscal meter so think of it as a guide as to what normally happens and if it starts behaving differently then investigate why.

If you have a specific issue I have not addressed in the above kindly clarify such. The S4 alternator is at best only just adequate when everything is stock and functioning correctly. Stick on a powerful amplifier and such can easily give the system a hard time.

justinporsche

Yes, so when i rev the engine up the voltage does not come up like it should if there are loads on

justinporsche

Ive got the alternator off, on the bench now. So, the small terminal for the excitation wire, should it have direct continuity to the case of the alternator or should there be some resistance. I have 21Kohms when measured this way. Im thinking this is normal.

Mrmerlin

what is the charging voltage at a steady 1500 RPM at the hot post? if its 13.5v or above then you have a good charging system,
measuring the voltage at idle is not a valid test. ESP with accessories turned on
NOTE also replace fuse 10
NOTE make sure the battery terminals are clean and tight.
NOTE make sure you have a good ground strap and engine to chassis ground
NOTE if the CE panel has not had a Deoxit 100 application then this should be done as well as the ground points above the panel

dr bob

Having experienced some of this --

-- The voltmeter in the cluster depends on Fuse 10, as Mrmerlin shares. Install a new fuse there, regardless of how perfect the old one looks.

-- Inspect and clean the connections for the cables at the jump post. Make sure the long nut there is tight after you use a small wire brush to polish the terminals. The primary cable in that front-of-engine harness is a bellweather of the condition of the other wires in the harness. If yours has cracked and heat-discolored insulation, it's time for a replacement. Most report that a large number of small electrical issues magically disappear when full current is available. The alternator sits in the middle of that harness. Improved (modern wire, larger primary conductor) harnesses are available from several of our favorite vendors. Greg Brown at Precision, Roger at 928SRUS, and I'm certain some others have them available too.

-- Clean the grounds, paying particular attention to the grounds above the central electrics panel, and the one by your foot at the front of the driver's footwell. Again, small brass wire brush to polish any oxidation from the ring terminals, snug the cleaned bolt in the cleaned pad.

-- Last but most important: Replace The Ground Strap At The Battery, and the one from engine to chassis while you have the tools out. Get a new factory engine-to-chassis cable (not at all expensive), use one of the upgrade tinned ground straps, both from our vendors. The upgraded battery ground strap is a little bigger, but most important it is made with tinned braid to marine standards so it's more resistant (sorry...) to corrosion.


------

Know that the dash indicator isn't perfect, but offers a great relative look at the charging system condition at the CE panel. Use your DMM to measure voltage accurately, use the dash gauge to see when things change.

Make sure your battery is in good condition; a tired battery will suck a lot of the available alternator capacity trying to recover charge, leaving less voltage for the rest of the system. It will also contribute to premature alternator failure.

My car sits with a battery maintainer connected whenever it's parked for more than a few days. The parasitic drain is about 35 mA on mine, but there's no reason to beat up the battery letting it drain even a little between drives. I lift the ground strap during hibernation season, still keep the maintainer connected.

FredR

At the moment it is not at all clear to me what you perceive to be wrong but you do appear to be of the mind set that there is definitely something wrong- why else would you tear into the alternator?

Presumably you are of the opinion that the current voltage characteristics are different from what they were previously during your ownership. Perhaps you can describe what you see compared to what you are expecting to see in terms of volts and revs.

GregBBRD

Check your charging voltage with a volt/ohm meter. The stock gauge is virtually useless, other than for "gross" voltage measurements. And as the car warms up, the volt meter can give you different voltage readings, due to car wiring resistance. Do what Stan (Mr. Merlin) suggests, remove the relay board and completely go through each connection, removing oxidation, tightening down the fuse holding contacts, and applying some sort of contact cleaner/oil/oxidation preventing solution.

Besides what you have done (and the above), here's what I do:
If the rear ground strap to the battery is original, replace it with an aftermarket ground strap (ever single car needs this ground strap replaced.) The original construction has the ends crimped onto the actual strap material. These oxidize and the the resistance goes up, as well as allowing intermittent connection issues.
Our replacement ground strap has the strap material under the end connector, to ensure good contact. It's the correct length and made from similar materials as the original, except assembled in a much better manner.
$25 for this replacement strap....guaranteed to be the best ground strap, on the market!

600rpms is a very tough idle range for some 928's. Most of the S4 vehicles will drop below 12 volts with the lights on, the fans running, and the A/C running (and with the engine compartment heat soaked.) At a traffic signal, the battery should carry the load and the charging voltage should recover as soon as the rpms increase. In traffic (longer periods of time with the car making under 12 volts), the voltage drop below 12 volts can become critical. One quick "trick" is to simply raise the idle speed to GT speed (800rpms) and see if this helps enough to "get by" with the stock charging system. A "GT" LH chip will work, although not calibrated for the correct fuel cure for an S4 vehicle. We can also program a custom Sharktuner PEM that has the only change being increased idle speed.


The original alternators are undersized for the current draw....and of a very old design, with the field wiring hidden in the case, plus only one fan (for the alternator) to cool the alternator. Add a "modern" radio or high current draw components and charging voltage can be very poor.
I took a modern design Bosch alternator that comes from a 997 Porsche (so it is designed to "live" high in a very warm engine compartment) with increased output, exposed field coils, and dual fans (plus a host of other features.) I designed and custom made a billet front housing for it, so that it is a direct fit onto a 928.
We've, to date, sold over 100 of these alternators, with 100% satisfaction.

This is the ultimate alternator solution....nothing else, on the market, gets close to this product!

GregBBRD

I'm not sure what this measurement means, but I just measured two Bosch rebuilt stock alternators, for you.
One had 23.68K and the other had 23.28K.

justinporsche

Thanks for the replies.
So, at 2500rpm with AC and headlights on, with voltmeter on battery posts, im getting 12.5 volts initially and drops down to 12 the longer it runs.
At that same rpm, with no loads, im getting 14 volts
Battery is new.
I just replaced the ground strap from the battery to body. No change.
I inspected the one from the engine to the body and took both ends off and wire brushed the surfaces. I dont see anything wrong with this ground.

Mrmerlin

Try another test point use the hot post make sure it’s been cleaned and tightened first
11 mm wrench

justinporsche

Thanks. Its the same. I took it apart and cleaned it but it was fine.

GregBBRD

On an S4, which continuously tries to get to high gear at the lowest rpm possible, that's not going to work, very well.
Have you "rebuilt" the relay board, yet?

FredR

Do you have a funky Hi Fi system by any chance? My 90S4 did and it behaved along the lines you describe. I was never sure whether there was anything lacking or whether it was just they way they are. My GTS performs better but then it does not have a funky Hi Fi to lug around and my engine harness is new.

Shark2626

How old is that rebuilt alternator? And are you certain that it’s the correct unit for your car? I guess I should add: Is it a remanufactured unit direct from Bosch or something someone built for you locally? What you are describing sounds like a bad voltage regulator on the alternator.