Fire in the Hole! But wait, no tach...

[Dangerous Doug]

Bob,
Fired up my Cobra for the first time on Saturday. What a thrill. Nice and loud with straight headers. The neighbors love me.

I noticed that the tach wasn't registering. I have an MDS distributor, but no MSD box. Using the ballast resistor. Meticulously wired both under the hood and under the dash.

Where do I start? I have a multimeter that picks up frequency. Should I check for pulse frequency at the coil and then at the tach? Is there something about my MSD distributor that isn't coordinating with the SW tach?

Doug

[trularin]

Distributor to coil should have what you are looking for. Then you need to make sure you have power and ground ( a good ground ) to the tach unit.

If you have a scope, place a prob to that connection and see if you get a waveform.

Hope this will get you started.

BTW, you might want to use shielded cable for your tach lead to the cockpit.

[Excaliber]

My tach, straight MSD dist, no box, was acting eratic. It was a loose ground wire from the dist, a black wire.

[Dangerous Doug]

Thanks guys. I'll start poking around.

[fkemmerer]

Originally Posted by Dangerous Doug

Bob,
Fired up my Cobra for the first time on Saturday. What a thrill. Nice and loud with straight headers. The neighbors love me.

I noticed that the tach wasn't registering. I have an MDS distributor, but no MSD box. Using the ballast resistor. Meticulously wired both under the hood and under the dash.

Where do I start? I have a multimeter that picks up frequency. Should I check for pulse frequency at the coil and then at the tach? Is there something about my MSD distributor that isn't coordinating with the SW tach?

Doug

Doug,

Check your ground on the tach first. I had a problem with my ERA Tach not working when the dash lights were on due to a marginal ground. Worked OK with the dash lights off. I ran a ground from the metal dash brace directly to the tach and the problems cleared right up. Hope this helps.

- Fred

[Dangerous Doug]

Good place to start. Thx. DD

[Dangerous Doug]

Pulled my tach out to check my connections. There are five connections on the back (and bracket) of the SW tach:

POS
NEG
SIG
Light
Ground is a tab on the bracket side

SIG goes to the (-) coil
POS goes to a fused power source
Light, well, that's the light connection
Ground is a shared ground wire from the fuel gauge
NEG ---> This needs to be connected to ground.

I had left NEG floating, having assumed the case and bracket provide adequate ground. They probably do, but NEG cannot be left floating. Tie it to ground.

Tach works fine.