comp94

upon install ing an msd ignition on my 94 i found that asr light stays on and that my tach needle drops below zero after 4,000 rpm. i read some where that this is due to inline tach filter and that buy by passing the inline tach filter would correct the problem. i was just wondering if any one knew how to do this and if so please help me out. :chevy

drive it

Not positive on your 94...but on earlier years the tach filter is bolted onto the back of the drivers side head-just cut and splice around it. Anyone confirm that 94 is in the same place?
And yes it can be "fun" to get at....

Rick93Z07

Unless you're running a supercharger, you are making a big mistake installing a high performance ignition on the marginal Optispark distributor. The rotor/cap dielectric will break down and the setup will be far less reliable. There is nothing to be gained.

Nathan Plemons

Just follow the wiring harness back from the coil, an inch or so back you'll find a white wire going into a small black box. The black box is the tach filter, just bypass it by splicing around it.

Nobody has ever proven that the MSD hurts the reliability of the opti. It is no worse on the opti than it is on any other distributor. I'll agree that there isn't much to be gained on a stock ignition but you won't be hurting anything.

The distributor portion of the opti is very sound, the optical part is what usually fails and is not at all affected by the MSD. Yes it will probably shorten the life of the opti, but the distributor will still probably outlast the optical part.

Rick93Z07

Huh?

The dielectric strength of the cap and rotor are compromised due to moisture & the bearings rust to failure. Higher voltages stress insulation & induce high voltage breakdown & carbon tracking. High voltage breakdown is the primary cause of Optispark failure, not optical sensing circuit failure. The misfire everyone experiences after washing their motor or losing a water pump or hose is due to high voltage breakdown in the cap assy. The low voltage optical sensor ckt is waterproof and can be submerged.

SHiNoBi LT4

is this true? if so im going to remove my crane hi-6. I never knew it could do this!!!

BlackonBlack

Comp94 - I just put a MSD on my 94 and my Service ASR light is on. And the tach drops down to zero when full throttle is applied.

Does your Service ASR light come on or just the ASR Active light?

Nathan Plemons

That's exactly what it does. You'll probably notice that it acts up when it passes 3000 rpm's. You'll need to bypass the tach filter and all of those problems should go away.

BlackonBlack

Nathan - I know you were talking about the tach problems, but when I bypass the tach filter will that cause the Service ASR light to go back off? Or is that a whold different situation?


[Modified by BlackonBlack, 11:29 PM 3/20/2003]

Nathan Plemons

Right. ASR monitors many things, RPM being one of them. If it's getting incorrect information because of the MSD box, bypassing the tach filter will take care of it.

I had the same thing happen when I installed my MSD digital 6. I knew that I would have to bypass the fitler but I fired it up without doing so anyway. It started up just fine but then acted real stupid when I rev'd it up. All was good until around 3000 rpm's when the tach would fall back to zero and the service ASR light came on. I don't remember exactly now but I'm thinking it might have also set a SES light and a Service ABS light as well. In any case I bypassed the tach filter and have never had another problem out of it.

BlackonBlack

Nathan - In addition to my service ASR light being on, my ASR does not work properly. Did your ASR stop working properly when you did your MSD?

comp94

blackonblack,
i had the same problems as you the tach jumping around and the service asr light always on. in addition the asr would not work at all. elimating the tach filter will cure all of the problems that you are describing. to do this you need find the tach filter, it is a little black box with two white wires and a black wire coming from it, that can be found on the front of your motor in between the water pump and coil. follow the wiring back from the coil and youll see it. next splice the two white wires together and leave the black one alone. then just secure the black box so that it dosn't flopp around and you should be good to go.

rick93z07
also in my dealings with the lt-1 (both f-bodies and y-bodies) the optical unit has been known to crap the bed. i can tell you this from experience . i have tried to replace just the cap and rotor a couple of times on a couple of cars and i still was getting no spark. what happens is that the water corrodes the optical unit over time and eventually it will fail. if you think about it ...why would they make this unit water proof when it is going to be completely enclosed ? the reason it did fail is because the engineers probally thought that the opti spark housing was going to be water proof the didn't anticpater it leaking. also how can it cant be water proof? the optical pick up that reads the crank position is exposed so how can it be water proof? it might be water resistent but i dont think it is water proof. But in retro spec i also belive that your high voltage theroy contributes a good deal to the dreded "OPTISPARK FAILURE SYNDROME". also i have come too relize that the newer vented style optispark cap and rotor off a 95 will work on the older style optispark.

my 2cents