Alternator Question

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09 Aug 2013 13:13 #15559 by GlennS
Alternator Question was created by GlennS
The voltage in my car continues to drop when I run the A/C unit. I have checked all my grounds and measured the voltage with the A/C on and off and the voltage is not holding. I am assuming the voltage regulator is attached to or integral to the alternator. Since I have made up my mind to replace the alternator I started looking for a replacement and encountered a huge dilemma. On Greg Nichols site there was a recommendation to buy a Bosch alternator, Vertex Auto sells a Bosch for $300.00.

However 928 Motorsports has a write up that says the OEM (115 amp) alternators are underrated and they sell a 200 amp alternator that they say is what these cars should have. By the way their 200 amp alternators are $700.00 bucks! My car is a 1980 and does not have many electrical loads. Before going forward I thought I would see if there is any wisdom out there on 928 alternators. Any advice is welcome.

Thanks,
Glenn

Glenn Simpson

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10 Aug 2013 08:17 #15560 by wallyp
Replied by wallyp on topic Alternator Question
First suggestion - make sure that you have an unworn, very tight alternator belt. Make sure that the alternator pulley isn't worn. The sides of the pulley where the belt rides must be straight - a step means that the pulley is worn.

If the belt and pulley are perfect, I have a different suggestion...

Find a good, local auto electric shop. This is a shop that repairs/rebuilds alternators, generators, starters, motors, etc. Not a standard garage or repair shop, not an auto parts store, but a specialized auto electric shop.

They can rebuild your alternator, so you know that it fits and is the correct alternator for your car. Discuss the situation with them - my rebuilder showed me eight or ten different regulators that could be used on the alternator.

One difference among those regulators was temperature compensation. A lead-acid storage battery is subject to Arrhenius' equation, which says that a chemical reaction doubles in speed for every ten degree Celsius rise in temperature. If the car battery is in the engine compartment, this leads to overcharging in the summer. To prevent this, regulators are available that lower the charging rate with increasing temperature.

Our batteries are not in the engine compartment, so don't get nearly so hot. Using a regulator that lowers the charge with temp can result in undercharging in summer heat, with AC on, lights on, and low engine speeds in heavy traffic. You rebuilder should be able to install a regulator with less (or no) temperature compensation, which will give better charging under such circumstances.

Wally Plumley
928 Specialists
www.928gt.com

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10 Aug 2013 12:47 #15561 by GlennS
Replied by GlennS on topic Alternator Question
Thanks Wally,

That sounds like real good advice. I went to a local electric shop today and learned that my battery couldn't handle a load test so I replaced it. So far it seems my alternator is OK. Before I pull the alternator off for a bench test I have 2 questions.

1) What should the idle speed be for a 1980 5 speed? Thus far I have not been able to find a source for the answer. My car is idling at about 750 rpm. A shop manual I have says the idle speed for an 85 should be 775 plus or minus 25 rpm (not sure if that is for a manual or automatic.
2) Should the idle speed increase when the air compressor is started?

The reason I ask these questions is after replacing the battery as soon as I increase rpm voltage kicks up and everything seems to be working fine.

Thanks again,

Glenn

Glenn Simpson

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11 Aug 2013 00:50 #15569 by 928mac
Replied by 928mac on topic Alternator Question
Glen, its obvious that you can idle a standard higher then an auto, and I am not 100% certain but I dont think the torque convertor will handle much more then 800 rpm in drive without causing extra heat (Heat kills automatics)
I have mine sitting around 775-800 but when I come to a light I ALWAYS take it out of gear and into neutral, then my voltage jumps up.
I also always manual shift my automatic. Its a pain in the ass in stop and go heat though.
I use the windows instead of the ac in town going slow, but with the revs up on the highway I use the AC.
I have a 200 amp Alt. from 928MS and a deep cycle battery and the voltage still drops. :confused:

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I believe I also have some ground issues showing up so even though most everyone seems to have similar issues, I dont believe it has to be this way.

Clean grounds and good wiring, good battery and good Alternator.

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Oh ya, I also have 4 - 70,000 volt coils firing my spark plugs, I'm sure that uses some power

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11 Aug 2013 21:21 #15585 by mm928
Replied by mm928 on topic Alternator Question
Hi Glen, not too unusual for early cars it seems. I have an 83 and had similar issues initially, but since I have fixed the major stuff and still see fluctuations when A/C is running as well as headlamps. Wally gives great advice, but in case you do not go that route, all I can say is what I did.
- Cleaned grounds of course
- Deep cycle battery just like Brad
- Also changed the battery cable going from the new battery to the starter to 0 qauge fine
wound copper! Get it from a welding supply, get super flex so that you can get it into
the right spots under the car! Otherwise you will be cussing at me!
- look for frayed wiring. I had to pull the wiring harness and have it rebuilt as it was
grounding out in places.
- I also had to replace the 14 pin connector on the passenger side of the engine as my car
would not start when it was hot...did the wiring harness at the same time and I have
had no issues since!
- Yes I did get a bigger alternator than original, I think the 115. Actually ours was a 90 I believe from the factory...quite low I think for an older car no longer in new condition. The 115 should work fine if the other stuff is ok as mentioned. Also there is a 130 model you can get from Bosch if I am not mistaken and a larger one than that - actually a GM Camaro alternator I think that works. Just do a search.

Our older model years seem particularly susceptable to low RPM so belt tension and idle can definitely be important. My idle is really low right now (around 450) but the rest is in good order so the car will not die and will hold it, at least for now....

Good Luck

1983 928S - 16v 4.7L L-Jetronic
Dark Grey w/ Blue Leather
85/86 exhaust manifolds
Y-Pipe
SS Hi-perf Cat
S4 springs
Bilstein shocks
Air pump delete
VIN: WP0JB0926DS861180
- - Isn't Life great? - -

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15 Aug 2013 20:05 #15646 by GlennS
Replied by GlennS on topic Alternator Question
Thanks Mike,

I chased grounds a few months ago. The wiring harness, running a new positive cable and upgraded alternator all make a lot of sense. I'll add them to my work list.

Glenn Simpson

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