Tachometer Rallye style 74 Roadrunner

[flamesoldier]

I bought a 74 RR restored except the dash items. My tachometer is bouncing back and forth on its own from 3,000 rpm to past 7,000. It does this on its own at idle. It sweeps left to 3000 and back to past 7000 as a constant rhythm. Fuses are good, sending wire is good, could it be a magnetic pick up distributor with hi performance coil causing it since the technology for that tac was built for points? Im completely lost and thinking of just buying a new classic industries replacement tach hut i dont want to get rid of original if its a simple fix.

 

[BeatersRus]

by 74,pretty much everything was Electronic ignition,so id be more apt to ask if theres a resistor missing in your application.
try asking Halifaxhops here on the forum.
( probably spelled that wrong,sorry )

 

[Bee1971]

Can I ask what ignition coil are you running

 

[flamesoldier]

Can I ask what ignition coil are you running

Accel 45,000

 

[Bee1971]

That really doesn’t tell me anything

Do you know the primary resistance on the positive side of that coil

Not saying the coil is your issue , but anytime I ran a .7 or a .4 OHM resistance coil like a MSD Blaster or equivalent, even with a ballast resistor , my factory tach did the same thing

Even with an Autometer tachometer , was bouncing all over the place

When I switched over to say a Standard or Pertronix 1.4 - 1.5 OHM resistance coil , the issue went away

Autometer wanted me to run a separate 10k ohm, 1/2 watt resistor in the signal wire itself

This was with a Factory type Chrysler ECU / Ignition Module mounted on the firewall
Magnetic pickup Distributor

 

[Bee1971]

One more thing , I should assume you’re running a factory or aftermarket Chrysler ECU / Ignition Module ?

Not an aftermarket MSD Digital

Thanx Scott

I should have asked that first

 

[flamesoldier]

running factory Chrysler ecu module and magnetic pickup distributor, my coil ohms1.4

 

[Bee1971]

Sounds good

I can’t remember who sells a replacement updated circuit board for those factory tachs

Worse case , keep your factory original tach if you decide to get it rebuilt - Damn thing in todays market is worth its weight in gold , dead serious

In my eyes the Reproduction Tachs 71 - 74 B Body look like **** with the 8,000 RPM face plate

And they have been out of stock for the better part of ? Or for sure a year or so

I have not looked lately

 

[Bee1971]

Peace - Click on link

MOPAR Products

 

[jimmy440]

My 70 Charger Tic Tac Tach does something similar but goes to 2000 RPM the drops to 0. Does this at every start; however, if I keep revs under 2000 Rpm, it works perfectly. Everything is factory as far as I can tell. Had Tach repaired by D&M, But they are saying problem is in the car. The car from factory did not come with TicTocTach, this was an upgrade....Any suggestions? Do I need a special Driver for Tach, etc.?