Asking this again, why is my tach all of a sudden showing double rpms

[sputnik 440]

My tach in my '67 Satellite is hooked up to a MSD ignition system. I had the stock "bullet" tach on the console modified to operate off a digital set up. I has run perfectly for the past 3 years. While driving back from town, it started reading twice what it should be. Does anyone have a idea of how I should troubleshoot? Is it likely the MSD, or the tach? How should I isolate the problem? Fix the problem? I did just add two electric fans with a temp controller. It required a lot of wiring for the relays and controller, but, the tach ran fine with all the rewiring I had to do......this included putting in a new fuse block. Again, everything functioned correctly after the install and rewiring.

 

[Oldreddodge]

It should have a switch on the back.for banger, six banger and V 8, maybe you bumped it to six banger

 

[Don Frelier]

You mean 4 banger I believe.

 

[Oldreddodge]

WHAT ? The

 

[dadsbee]

for, fore, four.. don't become KD !

 

[Don Frelier]

I wasn't commenting on spelling or grammar.
An 8 cylinder would fire at twice the frequency as a 4 cylinder at the same RPM.
So if the tach was set to read/display a 4 cylinder and it was connected to an 8 cylinder it would read 2 times the correct reading.

 

[dadsbee]

Yah... correct.. I saw where he typed FOR.. so guess I became KD!! LMAO

 

[greycharger]

So it worked, then malfunctioned all of a sudden. Either something burned up or became disconnected. Sun tachs have a resistor in them and you get 4,6, or 8 by the way you connect that Resistor. I don't know how resistors fail, but that is a possibility.
I would double check all connections, then reach out to whomever redesigned the Tach and if they have never seen this sort of issue, then MSD. I imagaine there is a wealth of knowledge about the MSD on this site, but only the shop that redesigned the Tach knows what they do.

 

[kiwigtx]

It can also depend on how the cables are routed - if the Tach wire is closely following the coil supplies, an induced current situation could occur. Are you running a Tach Adapter - or direct from the MSD Tach output to the tach?

 

[sputnik 440]

I am running it direct. Did not need the adapter with the modifications done to the tach internals. I will check the cable route, that could be of concern. I will check the wiring and switch for 4,6,8 Thanks to all.

 

[Billccm]

Do you have an oscilloscope? It's easy to confirm the correct MSD tach signal.

http://www.average-guys-car-restora...ing-tach-for-68-road-runner-project-beepbeep/

 

[sputnik 440]

Bill, an oscilloscope is one tool I do not have, nor have any access too. If I cannot locate the cause, then I may have to find someone that uses one and have them do the diagnostics. Thanks for the input.