Losing voltage at high rpm?
Thread Starter
Joined: Nov 2010
Posts: 808
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From: Toronto Ontario Canada
Losing voltage at high rpm? Ok guys Im having an odd issue I noticed recently when Im at wot.I didnt Have this issue with this motor in my last transam with the same wire harness being used.My problem is that when Im at wot for a good length of time..Im seeing the voltage drop from 13.9 cruising and then punching 3rd of 4th for some fun..it will steadily drop below and Ive seen it hit 12v.
The only thing I can think of is bad alternator,bad grounds or insufficient power cable. The alternator was replaced last year when my power steering pump leaked all over it and i have a truck alternator on the car now which has been great for 150amps.The power cable from my battery to the alt is 4ga car audio powercable ..from the alt is 4ga to the starter where my original main power for the car connects to. Each cyl head has 4ga grounds bolted to the frame and 4ga ground at the battery to the frame.All ecm ground connections look good at the back of the heads.The only difference from when i had this motor in my last transam was that I had the ecu main power fused at the battery directly with 10ga wire as opposed to now where its wired to the original fuse panel in the car which is probably 12ga wire at best lol.
Now its easy to run new wires for the ecu just cause it wouldnt hurt but Im curious if there are any different settings that can be adjust in the tune for the charging system?Last time on the dyno I knew the alt was bad and it wasnt charging anymore so it was no surpise of low numbers.but its been changed..any guesses or advice of what i should look for?
The only thing I can think of is bad alternator,bad grounds or insufficient power cable. The alternator was replaced last year when my power steering pump leaked all over it and i have a truck alternator on the car now which has been great for 150amps.The power cable from my battery to the alt is 4ga car audio powercable ..from the alt is 4ga to the starter where my original main power for the car connects to. Each cyl head has 4ga grounds bolted to the frame and 4ga ground at the battery to the frame.All ecm ground connections look good at the back of the heads.The only difference from when i had this motor in my last transam was that I had the ecu main power fused at the battery directly with 10ga wire as opposed to now where its wired to the original fuse panel in the car which is probably 12ga wire at best lol.
Now its easy to run new wires for the ecu just cause it wouldnt hurt but Im curious if there are any different settings that can be adjust in the tune for the charging system?Last time on the dyno I knew the alt was bad and it wasnt charging anymore so it was no surpise of low numbers.but its been changed..any guesses or advice of what i should look for?
What condition is the drive belt in, the belt tensioner?
At what rpm does the voltage start to drop, and how high are you spinning the motor, also what crank and alternator pulley is on the engine, stock, UD, or OD ..?
At what rpm does the voltage start to drop, and how high are you spinning the motor, also what crank and alternator pulley is on the engine, stock, UD, or OD ..?
Thread Starter
Joined: Nov 2010
Posts: 808
Likes: 62
From: Toronto Ontario Canada
Id have to say around 4k and up it starts to happen and the limiter is at 6700rpm.The belt is a few years old,the tensioner is about 3yrs old.The drive system is all stock fbody from there.
Any oily/dirty build up on the belt or stuck in the pulley groves(ps.,alt.,Crank)....?
The alternator pulley nut is nice and tight; correct?
Factory or aftermarket manual belt tensioner ...?
The alternator pulley nut is nice and tight; correct?
Factory or aftermarket manual belt tensioner ...?
Thread Starter
Joined: Nov 2010
Posts: 808
Likes: 62
From: Toronto Ontario Canada
The belt might have some build up,Ill have to check in the morning.Alt nut should be nice and tight as its still a less than a year old. I could just get a new belt regardless..2 min fix to change it out.
if the belt has any build-up on it then you likely have some also on the pulleys. I'd remove the belt and throw it in some hot soapy dish water and clean the pulleys. Then you eliminate any possibility that the belt is slipping, and if the voltage drop issue remains then you'll know the problem is with the alternator.
Thread Starter
Joined: Nov 2010
Posts: 808
Likes: 62
From: Toronto Ontario Canada
looks like ill need a new alt.changed the belt and it helped some.i did notice that the connections at the alt have build up on them so that cant be good either.
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Thread Starter
Joined: Nov 2010
Posts: 808
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From: Toronto Ontario Canada
Today I tried 2 brand new alts.....and my issue is still there.Testing further at idle I meter 14v.Once the revs pick up to 2300rpm the voltage will drop and the car is clearly running off the battery only.I also tested the pcm wire at the alt and its getting 9.8v when unplugged. I havent tried probing it and measuring the voltage while its connected.
Now previous alt indeed had a good amount of water damage and corrosion and the tech who I got the alt from thinks its very possible the water damage may have fired the circuit on the pcm and its not sensing the voltage drop to command charge at higher rpm.
Does anyone think this is my problem and I need a new pcm? Ive went over all the grounds and power cables..cleaned the ring terminals to be sure and retorqued them down.It would really suck to have to get a new ecm when Im in need of getting a retune also
Now previous alt indeed had a good amount of water damage and corrosion and the tech who I got the alt from thinks its very possible the water damage may have fired the circuit on the pcm and its not sensing the voltage drop to command charge at higher rpm.
Does anyone think this is my problem and I need a new pcm? Ive went over all the grounds and power cables..cleaned the ring terminals to be sure and retorqued them down.It would really suck to have to get a new ecm when Im in need of getting a retune also
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Verdad Gallardo The info that I have for my 99 LS1 F-body states you should get at least 10V or greater from the PCM to Alt. wire. You did disconnect it at the alternator and check it with the engine running; correct?
BTW, maybe you answered your on question..?
The only difference from when i had this motor in my last transam was that I had the ecu main power fused at the battery directly with 10ga wire as opposed to now where its wired to the original fuse panel in the car which is probably 12ga wire at best lol.
BTW, maybe you answered your on question..?
The only difference from when i had this motor in my last transam was that I had the ecu main power fused at the battery directly with 10ga wire as opposed to now where its wired to the original fuse panel in the car which is probably 12ga wire at best lol.
I can't really give you any tips or advice, but I was watching some stalled automatic acceleration videos on youtube the other day and noticed that when they went WOT and got around 4000rpm and up their voltage dropped to the yellow part on the gauge. Maybe the coil packs start sucking down that much more voltage or some sord of shut off that lowers alternator drag at WOT. I think the AC compressor does this at like 3000rpm or above or seems i read something like that. Anyways, Good Luck!
Thread Starter
Joined: Nov 2010
Posts: 808
Likes: 62
From: Toronto Ontario Canada
Couldnt reply back earlier.Im going to run new 10ga wire direct from the battery to the ecm on friday regardless.It was just too damn cold to go crazy fixing and testing things out in the cold. The ecm wire to the alt measured 9.8v unplugged from the alt while running but Ill have to check if it does anything different when its connected to the alt. ...maybe it drops in voltage in higher rpm.
My ecm is mounted on the passenger side underdash so ill check the connections there that everything is tight.I opted to get a new ecm from Frost with a tune just because my current ecm is doing some other weird things and this ecm came out from my previous car which had a major fire where it was located.I wouldnt be entirely surprised if some things on the board didnt take well to the heat.
My ecm is mounted on the passenger side underdash so ill check the connections there that everything is tight.I opted to get a new ecm from Frost with a tune just because my current ecm is doing some other weird things and this ecm came out from my previous car which had a major fire where it was located.I wouldnt be entirely surprised if some things on the board didnt take well to the heat.
Thread Starter
Joined: Nov 2010
Posts: 808
Likes: 62
From: Toronto Ontario Canada
My current ecm is the older 9354896 and im be getting a 12200411 to replace it.Supposedly the 411 is a better ecm with faster processor and more tuning abilities?
Thread Starter
Joined: Nov 2010
Posts: 808
Likes: 62
From: Toronto Ontario Canada
Ok..so seems like the issue is fixed for now.After trying 3 rebuilt alts.,replaced the starter since it was over 5 yrs old and had a fair amount of corrosion build up,tried 2 new belts.I paid the extra cash and bought a brand new gm truck alt and all seems well now. Dont know why or how but makes me feel relieved so far.I drove the car fairly hard and it held a solid 13.5v across the board.Plus the new starter cranks soo much more effortlessly than the stock ls1 unit did.
So i guess it pays to use quality gm parts sometimes with a warranty.
So i guess it pays to use quality gm parts sometimes with a warranty.
I'm wondering what the "exciter" wire was doing when
the output voltage dropped, and whether there is some
kind of load-shedding or protection scheme at play like
how the A/C is cut out at higher RPMs by the PCM.
the output voltage dropped, and whether there is some
kind of load-shedding or protection scheme at play like
how the A/C is cut out at higher RPMs by the PCM.
Thread Starter
Joined: Nov 2010
Posts: 808
Likes: 62
From: Toronto Ontario Canada
I dont know if there are any settings in the tune that can adjust the exciter wire.When I get my new frost ecm and strap the car down on the dyno ill see if the tuner can adjust anything if need be.