odd elctrical prob, i think

[project89]

alrigh theres the deal,last week my tach started to jump up and down like 500 rpms,even though u could clearly hear the motor stay at the same rpm.ive had this happen before and it was due to arcing plug wires,so that the first thing i check.nope there all good,so i pull the plugs to see if any are fouled,nope all a nice tan/brown color just like they should be.so i pull the dizzy cap(oh boy)well the rotor had cracked and was running off center causing the rotor to rub the top of the cap,so i clean out all the debris,and drop in a new cap and rotor,put it back together and its still jumping around.i checked the wires to the dizzy/coil/stock tach/and my aftermarket tach.there all good.the tachs run off seperate leads,and they both bounce the same amount.the mnotor dosent miss at all.the only thing it does is when i take off or am driving and the rpms go above 3 grand the tach starts wildly bouncing to almost 7,000 triping my revlimiter making the car backfire threw the ex.

im out of ideas on things to check.u guys have any idea on what else could screw up the tach signal/make the tach bouonce around like that,that i could have missed

[V]

ignition module or est wire grounding out

[project89]

thnx man i forgot all about the est wire,hmm the ign mod is about a year old(ill have to dbl check that though).
if the est grounded out wouldnt the wire melt? since it is a + wire

[V]

no it wouldnt melt, just last week i had the same odd issue with the tach, jumping mainly at 3k+. the computer threw a code 42, for ignition module, so i changed it, still no good, i then swapped in a diffeent dizzy, still no good, then i used a flowchart for troubleshooting code 42 and turned out to be the est wire grounding out. I had used a small piece of a coat hanger to hold a bunch of wires together about the height of thew valve cover on the pass side, but the heat apparently made the wire melt into the est wire, and when running the whole wire bunch would move around and the est wire would ground out through the coat hanger wire into the back lip of the valve cover, it was such a freak thing, but i pulled the hanger wire off and taped it up and its 100% fine now. Maybe not the exact thign in your situation but worth checking out.

chekc this out, the part that may help you is where it says probe the tan/black wire with a test light and 12v. thatll show if theres a short. This is for a 86-89 tpi car but wire colors were the same for my 90-92 setup just different terminal locations, but the white wire and tan/black should be found on yours as well.
http://www.chevythunder.com/est_code_42.htm

[project89]

thnx man,mine does it continously but still no codes poped up,but its something i will deff check.if thats nto the problem im just gonna swap out the whole distributor,im thinking maybe the plastic debris from the cap could have ****ed something up inside it

[BonzoHansen]

Perhaps a pickup coil.

[project89]

est wire is fine/coil checked out ok,im still waiting to get my new dizzy,im just gonna swap out my whole old dizzy with a new one,and hope thta fixes it

[BonzoHansen]

Notice I said pickup coil and not coil. You might not have seen that. I had one go bad and it did really weird stuff.

[project89]

Originally Posted by BonzoHansen

Notice I said pickup coil and not coil. You might not have seen that. I had one go bad and it did really weird stuff.

yeah i know its getting replaced with the new dizzy,this ones 18 years old so im just gonna get a new one