I noticed today that when I rev my motor on the road to over 4000 the tach wont stay up there and jumps around below 4000. the motor keeps revving fine but the tach just bugs out until the tranny shifts and the revs come back down.

what would cause this??

this is with the stock ignition hooked up by the way and I havent tried it with the accel hooked up yet.

thanks

 

ok I drove it all day today with the 300+ ignition plugged in and the tach was fine. what gives???

what would make the tach do something like this?

 

Wow -- this is a tough problem.

I have done countless mods including going over and rerouting much of the engine wiring harness and i still have this problem.

notice that my original post was 3+ years ago.

I read every post with "TACH" in the title since this post and cant find any ideas except for once vader suggested that bad grounds at the dissy can cause this.

looks like I am looking for bad grounds with my multimeter now.

 

I had the same problem when I was using an aftermarket "smart-box" for the ignition. Installed a tach filter, and the problem went away.

Where does your tach wire go? To the coil? Or the tach "outlet" on the smart box? (mine was problematic either way......)

 

white tach wire goes right to the coil.

And I actually get engine stumbling when the tach misbehaves.

 

You may want to try and change the pick up coil on your distributor. That may be your problem.

 

That wouldn't be the tach misbehaving then, it would be your motor, the tach would then apparently be working fine.

 

mine was having similar problem... I thought it might be the ignition module, but NO... I t was the factory pick-up coil... it was a little rusty and also 14 years old!... it fixed the problem... the AC Delco parts was like $14.00 at the auto parts store.

 

yeah man just rebuild your dizzy or did you get a new one? it like $100 in parts and your tach will be fine, then watch your speedo. Mine under hard acceleration will stop around sixty the tach will say 5500 in 3rd gear i will shift and it jumps into the 90's

 

entire dissy was replaced along with tach (dash cluster), coil and wires.

Only thing that hasnt been looked at is the wiring between the components in the harness. So I will look there.

 

the problem seems to have morphed into the following:

sometimes when I start the car it runs ok until I hit the gas and it wants to die. as long as I hold my foot on the gas it lopes very poorly below 500 rpm. when I let my foot off the gas it idles normally.

I then turn the car off and leave it and start it again and it runs fine.

 

Kinda Strange that I came across this because mine was doing the same thing not very long ago. Really started to get frustrated because I could not fix the problem, and it happened worse sometimes than others. I replaced the Pick-up coil and Ignition Module, but to no avail. But I finally found what solved it. Might help you too.

Without the right (or worn-out) wiring around the Dizzy and Coil, especially with a high-v coil, the high RFI from the coil can cause false triggering and all kinds of stuff. (Kinda like putting a magnet by your t.v, don't do it)

I found a wiring harness in Summit Online that has magnetically shielded wires, etc. to solve that. I installed it and the problem was fixed! I'm not sure if summit still carries the part, you could probably talk to a techie on the phone and find out a lot faster than surfing their site, but I'll look into it if you want.

 

Oops, just paid more attention to the set-up you have. If you have the coil-in-cap...then I'm stupid and disreguard that post. lol.

 

no -- I have separate coil.

Thanks for the info on the mag interference -- that sounds like a good lead I will check out.

 

Problem solved -- Bad MAF and Ignition Switch.