Voltage Experts...
Le Mans Master
Joined: Sep 2006
Posts: 6,050
Likes: 147
From: Phoenix AZ
2021 C4 of the Year - Modified Finalist
problem
Ya know,
every post has you with in gear or in neutral. and because it's an automatic, your foot has to be on the brake.......................
1. Just for grins, check your tail light bulbs sockets for a problem with the stop light/turn signal wires, chafing, etc.
2. Check your third brake light for any wire chafing problems as well.
3. check the stop light switch, connector, etc.
4. What else is tied to the stop light switch that operates by relay? What else is on the parking brake handle?
It seems that in park or neutral, everything is hunky-dory, but with your size 10 planted on the brake pedal, it goes low.
every post has you with in gear or in neutral. and because it's an automatic, your foot has to be on the brake.......................
1. Just for grins, check your tail light bulbs sockets for a problem with the stop light/turn signal wires, chafing, etc.
2. Check your third brake light for any wire chafing problems as well.
3. check the stop light switch, connector, etc.
4. What else is tied to the stop light switch that operates by relay? What else is on the parking brake handle?
It seems that in park or neutral, everything is hunky-dory, but with your size 10 planted on the brake pedal, it goes low.
Last edited by coupeguy2001; Nov 16, 2008 at 12:16 PM.
Thread Starter
Le Mans Master
Joined: May 2006
Posts: 6,774
Likes: 363
From: Midland TX
Ya know,
every post has you with in gear or in neutral. and because it's an automatic, your foot has to be on the brake.......................
1. Just for grins, check your tail light bulbs sockets for a problem with the stop light/turn signal wires, chafing, etc.
2. Check your third brake light for any wire chafing problems as well.
3. check the stop light switch, connector, etc.
4. What else is tied to the stop light switch that operates by relay? What else is on the parking brake handle?
It seems that in park or neutral, everything is hunky-dory, but with your size 10 planted on the brake pedal, it goes low.
every post has you with in gear or in neutral. and because it's an automatic, your foot has to be on the brake.......................
1. Just for grins, check your tail light bulbs sockets for a problem with the stop light/turn signal wires, chafing, etc.
2. Check your third brake light for any wire chafing problems as well.
3. check the stop light switch, connector, etc.
4. What else is tied to the stop light switch that operates by relay? What else is on the parking brake handle?
It seems that in park or neutral, everything is hunky-dory, but with your size 10 planted on the brake pedal, it goes low.
Instructor
Joined: Oct 2007
Posts: 214
Likes: 0
I always presumed that the variation in indicated voltage was due to the computer controlling the alternator for reasons not mentioned at all in this thread.
Fuel mileage, an alternator that is at full charge represents a fairly significant load on the engine and when trying to get every bit of fuel mileage and avoid the Gas Guzzler tax, the alternator could be outputing the absolute minimum amount of current needed to keep the car running.
Performance. See above. Engine output ratings are almost always done with the engine in a stand/bench dyno with the bare minimum accessories and no extraneous load. Why wouldn't the guys who programmed the ECM that controls the alternator to keep load at a minimum?
I see lots of measurements being made and some doubt about the DIC. The DIC voltage display is controlled by a computer, computers do what they are told to do. Put another way, DIC displayed voltage will be exactly what the PCM (or whichever *CM manages the IPC) tells it to be. The output to the PCM from the Alternator should be the same as the DIC output unless the PCM massages the data.
The Voltage gauge appears to be traditional, ie mechanical. Resistance is built in to damp the reading which is why it won't vary as much, and if anything it should read a little low if there is resistance in its circuit.
My Z06 has interesting fluctuations in the voltage which had me confused for a while. But after checking grounds, replacing battery with no change in behavior, I decided not to worry about it. I don't look at charging voltage in the DIC, I leave that to the gauge. Maybe that's ignorant on my part becuase I do have some electrical gremlins, but I think it's easier to troubleshoot some things when the clear problem is presented.
Just food for thought.
bob
Fuel mileage, an alternator that is at full charge represents a fairly significant load on the engine and when trying to get every bit of fuel mileage and avoid the Gas Guzzler tax, the alternator could be outputing the absolute minimum amount of current needed to keep the car running.
Performance. See above. Engine output ratings are almost always done with the engine in a stand/bench dyno with the bare minimum accessories and no extraneous load. Why wouldn't the guys who programmed the ECM that controls the alternator to keep load at a minimum?
I see lots of measurements being made and some doubt about the DIC. The DIC voltage display is controlled by a computer, computers do what they are told to do. Put another way, DIC displayed voltage will be exactly what the PCM (or whichever *CM manages the IPC) tells it to be. The output to the PCM from the Alternator should be the same as the DIC output unless the PCM massages the data.
The Voltage gauge appears to be traditional, ie mechanical. Resistance is built in to damp the reading which is why it won't vary as much, and if anything it should read a little low if there is resistance in its circuit.
My Z06 has interesting fluctuations in the voltage which had me confused for a while. But after checking grounds, replacing battery with no change in behavior, I decided not to worry about it. I don't look at charging voltage in the DIC, I leave that to the gauge. Maybe that's ignorant on my part becuase I do have some electrical gremlins, but I think it's easier to troubleshoot some things when the clear problem is presented.
Just food for thought.
bob
Instructor
Joined: Oct 2007
Posts: 214
Likes: 0
With an assistant in the car, you should be able to block the wheels with the transmission in drive and not have the car run off... Of course if idle is too high, or you or your assistant has a brain fart and blips the throttle... you could have a prob.. 
That will allow you to remove the brake light/pedal switch, etc from the equation.
bob
That will allow you to remove the brake light/pedal switch, etc from the equation.
bob
Thread Starter
Le Mans Master
Joined: May 2006
Posts: 6,774
Likes: 363
From: Midland TX
With an assistant in the car, you should be able to block the wheels with the transmission in drive and not have the car run off... Of course if idle is too high, or you or your assistant has a brain fart and blips the throttle... you could have a prob..
That will allow you to remove the brake light/pedal switch, etc from the equation.
bob
That will allow you to remove the brake light/pedal switch, etc from the equation.
bob
This afternoon I noticed that if I am going say 40MPH and apply the brakes neither of the voltage gauges show a drop until the engine RPM get under 1000 and then it will recover. I think the problem is that sometimes it dips further than other times and that is when the engine starts stumbling and I usually have to let off the brake to let the car roll a few inches and then it recovers.
Since I have now replaced EVERYTHING in the charging circuit except the harness, I am back to thinking that maybe it is another bad voltage regulator inside the new alternator? Either that or maybe the PCM is hosed?
Last edited by Choreo; Nov 16, 2008 at 07:20 PM.
Thread Starter
Le Mans Master
Joined: May 2006
Posts: 6,774
Likes: 363
From: Midland TX
Finally found my solution for most of the problems in this post.
See:http://forums.corvetteforum.com/c5-t...post1567886982
See:http://forums.corvetteforum.com/c5-t...post1567886982
