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Tachometer Jumping

El.Padrino

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Hi all,

I have a 68 standard gauge dash with a tachometer. Full electronic MSD ignition setup. Hooked up the tach per MSD instructions.

The tach seems like a reproduction (maybe OER). I can spot the green printed circuit board. So it should NOT need a tach adapter at all.

Without it shows a pretty accurate reading at idle and when I stomp on the gas. When I drive at a constant speed and rpm the needle jumps all over left to right.
Any ideas what to do?

I tried the tach adapter PN 8920 and with it's not moving at all.

IMG_8205.png
 
Try a known good tachometer. I had my 69 charger tach redone and I still had to use a tach adapter. Even though they said all my tach internals were VDO and capable. My bets if you have it wired right the tach is junk.
 
Got this answer from the Holley/MSD tech support:

"We have found some inconsistencies with the OER type tachs, they seem to be really sensitive to the signal. We've seen where they operate normally, some do not function at all, and others that seem to work as you stated. Sometimes the 8920 tach adapter helps and sometimes it doesn't.
The MSD Ignition box provides a precise 12V-square-wave signal."

So seems it's the reproduction tach..
 
'Full electronic MSD ign'. What does that mean? Are you using the MSD 'box', such as the 6A or 6AL ?
 
Is you tach getting it's signal input from the coil or the MSD?
The MSD signal is very stable and easy to read by any modern tachs. I'll post a link to a similar tach troubleshooting thread with videos.
 
These videos will help you understand the MSD signal for the tach.

Post in thread 'Tach issue' Tach issue
 
Tach signal is coming from the MSD.

Seems like there's no real solution: Either have a factorey OE tach and use the MSD tach adapter which will result in weird inaccurate rpm readings.
Or have the reproduced tach that can handle the MSD signal but you don't know before if your particular rep. tach will handle it correct (due to inconsistency in production).

Probably the best way to use an aftermarket tach
 
Probably the best way to use an aftermarket tach
Not everyone wants their interior to look that way. Some want the gauges in the instrument panel where they look original or at least, mostly stock.
I don't like the add-on tach or gauges that so many of us use. All the A body cars I've owned have been the non performance models that never got a tach or oil pressure gauge so cars like those, you have no choice but to use something like that. The 68-70 B bodies had a more performance oriented instrument panel.
I had a bouncing tach when I tried using an FBO ignition ECU. Some members here suggested that I use an inline diode in the line to the coil. For me, this didn't help so I pulled the FBO unit and went back to the MP based electronic box. I have an aftermarket Dakota Digital cluster, but maybe the bouncy tach issue the OP is having could be rectified with the diode?
 
Not everyone wants their interior to look that way. Some want the gauges in the instrument panel where they look original or at least, mostly stock.
I don't like the add-on tach or gauges that so many of us use. All the A body cars I've owned have been the non performance models that never got a tach or oil pressure gauge so cars like those, you have no choice but to use something like that. The 68-70 B bodies had a more performance oriented instrument panel.
I had a bouncing tach when I tried using an FBO ignition ECU. Some members here suggested that I use an inline diode in the line to the coil. For me, this didn't help so I pulled the FBO unit and went back to the MP based electronic box. I have an aftermarket Dakota Digital cluster, but maybe the bouncy tach issue the OP is having could be rectified with the diode?

I absolutely agree with you. I prefer the stock look without add-on gauges too. My last option will be an aftermarket tach. The idea with the diode sounds good, gonna try this. Will let you know
 
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