Tach not working

[junior636]

Alright guys I give up. Fresh rewire of whole car and my tach, the easiest thing to wire won’t work. I disconnected from msd grey wire and hooked straight to negative side of coil, still nothing. It was an old tach so I bought a new one. Still nothing. I have power to tach constant. I thought it might be a ground issue. Added a heavy engine ground and checked ground from tach, nothing. When I hooked to msd box grey again I get movement from tach during cranking but then nothing once the car fires. First tach was an auto meter, second tach is an auto gage.

 

[Dana67Dart]

Where are you getting power for the tach? Almost sounds like the tach power is coming from the start circuit side of the ballast

 

[junior636]

Where are you getting power for the tach? Almost sounds like the tach power is coming from the start circuit side of the ballast

I could try a different power source but it’s the same power all the other gauges are off from. I also verified it has power while the car is running.

 

[MadScientistMat]

When MSD says the black wire is the only thing that should go to the coil negative terminal, they mean it - the coil wires will either not trigger or damage a tach. Probably not damage though, it's the orange wire that's really hot.

Is this a factory or aftermarket tach? If aftermarket, what kind?

 

[junior636]

It was an autometer tach and now an autogage tach. Both say you can go to negative side of coil so does msd. I had it hooked up to msd originally. I think the msd box is bad.

 

[Oldmanmopar]

Shift it when it stops pulling for now.

 

[Bewy]

It is an MSD...................what???
6AL box, MSD Ready to Run Dist, #836457 HE module....Other?

 

[Dave999]

orginal tacho and many aftermarket need high voltage spike to trigger. they connect to coil negative and expect 1 spark per event producing 1 300-400 volt trigger spike per event . some work ok with multispark some don't (much like a timing light)

one of the output wires for tacho drive on many ECUs and ignition boxes is a low voltage upto 12 volt square wave, it will only drive a tacho designed for a square wave signal at this low voltage.

some have another output wire which just replicates a filtered high voltage spike from the coil negative think of it as a somewhat cleaned up version of connecting to coil negative, to get round multispark problems...

obviously getting the right connection is key, an interface for a 9 volt square wave signal will not be happy being exposed to the 400+volt spikes you get out of the coil negative

vintage sun tachos used a converter box. it connected to coil negative and a bunch of wires from the box connected to the sun tacho... this is not that common these days and finding any parts to fix is a pain in the butt.

if you mess up connections and get 12 volt on the tacho signal wire in some cases the car won't start.... in others you can break your ignition box

Dave

 

[MadScientistMat]

If this is an MSD 6 or similar, the tach output is a 0-12 volt square wave. Some tachs are going to need a high voltage spike to trigger. If you have one of those, you'll need an adapter - MSD has one, there's also Amp EFI's AXM-100 and a few others.
If the ignition isn't a 6 series, what is it?

 

[67Dart273]

LISTEN UP re: coil and MSD.---- MSD works COMPLETELY DIFFERENT from most other ignitions. With MSD, the coil negative is GROUNDED. The MSD sends high power PULSES to the coil POSITIVE, but most / all? tachs cannot trigger on that / those pluses plus MSD is just exactly what it says....MULTIPLE SPARK Discharge. That means it would confuse the tach if it WAS able to trigger.

What I would do: Remove the tach and temp wire it up to a car with a distributor and conventional ignition to see if it works. If it does, contact MSD. I BELIEVE some tachs need a separate adapter to work.

Otherwise if it worked earlier and not now, on your installation, either you screwed up the tach or you have something mis-wired

 

[junior636]

It’s an msd 6al, I’m going to switch out the msd box. It’s the only thing I haven’t ruled out.

 

[Dave999]

MSD stuff and tachos Page 6

https://documents.holley.com/5df77ecd9ef0fe1ef31596208efff64fe8293e63.pdf

 

[junior636]

The msd box was bad not my wiring……

 

[MOPAROFFICIAL]

Shift it when it stops pulling for now.

Exactly. The tach is mostly for reference

 

[junior636]

Exactly. The tach is mostly for reference

I thought this was the technical section. Thanks for your guys highly technical advice….