Crappy tach readings

[THOR]

I have heard that pontiac tachs can give inaccurate readings but I'm wondering....
I just now finally got a replacement tach to replace the rally cluster factory tach which was dead. New tach reads about 1800 rpm at idle. Then independtly double checked idle with a dwell/tach unit and it's reading fine...about 850 rpm at idle. Now, I could understand/live with 100 rpm or so but this difference of about 1000 rpms ain't cutting it. *???What gives? I got the unit at a local muscle car parts place after not being able to get one outta Ames for about 4 months

[theJUDGE]

Bruce,
Yikes.
I would say the replacement tach you got has issues.
In terms of factory tachs I have only used Hood Tachs & they have always tested out as being accurate.
The aftermarket tachs I have used (Autometer) have always been fantastic, but I understand you are trying to keep a factory look.
Again, I would say that new tach needs to come back out. :-/

[THOR]

Dang! I've been dying to have a working tach and now this happens. I was wondering if having a new stealthy pertronix ignition and a new flamethrower coil had anything to do with the inaccurate info. The new ignition was put in at the same time as the tach. Anybody have these kinds of problems?

[theJUDGE]

Bruce,
I have a Crane box, Pertronix, & Blaster coil on the 68.
I have a Pertronix on Da Judge, & "s'all guud". Both Tachs are dialed in.
Where did you get this tach again & did you use the factory wiring supply?

[THOR]

Hi Mark,
Got the tach at Evergreen Muscle car. Nobody else could ship one, every source I tried (Ames, Classic, etc) had 'em on backorder for quite some time last spring and summer.
Have new info now since my last post The tach comes from the same company that supplies just about all the aftermarket Pontiac parts houses. Spoke to the manufacturer.
They said that the tach is designed to take the factory signal and that in some cases when one is doing an electronic ignition changeover, some sort of filter is supposed to go on the wiring. I'm now trying to get info from Pertronix on this.
But since Milt did this, I'm going through him and am awaiting a reply. But dang, sounds like you've got the same thing on your '68 and you didn't need some sort of filter! I used factory wiring. Thanks man!

[THOR]

So, if anybody was curious...I checked with Pertronix on this.
They said that in some cases a pair of diodes need to be splice inline between coil and tach. They gave me radio shack part#s, told me which direction the stripes on the diodes go and bid me adieu. I will let you know how it goes when I get around to it. :P

[theJUDGE]

Hey Bruce,
just for the heck of it, take your engine dwell/tach unit & hook into the wire that that normally would feed your dash tach.
See if the proposed problem would also effect the reading of the dwell/tach when tapped into the wiring harness (alligator clip), instead of clipping it straight to the (-) side of the coil..
I still suspect that Evergreen Musclecar Tach. :-/

All the diode issues I have run into have to do with ignition run on at shut off, which mostly is affected using an aftermarket ignition box such as MSD or Crane.

[Icelander]

Having absolutely no experience with this type of tach I as the following question:

Is the tach switchable between 4, 6, and 8 cylinder operation?

Again, I don't know about these tachs, so I'm just asking the obvious questions.

Good Luck!
Kendall

[THOR]

Hi Mark and Kendall
Ugh :-/
A man needs to know what's up with his vitals, you know?
Mark...Thanks for these suggestions
Kendall, this tach is a reproduction of the PMD rally cluster tach so I don't believe there are any 6 or 4 cylinder applications for it, thanks for suggesting
Neat about this is I've located the actual man who
builds these tachs for almost all of the aftermarket parts houses, so if the tach has gone south on me, he will replace it or calibrate it for no cost. It's a pain to pull it out again though. I will stop whining now.