2001 Coupe LS-1 / A4

I am having a similar problem to what I had a couple years ago, but not exactly the same. Previously, when I would come to a stop at a red light the RPM would drop below 600 and the car would start to shake and the voltage would drop until I put the car in neutral. Things I did at the time...

• Tightened Solenoid connections on Starter (no change)
• Replaced Ignition Switch (slight improvement, but no fix)
• Replaced Starter & Solenoid with new unit (no change)
• Replaced Alternator with new factory Valeo unit (no change).
• Finally cleaned the throttle body / butterfly (fixed the problem)

The past month or so, I have noticed that the voltage gauge which normally reads about 13.4 - 13.7 starts to drop when I stop at a red light after about 30-seconds. The RPM does not drop like before and the car does not shake like before, but it starts to die when the voltage drops under 12-volts on the gauge until I put it in neutral. Once I put it in neutral the voltage comes back up above 13-volts and all is fine again. It only has this behavior after the engine has warmed up and is much worse at night with the headlights on and I am stopped for any length of time.

I have cleaned the throttle body like before, but it looked good to begin with.

One suspect is the alternator. Two years ago when I had my other problem, I had O'Reillys check the alternator and their tester showed "Bad Diodes". So... I took the car to the dealership and they agreed that I had an alternator problem and they replaced it with a new one under warranty (but I saw no improvement). So, I then drove the car back to O'Reillys and had them check the new alternator and again their tester said "Bad Diodes"? I decided this was too much of a coincidence to have two bad alternators - one with 39k miles and the other with 10 miles. That was when I discovered the fouled throttle body on my own which seemed to fix the problem and for the past 2 years and 8,000k miles of daily driving have always stayed above 13.4 volts on the gauge until the past month.

I have a 2.5 year old Optima Red Top which I had them bench test and they said it was good (and it has never been discharged as I drive the car every day).

So any idea what I may be looking at?

 

No offense, but I see a lot of parts changing, and no troubleshooting. Have you evaluated anything, using a meter...and schematics?

 

Did you try the idle relearn and also looked for air leaks...bridge tube, pcv valve connection tube and so on....

 

The voltage drop is not the problem but a SYMPTOM resulting from the poor idle. Your alternator has poor output anytime the idle drops LOW. Nothing wrong with it cause it was NOT designed to provide the correct voltages and current output below the standard IDLE RPM.

You need to troubleshoot the idle issue before you start worrying about the electrical.

First, what modification do you have and has your car ever been re-tuned? Read and post ALL DTCs. If there are a LOT, clear them and when the car has the issue, immediately read and copy down the DTCs and provide the results to us.

Look for vacuum leaks at any and all joints between the TB and MAF including the PCV Sysyem.

BC

 

Did they install the correct replacement alternator? Your 2001 A4 equipped Corvette requires a slip clutch pulley on the alternator. Without it, the symptoms that you have can occur, when idling in drive.

 

This happened to me after the LED swap on the HVAC, voltage drops very slowly, slowly enough that its bearable, but since i bought the car ive also thought it idled kinda low. My LS2GTO would idle at about 800RPMS. My C5 idles at about 600 IF that.

 

Have not done any parts changing is a couple years - did a few diagnostics back then before changing the parts mentioned. Have not done any meter testing yet this time around (yet).

So are you saying that 600 RPM is not normal idle? That is all I have been reading for at least the past two years?

I did get a Chuck Cow Mail Order tune just over a year ago, but did not notice any change in idle speed?

No Codes showing.

The idle RPM dropping was two years ago - I am not experiencing that now - it stays solid at 600 RPM as the Voltage slowly starts to drop - usually don't see any drop for about the first 30 seconds after stopping at a red light, after that it just starts to go down under 12 Volts over the next 10 seconds, then I put it in neutral to get the volts back up - but RPM stays at about 600.

Guess I could start looking for a sporadic vacuum leak.

Did the idle relearn after the Mail Order Tune about 14 months ago.


I am pretty sure they installed the correct one...

 

Stock idle rpm for all years LS1 / LS6 Cars is:

M6: 800rpm
A4: 650rpm (park/neutral), 550rpm (in gear)

 

I couldn't tell from your pics if your alternator has a pulley clutch or not.

If it's equipped with a pulley clutch, you will see a black hub that sticks out about 3/4" in front of the belt pulley on the alternator.

 

Then looks like my idle is right on the money (until the voltage starts dropping, but does sometimes takes 30 seconds - 1 minute idling before the drop starts). Double checked today while driving and it was 600 in neutral and 550-600 in gear.

A better pic taken tonight...

 

Your alternator has a pulley clutch, so we can now rule out the possibility that I mentioned earlier.

 

I still think the voltage issue is a result of a seperate issue that is effecting your idle speed. Have you read your DTCs when the problem happens??

Please clear ALL the DTCs and when the car has the issue, read the DTCs and post what codes are present.

What is the actual battey voltage at the battery when the problem happens. Dont know if you can make the problem happen when the car is stationary so you can measure it but knowing that voltage may help solve the issue.

BC

 

Follow Bill's advice listed above, then make a couple of resistance checks on the +12 volt and ground connections between the alternator and battery cables, as described below.

1. Disconnect BOTH cable connections at the battery. Disconnect the red +12 volt output connection at the alternator.

2. Measure the resistance from the alternator case to the battery negative cable.

3. Measure the resistance from the alternator output wire to the battery positive cable.

Each resistance measured should be less than .5 ohms.

 

I have not checked codes during an event, but it shows no history codes saved.

I may be able to test with the car stationery by hooking up a DVM and lying it on the outside of the windshield? After all the only time this happens is when the car is stationery and in gear after a period of time.




I'll see if I can get to this tomorrow.

 

The battery voltage should be nearly the same, inside the car, at the cigarette lighter receptacle. It's also present at the OBD II diagnostic port, if you have a code scanner that can display battery voltage.

 

Good point!

Anything that is turned on when the key is switched to ON could have a lower than battery voltage because it goes thru a switch or relay which could have a poor connection and drop some voltage.

 

Bill Curlee makes a good point. If the car was mail order tuned, I have a thought.

When I mail order tune cars, I never make changes to the idle stuff unless dictated by a cam or something like that.

Now, when tuning these cars with stock engines, you usually have somewhere in the neighborhood 6-8 grams
of air required to idle the vehicle. The problem is that with such a small number to work with any error in air
or fuel can be magnified quite a bit.
With all of the air intakes and variances in design, install, and manufacturing, a MAF TRANSFER FUNCTION
that works perfectly for one guy with a particular intake, might be off a few % for another guy.

I wouldn't spend any more time chasing voltage problems. My guess is that your fuel trims are off (likely too rich)
in the lower part of the MAF table. YES, the car may run great, but as the weather changes you may have issues
like this from a small variance in fuel.

Many of us tuners have been successfully delivering mail order tunes for many years. Most everyone is very
happy and we have very few problems. But, there is a small % of customer's cars that for whatever reason,
don't work EXACTLY the same way with tunes and mods as most other vehicles do, and it isn't easy to know
unless you have some datalogging tools or crazy side effects.

SOME WORDS OF ADVICE: Please don't take this as a sales push on you, but in the past (and not until
VERY recently) we did NOT have a way to see how our mail order tunes were doing in the field.

NOW, WE DO. It's called AutoCal and it's AMAZING. It allows us to either fix problems like this or prevent them
from happening in the first place.

I know you want a quick solution to your problem and purchasing a NEW ALTERNATOR makes good sense to you...

But when compared to the cost ($299) of the AutoCal, On this one, it might be a better shot at fixing the problem.

One last thing, GM Throttle bodies can tend to get gooked up with carbon and whatever.

Cleaning them with throttle body cleaner has been known to cute idle problems and dips like you're speaking about.

Consider the AutoCal and call me if you have futher questions.
Chuck CoW

 

Outstanding advice and Chucks AutoCal Tunes are fantastic. With AUTOCAL you can retune or have your tune checked as easy as doing some data logging and e-mailing the data logged file and getting a file back with the correct or updated tune and up-loading it to the PCM.

You have an Automatic so, you can get a spot on tranny cal. Chuck tranny calibrations are the best!

 

Good advice, and it's cheap and easy to do.

 

We spoke last December about the Autocal. I plan to do that with you, but me and everyone else I know uses a Mac - so as soon as I find someone with a PC I can rely on I will do it.

I can install Windows on my Mac as well, but this would be the only app I would have for it - so hard to justify buying Windows as well just for this one function.