Dand88944ls1

Hi, I have noticed my voltage dropping under WOT. I have read through search results and some have reported the PCM will cut the Alt exciter voltage under WOT. I have posted a picture of the data from my car, a Porsche 944 with LS conversion using a 2000 Camaro PCM.

Does anyone know if the PCM does in fact do this or do I have another problem? The car was running great. I have a hard time believing the battery voltage should drop this low even if the exciter voltage was cut for such a short time frame. The battery is new and voltage at idle and off full throttle is back up close to 14v.




Voltage Drop at full throttle

Thanks

jimmyblue

iTrader: (11)

I have speculated that cutting exciter bias could be
another trick like how the A/C compressor is cut off
at higher RPM. But I have never seen this confirmed.

Your running motor electrical load is higher than the
load you see at key on engine off. This (and perhaps
some sheet metal ground rise) could be why you see
an unusually low voltage.

Might try the trick of putting an indicator bulb in line
with the L terminal and from there straight to IGN.
Then the PCM can think whatever it wants?

Dand88944ls1

Thanks, I am not sure what you mean by the "L" terminal? I have looked at the grounds so far they look good. The logger shows the voltage starts to drop somewhere around 4500 rpm. The belt is relatively new and I think tight enough.

jimmyblue

iTrader: (11)

L terminal (on F-body Gen 4) is the lonesome wire in
the push-in connector on top. L for "lamp". Old cars
this biased the field until regulator takes over, and
when faulted lights the GEN or ALT indicator. If you
give it an appropriate current limited feed then you
will get the boot current. You can consider this as a
diagnostic measure, just to tell if the PCM feed has
anything at all to do with the problem.

Now, these alternators do fail and mine went out at
about 50K - never saw the high RPM issue (which can
be brush skip or simple wear), just went below useful
charging voltage one day with no warning.

Dand88944ls1

Ah, the exciter wire in the Alt. I'll have to try that. And yes the Alternators do go out a lot, this is a track only car and on average I think the alternators have lasted maybe 10-15 hrs. This one is relatively new and when they have failed in the past they don't make the correct voltage at any time.

I was hoping someone would know for certain whether or not the PCM actually did cut the exciter voltage.

Dand88944ls1

Ah, the exciter wire in the Alt. I'll have to try that. And yes the Alternators do go out a lot, this is a track only car and on average I think the alternators have lasted maybe 10-15 hrs. This one is relatively new and when they have failed in the past they don't make the correct voltage at any time.

I was hoping someone would know for certain whether or not the PCM actually did cut the exciter voltage.

RonSSNova

iTrader: (3)

Interesting post.
I've noted this on both my C5 and 5.3 Nova with a 99 Camaro pcm when I log ign volts.

Not rely when you want reduced system voltage is it?

Would be good to watch the volt gauge in the car to see if it drops as well.

Ron

Dand88944ls1

Watching the voltage gauge in the car is what made me go look for it in the data log file. I started noticing the dash gauge flickering and then after watching it a bit more closely I realized it was doing this every time I accelerated out of a corner or on the straights. So I dug into my data files and sure enough the system voltage is dropping as the rpm go up.

1 FMF

iTrader: (1)

i don't know for certain, but as an educated guess i would say no based on the following.
the one wire coming from the pcm to the alternator on the LS1 (1999-2002) is the generator turn on signal. I'm not sure exactly how they did it in 1998 but i know for sure having had a 1999 and now have a 2002 along with the service manual that explains all this. the pcm puts 5v on that "exciter wire" which is technically not an exciter wire in the traditional sense. the 5v signal is the turn on signal from the pcm to the voltage regulator that's internal to the alternator. the regulator is a microprocessor and when it sees the 5v the regulator then manages alternator field current to control output. when the voltage regulator goes bad or has a problem it connects that wire from the pcm to ground, and is how the pcm knows there is a problem.
also, the volt gauge in the dash cluster is not measuring voltage directly, that info is in a data packet sent by the pcm to the gauge cluster along with all the other gauge information to display such as rpm, oil pressure, and coolant. the gauge cluster is it's own little computer. the volt gauge is not like the old ones where you connect the gauge in parallel to the electrical system one hot wire and a ground and it tells you system voltage directly.
so i doubt the pcm would actually stop sending the 5v signal and turn the alternator off at high rpm when electrical demand for the ignition system is the highest. it makes no sense to turn off the alternator and run off the battery which results in 12.0 volts or less to the ignition coils. for best performance the ignition coils want system voltage to be around 14.0 volts, when voltage to them drops at high rpm you are losing power to them which results in spark failure.
so my guess is the alternator itself is failing at high rpm which if i had to guess would be from bad brushes or a bad commutator. at 5000 engine rpm the alternator is turning faster than that, if the armature in the alternator is not well balanced and the brushes are not riding on a surface with zero runout then it would not surprise me to see fluctuations in output voltage.

to reliably measure this you would want to get an analog volt gauge that gives good resolution between 10 to 15 volts so you can observe any voltage fluctuation by the needle moving. digital volt meters tend to not report numbers fast enough to see a fluctuation quicker than 1 to 2 seconds. i haven't tried cheapo harbor freight analog meters but i've mail ordered other makes for less than $20 and with any you can easily wire to a cig lighter plug and just plug in.

1 FMF

iTrader: (1)

oh and the other thing i forgot to say was use a scotch lock splice connector and tap off the "exciter wire" from the pcm to the alternator and run a wire from it to a high impedance volt gauge preferably analog. and measure that 5 volt signal directly from the pcm, if you don't see it dropping out at high rpm then the pcm is continuously telling the alternator to do it's thing and the problem lies in the alternator.

Dand88944ls1

1 FMF thanks for the comments. On my car if I remember correctly I only have one wire going to the alternator and it is the wire from the PCM other than the main wire to the battery, so that is the only place I can see that the alternator (regulator) would be able to get the field voltage needed.

Thanks for the comments on the dash gauge, and I will have to trace my wires as my dash gauge is not from a factory GM, it is a Porsche 944, and I think it gets it's signal from the system (battery to ground) not the PCM output. Good suggestion to check the exciter wire voltage during acceleration.

Since the Alternator is a rebuilt, either O'Reilly's or Autozone, your comment about it losing the brush contact at higher RPM is something to consider. It is sounding like there is no pre programmed cut to the alternator on voltage on full throttle built into the 2000 PCM. Which means I probably have an alternator or wiring issue somewhere.

RonSSNova

iTrader: (3)

My Nova has a volt gauge that simply measures batt voltage from the main buss that feeds all accessories. It's not in a location that I'd care to watch going down the track. :-)

Also, my balancer has a 25% under drive.

So, for me I really want to know what the main system volts is doing during a run. The fuel pump draws 35 amps, I need it to see full charging volts.

I'm glad the OP brought this up.

My C5? All bets are off. Those things are electrical nightmares.!

Ron