Midnight89

I've searched the archives and still can't get comfortable with my situation...

My 89 Auto just came back from the shop: new tranny, hypertech coil, rapidfire plugs, conrad cap, taylor wires.

I believe that I'm having a problem with the charging system. My car now idles at 500 rpm (which seems a bit lower than usual). When driving, even with the major accessories on, I maintain around 13.7 volts. As soon as I stop and the motor idles, the volts will drop to 12.5-12.8. I've seen it go all the way to 11.5 with the rear defrost on. I just changed the alternator (105 Amp GM Rebuild), and it didn't remedy the problem.

Should I try another alternator? Would it be best to go to a 130 or 140 amp alternator? Could I raise the idle so that the charge wouldn't drop as low?

CFI-EFI

It sound like the charging system is working properly. Just like your engine makes it's rated horsepower at a given (elevated) rpm, so does your alternator. It simply isn't turning fast enough to keep up with your accessories. Your idle IS low. I would get it back up to spec. If it doesn't cure the problem, it certainly will help alleviate it somewhat. Good luck, and...

RACE ON!!!

scorp508

You don't happen to have an underdrive pully do you? I do and I still don't get voltage that low at idle. Always 13.7 or higher. I would have your battery tested also.

jfb

First, your idle speed is too low and should be bumped up. Your voltmeter readings are normal and for such a low idle speed the 12.5 reading and 11.5 with the rear defroster is also normal. At normal idle speed , the alternator cannot deliver its rated current and heavy electrical loads forces the battery to supply current and that is why you read 12.5 to 11.5. The rear window defogger draws 24 amps! C4 alternators at highway speed deliver 14.7 volts cold and this drops to 13.7 hot. Your dash voltmeter reads the voltage after the ignition switch which has about 0.3 volts drop across its contacts, so your dash voltmeter will read 14.3 cold and 13.3 hot.

okinawa86vette

That hypertech coil might also be drawing some extra amperage at idle that your alt can't keep up with, especially with the lower than normal idle. Check the archives to correctly set the idle. It's not just a matter of turning the set screw on the throttle body.

Chris

Midnight89

Thanks for the input fellas.

Scorp...I do NOT have the underdrive pulleys. I'd love to put them on the car (I have a March Power & Amp set), but I'm already having charging problems without the pulleys!!

It seems that I need to adjust the idle. Anyone out there have experience with adjusting the idle on an 89? This would be a first for me, so any specific tech information would really help. I'll go check the Haynes manual, but it sure is easier to understand when I hear it in "layman's" terms.

cash's90

My '90 would indicate a low charge rate under load at idle. I disassembled and cleaned the heavily corroded contacts on the B+ terminal post and the charge rate improved to normal. To find the B+ terminal post, just follow the cable connected to your battery + pole to the terminal location. It should have several smaller wires connected to it. I used Permatex di-electric grease during reassembly to avoid future corrosion. :steering:

Midnight89

OK, I checked the Haynes manual and it tells me that the ECM controls the idle and that it isn't as simple as turning a screw!!

Is there an easy way to bump up the idle?

dgoodhue

Their is a procedure for setting minimum air (involves IAC) and TPS. When these are set properly you ECM can control teh idle properly. Try doing a search this topic should come up or O or someone can type up the procedure.

CFI-EFI

Very likely you have carbon build up in your TB. A good first step would be to remove the TB and clean it out thoroughly. Pay particular attention to the idle air passages and the IAC. Then set the minimum idle and go from there. Good luck, and...

RACE ON!!!

Midnight89

Score two points for CFI-EFI!! Just wanted to let you know that I cleaned the TB today. It was horrendous inside!! Not anymore....shines like its brand new, and it fixed my idling problem. Hence, my charging problem has disappeared.

Just wanted to say, thanks for your help....I could've never fixed this on my own.

86PACER

I agree with all the posts. Here is the complete procedure on adjusting your idle. Your altenator will not make it's full output at 500rpm. NOTE: Disconnect your EST connector while doing this, the tech tip left this step out, the Shop Manual highlights this step. https://www.corvetteforum.com/techti...D=55&TopicID=2


[Modified by 86PACER, 3:22 AM 3/14/2004]

CFI-EFI

Hey, Great! I'm happy if I was a help. I'm even happier, that you've found and solved the problem, without it costing a bundle. As far as *I'M* concerned, that's mostly what this forum is about. I appreciate the feedback.

RACE ON!!!


[Modified by CFI-EFI, 10:43 AM 3/14/2004]