Bret928

Kind’a PO’ed with a local shop, that I took my 78 Shark to look into my Charging system problem. Anyway they called me and told me that the Alternator was bad and it needed to be replaced.

Well they swapped it out and told me the car was ready to pick up. But when I got the car home I noticed that the dash volt meter was reading low 10-11volts. Knowing it wasn’t the most accurate, I took some measurements with my multi-meter and sure enough the charging system wasn’t working. Measured an average of about 11volts, but would dip to a low 9.5v and a high of no more than 11.5v.

Well just as I was about to drive the car back to the shop things got really ugly as it abruptly stopped running! Attempts to re-start yielded noting. Heck even the hazard lights wouldn’t work. Took some measurements under the hood and had "ZERO" volts with the ignition on. Now I could have investigated further – but I was so mad by this time that I called the shop to get my pound of flesh & report what had happened. Got a couple of "sir-yes-sirs" & they quickly dispatched the technician who worked on my car and a driver to restart the car & bring it back to the shop.

After the tech arrived he prepared to jump start the car & as he was removing the battery cover we observed little sparks emanating from under the cover. Once he got the battery cover off my multi-meter that I had connected to a couple of test points jumped up to 10volts! Ah-ha! It appeared that when the last tech re-connected the battery after installing the new Alternator he left an auxiliary wire (old car alarm I suspect) orientated upward & that is what made contact with the battery cover and caused the abrupt loss of power.

Anyway after jumpstarting the car I showed the tech the reading on my meter as it was obvious that the charging system still wasn’t working. As they drove off I called the shop manager to report what was discovered & what we observed. Just incase the tech doesn’t tell the story properly.

So now I’m sitting here waiting for a call back to tell me what they find. Personally I suspect that the Alternator wasn’t the real charging system problem and whatever is wrong either took out my old unit or it wasn’t bad at all. Hopefully I’ll know more today.

docmirror

If you have a volt-ohm meter and know how to read it, you're probably ahead of the shop you took it to. My advice, get your old parts reinstalled, get your money back(if possible), take it home, get out the books and repost. We'll walk you through it. (by "we" I really mean Alan, with assists from the rest of us).

78s aren't very complicated, and the charging system is pretty simple too. Just my 2 cents...

Doc

neilh

Bret,
I've been down that path as well. I have a spare regulator for a marschal alternator if you want to give it a try.
Neil

j.kenzie@sbcglobal.net

Bret,
It's still possible that the shop was correct about a bad alternator/regulator (ask them how they tested it) and the battery may be toast as a result of lack of proper charging. If they are competent they will figure that out. If not you should get as much info and old parts together as you can and hopefully one of our electical experts can help you. Good luck.

Bret928

I just got off the phone with them and they're telling me that the battery wasn't secured and they are looking to make sure it is properly tied down. I told them it is pretty secure already but I don't think there would have been a problem if the tech had reconnected that AUX-wire properly.

Anyway the guy told me that they would be looking at the new Alternator for any defects and also check the regulator. My question is why they didn't do that the first time.

That said I will be requesting they give me back my old Alternator and don't want to pay a core charge.

neilh

Also make sure they correctly reinstall the cooling duct and pipe on the alternator, it needs all the cooling help it can get!

AJK

Sorry if I am hyjacking this thread, but your comment about the alternator needing all the cooling it can get brings to mind what I have noticed lately. I have one of those gages that reads about 12V on the dash and 13.5 at the battery terminal but it also seems to respond to temperature. As I start out on a cool morning it will read about 12.5 V, but then drop to about 12V as things warm up. Is this a normal response to changing temperature?

j.kenzie@sbcglobal.net

Mine does the same. I thought it was normal charging behavior. Just like a trickle charger, it reads higher current (here voltage in the car) as it replinishes the battery from the starting load, then drops to a lower level. I think I read that here and it made sense at the time.

dr bob

Interesting. Actually, the current flowing to the battery is a function of holding a constant voltage at the alternator end of the cable, and allowing the battery at the other end of the cable to absorb as many amps is it needs to maintain the voltage at the alternator end. So the 'charging harder' at a certain time of day would cause the volts to read lower when that happens, not higher.

Any other thoughts from the electrical gurus??

John Struthers

Betting on the regulator...yep even on a replacement alt.
You won't get your old parts back as the alt. is a core charge item
and either has been or will be put back in to the supply chain for repair.
With a chev/ford/mopar the failure rate on a rebuilt alt. is so low that it
doesn't rate a percentage point. So, where you might call the shop sloppy by
not checking the alt. before installation consider their past experience installing
near perfect rebuilds on other marque's. Personally, a few years ago, I was juggling
3-4 alt's at the same time and went thru 5 before getting one that worked. And that one
I had to swap the pulley to have one working alt.
Auto part's store rebuilds for our sHARKs are available and comparatively cheap,
however, they do NEED to be checked at a AUTOMOTIVE ELECTRICAL Repair Shop.
If they o.k. the alt. you can feel confident that it will work after install.
Wally constantly harps on grounds, connections, etc... and I have to agree.
Find your chassis/engine grounds, follow your leads from battery to alt/starter.
If they are grungy, oily ... disconnect battery and start cleaning!
Even old, fat sHARk drivers can find a spot on the curb to get a little
height -belly room-for inspection. For under $75.00 you can find a set of jacks and
stands for safer/higher work would be better. You don't need to be an ace wrench
to to some PM. Good luck.

Shark Attack

isnt there a resistor on the back of the gauge cluster that also plays a roll in this?

Alan

Humm - interesting...

Certainly the battery is not charging if you see well sub 12v when running.

Prime possibilties are Alternator, Regulator, Battery and wiring/connections from alternator to/from battery (and or their grounds)

Does the battery take a good charge with the car off/disconnected?
do you have any issues starting the car after charging the battery?

The total voltage loss at the jump post is interesting - suggests a dead battery (won't hold any charge) or a serious connection issue. The small gauge wire in the battery box is likely a 'red herring' since a 928 battery in good conditon would have no trouble melting a 14/16AWG wire that short circuited. I'd rather suspect a failure (corrosion) of the connections to the battery posts or the battery ground strap - test this by connecting a booster cable between the battery ground post and the car fame - say ground point wingbolt...

I suspect there is still an alternator/regulator issue - but its possible that if you have a faulty battery or battery conection - this issue could have fried both your old and new alternators/regulators... However the shop should probably have detected the other fault...

Test the battery at a store with a large carbon pile test load - personally I'd take the ground strap with me - leave it on the battery and run the test through the ground strap - flex it as the test runs... you'd be suprised how often that big fat strap corrodes to the point it is an intermittent open circuit - once they deteriorate they fail fast... moving the battery or even opening the box can change how it acts...

Alan

John Struthers

Shark,
Yes I believe it's a 3ohm resistor.
Aka an EXCITER.
Could be the cause but very rare.
I haven't looked at the schematics, but have a complete spare pod ... the resistor/exciter appears to be wired right behind/into the voltage meter. With the pod off it sticks out like a sore thumb.

docmirror

Resistor aka exciter. That is the usually the 'kick' circuit for the alternator field. I believe that's the function on the early 928 as well. Any opens in that circuit will not energize the field of the alternator and it will not start charging. Late GM alternators had a one wire connections, where the kick circuit was included after the diode pack to get the field going.

As for the voltage difference on the system, I too notice that my voltmeter shows a slightly lower value after driving for a while with a constant load established. Since I don't know where the voltmeter actually does the sensing, I would guess that it is very near the source(alternator) and not the load(battery). In this case, the voltmeter acts more like a wheatstone bridge meter, or a galvanometer, where the apparent voltage at the alternator is high to maintain a decreasing current into the load. As the battery charges, the voltage needed to 'balance' the current draw decreases, and that decrease is seen on the voltmeter.

It seems a poor location to sense the voltage to me. When I use the AC and the lights, my voltmeter shows barely 12V. However, with the engine running at 1800RPM, the voltage at the battery is up around 13.5V which is correct. Another indication that the voltmeter is acting more like a load meter, than a charging system reserve meter. Perhaps that's the way Porsche intended, but it seems strange to see the voltmeter change so dramatically under load and no load conditions. In US cars, the voltmeter generally sits right on 13.5V no matter what. I suspect it's sensing near the battery, and is well sunk from the other loads.

Of course an ammeter would have been preferrable, but it's more costly to implement.

Doc

Alan

Doc,

Actually although the absolute voltage reflected may be offset from reality (as many have seen) any voltage variations you see are still likely close to reality across the system since the wire size to the battery is pretty big and the most common effect you see is the alternator collapsing to a lower voltage under a serious load @ low RPM - it just cannot generate enough power.

The alternator/regulator is essentially designed to maintain a system voltage
(approx 13.8v) one that will also guarantee charging of the battery - but its not designed primarily as a battery charger so it certainly is not a regulated current type charger - except that is limited as to how much max current it can source (varies with RPM).

The natural battery voltage under marginal load is approx 12.6v (or less) - whenever the battery voltage is above 12.6v under system load - you can be quite sure the alternator is generating and supplying the system load and driving the voltage level you see... and the battery is receiving charging current, whether it's actually being charged depends on its health.

There are also various types of battery construction and some tolerate or benefit from higher or lower charging voltages better or worse than others. Some US models have higher system voltage - some up to 14.8. Some battery types designed for these vehicles (e.g. Delco Fredom II) require a higher voltage (14.8v) to fully charge while others may overheat and be damaged due to excessive charge current at these same voltages. Swapping to a GM style alternator or battery could cause some issues as a result (including for an alternator different voltage/current performance at idle).

A battery ammeter would tell you when you are demanding serious charging current but alone won't tell you if the alternator or battery are healthy - it is in the nature of the system that sometimes the battery needs a serious charging current and at other times it does not. High or low battery current in itself does not indicate a problem or lack thereof... A smart system that monitored rpm, system voltage, battery current, additional system loads - and time since starting could give you a very good idea of your battery health.

Almost makes me think of designing one....humm. It is rather car specific and invasive to install though.

Alan