Hello everyone,

I recently purchased a '94 6-speed to add to my '89 auto that I currently have (and will probably soon be selling). I'm trying to work out a few problems I've been having. First off, I had an intake manifold leak which I just got done fixing up, but the following problems remain which I am not having any luck diagnosing.

1.) At high rpm's, there is a vibration. It is not driveshaft or u-joint related because it occurs even when the car is parked and just revving. It is the same with the clutch engaged or disengaged as well. Also, I have removed the serpentine belt to confirm that the vibration isn't caused by any of the accessories and it is still there. My best guesses till now are the harmonic balancer, misfire, or unbalanced flywheel. This is my first experience with a manual transmission, so I don't know how likely the flywheel problem is.

2.) When revving my car at high rpm's my ABS light sometimes comes on which makes absolutely no sense. The only thing that I can think of is on my '89 corvette, the ABS light was the first thing to come on when my voltage was low (~9 volts - this is a story all in itself, I went through alternators left and right with the competition sound system I had installed). This got me thinking that I was probably losing voltage at high rpm's for some reason (even though the gague doesn't show it); the only thing I could think of is there is a loose ground somewhere that is disconnecting for a split second when the engine torques. Other than that, I'm clueless...

Any info would be greatly appreciated! Thanks for your help guys!!!

 

ttt

 

Re:vibration- A really, really long time ago, I had a 63 Ford with a similar problem. After what seemd like months, it turned out to be an slightly unbalanced clutch pressure plate.
Re:voltage - On my 91, the engine would cut out at high RPM, alt light and ABS light would come on. The alternator finally died a few months ago and when I replaced it the problem went away. You might check to see of the alt is outputting proper current at hi RPM's.
Good luck

 

Thank you sub, i just took out the battery and tray, and it looks like there was a leaking battery at some time because everything is rusted and corroded. I just cleaned up the ground that was in that area caked with rust, so hopefully i am getting closer to a resolution. I have thought of replacing the alternator and battery as well, but wanted to get some more confirmation before dropping the $$.

 

Possible vibration causes:
for some reason known only to GM the LT1/4 motor balancer hubs are not keyed. If for any reason your balancer has been removed in the past it maybe out of position or it may have slipped/rotated on the crank nose by itself from hard driving.... to check proper position..put the engine at TDC...that is the number one pistin should be all the way up..use a positive stop tool or simply pull the spark plug put your finger over the whole and have a freind "bump" the motor over until your finger is displaced by the compressed air as the piston comes up to the top...now that you have TDC...look at the balancer there should be a knotch/arrow/tit on the hub that is pointing to 12 o'clock position..if not the balancer has moved and hence your vibration issue. A few fixes ..buy a keyed balancer hub from Thunder racing (http://thunderracing.com/catalog/?ac...vid=4&pcid=239) or if you can adequately mark yours have a machine shop broach a key way in or get the expensive fix; a whol;e new hub and balancer setup from ATI ..if it is at 12 o'clock your pressure plate and flywheel assembly maybe off balance a little... a good trannyshop or if you do your own work a good machineshop can balance it for you
If you have found that the altenator is dropping off at hi revs..check the tensioner pulley and spring it maybe worn and causing the belt to skip off the altenator pulley as the revs move up... good luck

 

UPDATE:

I have replaced the battery (needed it anyway) and cleaned up a ground that was on the frame rail beneath it. Just for the hell of it, I tried starting it and revving in a few times. The ABS light is no longer coming on, but the ASR light is. I believe it is actually activating also, because when the light comes on, there is a slight dip in voltage and I can hear some kind of motor sound coming from the engine compartment ( I am assuming it is the ASR motor). Also, it only happes in the 4500-5000 rpm range when I "release the accelerator". I can bring it up to this rpm range gradually without a problem, but when I let go of the throttle is when I get the ASR Active light and the slight dip in voltage. Any ideas????

 

ttt

 

Now that the ABS light is not coming on anymore after cleaning up the ground and replacing the battery, I'm starting to think that the ASR Active light coming on above 4500-5000 rpm is normal operation. Does anybody feel like wasting $1 of gas warming up their 6-spd LT1 and revving up to this level to see if the light comes on. I don't see the purpose of it if it is normal, but I'm trying to be optomistic.... Thanks, and I'll be glad to mail out $2 for your gas and wear and tear!

 

One final thing, when looking closely at the tach, the light comes on at exactly 4200 rpm every time... and the tach dips ever so slightly when it comes on...

 

ttt

 

last time...ttt