Digital Tachometer Issues

[hoeser]

I recently installed a pair of Prosportgauges 4 digit Digital tachometers in my 1969 Chris Craft Commander 31' - The boat is equipped with twin 327Q engines. I connected the tachs to a switch and the common groundplate below the console, when the switches are on and the engines are off, they light up just fine. I connected the tach signal wire (green) to the negative side of the ignition coil... when I turn over the engines (as they are turning) it registers about 160-200rpm which seems normal too me but as soon as the engines start the tachs go all whacky, flashing erraticly and registering random numbers... can anyone enlighten me on whats going on here or if I have done something wrong?

Any help would be greatly appreciated.

 

[calwldlif]

Re: Digital Tachometer Issues

Thats curious.
Did you try the existing tach
leads to your analog tachs?
I ASSuME that you have the new tachs
setup right?(8cyl)
The wire you ran to the neg side coil,
what gage and sheathin?
Short of being tachs incompatable
with points?
Test and lets us know

 

[hoeser]

Re: Digital Tachometer Issues

The analog tachs are mechanical (cable driven) and are no longer connected. The new tachs are configured in 8cyl mode. The wire run to the negative side of the coil is 14gauge stranded copper with a green jacket, it's automotive grade. The tachs are "supposed" to be compatible.

I was thinking last night, could the fact that the tachs are using the common ground for the accessories (which are on a SEPERATE battery from the two motors) cause this issue? I may try grounding it to the same battery the engine is on. I somehow think I'm screwed though.

 

[calwldlif]

Re: Digital Tachometer Issues

Mechanical tachs?
A Vette ? Kewl
How long was the cable?

I won't be surprised if using a different bat
was causing the prob.
A good ground is surely needed.

I am sure you'll get it right.

 

[hoeser]

Re: Digital Tachometer Issues

Yep, ancient cable driven mechanical tachs... think "speedometer from your dirtbike" type of rig. I'm going to try grounding to the battery negative but any other info would be cool. I really don't get why it works fine when the engine is turning but the thing isn't started yet. The dead mechanical cables are roughly 6 to 8 feet in length which is about the same as the wire I'm running to the new tachs.

 

[ron7000]

Re: Digital Tachometer Issues

with these new tachs, if they register correctly at around 200 rpm when cranking the motor then they most likely are compatible and working properly, and wired properly. If not they wouldn't register to begin with, your battery grounds should be connected together already. What comes to mind is RF noise, maybe you have a leaky spark plug wire or alternator noise affecting the gauge, and that would only happen once the engine is running. Also make sure you have resistor spark plugs, and RFI suppressing spark plug wires. You could also try disconnecting the alternator wires and run the engine for a minute, but make sure you insulate the leads so they don't ground out and arc. Your battery won't charge but that's ok, you're only running it for 30 seconds. If that fixes it then you know where to look.

 

[hoeser]

Re: Digital Tachometer Issues

I see where you're headed with that and I'll have to examine a bit. I'd be surprised if that was the case... all that equipment is quite a distance from the coil. These are large engines and the coil is mounted on the far corner of the alternator and the plug wires are a good 2 or 3 feet away from the coil wire. One thing I did notice is that the left engine has a different (and seemingly newer) coil on it. The left engine once started just basically flashes the gauge and gets it to go all wonky. The right engine does the same however it actually reports random numbers instead of just 0 when running. Bizzare. I still have a fair amount of loose wiring around as I haven't routed and tied it down yet and I should be able to move the wire close to the potential RF sources and see if the problem worsens or gets better. Thanks for the info.

 

[calwldlif]

Re: Digital Tachometer Issues

If the tachs are easy to unmount,
take them to the engine.
run a short good wire to the neg
side of coil and other grounded to the motor.
If they still read wrong they are bad.

 

[Silvertip]

Re: Digital Tachometer Issues

Wait a minute. If the tach ground is wired to a battery different than the engine It can't see ground unless its getting a ground-loop through some other instruments. Put the tach ground on the engine battery and all should be well. The random numbers are very likely due to interference (ignition, alternator, and other sources) because of the "no ground" situation.

 

[hoeser]

Re: Digital Tachometer Issues

YAR! I hope that be the case. I'm going to extend the ground down to the battery when I get to the boat tonight and see if that fixes my issue... if it does I'm gonna need some more wire